Gisela & Joe Noci's Posts (23)

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We are designing a new Fixed Wing plane, all composite again, but this time with no landing gear, using a single peg bungy launch, with a parachute recovery. As w are fitting this Gimbal to this plane, we want the nose clear of motors and props. I wish to use a tail boom mounted co-axial prop, but do not approve of the possible options around - noisy and short lived gears, or Gimler belt and toothed gears, etc.All a bit complex, and awkward to locate and then house in the fuselage.

The very Nice Gimbal - Day and Thermal IR Night cameras, 640x480 Pixels. Fully Gyro Stablised. Serial port to drive and control.

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So, I decided to modify a few Brushless motors and make then truly co-axial, with integrated folding prop mount, really beefy bearings, and a nice through hole for tail servo leads and lighting.

I used two HACKER Motors, A40 series, 50amp constant, one of 410kv to spind a 17x15 prop, and one of 610kv to spin a 16x8 prop, and an AXI 4120 series, also 50 amp, of 465kv for a 17x13 prop. Flight test will show which of these is best for our application.

How did I do it? Well, pictures are the best, but essentially I cut of the outrunner bell ends, machine out the inner Aluminium ball race holder, and ream out the hole to 10mm. Then I make a new bell housing, with double beefy bearings at one end, and a folding prop holder. All bonded together with locktite.  

The end results with regard to weights -

The weights are done using a standard motor, fitted with prop holder, fuselage mounting, etc, and then compared to the final weight of a modified motor.

Motor                              Standard weight             Modified weight

AXI-4120-20 465kv                  345g                            342g

Hacker A40-12S-V2-610kv      240g                             243g

Hacker A40-12S-V2-410kv      317g                             310g

As seen, there is no penalty in weight for this mod, even with a 110mm long 10mm diameter stainless steel tube through the motor centers...Through this tube is passed all the wiring for fuse to tail connections. The tube is then fitted to a holder on the fuselage, and the tubular tail boom ( actually an ALIGN 'copter tail boom - 20mm diameter) fits on the other end of the motor shaft, with a small aluminium bushing ( 4grams...)

The pictures:

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The Nampilot.....

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The Namibian Department of Civil Aviation Presented the first pass of the Unmanned Aircraft operations and requirements Draft Regulation on the 17th March 2016. 

The presentation was held at the Ministry in Windhoek, during most of the morning, and was to a full audience, invited by the DCA, as representative of the UAV manufacturing industry and commercial users in Namibia.

This presentation was the first of a group of meetings to take place over the next few months, massaging and refining the draft to a form acceptable to as many parties as possible.  

The ICAO Articles ( actually DOC-10019) form the basis of the minimum requirements, but DCA is shown to be  extremely open, willing and fully prepared to discuss and negotiate around the draft presented.  

In all , the draft presented is a totally enlightened, practical and realistic proposal.

Since these are drafts, not for public dissemination as yet, I am not going to give deep detail, but some basic elements which cut to the core are given briefly below. I have not detailed all the ruling elements associated with each , so please don't take things out of context here...there are many rules regarding where you may fly, where not, max heights, proximity to people, aircraft, buildings, etc....

This text is also not meant to give insight into the details as yet, but to present the fact that Namibia is now also on the regulatory map, and in a very positive way!

UAV's below 2kg may not be used for commercial gain, and as such need not be licenced, nor its operator.

This allows all the recreational user to play to hearts content. The group present did however feel that all these systems should be placed on a DCA National Register - simply to aid as a deterrent to the fun flyers, for, for example,  making their own movies at sports events, etc, while the commercial operators at the event are grounded due to these operators. ( there are a number of other reasons as well, but not now...) 

All UAVs above 2kg up to 50kg shall be licenced, as shall the pilot/operator.

There is some leniency allowed, but for commercial operators, this shall hold. The UAV shall have full documentation, user manual, operator manual, log book, maintenance manuals, Training manuals, as applicable. The supplier/mnfr shall execute the applications for license.

The operator shall be licensed according to the task - the license will limit operation to that function - eg, sea patrols, real estate photography, photogrammetry, etc. Operational flight areas shall also be defined in the application. For example, if the UAV is used for anti-poaching, only in a national park, over sparsely populated areas, not near a registered airfield, then the license will be very simple. In this case, the operator may possibly not need the UAV flight school training - the Mnfr/supplier training may suffice. He may also not need the VHF Coms license. Someone doing survey work in an urban area, near a registered airfield, etc, will require the full Monty.

Operators shall also be licensed per aircraft type - a license for a fixed wing aircraft shall not allow rotary wing or multirotor operation - a new license is required. Type conversion license are required between different types of similar craft.

All Accidents and Incidents are to be logged and reported to DCA. 

Foreigners visiting Namibia will have to show  verifiable licensed approvals from their home country before being permitted to bring in and operate any UAV in Namibia.

The draft is in great detail, around 90 pages, but has to be to most refreshing UAV regulatory document I have seen!

The group present at the presentation forms a committee that will now continue the discussions and negotiation with DCA in the coming months. I believe the outcome is going to benefit UAV's and their application in Namibia tremendously.

Well done DCA and well done Chris Gundu, Prime Lead at DCA on this grand and brilliant effort!

The Nampilot...

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We have just completed a flight test program of our MK-II SurVoyeur UAV System after having fitted the new Autopilot/IMU. This autopilot replaces the 32bit AVR based autopilot in our preceding UAV's, with a view to giving a big boost in performance, shorten IMU cycle times ( from 200Hz), and eliminating all the component obsolescence issues of the AVR based modules.

The autopilot is again a single module, but now contained all the video switching for up to 4 video sources, switchable from the ground station, all the DC-DC convertors for up to eight Medium size Digital servos, all the power supplies for Datalink and video transmitter modules, as well as all power conditioning for the EO-IR stabilized gimbal fitted to the MK-II system.

There are now 6 serial channels ( RS422/RS232/TTL configurable) , 11 PWM outputs, Analogue inputs for Laser Rangefinder. Ultrasonic Rangefinder, ESC/Motor Amp measurement, and for a two cylinder Gas engine CHT measurement ( using thermocouples).  External Air temp is also measured to do Static Pressure compensation for Baro Alt.

The module is also fitted the Measurement Specialities 24bit Static and Dynamic pressure sensors. In addition, a further 6 (!) digital static pressure sensors are fitted. 

These are :

MS5611  ( two fitted in different locations)

MPL3115

LPS331

BMP180

LPS25H

The reason for this was really to test the performance of each of these sensors, and as they cost very little, we decided to include them all on the final modules.  Since most actually all work quite well, we are implementing a redundancy selection scheme to select working sensors from failed ones, just because they are there...

The sensor we are most unhappy with is the 24bit Measurement Specialities device the MS4525HDR. It is so complex to manage, each sensor serial number is supplied with a special set of coefficients that are used in a complex set of computations to do the pressure/temperature computation. These coefficients have to be stored on-board the autopilot which means each software load has a specific module particular to that board. A config management mess...since the coefficients are not stored within the sensor by the manufacturer, but on a piece of paper supplied with the sensor. Nuts!

And then the sensor performance over the range 0 to 60deg C is not great - it varies by more than 5millibar ( 40meters!). So, on top of its complexity, we still have to do a 2nd order polynomial temp correction to get better than 0.5millbar variation over that temp range. It does not end there - it is also very susceptible to vibration - in the plot below, you can see the wild oscillation  ( blue line) as soon as the wheels start to roll, and how the pressure reading goes smooth again as the wheels lift off the ground : From vertical lines 2 up to 4. The graph is in pressure, so the graph shows reducing pressure as the plane climbs from vertical line 4. The oscillation is a peak to peak value of 0.9millibar - nearly 8 meters! Certainly cannot use this sensor to know if you have lifted off the ground! We use the LRF...

