SBAS use for differential GPS

Over here in the States the SBAS (Satellite Based Augmentation Systems) are the FAA's WAAS satellites. They can be used by any Trimble Survey receiver for differential, but that has nothing to do with RTK or the RTK engine. If you use the right coordinate system in Access WAAS can give you about 2' Hz and 4' V pretty consistently.

I believe that the European counterpart to WAAS is called EGNOS, and should be able to be used in the same manner. Set up a Survey Style, in Rover Options change the survey type from RTK to RT Differential, and select SBAS. When I set one up for WAAS I turn off GLONASS and set my precisions to 3' Hz and 6' V. Then I can use Auto Store for Topo just like with RTK.

sorry - to clarify, by differential GPS I mean RTK with a base station.

squowse, post: 352189, member: 7109 wrote: sorry - to clarify, by differential GPS I mean RTK with a base station.

Got you - when we say "differential" - at least here in Louisiana where we've surveyed with it for decades - we're talking about L1 C/A code meter-level differential; when we say RTK we mean fixed ambiguity carrier phase.

I know you can get close to RTK accuracy with an R10 and the Trimble RTX service, I think they rate it at 4cm. As far as SBAS is concerned, I'm pretty sure the best you can do is a couple feet, at least using our WAAS satellites. I know less than nothing about EGNOS though.

Lee D, post: 352190, member: 7971 wrote: Got you - when we say "differential" - at least here in Louisiana where we've surveyed with it for decades - we're talking about L1 C/A code meter-level differential; when we say RTK we mean fixed ambiguity carrier phase.

I know you can get close to RTK accuracy with an R10 and the Trimble RTX service, I think they rate it at 4cm. As far as SBAS is concerned, I'm pretty sure the best you can do is a couple feet, at least using our WAAS satellites. I know less than nothing about EGNOS though.

Yes sorry you;re right - DGPS is mostly used meaning that.

I was just wondering if the L1-C/A code they transmit can be used for RTK (and static) calculations in the same way as a GPS satellite.
Or is there no point in tracking them at all, if you are not using the SBAS correction transmission?

Good question - I don't know if the RTK engine uses it or not. They do of course have the check box in Rover Options.

As far as for static processing, I've never been aware of the WAAS observations being logged, nor have I seen them in a RINEX file or Baseline Processing Report.

Ashtech's Blade technology used any SBAS for ranging, however it is L1 only, but since the SBAS satellites move very little it is more useful for fast fixes with L1/L2 than for ranging. I believe it was most important in the ProMark 3 L1 only RTK. It was in Ashtech receivers from the Z-Max on up and was broadcast for RTK use.

I am unsure which of the Spectra Precision receivers now use it. I believe it is also used now in some of the farm equipment GPS receivers.

IT WILL BE MUCH MORE VALUABLE WHEN THE L5 SIGNAL IS ADDED TO THE WAAS SATELITES.

Years ago I had a RINEX file with it for curiosity purposes but never had a receiver capable of doing it myself.

Paul in PA

Paul is correct. The Ashtech receivers (PM800, PF500, PF800, PM120, PM220, SP80, SP60 and even the PM3) do use L1 in the RTK calculations. And it is fairly valuable because there is no Doppler; I have been told that each SBAS SV is worth two regular SV's. I believe the modern Trimble boards (BD970, BD930) also do ranging to SBAS.

A good receiver might make 0.3 meters with WAAS (after some time). I am told that EGNOS is 0.5 meters. WAAS is IGS08 current epoch, so one must do a translation from IGS08 to NAD83 2010.0.

Mark Silver, post: 352241, member: 1087 wrote: WAAS is IGS08 current epoch, so one must do a translation from IGS08 to NAD83 2010.0.

0.3m is possible after a long enough convergence time. What I'm seeing day-to-day when I use the ITRF - NAD83 transformation supplied by Trimble is around 2' or so Hz.

Mark,

I never inquired, where do you get SBAS ephemeris data, or do they just send an XYZ location?

