Via Hackaday:
Where most GPS receivers only look at the data coming from the GPS satellites orbiting overhead, the Piksi uses another technique, real-time kinematics (RTK), to determine the receiver’s location with exacting precision. The basic idea behind RTK is to look at the carrier frequency of the GPS signals at 1575.42 MHz. This frequency has a wavelength of 19 cm, compared to the alternating 1s and 0s of the that are transmitted at around 1 MHz, or about 300 meters between each bit. While centimeter-level precision isn’t possible with only one receiver, two of these Piksi boards – one base station and one on a vehicle, connected via radio link – can make for a very exacting high-accuracy GPS receiver.
Comments
Yep, if you add all the options we need to get RTK working on a copter or perhaps on a rover (100 Hz RTK rate and advanced multipath filtering), you end up at about 18 000 $ for a single module, and you'll need a second one for the ground station.
In this case the only remaining option for better accuracy seems to be an Omnistar subscription. (needs an expensive two channels GPS as well).
Without ground station, using carrier phase on the rover will only give relative precision enhancement, not a better absolute precision.
It seems to me that carrier phase would ask for a very fast acquisition GPS frontend to avoid cycles slips.
And for a rover a few centimeters above the ground, multipathing will be even more challenging with a carrier phase solution. I doubt it can work.
There are other problems poping up implementing RTK on a UAV :
On a copter we have the banking angles problem, that can induce fast phase jumps. Another problem is signal interferences (FPV video transmitter, RC, telemetry...). For RTK to work, the signal must be perfectly clean.
Yet another problem : RTK or other carrier phase solutions convergence can ask for about 30 - 40 minutes... Not very convenient. And do not forget the time needed to get the base station position, something like one hour to get a good static solution convergence.
Without serious antenna mounting and interferences isolation, it seems that an RTK solution on a UAV will perform worse compared to the ublox stock solution.
According to this report, the RMS precision of a RTK solution on a UAV could not be better than 1 meter.
http://www.asl.ethz.ch/people/slynen/personal/student_projects/2012...
Before spending large amount of time on a low cost RTK solution, i think that it would be interesting to try a high end commercial solution on a rover and on a copter, and see if that really works.
Here is an interesting table comparing DGPS vs RTK solutions for farming GPS applications.
http://www.agri2.com/index.php?page=accuracy-en
I still think that if we can get DGPS working correctly with a local base station, it will be enough for our use with about 10 cm precision and without the hassle of RTK phase jumps.
For those that have not backed (or who aren't following) the Piksi Kickstarter Project yet... they just released an update: "I know a lot of our backers are flying APM based systems. I wanted to let you all know we are teaming up with 3D Robotics to integrate Piksi with APM and make sure that ArduPilot is one of the earliest supported systems."
Sounds exciting to me!!
@monroe
Why dont you start a kickstarter ?
Dan, is it really necessary to use RTK, when DGPS with a local base station can give about 10 cm accuracy and seems precise enough for our use ?
If 10 Hz RTK could be ok for farming, i doubt it will work on a copter with less than 100 Hz acquisition rate because of the small wavelength of the carrier (about 20 cm). 100 Hz GPS modules are not really cheap.
To avoid phase jumps, i think that the max distance between two consecutive acquisitions should be not more than 10cm (1/2 wavelength). At 10 m/s, 0.01 second is needed to get a 10 cm move. This translate to a 100 Hz acquisition rate to keep a 10 cm precision at 10 m/s.
At 30 m/s, a 300 Hz acquisition rate would be needed to keep RTK working.
The advantage of DGPS is that it is using a pseudorange only solution, so the problem of phase jumps and fast acquisition rate L1 / L2 expensive receivers are discarded.
The only concern with DGPS would be correction latency, i can't find practical informations about the correction latency when DGPS is used with a local base station. Do you know what we can expect here with RTKlib, and if the RTKLib DGPS solution is reliable ?
hi guys, exciting stuff here... personally I want the RTK for locating my ground targets at a reasonable price.
I am paying big dollars to have surveys done and of course timing etc gets in the way....
I would love to have something that gets me a base station, and rover with sub 5cm accuracy....
I think 900$ is still pretty high for what you get... however you have to pay for the software and need to be confident it will work. I would buy one but all the developemtn boards are sold out.
Steve
W.D, do you have an idea about the amount of correction latency we can have on a DGPS system with a local beacon and RTKlib ?
If it's a couple seconds, then it could enhance navigation, if it's in the 10 seconds or more range i'm afraid that DGPS will not be so useful, or an advanced prediction algorithm will be necessary to reduce DGPS latency.
http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=509111&am...
Using RTK instead of DGPS seems out of reach for a copter. Too much vibrations, banking angles and fast moves. It would certainly ask for a costly L1 / L2 high end GPS receiver to filter out carrier phase jumps.
I salute any initiative in the tech realm, particularly those involving DGPS but it seems you could buy two Ublox LEA-6T receivers ($149/ea), two transceivers ($100/ea) and have quite a bit left to provide interconnections. If you want to go low-ball buy two Ublox LEA-4Ts for $20/ea - you'll be ok if you're working outdoors.
Then, download rtklib and you'll be good with the results.
The issue with not stitching is that often the producer (farmer) wants to feed the information remote sensed into other ag-focused software packages for things like variable rate spraying.. if you are just looking at a bunch of 400ft x 600ft images taken from 400 feet up at 12MP than its hard to get a understanding of what is going on in the field as a whole. Individual images also make it hard to understand where that image is taken and get your bearings straight.
You are most likely right on the cheap ndvi camera like that kickstarter one. More than likely you won't be able to get the precision that you would want.. and without some sort of calibration/reflectance panel usage then week to week sorts of analysis just won't work very well due to atmospheric condition differences.
In theory you could not stitch and just have a bunch of images.. the cheap camera isn't going to be geo-tagging those, so you will have to post process the imagery using some sort of gps logger / flight logs, which likely won't match up very well with the camera's clock (assuming it has a clock)... which is the only way to know which tag goes with which image.. and if the camera has a clock then they likely aren't that in sync.. and at the speed you likely captured the imagery at, the tags could be way off the mark... not to mention the already +/- 30 feet gps precision issues.
Seems just problematic and a lot of work and likely not very ideal results the end up taking a lot of extra time to deal with each time you go out and fly... if your time is plentiful and free than great.. but not sure if it pays to be cheap in the end on things like this.
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