Measurement Specialties just snubs us - we are to small for them to be concerned about our 'bad' report...

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The IMU is fitted with two MPU6500, each running a full IMU at 400Hz, but with different filter coefficients within the Kalman filter - this helps tremendously in the elimination of IMU bias or drift during high vibration, such as during taxi on rough ground.  An HMC5883 Magnetometer is also fitted as a separate module, able to be located in a clean environment on the A/C.

ALL sensors are fully temperature compensated from 0deg C to 65deg C

The host processor is the STM32F427, running at 140MHz.

Failsafe is managed by a separate AVR processor on the module.

We also used large connectors so that cables are more easily crimped, more reliable, and fit securely.

( see my previous blogs for info on the previous autopilots)

The test aircraft was fitted with is full payload - an EO-IR stabilised gimbal ( Sony EX11 block cam, and FLIR QUARK IR CAM), and a 16Mpixel fixed down looking stills camera, snap and focus controlled by the autopilot.

The Laserman ( Lightware) SF10 laser rangefinder is also fitted as the main landing aid sensor, as visible in the above photographs. 

Here are some photos of the autopilot module:

Top view of full board:

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These are the two module in the middle, containing the inertial sensors and all the pressure sensors. They are separate , small PCB's, so that they can be easily exchange for ones fitted with newer devices, etc.

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The flight test were very successful, the new autopilot performs very well, and the IMU worked as designed...

The launch and landing plots below show the Height above ground from Baro Pressure sensor and the laser Rangefinder height to ground.

This is the auto-launch plot:

The image is not very clear, but resolution on the blog post does not really allow any better...

The green is the Rangefinder, which works to 60meters, but we limit it at 40meters, since we are not interested in range above that.

The red is pressure height AGL.

The vertical delimiters are 2.4seconds apart. Wheels up are at the 4th vertical.

The 'step' in the red line at middway between 9 and 10 is where we command a change in datalink TX power from 1milliwatt to 100milliwatts which results in a short link loss period.

A nice smooth climb of the A/C. The left scale is 4meters per horizontal line.

What is very evident is the lag in pressure altitude measurement during the climb - the lag is almost a half second. The air pressure measurement is not filtered or smoothed at all, so it is purely the digital sensor internal oversampling delay.

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And the Auto-landings , below, are a thing of beauty!

Red again is pressure based height AGL, green is LRF.

And again, the step middway between the 1st and 2nd verticals is the switch from 100milliwate back to 1milliwatt on the datalink , because we land about 10meters away from the ground station, and 100milliwatts saturates the RX at that range - the Xbee 868MHz modem are VERY sensitive - we get nearly 30km range at 300milliwatts with 1/2 wave dipoles.

The landing is a circle, starting at 100meters height, and spiraling down, till about 20meters AGL ( vertical line 6 approx) and then unrolling, straightening out onto track to touch down at a GPS point ahead. It is seen that the LRF and Pressure height track reasonably well, but the pressure sensor lag is still evident.

At vertical 11, we are at approx 6 meters height, and we begin final landing control, at 4 meters we kill throttle, and start the flare maneuver, which completes at 0.4meters above ground, and a smooth gentle touchdown at just after vertical 16, at about 0.5m/s sink rate. After 14 such landings the min sink rate achieved was 0.3m/s at touchdown, and the max was 0.7m/s. Windspeeds during the 14 flights varied from 4m/s up to 13m/s.

We never managed this reliably at all with pressure only landings, and with the previously fitted ultrasound sensors we often had 'hard' landings, due to the fact that the switch from pressure height to actual ultrasound height would only occur at 5meters AGL or so - the typical range of ultrasound sensors over rough ground. With laser we switch to the LRF height at 30 meters already, and are able to compensate for any pressure sensor drift in good time.

The system is now totally hands off....Push the button and go..

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The NamPilot...

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After after almost 9 months of application of the SurVoyeur MKII UAV system in the Etosha National Park here in Namibia, The Park and Ministry have added further to their capabilities. Two more Pilots have also completed there training this week, each on both the SurVoyeur MKI and MKII systems. This will enable the Park to fly simultaneous flights in separate areas, day and night if required. The fully automated systems have day and night ( Thermal) imagers and the full autonomy in launch, flight, target designate and track, as well as autolanding, ensure the best survivability in this harsh environment.

Pilot effort in flight control and management is also vastly reduced, a huge plus in the National Parks.

The preceding 9 months of system use has shown many successes and preemptive flights have been shown to act as strong deterrent to would be undesirables, leading to the latest additions.

Congrats to the two new Pilots , Shayne and Isaskar, and their new systems!

Nampilot.

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Pictured above is a twin tail boom Pusher Composite UAV made by Paramount Advanced Technologies. It was fitted with our autopilot and flight control software and we spent a few weeks flight tuning the system and implementing full auto takeoff and auto landing modes.

About 4m wingspan, pusher prop, 4 flaps, split elevator, flies like a dream!

These two modes used the Lightware SF10 Laser Rangfinder as the source of height above ground for the Wheels up detection, and for the landing approach and landing flare modes. Still a little work in getting touchdown closer to the target point - but landings are smooth and safe..

The SF10 performs flawlessly, giving cm accurate height data from 60meters AGL and has enabled the auotlanding function to be easily implemented.  This airrcaft in its test guise weighs 25kg, with electric motor drive. For those interested in more info on the UAV, contact Paramount Advanced Tech. in South Africa - they are on the web..

Here are two videos of the landing, approach and touchdown. You will notice that the UAV approaches the landing strip, and flies on a ways if the touchdown would have been short, and then does finals with the flare and a smooth touchdown.

Nice and Slick!

The videos are low res - takes too long to download for viewing here in Darkest Africa!

http://youtu.be/aV8eXD8_MuA

http://youtu.be/SHmpS1vexVM

Thanks Laserman for the SF10! ( see www.lightware.co.za)

The Nampilot

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Seeing Andrew Tridgell's post on the SF02 LRF used for landings on APM, I thought I would post some flight logs of our landings using the same rangefinder.

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NOTE - Log sample rates are 120milliseconds - each vertical line in the graph is 2.4seconds from the next.

The top chart shows Baro height (ASL..) . Our SurVoyeur A/C landing commences with a spiral down to 80meters AGL, and then lines up with a computed descent point, down a parabolic curve in a circle, to exit the circle at a point approx 20meters above ground, and lined up with the touchdown point.

Point - 1 above is the beginning of the spiral from 80M AGL down to the 20meter spiral exit point.

Ignore point -2 - that is where the datalink power is switched from 300mW to 20mW - short link loss during this period.

Point-4 is where the LRF is enabled - measures 38meters AGL - we switch to laser height here from ( always erroneous) Baro height. Pressure Alt is always in error - 0.5mbar is around 4 meters, so that bad for the touchdown. In a half hour flight, baro pressure can easily change by 0.5mbar...Temp compensation of static pressure sensors to give better than 2meter error is not simple, and even 2 meters error is not good for touchgdown.

Point-6 is the 20meter AGL spiral exit, lining the A/C onto the straight in landing approach, aiming at a GPS point.

A throttle from airspeed error and throttle from height error control loop manage throttle in this part, till touchdown.

In the top graph, the RED line is the commanded height to fly down - the green is the actual A/C height from Baro pressure.

In the lower graph the RED line is the LRF range height, and the green line from point 6 is the height line we wish to follow onto the flare to ground. This green line is computed form a desired descent rate, and manages the A/C height accordingly. Lots of gain scheduling in the height, speed and throttle loops happen from point-4 to point-5, the touchdown point. At point 5, the A/C flares in the final touchdown, with a descent rate of around 0,25m/s

In the graphs below, Point-1 is the start of the spiral down from 80meters AGL, 2 is the link power change point, and 3 is the flare in the pitch angle for the flare to touchdown - all works smoothly.