Paul in PA

There are SBAS subscription services that provide almost RTK results but not quite- I think in the 0.2-0.5 meter range. It's never really been a survey product but more of a precision ag or marine product. The advent of RTN has reduced the demand for such services on land.

Lee D, post: 352188, member: 7971 wrote: Over here in the States the SBAS (Satellite Based Augmentation Systems) are the FAA's WAAS satellites. They can be used by any Trimble Survey receiver for differential, but that has nothing to do with RTK or the RTK engine. If you use the right coordinate system in Access WAAS can give you about 2' Hz and 4' V pretty consistently.

I believe that the European counterpart to WAAS is called EGNOS, and should be able to be used in the same manner. Set up a Survey Style, in Rover Options change the survey type from RTK to RT Differential, and select SBAS. When I set one up for WAAS I turn off GLONASS and set my precisions to 3' Hz and 6' V. Then I can use Auto Store for Topo just like with RTK.

Lee D - that is very informative (I have been a long time lurker of this site and have learned a lot about Trimble from you, so thanks for all of the years of knowledge you have supplied us all). The company I work with continues to rent some kind of Geo handheld for wetland flag locations. I told them about what you said and now they want to know if the RT Differential data can be post processed? I have access to TBC and know how to process static and kinematic data. Will TBC let me post process the WAAS data? Thanks!

Assuming you log the RT differential and/or SBAS data in real time for later post processing, RTKLIB will allow you to post process with them. RT differential must be rare these days in the USA now that most of the stations that were previously broadcasting it have been shut down.

I've never tried bringing Geo data into TBC but as far as I know there's no way to process it there. My knowledge of Geos is extremely limited though.

In the early days RT differential was done using the Coast Guard Navigation Beacons and yes, most of those have been shut down. But WAAS is much more robust than the beacons were, the only advantage the Beacon stations had was that they broadcast in NAD83 and so there was no conversion necessary.

Sbas sats are used in sokkia's L1 rtk solution with GSR1700ISX receivers. At least this is written in the manual.
I want to enable sbas in rtklib but i dont know how to set up the receivers (base-rover). I suppose the base should broadcast the sbas info and the rover use it in the solution.

Lee D, post: 444305, member: 7971 wrote: I've never tried bringing Geo data into TBC but as far as I know there's no way to process it there. My knowledge of Geos is extremely limited though.

Lee - thanks for the reply. I didnƒ??t word my question properly. Can the RT Differential data collected in Access be post processed in TBC? The company is trying to achieve ~meter accuracy. Thanks again!

I've never tried it, but if you create a Survey Style and set the rover option to RTK & Infill you have the option of using WAAS and also recording raw data at whatever rate you choose. So yes, you should be able to get WAAS accuracy in the field and then post process.

Bear in mind that if whatever base data you're using as a reference is broadcasting NAD83 coordinates then the base and WAAS are not in the same system and will have about a two meter difference if you fail to account for that.

I am a little unclear how the various services from the vendors Such as from Trimble http://www.trimble.com/Positioning-Services/CenterPoint-RTX.aspx?tab=How_it_Works or http://www.terrastar.net/partners.html who is the provider for NovAtel, Septentrio and Leica via the Inmarsat satellites. I consider this to be a SBAS like correction service, but that may not be technically correct.

All I believe somehow provide a RT PPP solution, but there may be more to it than that, I don't think it is a "fixed" solution in the traditional sense because accuracy seems to be around 4 cm, it seems there is a convergence time required at the outset, leading me to the PPP theory. Maybe other posters can enlighten or correct me on this, but for certain tasks we do, even the 4cm would be fine and not having to depend on cell comms in the bonnies would make this very useful.

SHG

Shelby - Center Point RTX is a satellite based correction service that broadcasts states space representation corrections (excuse me if my vernacular or use thereof is a little off); in other words, ephemeris and clock corrections, etc. It's a subscription service and as I understand it the convergence time to 4cm is about 20 minutes. However, if I'm not mistaken you can also initialize on a known point, just as you would PPK or RTK.