Oliver Huang's Posts (6)

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With enough satellite signals, single frequency RTK receiver works well under open sky environment. Of available ones on the market, we have yet to see one that works well in environment with surrounding trees and buildings. The above video shows a new level of performance with our soon to be released update firmware enabling to work under such condition.

The recorded NMEA is replayed using RTKplot, overlaid on video showing the test environment. Green dots represent RTK Fix points; orange dots represent RTK Float points. It showed RTK Fix being maintained in environments where other low-cost RTK receivers would change to RTK Float or have jumps. Even when changing to RTK Float, accurate positioning is still being maintained without any jump. This more robust performance is coming closer to what people expect with more expensive dual-frequency RTK receiver for autonomous applications.

Currently this is using our S2525F8-BD-RTK GPS/BDS receiver. We would like to make similar improvement to our S2525F8-GL-RTK GPS/GLONASS receiver, but require some assistance logging 20 minutes of static data on 3 separate occasions using two survey grade base and rover antennas that covers L1 GPS/GLONASS in Europe and North America where there is EGNOS and WAAS signal respectively. 

If anyone with suitable survey grade antennas living in North America or Europe kind enough having interest to offer assistance, we would be happy to supply a pair of our 10Hz capable S2525F8-GL-RTK receiver evaluation boards; please send me a message. Thank you !

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Holidays Greetings!

There is much interest in seeing demo of our $50 pre-order NS-HP RTK receiver on a quadcopter, so we tested last week on a less windy day (with up to 4.5mph wind). A compass module and 5Hz version of NS-HP RTK receiver replaced the original GPS and compass inside the 3DR Iris. These two videos show the result we obtained with default values without any tuning.

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Affordable RTK Capable GPS Receiver

We are pleased to announce availability of NS-HP, a $50 RTK-capable high performance GPS receiver. At 2 / 4 / 5 / 8 / 10 / 20 / 25 / 40 / 50 Hz update rate, NS-HP is a high-performance GPS receiver suitable for traditional GPS applications. At 1Hz update rate, NS-HP can accept RTCM 3.x message type 1002, 1004, 1005, and 1006, or SkyTraq raw measurement data from a base station to perform carrier phase RTK processing, achieving centimeter-level accuracy relative positioning.

When set to base mode, NS-HP output GPS carrier phase raw measurement data, the base antenna location can be set using binary command or can be self-surveyed.

Below figure show road test RTK performance of NS-HP driving on a road with clear view of the sky, back and forth on the inner lane 3 times, each time using one of three different antennas placed at the same location on the roof of a car. The antennas used are Harxon HX-CSX601A survey grade antenna, Tallysman TW2710, and a low-cost tri-band antenna. The baseline varies from 1.6Km to 2.5Km. Each color represents testing done using a different antenna. Although we tried to keep the vehicle on center of the lane, it’s not easy driving manually; the figure shows variation less than 40cm driving on the same road 9 times.



Below shows result using a popular brand normal GPS/GLONASS receiver; as expected the variation is much greater than an RTK receiver.


Static test scatter plot of 5 minutes RTK fix solution w.r.t. baseline is shown below; with centimeter-level precision.


Minimum hardware connections needed for NS-HP are:

UART1 TX1:    to output NMEA result

UART2 RX2:    to input base station data (not needed for non-RTK use)

VCC33:            3.3V power

GND:                ground

RFIN:                connect to GPS antenna


Using a Bluetooth serial module (2.1 EDR) connecting to TX1 and RX2, running NTRIP client software (Lefebure) on Internet-connected Android smartphone, setting smartphone Developer Options to allow mock locations enabling use of external GPS, one can immediately upgrade their Android smartphone GPS with high-precision RTK capability, one step toward "The Democratization of Accuracy (http://blogs.esri.com/esri/esri-insider/2015/09/03/the-democratization-of-accuracy/)”, an interesting blog by Brent Jones. Mobile GIS data collection on Android smartphone using cloud-based software (http://www.terragotech.com/products/terrago-edge/gps-receivers) would be possible.


Connecting RX2 to wireless radio to receive base station data, the NS-HP TX1 NMEA output can be used for outdoor precision machine control in the fields.


For base station data, it can come from public/private RTK base station within 10Km retrieved using a NTRIP client, public/private RTK base station that provide virtual base station (VBS) service then distance can be relaxed, or using another locally placed NS-HP in base mode.


The only catch with NS-HP is that, it’s GPS-only, time to RTK fix (TTRF) can vary w.r.t. GPS satellite availability and orientation as below figures show. We in Asia see many GPS + BDS satellites combined, TTRF is consistently much shorter with GPS/BDS RTK receiver. For GPS/BDS RTK receiver users in the US, due to currently very few BDS satellites visible, GPS/BDS RTK will behave like GPS-only RTK; users will see performance improvement over the years as more BDS satellites are put into orbit from 2016 ~ 2020, until BDS becomes fully operational in 2020.