The oscillation in the A/C pitch angle in the lower graph (RED) at the touchdown, is from A/C pitching as it rolls over the rough terrain - a gravel plain with golfball size gravel...in the desert here. A/C is a tail dragger, so resting pitch is plus 12 deg, as seen in the final pitch angle below.

The lower graph - GREEN is the commanded pitch angle, RED is the A/C actual pitch angle.

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The SF02 Laser Rangfinder works VERY well indeed - it does not compromise on range , nor does it have any limitations related to the type of surface from which it will work thanks to some smart and innovative design. ( ok, a mirror might not work..).

We used to use a SESCOMP ultrasonic rangefinder - which is a very good product, accurate, no noticeable wind disturbance effects, etc. However, the reliable working range is limited to 6meters max, and if you have a pressure ALT error of say 0.6mbar either way, an error of around 5meters, you introduce major pitch disturbances into the final flare due to a major step change between pressure alt and Ultrasound height when ultrasound is detected. This is at the worst point, where you are close to the ground, etc. The laser fixes all this!   

There is a new laser from LightWare.co.za, the SF10/a, with serial, I2C and analogue outputs, its smaller , and very neat. We intend to fit these as standard now on all our autoland airframes.

I would beware of options with smaller lenses and lower power emitters - they will not work reliably on 'all' surfaces' at expected ranges - from costly experience...

See my previous blog on the LRF for more info on its fitment to SurVoyeur, etc

http://diydrones.com/profiles/blogs/laser-rangefinder-flights-tests-in-namibia-1

From the NamPilot..

Below the Ultrasound and Laser rangefinders                                                        Below the new SF10/A

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This is a reasonably compact, linearly polarised and well behaved antenna, with a gain , when well made, of around 1dBD    ( 1dB more than a dipole). It is smaller than the Big wheel and the Skew Planar wheel, and I have used it very successfully on 868MHz on our SurVoyeur UAV.

It is derived from a 1/2wave ring, located some distance above a large conducting ground plane. The ring is at a point on its circumference shorted to the ground plane by means of a conducting copper strap. The coaxial cable feed line has the center conductor attached to the ring a short distance away from the shorting strap, and its braid shield connected to a corresponding point on the ground plane. directly below the center conductor connection. The Antenna radiates from the gap between the ring and the ground plane - a Slot radiator antenna. Since the ground plane is inconvenient, we can 'roll' it up into a cylinder, the same diameter as the ring, located coaxially directly below the ring. The antenna performance remains intact. However, the long cylinder is just as inconvenient, so we shorten it till its length is the same as the thickness of the wire forming the ring, and the end result is the picture above. Two concentric rings, spaced apart by plastic insulators, with a 4mm wide copper shorting strap, and the feed coax in place. When the dimensions are proportioned correctly, the antenna gives 1dB gain over a dipole, and when laid horizontally, has a vertical radiation pattern identical to the dipole, with a slight null in the direction of the shorting strap.It is linearly polarized.

For 433MHz, the antenna dimensions shown will give an SWR of 1.1:1 at 433MHz, and 1.25:1 at 429MHz and 437MHz

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Seen from the side :

The rings are 100mm diameter, 2mm clean copper wire, no plastic insulation.

The plastic insulators can be Servo push rod tubes, or any plastic tube or solid rod, about 6 to 10mm diameter, and about 20 to 25mm long, with 2mm holes drilled in the sides, 15.5mm apart, to take the wire rings. 

To make the rings, take the copper wire, place one end tip in a vice, and with a pair of pliers clamping the other end of a 1meter long piece, pull hard and gently to stretch-straighten the wire. Find a former ( coffee tin, etc) exactly 100mm diameter, and wrap the wire around the former overlapping the wires. With sharp sidecutters cut through both overlapping wires, to make a butt joint in the ring.

Remove the wire from the former, slip over the plastic spacers ( at least 4 prefer 6) and then, using  a 10mm x 5mm piece of thin copper foil, wrap it around the but joint and solder the wire ends together this way. 

Make two such rings, feeding the second ring gently through the second set of holes in each spacer before butt jointing the ends.

Then position the two butt joints over each other and connect a shorting strap of copper foil, 18mm x 4mm wide, from the top ring to the bottom ring. solder the bottom connection, and then the top, ensuring the strap is straight and taut.

Now connect the inner conductor of the 50ohm coax to the upper ring, 9.5mm away from the center of the shorting strap position. then connect the braid of the coax to the lower ring, at the same place.

The shorting strap end of the antenna is low impedance and low voltage. the direct opposite end of the ring is a very high impedance, high voltage point, sensitive to the proximity of auy other objects, servos, wires, etc, as are the end tips of a dipole, for example. Objects close to that region will affect the tuning...Do not place any support plastic pillars  past the two pillars farthest from the coax feedpoint, as depicted in the last image at the bottom of this blog.

                                             Here you can see the shorting strap location.

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No spacers past the two on the right of this image...

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It is convenient to use tubes for the spacers, as you can fill them with hot melt glue to fix the spacers and rings in place, forming a rigid structure.

The antenna works equally well right side up, or upside down....

Tuning the antenna :

This is a sensitive process, and you will need and RF source at the right frequency, and a decent SWR meter.

Connect the rf source to the SWR meter, and the meter to the antenna via the intended cables.

There re two adjustment to be made - feedpoint impedance, and frequency of resonance.

Feedpoint impedance is adjusted by moving the coax connections to the ring ( the inner conductor and the braid) either closer to or further away from the shorting strap. Why does this work? Well, the shorting strap is a short, so impedance at the strap is 0 ohms.  The opposite side of the rings circumference is a high impedance point, many thousands of ohms, from an RF standpoint. So, somewhere in-between 0 and 'thousands', will lie 50ohms - that is the point you seek, and it is very close to the short.

Frequency adjustment is done by squeezing the end of the two rings opposite to the shorting strap together to lower frequency, or apart to raise it. In other words, the spacing of 15.5mm is reduced or increased appropriately.  If you find you need to increase this spacing by more than 2mm or so at that quadrangle, then the rings are to long in circumference. Cut 2mm out at that point, and join with the wrapped foil trick again, SOLDERING WELL...and tune again.

When you achieve lowest SWR by squeezing, then adjust the coax spacing to the short, try closer or further, for lowest SWR. Work back and forth between these two adjustments, till no more improvement occurs, and you should strive for an SWR better than  1.4:1. 1.2:1 is easily achieved.

This antenna works very well. I have achieved nearly 30km with 300milliwatts on 868MHz, with one of these on the aircraft at 100meters AGL, and a sleeve dipole on the ground at 2meters AGL.

Here are some pics of the 868MHz implementation , the rings made from FR4 PCB material.

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This is the datalink and video tx module used on our SurVoyeur aircraft. The ring of copper is the 1/2wave ring radiator ( the other ring lies below it)

The Skew planar wheel is for 2.4GHz Video TX.

To the left of these antenna are the datalink modem and the video TX module.


                                       Here are views of the shorting strap and coax connection

                                                                    to the rings

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Joe

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Dipole style antenna for 433MHz

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Hugues and a few other folk have been looking at using dipoles on 433MHz, I presume this to be the telemetry or RCS radio frequencies used by them. 

I have been playing with some concepts here to try come up with a repeatable implementation with reasonable performance. The main aim is to obtain a good match to the feedline and RX/TX equipment, and to ensure that the dipole radiation pattern is preserved as best possible. The idea is to use a vertically orientated sleeve dipole on the ground as this could be housed in a plastic tube with the radio connector at the base of the tube. On the aircraft such an antenna is rather unwieldy since it is around 400 to 500mm tall, so a conventional dipole coaxed into a V form is suggested. This can be cut into the fuselage and vertical stabiliser foam, or simply taped to the foam on the outside for tests. It could also be taped onto the horizontal stabiliser, one element on each, with the coax down the center of the fuselage, although the signal polarisation to the ground vertical is not optimum this way. If this were the prefered installation, then a similar 'V' could be used on ground.

