WAAS 135 & 138 navigation message

[ricky22]

Hello,

I've read in multiple online sources that two WAAS satellites, PRN 138 and 135, transmit both DGPS correction data and standard GPS navigation message, which means they can be used for a fix, in addition to the "normal" GPS satellites. PRN 133 on the other hand only transmits correction data, but not the GPS navigation message. My question is, is that correct? If so, how is that possible? AFAIK, commercial GEO communication satellites don't have atomic clocks on board. Neither is their position maintained to the same accuracy as GPS satellites, using multiple ground tracking stations, radars, lasers and what not.

My second question is, do SBAS systems like WAAS, EGNOS and MSAS transmit correction data for other GNSS constellations, like GLONASS and BeiDou?

Many thanks,

Patrick

 

[ricky22]

Hello,

I think I found partial answer to my first question.

"It is evident from their groundtracks that PRNs 135 and 138 are in more tightly controlled GEO orbits than that of PRN 133 whose orbital inclination is now at 3 degrees compared to the others each at less than 3 arc minutes."

That would mean that precise tracking to the same standard as the MEO GPS satellites is not necessary. Secondly, unlike with the GPS satellites, apparently the WAAS navigation message is generated on the ground, at a precisely known location of the ground station, sent to the WAAS satellite in the C band and repeated using bent pipe (had to look that up). That might account for the apparent lack of need for the atomic clock onboard, although I'd appreciate if someone could confirm.

The answer to my second question is as follows:

Currently operating SBAS (WAAS, EGNOS, MSAS, GAGAN) only support GPS. In the future, EGNOS, SCDM (Russia) and SNAS (China) will support GPS plus the countries' own GNSS systems (Galileo, GLONASS and BeiDou respectively). KASS (South Korea, in development) will support only GPS initially, then expand to include GLONASS and GALILEO.

Thanks,

Patrick

 

[ndoggac]

I can confirm.

For the GEO's, the accurate clocks are at the Ground Uplink Station (GUS), not on-board the satellite. The correction message however, is formulated (and verified for safety) at the WAAS master stations (ZLA, ZDC, ZTL) and relayed to the GUS's for up-link to the various GEO's. Both the formulated message received from the WMS, and the clock timing code generated locally at the GUS is relayed via C-band signal to the GEO. The hardware on-board the satellite translates that to the L-band down link (same as GPS) for broadcast to the user (the bent pipe).

You are also correct on the orbit of AMR being the main reason for lack of ranging on that GEO. AMR will be decommissioned from the WAAS system within the year. The satellite itself (Inmarsat 4F3) will remain in service for other means, just not as part of WAAS. The new WAAS GEO (PRN 131) which is currently in on-orbit testing will replace AMR. The new GEO (Eutelsat 117 West B @ 116.8 degrees west longitude) will provide GPS ranging in addition to the correction message data link.

There is also another WAAS GEO being launched soon (SES-5), but will not come online until 2019. It too is planned to provide ranging service.

Technically speaking, the WAAS design can support GLONASS corrections as well, but the politics of that decision are another matter.

 

[ricky22]

Thanks ndoggac!

 

[ricky22]

I think you meant SES-15, not SES-5. Due to be launched on May 18 to 129° W. This and Eutelsat 117 West B (WAAS GEO 6 and 5 respectively) will replace two WAAS GEO payloads that are approaching the end of their service leases, so the number of active WAAS satellites will remain at three.

 

[ndoggac]

ricky22, thanks for the correction. Indeed it is SES-15. That's what I get for typing on mobile. Original launch date was April 4th. Strikes in French Guyana delayed the launch.