NS-HP uses carrier phase measurement to achieve centimeter-level accuracy. Compared to normal GPS receiver, NS-HP is much less resistant to noise, noise level that normal GPS still function reasonably well the NS-HP likely will not able to get RTK fix but will have float or single solution, giving similar accuracy as a normal GPS receiver. Thus for UAV application, unless hardware could be redesigned with shielding to minimize noise radiation, ad hoc method might be needed to get NS-HP to work on a UAV (including placing antenna at different location, using RF absorber, changing board-level module to other brands that doesn’t interfere...).


Other known issues with NS-HP for RTK applications are:
1. Single-frequency RTK works within 10Km baseline
2. Operate only under open sky
3. Require 6 or more satellites above 15degree elevation angle with good satellite geometry
4. Require signal over 38dB/Hz


NS-HP is available for pre-order on the NavSpark website. First batch of 50pcs pilot production ships out on December 15.


Thank you for your interest!


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Antenna: Harxon HX-CSX601A
Receiver: SkyTraq GPS/BD RTK receiver
Baseline: 1.7Km
Max Speed: 81.2Km/hr
Blue point: single solution
Yellow point: float solution
Green point: fixed solution

RTK is well known for land survey and automated precision farming applications, which are low speed or static. With emergence of lower-cost single frequency RTK receivers for less demanding applications (cm-level accuracy yet with shorter baseline not beyond 10Km and longer time of around 2 minutes to reach initial cm-level accuracy solution) we are interested find out how it performs under dynamic situation.

A road test was performed on a car; we see that it could maintain RTK fixed solution (meaning cm-level accuracy) up to 81Km/hr; it’s over speed limit of the road on which we are testing, not the limit of the receiver though.

Currently we are still using survey grade antenna to first understand performance capability of the single frequency RTK receiver, not to be limited by performance of smaller antenna in these early tests. After more testing done understanding the performance capability, we'll try smaller antenna on quadcopter!


More information can be found here:



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RTK DGPS GPS Comparison


We were running some tests on RTK / DGPS / GPS these two weeks. Thought it might be interesting to share our results in image format, although the accuracy numbers are already well known.

Test setup is as below:




Below is RTK result from 3 different days; black:20150716, green:20150721, white:20150723

As RTK accuracy is centimeter level, they overlap pretty well at this zoomed-out view.


White line RTK result is used as frame of reference to compare DGPS and GPS results



Below is DGPS result from 2 different days; red:20150716, cyan:20150723. Result is quite good since distance from DGPS base station is only 1.5Km.


Below is GPS result from 2 different days; red:20150716, cyan:20150723.

The shifted red track is due to ionosphere error that changes over time, which accounts for major error in a typical 2.5m CEP accuracy GPS receiver. It just happened that ionosphere error was less on 20150723, resulting in the cyan track closer to the correct white track.

What’s noteworthy is that the two rounds made on each day overlap quite well, meaning that the GPS receiver is fairly precise, just that the accuracy is still 2.5m CEP.


Below is 20150723 result with another popular GPS receiver brand for comparison. 


Overlapping all previous plots, below figure results; giving a glimpse of potential GPS error on day to day basis.


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Single Frequency RTK Receiver for UAS


Recent developments in using single-frequency dual-satellite navigation system for RTK application (L1 GPS + L1 GLONASS, or L1 GPS + B1 Beidou), with more satellites overhead at any given time resulting in much higher availability getting ambiguity fixed solution, it has brought lower-cost single-frequency centimeter-level accuracy RTK receiver closer to reality. Using GNSS Radar, below figures show what satellite availability might be like over a 12 hour satellite orbit period at different locations. 









In East Asia, Australia, and India, there are more GPS + Beidou satellites over the horizon than GPS + GLONASS satellites. In Europe and South Africa, slightly more GPS + GLONASS satellites than GPS + Beidou satellites. In North America and South America, number of visible Beidou satellites is low.

With goal of developing a small low-power single-frequency RTK receiver module, for ease of debugging, initial development of RTK software is done on a PC, with below recent results:







Although above image looks wobble a bit, the result on the athletic track should be good, considering it’s not easy to hold the antenna pole straight and perpendicular to the ground without shifting the antenna slightly.

For comparison, below is from jogging around campus, logged using Nike+ using Galaxy Note 3; four passes over a same route, with fifth path making the smaller round.




With more available satellites using two satellite constellations, time to resolve integer ambiguity achieving fixed RTK solution is much shorter than using single GPS constellation alone (90 sec with 7Km baseline in above example), and it has high availability at any time of day. Above test results show exciting promising potential for emerging applications requiring high position accuracy using lower cost single frequency RTK receiver.


A complete SMD receiver module prototype about the size of 25mm x 25mm will be developed, beginning schematic design next week. A separate interface adapter board will have accommodation for wider power input range, data logging, and other interface if needed. Now at beginning development stage of the target hardware, if there are developers interested in adding software support making this RTK solution workable with Pixhawk or Ardupilot, would be happy to try accommodating the needed hardware interface on the board design. Cheers!

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