I have not indicated all the final construction details, such as the tube into which the sleeve dipole could fit, or a substrate onto which the V antenna could be fitted for robustness when used for ground antenna - I can help with further ideas and suggestions in this regard if need be - it is just a little difficult as I do not know what materials may be available to each in his part of the world...

Onto the antennae:

The sleeve dipole resembles a section of coax cable with a 1/4wave section of the braid folded back onto itself, over the insulation jacket. This leaves a 1/4wave section of the center conductor at the top, and then below the 1/4 wave braid.

This forms the dipole and the braid section forms both the one half of the radiator, and a balun to suppress unwanted current from flowing down the outside of the coax shield and degrading the radiation pattern of the antenna.

However, the performance and matching of such a 'folded braid' sleeve dipole is not optimum.  A more optimum sleeve is where the inside diameter of the sleeve is 2 to 4 times the outer diameter of the coax cable, and when around 4 times, the sleeve should be around 0.21 wavelengths long. With this in mind, an antenna was constructed and tuned to see if this would be easily reproducible. The first used the 'folded braid' method, and then a further three employed brass tubes as the sleeve, of 7mm, 9.4mm and 11mm internal diameter. In each case the sleeve length started out at 0.26 wavelength.

In each case the antenna was tuned by trimming the length of the top vertical element till resonant. Then an RF current probe with a spectrum analyser was used to measure the currents flowing in the braid of the coax , a 400mm long section below the sleeve on the antenna. Then the sleeve was trimmed in length by 2mm, and the top element re-trimmed for resonance, and the currents measured again. This process was repeated on for each diameter sleeve tube.

The results indicate that for the thinner diameter tubes ( folded braid being the 'thinnest') the SWR would remain above 1.5:1, and the common mode current on the coax outer shield was still high. As the tube diameter increased, the tube would need shortening, the top element lengthening, and the SWR would improve. Simultaneously, the coax currents began to reduce dramatically. The 9.4mm diameter tube showed excellent results, with SWR of 1.15:1 at 434MHz, a tube length of 145mm,( around 0.21 to 0.22 wavelength,)  a top element length of 182mm, and the coax current was less than 7%  of that measured on the folded braid version. The 11mm tube showed no worthwhile improvement, so the 9.4mm tube is chosen as the optimum.

Dimensions are indicated in the drawings below: 

The construction is as follows - the tube has a small brass nipple , or disc, with a hole in it dimensioned to pas the coax braid. This nipple is soldered into the top of the tube. Inside the tube are three plastic spacers, through which the coax passes, keeping the coax centered in the tube. The end of the coax is tripped of the insulation for about 2mm for the center conductor, and then the braid cut back to expose around 2mm on the insulation of the center conductor. The outer braid insulation is cut back about 6mm and the braid then enters the nipple, with the exposed center conductor protruding. The braid is then soldered to the nipple, and the top element soldered to the protruding coax center conductor.

different sized tubes with top nipples                                                  Nipple and coax prepared

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Nipple soldered to coax and braid                                                       Exploded view

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Plastic spacers fitted

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The following images show SWR and Smith chart data for the final antenna. Note that this antenna is not fitted into any housing or tube. I fitted it into a length of 20mm diameter PVC conduit tubing to measure the effect, and it is quite dramatic. Therefore, if anyone wished to package is so, please let me know what the tubing that you wish to use is - I will try to obtain something similar, and retrim the elements to compensate for the tube shortening effect.

3689597275?profile=originalSWR is 1.15:1 with no plastic overtube.

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SWR is 1.04:1 at 425.35MHz - a big change with the plastic over tube.

SWR at 434MHz is now 1.8:1 with the overtube.

The V Dipole is made with a 1.4 wave section of the same coax serving as a balun to suppress the common mode coax currents. It is seen as the parallel section in the photo at the beginning of this blog.

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here the antenna is taped to a tall block of polystyrene, which does not affect the antenna characteristics, while taking measurements. The balun is clearly visible.

 

   3689597327?profile=originalhis shows how the balun is terminated at the top -

the main coax shield connects to the left element.

The main coax center conductor connects to the shield of the balun and to the right hand element. The other shield end of the balun section ( photo above) connects to the main coax braid at that point.

The length of the balun is the same as one half dipole element, close to a 1.4 wave. Measurements were taken with the current probe with and without the balun - with the balun current levels were almost 22dB less, a significant amount.

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This shows the balun shorted to the main coax at the balun bottom end.

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Here are SWR Plots:

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SWR is 1.01:1 at 434MHz

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In this case the bare copper wire dipole elements was replaced with 'servo lead wire' - wire covered in plastic insulation - the resonant frequency has moved down considerably.

SWR is now 1.07:1 at 422MHz.

You cannot just put any plastic over the wires without re-tuning.

Also, 2mm removed from the wire ends shifts the frequency up by 1MHz - it is sensitive to adjustment! 

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This Smiths Chart plot shows the excellent match of this antenna - no reactance and a good 50 ohm match.

The 50 Ohm match is achieved by the V shaped elements - bending them into the V form lowers the feedpoint impedance in this antenna. The element show very good match for inter element angles between 100 and 115 degrees, typical of V antennae.

The elements can be bent upwards or downwards ( away from or towards the coax feeder), as required by the installation.

Tapping the antenna elements to an EP or polystyrene aircraft frame or wings will have no effect on the element length or SWR. However, placing the elements against any fibreglass or plastic ( PVC, etc) surfaces will affect the tuning detrimentally. The shrink iron-on cover materials used on model planes will have no effect either.

I hope this will be of some use to all - if anyones ends up building any of these ideas, let me know if I can help with re-tuning for you choice of materials and mounting methods - I will try!

Joe

The Nampilot.

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3689584418?profile=originalJeff asked about adding a 'Balun' to a simple dipole, so I thought to disertate a little once more on the subject...

The purpose of a Balun is to allow the connection of a coaxial feed line ( unbalanced feedline) to a balanced antenna , such as a dipole.

Picture at left is a Circularly polarised turnstile antenna. Although this is for 75ohm coax, similar strusture are possible for 50ohm as well. Significant in this particular design is the fact that the 75ohm phasing cable section serves the function of the Balun as well. 

What we are trying to achieve with a Balun, or its electrical equivalent, is to prevent common mode currents from flowing down the coax feedline. These currents are induced when a dipole is simply connected directly to a coax feedline, one element to the inner conductor, and one to the outer. The currents flowing in the dipole elements result in the electromagnetic wave being generated and radiated by the dipole elements. However, the current flowing in the element connected to the coax outer sleeve has to return to the generator ( the transmitter) and so does this by flowing on the outer sleeve of the coax cable. This current induces radiation from the coax outer sleeve, thereby distorting and destructively interfering with the dipole radiation pattern. This current can also return all the way back to the transmitter, and induced energy flows in adjacent cables and looms - In a small UAV with wiring close to everything, this sometimes manifests as servos twitching in sync with the video or datalink transmissions, etc, among other phenomena. 

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The balun on the left could be used to feed a folded dipole. Such a dipole has a feedpoint impedance of around 300ohms, and this balun exhibits an impedance step-up ratio pf 4:1. SO a 75ohm feedline would result in a good match to the 300ohm dipole, with a unbalanced to balanced trasformation in the deal.

However, there exist a number of alternative 'baluns' that can be used where an impedance transformation is not desired, a sort of 1:1 Balun.

Note that these are not really baluns in the true sense, but actualy perform the same duty by acting as a choke for the RF currents flowing on the coax outer sleeve.

