NavSpark

Contents

Quadcopter + RTK flight test on a day with 7kmh wind:

https://www.youtube.com/watch?v=mFhKbGHDNtg

https://www.youtube.com/watch?v=FA8eI41Mw44

How to track movement of race car on track, drone maneuver in sky, or athlete sprinting on football field for performance analysis?

A simple way may be using RTK GPS.

But how accurate is it?

For accuracy and consistency check, we used one antenna with signal splitter feeding signal to two $50 NS-HP RTK capable GPS receivers, driving on open sky road to log NMEA output for analysis. NMEA result when converted to KML and measured on Google Earth, maximum deviation of two tracks is less than 1 centimeter!

With such cost-effective precision measurement tool, how can you make use of it to improve your performance measurement?

NMEA log files and converted KML files for this experiment is available here: http://navspark.mybigcommerce.com/content/Two_NS_HP_Cmp.zip

High precision Bluetooth RTK GPS receiver on market cost many thousand dollars. They work with smartphone, boosting smartphone’s internal meter-level GPS accuracy 100X to centimeter-level accuracy using external RTK GPS.

Using a affordable RTK GPS receiver, a GPS antenna, and a Bluetooth serial, one can have this high-precision RTK GPS for smartphone under $100.

Below shows simple hardware hookup needed:
green wire: 3.3V
white wire: ground
yellow wire: NMEA from RTK GPS to Bluetooth serial
blue wire: base station data from Bluetooth serial to RTK GPS

On software side:
Install Lefebure NTRIP Client from the Android Play Store

Select Settings Option

Select NTRIP Settings.
Set Network Protocol to NTRIP v1.0
Set up IP, COM port, user name, password of accessible base station providing RTK service. Select suitable data stream available that match user name and password entered.
A article on available public RTK base station in USA is available here: http://gpsworld.com/finally-a-list-of-public-rtk-base-stations-in-the-u-s/

When done, change to Receiver Settings page.
Receiver Connection choose External via Bluetooth. Before selecting Bluetooth Device, enable smartphone Bluetooth, scan if the Bluetooth serial device has not paired before. After pairing Bluetooth, for Bluetooth Device you can select Bluetooth serial used for RTK. You can decide if to save GPS NMEA data to file or save NTRIP base station data to file. Select GPS Mock Locations to allow other App to use RTK GPS.

When done, change to Display Settings page.
Change Info Box1 and Box2 to show longitude and Latitide so you can see position coordinate changing.

When done, go to top level screen and click Connect. NTRIP Client first try to connect with base station over Internet. When connected, no GPS fix available it shows Invalid and zero for latitude and longitude.

After getting position, it first show normal meter-level accuracy GPS position fix, with 10 satellite in this case.

After 10 seconds it shows FloatRTK, meaning RTK receiver is trying to resolve integer cycle count number for each satellite in order to get centimeter-level accuracy.

After getting to centimeter-level accuracy, it shows RTK,

If you run GPS application, such as GPS Essentials, in the Satellites window, one can see position accuracy estimate is 0.02 yard.

If using internal GPS of Samsung Galaxy Note 3, position accuracy estimate is 3.3 yard.

Now one can proudly own survey grade centimeter-level accuracy RTK receiver working with his smartphone, like those couple thousand dollar Bluetooth RTK receiver on the market: http://www.terragotech.com/products/terrago-edge/gps-receivers !

Testing is done on roof of building. 3.3V from USB Serial breakout board used to power RTK GPS and Bluetooth serial. All 3 breakout board can be put into protective 3D printed enclosure with UFL-to-SMA connector sticking out like those professional Bluetooth RTK receivers, have option to use better antenna if needed.

Lefebure NTRIP Client
https://play.google.com/store/apps/details?id=com.lefebure.ntripclient&hl=en

GPS Essential
https://play.google.com/store/apps/details?id=com.mictale.gpsessentials&hl=en

25mm x 25mm S2525F8-BD-RTK single-frequency cm-level accuracy RTK module is nearly ready.

Below is a test board that allows receiving base-station RTCM 3.0 data from Bluetooth 4.0 over smartphone's 3G/4G network running ntrip, the rover module receives global positioning satellite signal, performs on-board RTK processing to generated cm-level accuracy position data by itself, and output the result over Bluetooth 4.0 to other applications. 

Alternatively another carrier phase raw measurement module can be used in place of the base station for high-accuracy relatively positioning applications such as precision farming, grass cutting, or UAV multi-rotor precise landing.

A new evaluation board is being prepared with interface options of Bluetooth 4.0 or USB 2.0 or pin-header for both input and output. The UART pin-headers can be used to connect to other wireless interface modules when needed.

Dynamic Testing

Antenna: Harxon HX-CSX601A
Baseline: 1.7Km
Max Speed: 81.2Km/hr
Blue point: single solution
Yellow point: float solution
Green point: fixed solution

The dynamic test on the road, being able to maintain fixed solution at speed over 80Km/hr, shows promising result for precision navigation guidance beyond low speed agricultural and data collection applications.

