Improving Ublox Accuracy for Automatic Landings

[Peter Hollands]

Of late I have wanted to do more automatic landings.  Automatic Landings require accurate altitude. However my experience with Ublox altitude is that it has never been accurate. I have seen altitude drift by over 50 meters during a flight (verified by on board video against telemetry). By contrast, some people in the USA seem to have an altitude accuracy typically of around 2 meters. I  have been reviewing the Ublox code and configuration for myself 60 miles West of London in the UK. 

Here are a few notes:-

1. The Ublox code in MatrixPilot used to setup the  SBAS (Satellite Based Augmentation System) reception in Ublox. However, the software interface changed in version 8 of Ublox, and so MatrixPilot currently does not set up SBAS correctly for version 8 Ublox devices onwards.
2. You can setup the correct configuration of SBAS using the Ublox configuration tool called U-Center. Then, if your ublox supports flash memory, you can save a new permanent configuration for SBAS that is independent of MatrixPilot settings. This is the current workaround.
3. The generic multi-national version of GPS is called GNSS (Global Navigation Satellite System). You can configure your Ublox to pick up satellites from the American GPS system, the Russian Glonass system, and the Chinese Beidou system.  By default, my Ublox was configured to pick up GPS, SBAS satellites, GLONASS, and QZSS (Effectively an SBAS system for the Pacific near Japan). For a number of years the European Galileo system has been in a test mode while the full set of operational set of satellites are launched.
4. The Ublox version 8 manual, recommends that you specifically choose the most appropriate SBAS system depending on your location, and switch off all other SBAS systems. In Europe and the UK the best SBAS system is EGNOS (European Geostationary Navigation Overlay System). I have ensured that the American WAAS SBAS system is switched off in my configuration. 
5. I enclose a photo of my computer in the garden (back yard). (enclosed below). I was not able to test in my house even though I was using the mRo (Jordi) large and super sensitive GPS unit (Neo-M8N) board; it could not pick up any SBAS satellite in my house. I ran u-center under Windows 10 under Parallels on a 2012 Imac running MacOS High Sierra. It works fine. The GPS unit has a USB socket which enabled me to plug it straight into my computer (No FTDI required).

I attach a screenshot of my GNSS configuration, along with a sky diagram of he satellites being received. EGNOS has a designated number of S123 on the screen. Satellites beginning with the letter G are GPS, while R are Russian Glonass.

There is a screen which shows for each Satellite being received whether SBAS corrections are being applied. (See below for u-center_sbas_status_by_sat.jpg) I was interested to see that SBAS corrections via EGNOS are only corrected for American GPS satellites. The Russian Glonass Satellites are not corrected. This raises the question as to whether it is best to turn Glonass reception off Glonass when using SBAS ? 

The Sky view of antenna reception of satellites showed me that the EGNOS Satellite (GeoStationary), is only about 20 degrees above the horizon to the South East. This probably explains why it cannot be received in my study in the North West of my house. (Too many walls between me and the Satellite).

The top right window in U-Center shows whether I have a 3Dfix ( which means SBAS is not working) or "3D/DGNSS"  (3d Differential GNSS meaning SBAS is working).

I was able to verify that SBAS was working in the garden with this status.

With SBAS working I then watched the altitude wander by about up 10 meters and then down 20 minutes over a few minutes while the receiver was sitting still on a small table in my garden. So I have not achieved a good, reliable altitude reading.

I hope that my exploration of Ublox SBAS settings will be helpful to others.

Please feel free to add to this thread if you have tips on configuration, or experience a much better accuracy of altitude in your location.

Best wishes, Pete

[Guy-François MESNIL]

Hi Pete,

I beleive that you cannot reach an altitude precision better than some meters, especially if you are descending for landing, that means without long term filtering.

I suggest two solutions :

1. Use an altimeter, like the LIDAR if you need high measurement rate or ultrasonic device if not;
2. Use a differential GPS with a static base near the landing place.

I always enjoy to read you,

Best wishes,

gfm