3689595987?profile=originalThe left image is often referred to as the 'Bazooka' Balun. It is a 1/4 wavelength of tubing, snugly fitted over the coax sleeve insulation, with the bottom end of the tube soldered all the way around to the coax sleeve braid. The top end of the tube is open and insulated from the rest of the antenna. This works by the 1/4 wave section forming a short circuit to the flowing currents at the base, and a high impedance at the top, choking of said currents.

The following are variations of the theme:



Where in the Bazooka Balun a tubular sleeve surrounds the Coax, a single 1/4wave length of conductor can be substituted in the following manner.3689595838?profile=original

The Bazooka balun is preferred and is more efficient. 

In order to not distort the antenna radiation patterns ans not cause EMI with other on-board electronics, it is always desirebale to use a Balun type feed for balanced antenna such as dipoles, Turnstiles, etc. The examples shown can be used with 1:1 and 4:1 impedance match for all dipole types.

The Nampilot.

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The Science of Successful antenna design

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As promised, James, Brad and Hughes, and anyone else interested, some info on turnstiles and the methods of antenna matching.

James made the following statement:

I came across this link http://www.fpvmanuals.com/category/manuals/equipment-manauls/antennas/  which explains how to build what they call a turn stile antenna.  Could anyone try and explain why this antenna would work as I had really poor performance with it.  I ended up using a simple cross dipole on the RX side with much better results.

I gave the start of an answer in a long reply which can be found in his post : 433 UHF LRS Antenna "Turn Stile"

Some further questions where asked regarding the methods of match the antenna, and the antenna tuning, so I will try to elaborate. 

First some fundamentals.

When I speak of antenna radiation it also includes the reverse, that is, the antenna 'collecting' radiated energy from the ether. 

Any piece of wire will radiate energy when connected to a source of RF ( Your transmitter or receiver). How WELL it radiates that applied energy  is dependent on only two factors-

  - That the antenna is resonant at the same frequency as the radio signal applied and

  - That the feedpoint of the antenna is matched to the impedance of the attached transmitter energy source.

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Maximum energy tranfer between source and load ( the antenna) occurs ONLY when the load and source impedance's are equal.

How well the antenna radiates that energy in any or all directions, if the above conditions are met, is then only dependent on the antenna design, shape, or style. For example, a simple vertical 1/4 wave radiator will radiate a doughnut shaped pattern, in all direction of the compass, with low energy upwards and downwards, in the direction of the antenna element.

A yagi type antenna, on the other hand, ( such as your vhf or uhf TV antenna) focuses the energy in a single direction, as would a torch. Antenna do not have gain. They focus the energy to a greater or lesser extent, in a direction of design, but do this by robbing energy from other radiating directions - Your torch puts more light out the lens end, with nothing out the rear end. If you remove the reflector from behind the torch bulb, the light is radiated in all directions, omnidirectionally, but is much weaker at any distant point than the focused beam.

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Antenna radiation is polarized; that is to say, the radiated electromagnetic wave has a net polarization plane. This is usually either linear or circular. Elliptical polarization is also found, but that is merely a mix of the two former types.

A 1/4wave vertical antenna will radiate linearly, with vertical polarization. Placed on its side it will radiate horizontal polarization.  A Helical antenna ( looks like a coil of wire wound in a screw fashion) wound clockwise when viewed from behind will radiate clockwise circular polarization, and vise versa.

For reception of maximum energy, the two antenna must be identically polarized. There is a massive loss of signal ( easily some 30dB, although the theoretical loss is infinite) if one attempts to receive a horizontally polarized signal with a vertically polarized antenna. Similarly, there are massive losses if trying to receive a circularly polarized signal with an antenna of the opposite circular sense.

The odd man out is that there is only a 3db loss between an antenna that is circularly polarized and one that is linear.

Why would one use circular polarization?

If the two antenna in question could not be made to maintain similar attitudes, such as one in a pitching , rolling aircraft, then there would be unacceptable signal losses as the aircraft banks and pitches. So you could use a vertical on the aircraft, and a helical or turnstile, or similar, on the ground segment. This way you would only ever experience a 3db maximum loss. ( all assuming good line of sight view). Or you could gain back the 3db loss by fitting a similar circularly polarized antenna on the aircraft, giving the best of both worlds. But you actually gain more than that with circular polarization at both ends.

Assume first that the two antenna are simple vertical monopoles, radiating vertically polarized signals. When you are flying, at the flying club, etc, you are probably near some metal structures, the 'hanger' , cars and other vehicles, etc. All these structures reflect the same energy you are trying to receive. In addition, when the aircraft is low and far, the RF transmitted by the A/C antenna follows two paths to your receiver - one directly, and one via a reflection from the ground, somewhat midway between you and the A/C.  What happens to the reflected wave is that the polarization is changed in unpredictable ways. Your receiver ( and antenna) does not know or care where the received energy comes from, so it receives this reflected energy as well. These multitude of received waves add constructively and destructively with the main received wave, causing large, short duration, signal drop-outs - a sort of 'flutter' in the signal.

If both antenna are circularly polarized, however, the picture is quite different. When the circular polarized waveform is reflected , it REVERSES its polarization. When this reversed polarized signal arrives at your receiving antenna it is largely rejected and hugely attenuated, so interfering minimally with the main received signal.

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On to Issues of resonance and matching.

To repeat a little in my post to James:

Most simple linear antenna are either of the monopole or dipole form. A single monopole ( 1/4 wave vertical for example) or a single dipole will only radiate linear polarization.

Any antenna is only resonant when it is exactly the correct length AT the frequency of operation.( this does not apply to the class of broadband antenna, such as helical antenna, etc. The helical will easily cover an ocatve with good performance).

At resonance the antenna will exhibit its characteristic feedpoint impedance. Feedpoint impedance is expressed with two terms, the pure resistive part, and the reactive ( j operator) part.

Most transmitters and receivers terminal impedance are made to be 50ohms resistive, or very close to that. So it stands to reason the antenna must also be 50ohm resistive to have max energy transfer.  However, none of the antenna are that obliging, so we have to do some feedpoint matching to meet the criteria.

A 1/4wave vertical monopole over a ground plane has a resistive feed point of around 75ohms. A half wave dipole is around 72ohms. As with resistors, placing two dipole in parallel as in the IBcrazy turnstile, will result in a feedpoint impedance of 35ohms.

A 75ohm feed connected to a 50ohm coax and transmitter will exhibit a 1.5:1 SWR ( the ratio of power going out to power reflected). A 1.5:1 SWR means that approx 3% of your transmitter power is not being radiated. ( 30milliwatts for a 1watt transmitter). That is not so bad, and we can live with an SWR of 1.5:1 in most cases.

The turnstile antenna is a pair of crossed dipoles, fed 90deg out of phase with each other, thereby generating circular polarisation. You CANNOT simply connect the dipole in parallel at the coax feedpoint though. Apart from the halving of impedance ( which we decided we can live with) the radiation pattern and polarization of the antenna will be totally destroyed by unwanted radiation from the coax cable. The RF energy, at the dipole connection point, 'leaks' out and currents then flow down the outer shield of the coax. As mentioned previously, any piece of wire will radiate RF energy, and so the coax radiates this energy, and the radiation again adds constructively and destructively with the main antenna radiation, causes complete distortion and signal nulls in the pattern. This radiation from the coax MUST be prevented.

This is done by means of a Balun transformer. - which is is an acronym for 'Balanced to Unbalanced transformer'.

A dipole is a balanced device - it is electrical equal along each element, outwards from the feedpoint. It therefore requires that the feedpoint be fed in a balanced fashion. Coax cable is an an balanced feeder - the shield is at ground potential, while the inner core carries the energy. This effectively ( oversimplifying a little) connects the one dipole half to the 'live' core, and the other half to 'ground' unbalancing the dipole. This causes currents to flow on the coax outer shield, and distortion of the dipole radiation pattern.

Baluns can be constructed from coax cable, but the accuracy required in coax cable length ( they are normally length multiples of 1/4 wavelength) is very critical, especially in the GHz range - 0.5mm can have a great effect.

