SBAS support

[Костя Стаханов]

In my MOTO G75, SBAS support is not officially declared, but in the application I see the following picture (in the screenshot). Since there is no letter U (used), does this mean that the smartphone simply sees the satellites, but does not take them into account?

[Sean Barbeau]

Correct, if there is no "U" for "Used" flag, then the signal is not used in computing the position.

Sean

[NLTrams]

There are two things to look at, I think.  (1) Is the SBAS satellite ranging information used in the position calculation (like any "normal" GNSS satellite)? And (2) are the corrections supplied by the SBAS satellite used in the position calculation? 

My guess (with no documentation to back it) is that "U" next to an SBAS satellite would indicate that the ranging information from the SBAS satellite was used in position calculations.  This generally doesn't happen (geostationary SBAS satellites are in a much higher orbit than other GNSS satellites so signals are weaker, also their orbits are less well defined / measured than GNSS satellites).

As far as I know there is no indication in Android of whether the corrections supplied by an SBAS satellites are used in the position computations (i.e. whether a Fix is SBAS-corrected or Autonomous).

[ndo...@gmail.com]

The only SBAS to provide ranging signals from their GEOs are the USA's WAAS, with healthy ranging UDREIs (a health bit in the SBAS 250bit message) designating it usable for ranging).  That being said, I've never seen a WAAS GEO utilized in a position solution on an Android device.  Believe its a limitation of both the chipsets and the Android API (ie it would have to be implemented in both).  GEO ranging can make a big difference for aviation receivers using them in their position solution, especially when GPS satellites in view are diminished.

Every other SBAS has their UDREIs set to "Do Not Use" for ranging purposes.  They use the GEO solely to send the modulated correction messages (250 bps) which does include the almanac (for all GEOs in the SBAS) and ephemeris (for only itself - the broadcasting GEO).

Every SBAS only sends corrections for GPS satellites.  There are no corrections sent for Galileo, Glonass, Beidou, etc.  This likely will change in the near future, probably lead by Europe's EGNOS as they go v3, which I believe includes multi-constellation.

PRN 134 (shown flagless above in the image) is KASS (Korean Augmentation Satellite System).  They are now operational for aviation purposes.  There are five SBAS operational and approved for aviation use currently:  WAAS, EGNOS, MSAS (Japan), GAGAN (India), KASS.

https://www.gps.gov/technical/prn-codes/

Several SBAS are starting to send correction messages on L5 as well, for a dual-frequency SBAS solution.  About a year ago they released the MOPs on the RTCA site defining how a DF SBAS user receiver should operate.  WAAS should likely have a test signal OTA in the next year.  Dual frequency SBAS coverage will be limited until the GPS constellation is fully modernized to all DF (L5) capable satellites.  During that time, GEO ranging will be more important to availability for aviation receivers.  DF SBAS coverage could be improved much sooner if an SBAS goes multi-constellation with Galileo.

[NLTrams]

Thanks ndo...@gmail.com - that's good information.  

I've seen WAAS satellites being used for ranging on Survey-grade receivers (probably more than 10 years ago), I wasn't impressed by the results but that could just have been GNSS firmware issues at the time.

[ndo...@gmail.com]

NLTrams: "I wasn't impressed by the results.....firmware issues..."

I've definitely seen my share of receiver firmware issues, so you could be right in that regard.  However, remember that SBAS's main focus isn't accuracy for users like surveyors who need cm level accuracy via RTK or PPP.  Its main goal is integrity & safety for aviation.  Bounding the uncertainty of the navigation error so well that you get a 10^-7 probability of hazardously misleading information that could cause a crash.  It allows the user receiver to eliminate much of the error, but then to also calculate a protective bubble of certainty around the airplane.  If that certainty bubble is small enough, you can use your calculated position to fly a published precision approach to a runway, or other similar published air procedures.

That being said, SBAS does improve accuracy rather well.  In Quarter 4 of 2024, it improved GPS 95% errors of 4m Horizontal & 8m Vertical (page 23), down to 1m Horizontal & 1.7m Vertical (page 52 & 53) for free to the public (other than taxpayer money that is).  Most of that GPS error is due to ionospheric error, since we're nearing the solar max and had a couple major storms in that time period.  SBAS models the ionosphere and corrects for it very well for single frequency users, except during only the strongest solar storms where SBAS simply reduces its availability accordingly.  You also get the integrity benefit, certainty that the GPS satellite is performing within spec so you don't utilize it blindly even if its sending you trash.  SBAS will correct for the incurred error, or set the satellite to "Do Not Use" if the GPS satellite has bad enough issues.  WAAS has been operating since 2003 without major issue.  It is very popular with the GA pilot industry, and is starting to be adopted widely by large airlines.  It has worked so well, that other countries followed suit to build their own regional versions.

On a separate note, that image above does not seem realistic to me.  Perhaps it's a composite image?  First of all, PRN138 (the ID) is owned by USA WAAS, but the physical GEO no longer exists, and it hasn't broadcast a signal in years.   Also, those GEO PRNs are so widely dispersed around the earth that you'd never see all those in view from any single location.  If your phone is showing you that SBAS GEO status currently, then something is really wrong with your phone.  PRN131 WAAS covers nearly all of North America, but you're also seeing Russian, European, Japanese, Korean, and Indian GEOs....it is physically impossible.

[Sean Barbeau]

>PRN131 WAAS covers nearly all of North America, but you're also seeing Russian, European, Japanese, Korean, and Indian GEOs....it is physically impossible.

That's a really good point. The SBAS signals also show a signal strength, indicating a live signal.

k.stah@ could you please open an issue for this at https://issuetracker.google.com/issues/new?component=313183&template=0 with the screenshot and device info?

Thanks,

Sean