We are setting up fixed IP for our base-station, to allow more robust RTCM 3.0 transmission over the Internet for longer baseline faster speed dynamic testing.

Aug 27, 2015

Below is test result we got earlier showing performance of our single frequency RTK receiver in terms of baseline distance.

The rain is pouring heavily today. Not knowing how single frequency RTK would perform under such condition, our engineer went out and did some testing.

Rover Antenna: HX-CSX601A
Test Environment: Road with open sky view

Static Testing with 4.5Km Baseline: It took slightly more than 4 minutes to get fixed solution; which is much longer time than earlier results on a sunny day.  

Dynamic Testing with 4.5Km Baseline: On section A the car drove on adjacent 3 lanes separated roughly 2 meters apart, on section B the car drove on the same lane on each pass. The result shows mostly float solution with small number of fixed solution. What's remarkable is that the 3 tracks on section A are distinctively on separate lanes roughly 2 meters apart running in parallel, and the 3 tracks on section B roughly overlap; a result similar to getting all fixed solution while actually it's float solution most of the time. It's operation under such non-ideal conditions that typical users will encounter and pushing to get good performance out of single-frequency RTK that we have been spending time optimizing.

Sep 5, 2015

We received a compact active helix antenna sample for high precision application. Incredibly when mounted on roof top for preliminary testing, it does receive signal very well and can get RTK fixed solution. This antenna + S2525F8-BD-RTK module + Bluetooth + battery configuration, receiving RTK correction data via NTRIP over smartphone's Internet connection and make the NMEA output position data available to other smartphone App, might be able to serve as a portable RTK receiver for anyone seeking 100X better position accuracy than what their smartphone offers (from meters to centimeters), or for data collection usage as mentioned in this interesting "democratization accuracy" article by Brent Jones.

More testing will be done later with this interesting antenna. Below is how it might fit with SMA connector device:

Some customers asked about availability of getting fixed RTK solution with NS-RAW. Although the answer may vary depending on location, we did a 12 hour experiment using host-based RTK receiver we are developing, it sheds some light on possible fixed solution availability using other RTK software.

For GPS-only, we see average of 73% fixed solution availability over 12 hour GPS satellite orbit period. The setup uses HX-CSX601A antenna.

Although NS-T generated frequency lock accuracy is better than 10^-11, there is phase jitter. This experiment tries to clean up phase jitter with $2 PLL : http://www.digikey.com/product-detail/en/PI6C4511WE/PI6C4511WE-ND/1305452. PLL used has output minimum of 2X input frequency, so 10MHz input will have 20MHz output, 19.2MHz input will have 38.4MHz output. Jumping wire to test, there is severe ringing on the output signal, but will see phase jitter cleaned up. PLL properly mounted on PCB with impedance matched should greatly reduce ringing. If using this PLL, NS-T need to be programmed with 1/2 intended frequency, so PLL output will be 1X intended.

PLL Setup:

Original 10MHz output having jitter:

20MHz output after passing PLL:

Original 19.2MHz output having jitter:

 38.4MHz output after passing PLL:

If anyone is interested in 1PPS phase-locked programmable frequency output of NS-T precision timing mode receiver, below are videos for 10kHz, 100kHz, and 10MHz.

http://www.ebay.com/itm/DDS-Signal-Generator-Module-AD9850-0-40Mhz-Sine-Square-Wave-/321497693136?pt=LH_DefaultDomain_0&hash=item4adac17bd0

Key difference from above low-cost DDS generator module is: Low cost DDS generator uses 125MHz +/-20ppm crystal oscillators, synthesized 10MHz still has +/-20ppm variation, that is +/-2E-5 accuracy. NS-T generated 10MHz clock signal has measured pulse difference on order of +/-5 pulse over 16 hours for example, that is +/-1E-11 accuracy (5/3600/16/10e6) while GPS is operating. Thus frequency accuracy is quite different between these two product.

10kHz Output Phase-Locked to 1PPS

100kHz Output Phase-Locked to 1PPS

10MHz Output Phase-Locked to 1PPS

We have been working on Beidou carrier phase raw measurement output for some time. Below is exciting test comparison showing for single frequency low-cost receiver, more satellites has higher ambiguity-resolved fixed rate. GPS L1 + Beidou B1 combination gets more satellites, helpful especially in urban environment. The test is done using not yet released RTKLIB with Beidou capability, collaborating with Tokyo university.

Receiver Module: S1722F8-BD-RAW

Antenna: HX-CSX601A (base and rover)

Post-Processing Software: RTKLIB

Test A: roof-top, baseline 13 meter, time = 5 minutes

Test B: baseline 800 meter, time = 15 minutes
There are 2 high buildings in front of rover, blocking some satellites in this direction.

At any time, GPS + Beidou combination has more satellites available in Asia Pacific region, giving higher ambiguity resolved fixed solution availability.

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Some customers ask us for precision timing mode NavSpark. We added NS-T to web-store, http://goo.gl/SjYGoI. It has additional 1Hz ~ 19.2MHz programmable frequency output phase-locked to 1PPS.