The turnstile is not new - it is some 50 to 60 years old, and is well researched and published. Up to the VHF and lower UHF region , the coax balun, with embedded impedance match transmission line transformer, is used, along these lines:

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For the higher microwave frequencies, a plumbing type version is more appropriate. This is called the spilt tube or split sheath balun, and looks like this when used as a feed for a pair of crossed dipoles.

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The balun and feed match consists of an outer and an inner tube. The ration of diameters D/d is chosen to give the desired impedance:

D/d = 1.86 for 75ohms, and 1.5 for 50 ohms.

Typically the outer tube would be around 8mm for use at 2.4GHz.

In order to obtain circular polarisation, I mentioned that the two dipole have to be fed 90deg apart ( phase quadrature).

This can be done as in the coax balun version above ( inserting an extra 1/4wave length of coax in the leg to one dipole gives an extra electrical wavelegnth of 90 degerees). 

Or, this can be achieved by slightly lengthening the one element ( becomes more inductive) and shortening the other( becomes more capacitive) - this also introduces the required phase difference between the elements.

This can be seen in the images above - the one element is typically around 0.21 wavelength per half, while the other is around 0.25 wavelength. One short and one long element penetrate the outer tube and are connected to the inner tube, while the opposite pair of elements are connected only to the outer tube. The outer tube is split or slotted ( 0.5mm width slot). The slot is approx 0.23 wavelength long.

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The relationship in length between the two dipoles is critical, typically this would be measured on a network analyser and the feed impedance of each element set to say R+j45 ohms ( longer dipole) and the other to R-j45 ohms. This will give the correct phase relationship between elements. A half mm variation can have a great effect, turning a good antenna into a mediocre one..

The last image above shows a teflon tube - this is inserted in the tube from below, and fits snugly inside the outer tube, and over the inner tube. This is then slid up and down to adjust the 'R' part of R+-jX, till the match is a good 50ohms. This does not affect the antenna radiation pattern or characteristics. Obtaining a 50ohm impedance match can be done by trimming the element lengths as well, at the same time destroying the antenna radiation pattern and circularity.

And that is why it is not so simple to do at home, and why the 'Hobby King'  et al variants sold everywhere are mostly trash..You will probably achieve a few km range with those- remember, any old piece of wire will radiate -  I easily  achieve 15km with 500milliwats at 2.4GHz using two split sheath balun , properly matched and trimmed, crossed dipoles..

For those interested:

References are - RSGB VHF/UHF Manual - page 8.45

Modern Antenna Design - Page 255

Here are some images of my split sheath balun crossed dipoles..

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Joe

The Nampilot.

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We have developed a version of the SurVoyeur mk-II, the 'c' version, which has a new nose housing a gimballed, stablized EO and IR camera combination.

This gimbal houses the FLIR QUARK 640 X 512 pixel Thermal camera, fitted with a shutter to enable flat field correction,  and also houses the Sony EX11 Colour Block Camera.

Both camera are fully controllable from the ground station, colour modes, zoom where applicable, AGC, etc.

The gimbal is stabilized and fully steerable from the ground station. We are currently working on making accurate fitment jigs to bore-sight the cameras to the IMU, to enable accurate line of sight determination to allow automatic tracking of designated 'targets'.

Here are some photos of the various parts of the Gimbal, and the Gimbal fitted to the aircraft.

The two cameras with the QUARK shutter mechanism:                                   The assembled Gimbal:

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Pre-assembly:

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A short video of the gimbal in motion:

https://www.youtube.com/watch?v=0xlB0HXT520&feature=youtu.be

Flight times are 70minutes at sea level 25deg C, 55 minutes at 1600m ASL, 40deg C.

Additionaly an 18mpixel Canon stills camera, stablised in roll, is fitted in the belly bay , under the wing, as was in the original mk-II aircraft.

search for SurVoyeur on DiyD for other blogs on mk-I and mk-II if interested...

The Nampilot.

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Gabriel Shatumbu, A qualified Pilot, receives his competency certification for the SurVoyeur mk-II UAS .

Etosha National Park in Northen Namiba received their first Wide Horizons SurVoyeur mk-II UAS yesterday. The Pilot completed his training on Friday. The system consists of the 2.6meter wingspan SurVoyeur mk-II aircraft , stabilized steerable Infra-Red gimballed camera ( FLIR TAU-2) and an 18mpixel stills camera also with video downlink.

The Aircraft is capable of 60minute flight at 1500meter ASL @ 35deg C @ 18m/s, and 75minutes @ 14m/s.

The aircraft is fully autonomous - auto launch and land - and has a video and datalink range of 20km.

The video receive ground segment consists of a 14dBi gain Helical antenna, fitted within a tracking system, tracking the aircraft in flight.

The Pilot completed 4 days of intensive training with 9 flights day and 6 night. The Pilot also is a qualified civil Pilot and a Senior Warden at the Etosha National Park. He was well at home with the system and its operation.   Since his arrival with the system in the Park on Friday afternoon he has already flown 3 flights.

The system is to be used as an aid in day and night patrols of 'hot spots' in the Park and on the critical boundary fence lines. If it proves to add value to the process, the Park intends to supplement the system with at least 3 more systems.

We hope this will help in even the smallest way to aid the Good People in these parks in there plight against poaching.

                                       Mk-II in its case, and then assembled on the stand  in its case.

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               Ready to Launch..

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Pilot at the GCS

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Pilot filling in Log Book after flight - Tracking antenna in the background

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The NamPilot.

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How have the Mission Planner Developers managed to get around the Restrictions laid down by Google Earth regarding the use of Google Earth/Maps imagery and geo data for the Mission Planner software?

 

It appears that the use of any bulk downloaded , offline stored tiles, tiles stitched together, etc is illegal?

It also appears that the use of these maps to provide navigation and guidance for any autonomous vehicle is illegal. 

It also appears the use of cached maps data is illegal other than 'to improve the user experience while having not internet connection' but for short term only. Storing cached maps for more than 30 days is also illegal.

It seems they have done everything to make it illegal and impossible to use the content in our type of applications!

Has Google provided special dispensation to permit its use by Mission Planner , and this community, for this Open Source application?

I have dug deep on the DIY-d forums and have not been able to find any guidance on this issue. I am not sure if Google would litigate in these issues, but they have the might to pull it off....

Some excerpts from from Google terms and conditions:

http://www.google.com/help/terms_maps.html

2. Restrictions on Use. Unless you have received prior written authorization from Google (or, as applicable, from the provider of particular Content), you must not: (a) copy, translate, modify, or make derivative works of the Content or any part thereof; (b) redistribute, sublicense, rent, publish, sell, assign, lease, market, transfer, or otherwise make the Products or Content available to third parties; (c) reverse engineer, decompile or otherwise attempt to extract the source code of the Service or any part thereof, unless this is expressly permitted or required by applicable law; (d) use the Products in a manner that gives you or any other person access to mass downloads or bulk feeds of any Content, including but not limited to numerical latitude or longitude coordinates, imagery, and visible map data; (e) delete, obscure, or in any manner alter any warning or link that appears in the Products or the Content; or (f) use the Service or Content with any products, systems, or applications for or in connection with (i) real time navigation or route guidance, including but not limited to turn-by-turn route guidance that is synchronized to the position of a user's sensor-enabled device; or (ii) any systems or functions for automatic or autonomous control of vehicle behavior; (g) use the Products to create a database of places or other local listings information.

AND:

https://developers.google.com/maps/terms

(c) No Navigation, Autonomous Vehicle Control, or Enterprise Applications. You must not use the Service or Content with any products, systems, or applications for or in connection with any of the following:

(i) real time navigation or route guidance, including but not limited to turn-by-turn route guidance that is synchronized to the position of a user's sensor-enabled device.

