SMARTNAV L1 RTK field test scenario - Results

SMARTNAV L1 RTK field test results, as promised. Please have a look at this previous post before reading.

The reference trajectory will be displayed in dark blue whereas SMARTNAV's output will be displayed as : 

- red for "single" solutions (when RTK is not available)

- yellow for "float" solutions

- green for "fix" solutions (maximum accuracy)

So first we will review all the points listed in the previous post.

  • Height differences

It is common knowledge that GNSS tend to output poor vertical position accuracy due to the low vertical spacing of satellites. We wanted to check whether RTK could enhance this.

Standard deviation is about 4 meters. This is not bad considering the "low VDOP" of satellites and the measurement conditions.

  • Bridges and tunnels

The idea behind this test was to monitor how receiver would deal with successive acquisitions/losses of satellites.

Receivers loses lock completely under bridges and tunnels. Reacquisition is quite fast and even if there are some glitches convergence is fast.

  • Large roundabouts

The trajectory is very good here, due to the open sky view. But it is difficult to measure repeatability under those conditions.

  • Urban canyon

The most difficult environment, really small roads with high buildings.

Here the output is not correct at all. The receiver has a hard time calculating the solution. Even the reference trajectory is not precise at all. This is normal considering the conditions, glitches go up to hundreds of meters.

  • Leafy streets

We wanted to know whether trees affect position stability or not.

 

At the moment we drove under the trees, they were masking the sky completely. We can easily see that solution is not robust in this kind of area.

  • Repeatability

Repeatability was difficult to measure with the car, because it is difficult to make sure to follow the same path several times in an urban area. So we did another test. SMARTNAV was placed on a square table, calculating real-time RTK solutions. Baseline was about 10 meters. SMARTNAV was moved around table's edge several times, always following the same path.

It is very easy to reproduce the same path. Here the accuracy is under 5 cm.

  • Accuracy/precision

In open sky conditions, accuracy is always better than 20 cm.

However, when conditions are difficult, offsets as high as 1 meter can be observed. Offsets tend to appear after a glitch is observed. A time of convergence shows up after a glitch to come back to the reference trajectory.

In stationary, this is the position calculated by SMARTNAV.

This is the position calculated by L1/L2 AsteRx (without inertial hybridation).

 

There does not seem to be much precision difference between the two.

  • Availability

NRTK (SMARTNAV real time) availability : 

- Fix : 27%

- Float : 62.5%

- Single : 10.5%

L1/L2 (post-processed) availability : 

- Fix : 50.6%

- Float : 46.1%

- Single : 3.2%

L1/L2 is much more available than L1, which means it will be more robust in difficult environments or longer baselines.

  • First conclusions :

- RTK in urban areas is not a pipe dream

- reacquisitions are fast

- excellent precision/accuracy in open sky views

- still quite strong in "medium" environments (could be enhanced by hybridation with an IMU or a better software cycle slip detection)

- not reliable when sky view is heavily obstructed

- L1/L2 does not look more precise than L1, but it is more robust and available

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3D Robotics
Comment by Chris Anderson on June 27, 2016 at 10:26am

Impressive! Is this natively supported by Pixhawk yet, or does it still require its own radio link?

Comment by Kevin Lopez Alvarez on June 28, 2016 at 12:22am

Yes it can be used with the "Inject GPS" function in Mission Planner, even though it might be a good idea to have a separate strong link for the differential corrections (10 kbps).

Comment by MAGnet Systems on June 28, 2016 at 7:01am

@ Chris, My 2 cents here in case someone finds it useful for RTK applications: The Microhard modem we’ve been discussing a few threads back has 2 individual serial ports, each one with adjustable baud rate. It also has one Ethernet port. One serial port can be used for the Pixhawk telemetry and the second can be used for the RTK data while the IP port can be used for video (optionally). We use the second serial port of the modem for extra serial sensors but the RTK GPS data is an excellent fit so you don’t have to use a separate radio, additional antennas, etc. 

Comment by kiki boy on June 28, 2016 at 7:12am

According to ublox specs we could have 10Hz with setting only GPS in M8T ( P7/35 => https://www.u-blox.com/sites/default/files/NEO-LEA-M8T-FW3_DataSheet_(UBX-15025193).pdf )

Does this data you mention @Kevin Lopez Alvarez ?

Comment by Kevin Lopez Alvarez on June 29, 2016 at 4:16am

For this test, rover has been working at 5Hz and base at 1Hz. 1Hz also for reference trajectory. Ad yes, M8T can go up to 10 Hz.

Comment by Kevin Lopez Alvarez on June 29, 2016 at 7:14am

@Chris, the integration is described here.

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