(ii) any systems or functions for automatic or autonomous control of vehicle behavior; or

(iii) enterprise dispatch, fleet management, business asset tracking or similar applications. If you want to engage in enterprise dispatch, fleet management, business asset tracking, or similar applications, please contact the Google Maps API for Business sales team to obtain a Google enterprise license. (If you are offering a non-enterprise implementation, you may use the Google Maps API(s) to track assets such as cars, buses or other vehicles, as long as your tracking application is made available to the public without charge. For example, you may offer a free, public Maps API Implementation that displays real-time public transit or other transportation status information.)

Can anyone throw some light on this please? How are the MP users protected, if at all?

The Nampilot..

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Pre Flight check List of system Integrity

How we do pre-flight safety and validity checks prior to beginning autonomous flight:

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I must admit I was a little flabbergasted at the realization that most of the 'autonomous' flights reported here are in fact relatively 'blind' - I was really under the impression that the 'check-list' concept that now appears as a forum of bright Ideas was a fundamental implementation in the GCS system and a basic part of all operation - Imagine my surprise when I discover that  such pre-flight safety and integrity test are not a prerequisite to launch....

In our SurVoyeur system we have a very extensive pre-flight check procedure - I have attached an excerpt from our user manual which gives some of the basic tests that are performed before the system will allow any launch command.

These tests include:

ALL sensor tests ( Gyros, Accelerometers, pressure sensors, GPS, ultrasound rangefinder for autoland)

All moving surface tests ( flaps, ailerons, rudder/elevator)

Throttle

Payload ( camera - focus/snap - video camera)

Ground antenna ( tracking antenna positioner, etc)

There is also a pre flight mechanical checklist the user must manually fill in, as well as a post flight checklist, to check for any mechanical damage, servo damage, etc.

A battery checklist has also to be filled in - cell voltage after flight , with a minimum 1 hour ' rest ' period before measuring, ( shows very quickly if a single cell is starting to fail), and the flight time and Ah recharged must be filled in.

Perhaps some of this is onerous for a 'hobby', but it lays solid ground for any authoritative investigation into any flight incident.  

We also record the entire flight and ALL A/C attitudes and flight path, all outputs of all PID control loops, battery, throttle settings, etc, all to aid post flight incident analysis.

At least we would be able to stand up against any investigative authority and have a fighting chance...

Here is the doc, for interest - it is just a small extract..

I believe this sort of approach - tailored to suit the end use, of course, should be mandatory, if only to breed good manners and safety within users.

Pre_Flight.doc

The Nampilot.

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Laser Rangefinder Flights tests in Namibia

                                                          SF02 Laser Rangefinder Flight tests:

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I recently purchased three of the SP02 Laser Rangefinders from Laserman, and have fitted one to one of our SurVoyeur mk-II planes. I had intended to fly earlier this week, but was beset with PC hard disc failures so spent a few unpleasant day resurrecting PC's...

Anyway, back on line and we flew today with excellent results.

The rangefinder was fitted in a housing and mounted in the internal bay of the aircraft, with the lenses looking out the fuselage underside. Here are some pictures:

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            Rangefinder lenses from under fuselage

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Rangefinder connected to autopilot

Rangefinder fitted internal to Fuselage

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Fuselage under view with rangefinder lenses and ultrasound ranger visible.

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The Plot below shows the SF02 rangefinder plotted against Baro Alt ( as a function of height above ground) and the Ultrasound rangefinder)

IMU-Height is Baro_height above ground

LRF Height is the SF02 height , active from 30 meters AGL only.

Ultrasound is the Ultrasound sensor height AGL, active from 6 meters AGL only

The SF02 performs very well. The terrain was soft desert sand, with patches of very coarse gravel, interspersed with desert brush and bushes.

At the price , this is a brilliant piece of equipment, and cannot be beat!

We are working at replacing the Ultrasound sensor with the SF02, which will allow vastly improved autoland, completely eliminating any destabilization due to errors and differences in baro-alt and actual height above ground when starting the landing flare.

Thank You Laserman!

Joe

The Nampilot

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Pitot Tubes and Air Data measurements.

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I was asked on the 'Advances in Airspeed Handling' forum to give more info on this, so here is some practical info on Pitot tubes and sense piping for Airdata measurement systems.

In general, strategically located air inlet ports are required to sense both static and dynamic air systems, feeding said pressures to the relevant sensors.

Static pressure sensing is used to compute the Barometric Altitude, and must be as immune to airflow as possible.

Dynamic pressure is related to the flow of air at the inlet of the dynamic pressure sensor, as induced by the aircraft motion through the air. This pressure is very small for low air speeds ( sub 40m/s, say) and is measured by a differential sensor. Such a sensor has both ports exposed to the static ambient air, with one port having the pressure caused by movement added to the ambient pressure. This way a low pressure sense element can be used to sense the difference between static and dynamic pressures. However, any variation on the static port input side will be seen by the sensor as a change in differential pressure, and a change in calculated airspeed will result.

It is therefore important to ensure that the static pressure sensed does not vary due to aircraft attitude, wind and wind direction, etc. Measured static pressure should only vary as a result of altitude and temperature.

These requirements place considerable constraints on the design and location of the air inlet ports for both airdata sources.

The dynamic inlet port is normally a suitably shaped orifice, facing the oncoming air. This orifice is a hole into a tube, the tip of which is rounded to coax the oncoming air to neatly part without creating vortices at the inlet. At low air speeds the 'rounding' is not critical - a hemisphere the diameter of the tube is acceptable.  At high airspeeds the shape is critical, becoming more pointed. The dynamic port is most accurate at an angle directly facing the oncoming air. Pitching or yawing the tube in the oncoming air reduces the resultant pressure with ensuing airspeed changes.

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The static pressure ports are normally orifices directly side on the the airflow. These are often combined co axially in a Pitot-Static Tube, with the holes spaced evenly around the circumference of the tube, at least 10 to 15 tube diameters rearwards of the probe tip. To close to the tip results in tip vortices affecting the static pressure at the hole entrances.

Such a tube can be located in the wing tip or in the fuselage nose ( pusher prop) but the static port holes must be at least 40 to 50 tube diameters from the wing leading edge or fuse nose, to not be affected by the airflow.

The picture below shows a working tube, the larger diameter one. The thinner diameter tube static sensing performance is poor due to the static inlet holes being far to close to the tip.

The static inlet holes are spaced around the tube so that the aiflow 'balances' out when the tube is not facing directly into the airflow, eg, when pitching or yawing, or with side winds. Higher pitch/yaw angles do however result in erroneous measurements. 

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An alternative static air sense port can be located directly on the straight sides of the fuselage, preferably two ports directly opposite each other, and joined in a T. 

These pictures show the making of such a tube setup:

This is the long tube, the end of which will be flush with the fuselage sides. left and right. The nick in the middle is where the hole will be, into which the T tube is soldered.

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This is the T Tube, with the scalloped end.

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These are the two tubes tinned and ready for soldering

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Now soldered together:

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This T assembly is inserted into the fuselage, from the insight, left or right side first, and then bonded in place with the ends of the tube flush with the left and right side of the fuselage. The T piece is then piped to the static and dynamic sensors.

The tube ends MUST be on a regular surface part of the fuselage, ie, not directly behind or in front of any protrusions, bumps , landing gear, etc. Also not on a tapered part of the fuselage. All these will cause vortices and pressure variations at the tube tips, rendering measurements worthless. The principle relies on a smooth airflow past the tube orifices, and if any side wind is experienced, the air enters one hole and exits the other, with little or no pressure change in the T part of the tube.

This is a picture of my SurVoyeur aircraft fuselage, showing 3 positions where I placed this tube to do measurements to see effects of the chosen position.

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Location 1 is no good - was on the tapered part of the fuse and pressure changed with airspeed.

Location 2 is good, on the flat portion, and forward of the landing gear vertical struts.

Location 3 is no good - it is on the flat portion, but the landing gear vertical stut ( only 5mm thick) creates sufficient disturbance to cause significant variation of pressure with airspeed ( of the order of 0.3mbar - 1mbar = approx 8meters .)

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If anyone is interested in more detailed info, let me know.

Joe

The Nampilot.

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SurVoyeur mk-II - From the Nampilot..

        Here is the latest addition to our Namibian stable

                            - SurVoyeur mk-II.

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2.7m wing span, Tapered Clark Z

Fitted with IR gimballed camera ( FLIR TAU-2 in own gimbal, around 700grams)

4 Cell Lipo, 10,000m mAH, 500KV motor, 15X12 prop

approx 70minutes flight time at 5kg @ 18m/s airspeed, 92minutes @ 14m/s.

Fully Auto-launch / Auto land, fitted with Nampilot Autopilot

Fully Composite, Carbon Spread Tow wings, with flaps and ailerons.

landing touchdown airspeed 8m/s @ 10deg angle of attack.

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You May Remember the mk-I plane--

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Similar to mk-II, but 1.8m wingspan, straight wings, no flaps. See my old blog on the SurVoyeur A/C...

Mk-II wings were made from molds, made from accurate wing plugs. Fuselage is also made from molds.Flies like a dream!

Currently in use by the Namibian Ministry in Night missions in an anti-poaching role in the national parks.

Joe,

The Nampilot.

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Some developments on 2.4GHz video link and 868MHz datalink antenna systems:

          Measuring the 2.4GHz Antenna SWR

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                         The Ground Tracking Antenna

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This blog shows an integrated 868MHz xBEE datalink antenna and tranceiver and a 2.4GHz Video downlink antenna and transmitter as a single modular unit. Feed it 3.3volts and video as well as the autopilot datastream.

Also shown is a ground tracking antenna system. Two antenna are contained in the head - a multiturn 13dB gain helical antenna for the 2.4GHz video downlink, and a Diamond loop , over the ground plane, giving 8dB gain, for the 868MHz datalink. The antenna base contains a Lipo battery and fits onto a tripod. The base unit is then lined up with North, and the unit tracks the aircraft in flight using the A/C GPS position. Cable wind/unwind software is also implemented to allow the unit to unwind for example when flying circles around the GCS - this process follows a computed geometry to ensure minimum time pointed away from the A/C, and uses the aircraft flightplane to estimate the postion to slew to to start tracking again, if the link were lost.

We have just completed range tests, first with the aircraft mounted on a tall pole, on a high ridge in the desert here in Namibia. The aircraft was able to be rotated 360deg in azimuth and +- 25deg in elevation. Tests were conducted with the ground tracking antenna at 10km from the aircraft position.

Video TX power was 27dBM @ 2.432GHz, into the Skew Planar Wheel. Datalink TX was a 868MHz xBee Pro- range was tested at 25milliwat TX power ( can be set to 1mw, 25mw, 100mw, 200mw and 300mw - 1mw at 10km did not work)

Video was perfect picture with the aircraft at any azimuth or pitch angle ( combined) except for the Aircraft directly facing the ground antenna, at 0deg pitch - the A/C antenna is shielded by the avionics, the on-board IR gimballed IR camera, and the Lipo batteries. Tilting the A/C up or down 5 degrees brought the video back with perfect picture.

Datalink comms was constant with NO dropouts at 25mw bothe ends, regardless of the A/C attitude and azimuth heading. The aircraft was also fitted with 2.4meter carbon fibre covered wings - 315mm chord, and a carbon V tail ( very similar to my mk-I SurVoyeur A/C , only greater wings span and carbon wings.- see the SurVoyeur blogs..)

The Datalink antenna consists of two rings, spaced apart, and is a Slot Antenna, ie, the radiation takes place from the slot formed between the two rings. It is a narrow band antenna- not more than 20MHz bandwith for 1.3:1 SWR - at center frequency it is 1.05:1 SWR.  The first rings were done in copper wire to determine the antenna factors, and then a PCB version made on the router, at various sizes, to determine the size reduction factors due to the PCB dielectric constant.

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Wire Rings Versus PCB Versions

An Integrated model was then machined on the router - this module consists of the antena rings and is fitted with the xBee datalink module and serial interface electronics.

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This module is later fitted with the Skew Planar Wheel 2.4GHz antenna, and with the video TX module.

The Skew Planar Wheel was constructed thus:

Pre-Cut element wires:                                                                       Wires formed in a 'stretch' jig.

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                               Wires Stretched and removed from jig

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Wires Formed.

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                              Coax End pieces about to be fitted

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Coax End Pieces soldered to coax outer and inner

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                Antenna elements fitted to the mounting Jig and soldered to the coax ends

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The 2.4GHz antenna is then fitted to the 868MHz module:

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The Video Transmitter is fitted on the underside:

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This assembly is then fitted into the antenna bay of the composite fuselage:

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SWR measurements were taken in-situ to determine any de-tuning:

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SWR AT 2.44GHz:: = 1.13:1

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The ground tracking antenna basics follow:

This Consists of a base unit containing the Lipo Battery, the drive electronics, an azimuth servo drive and feedback mechanical system and a yoke carrying the antenna head. The antenna head contains the 2.4GHz helical antenna, the 868MHz datalink Diamond loop antenna, the Video receiver, the xBee datalink tranceiver, and the elevation servo with position feedback.

The Head unit closed:                                                                   The base unit guts:

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Azimuth Mechanics and feedback POT

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                 The Helical and Diamond antenna in the head unit.

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We then flew a max range test to see what a typicall max range might be - Video was lost at 39km, and datalink was still error free with 200milliwats at that range...Video was 'snow free' ( P4 signal) at 32km.

Joe

The Nampilot....

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Ball Turret Gimbal for small Cameras

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Here are some photos and a short video of a 80mm diameter fully articulated turret Ball, currently fitted with the FLIR TAU-2 IR video camera. 

Camera and lens is approx 100grams. Complete assembly is 220grams.  Camera line of sight view is from 15deg below horizontal, full 360deg coverage. The Ball can be fitted with the smaller Sony Block cameras as well. Ball is 78mm OD - see the dimensions added to the one image right at the end.

The complete structure is made from composite. Male and female molds were machined from Polypropylene for the two half-domes. Timing gears and belts all from RS Components in South Africa.

From start of 3d CAD model to this prototype was 2 weeks.

The Gimbal is currently fitted to our SurVoyeur Aircraft, ready for Night Flight trials with the Ministry of Nature Conservation here in Namibia.

Here is a YouTube video of it working:

Turret Gimbal Vdeo

all Underside                                                                                              Assembly Side View

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FLIR TAU-2 camera

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Around 10 days ago I started a debate on wing Aerofoil choices to improve flight range rather than endurance. A number of folk joined in with ideas and suggestions, and the choice was eventually made - The Selig S7075 at 9% thickness.

Many Aerofoils were simulated and the S7075 choice seemed a good one,  so I proceeded to make a prototype set of wings to begin some trials.

The 'original' wing is a Clark Z Aerofoil, 316mm Chord and 1.8meter span. Fitted to our SurVoyeur aircraft with an all up weight of 4.8kg ( a heavy camera...) we achieve around 35km flight range at 21m/s airspeed on two 4cell lipo packs, each 5000mAH.

The 'New' wing is the S7075, chord 315mm, and span 2meters. The wings are CNC foam cut cores, vacuum bagged with Carbon cloth and mylars top and bottom. Ailerons are almost full wing length, but split so that the inboard can be used solely as flaps to test the limits for low speed autolanding and autolaunch.

Flight trials will start in the next few days.

For all those who were part of the discussions, here are some photos of the process and the results...

Regards

Joe

The NamPilot...Swakopmund

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