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 SS III receives SBAS data but makes no corrections

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beechfarm Posted - 13 mars 2006 : 09:43:41
I am testing my new Holux GPSlim 236 (SiRFStar III + GSW 3.1.1) linked to SiRFDemo (v3.81) by


I can "see" the 3 EGNOS satellites (PRNs 120, 124 and 126) with signal strengths in the range 28 to 38 db-Hz and I specify each of them in turn. My excitement rises as I watch the dark blue icon on the 'Radar View' turn to light blue. This must mean my receiver is reading intelligible signals.

My excitement turns to disappointment when I open the 'DGPS View' dialog box. The only field containing new information tells me that DGPS source is SBAS. There is no new information in any of the 12 records of the 'GPS corrections' record set. The 5 fields of the 'SBAS' record are blank (So are the fields of the Internal Beacon record).

My spirits drop lower as I watch the scatter pattern develop in 'Map View', showing the apparent wanderings of my stationary receiver. These finally overflow a 15m radius circle, indicating no improvement over the non-SBAS situation.

I check the EGNOS website. The Dutch ground station is currently using PRN 126 for SBAS correction and its scatter pattern for the day would easily fit in 5m dia circle. I try PRN 126 again but nothing changes and I still have a light blue icon.

Is there a hardware problem or am I misunderstanding the software?

Can the Gurus of GpsPasSion help please or should I start a dialogue with the receiver makers?
15   L A T E S T    R E P L I E S    (Newest First)
musky Posted - 19 mai 2007 : 16:29:58
Yes, It all smells like a coverup....
HIPAR Posted - 19 mai 2007 : 06:45:34
With my BU-353, I see PRN120 momentary flash on the SIRF Demo radar view screen. It never
stays locked long enough for me to check if any corrections are applied. I haven't
seen the US PRNs yet.

They are very sensitive about probing WAAS questions at the USGlobalSat support forum
and have deleted several of my posts concerning WAAS. I personally don't think it works
properly and they don't want anyone to know that.

--- CHAS
nemogps Posted - 13 mai 2007 : 16:24:24
I think I have finally gotten to the bottom of this with some advice from Dennis Gröning. He suggested that I turn message 27 off (DGPS Status) and turn message 29 on (Nav Lib DGPS Data). I had tried this before, but saw no difference in the dsiplay, which when MID 27 is used shows a constant 14.27 meter correction for all SVs.

Looking at the logged commands I saw that MID 29 does indeed show that corrections are enabled and that different corrections are shown for different SVs. However, I did not see a correcton for *every* SV being used in the fix.

If I restart SiRFDemo with message 27 disabled and 29 enabled, nothing shows up in the DGPS view window as if it does not report message 29. I believe in one of the GPSPassion forum threads, Carl from SiRF said that the SiRF III does not report corrections even when they are applied. I guess they *are* reported, but not with the MID 27.
nemogps Posted - 12 mai 2007 : 16:00:13
How do you know it is not making corrections? Carl from SiRF says that in SiRF III they no longer report the corrections, but they are being made to the position data. He also says the corrections make little difference in the readings, especially in real world conditions where you can expect 10 meters of wander. I guess there is no way to confirm other than to test a given unit with SBAS on and SBAS off to see a difference in the scatter of the data. I have done some testing with my unit and I can't tell by looking at the data if SBAS is on or off, but this is from inside a house and the errors are not small to begin with. It would take a special program to analyze the data anyway as I don't know if you could pick out a smaller difference in a data set with a larger standard deviation.
nemogps Posted - 12 mai 2007 : 15:37:02
I can't imagine that raw GPS location measurements with a standard deviation of a couple of feet is remotely possible! I suspect that you are seeing averaging at work which will filter out the wide variations in location measurements. Try SBAS/non-SBAS measurements using the same device. I expect you will get similar results in each case using the same device. Also see if you can find a control to enable/disable averaging. What are you using for software? If you are on a laptop, you can download either GPS NMEA Monitory or USAPhotoMaps. Both of these programs will plot the measurement each second on X-Y coordinates so that you can see your results. This makes it very clear how good or bad the wandering is. USAPhotoMaps can be used even if there are no maps for your area. In fact, for this I use it in high resolution "Urban" mode and there are no Urban maps for my area. But the Urban display plots with 0.25 m/pixel so you can more clearly see the results.
pyxis Posted - 12 mai 2007 : 14:38:41
I am using a Globalsat BR-355 (a PS/2 or USB mouse GPS) which operates very readily in DGPS SBAS mode. It can maintain a standard deviation of approximately one to two feet in both Lat and Long for an observation period of 6 to 10 minutes. I consider this to be quite good considering that the NMEA output resolution is approximately 7 inches. My Pharos iGPS-500 does not operate in DGPS SBAS mode and consequently will generally meander around 1 to 2 meters during a 10 minute observation.

I have been experimenting with DGPS for about 10 years using two Garmin beacon receivers and being in receiving range of four NAVCEN USCG stations.
nemogps Posted - 11 mai 2007 : 15:01:31
Can you explain this? I am seeing any of three WAAS satellites with SNRs up to 36 dBHz. It is reporting that the SBAS satellite tracking status as either 2D or 3F which are both good enough to receive corrections. The post from said, "unlike SiRFstarII receivers, it does not output the actual corrections". I have no idea why they would disable the display of the corrections in the SiRF chipset, but he is claiming that the SBAS corrections are being used, but the actual corrections are not being reported. He is also saying that the amount of correction is so small that you are not likely to see it under any condition except ideal.

Sounds fishy to me, but Carl is not the only "expert" saying this. I see in other GPS module data sheets I have the same small improvement to accuracy using WAAS. Perhaps we need to start dispelling the myth that WAAS is needed to get an accurate fix?
musky Posted - 11 mai 2007 : 12:59:27
It may appear to be in WAAS mode (SirfDemo will report it's enabled), but chances are it's not really making any corrections. I call it "Broken WAAS enabled".
nemogps Posted - 11 mai 2007 : 05:53:48
I finally got WAAS to "work" on my Holux 240 receiver. I had it enabled by using the GPS NMEA Monitor program, but I could not detect that WAAS was working. I got an email from Dennis Gröning that explained in great detail how to do the same thing using SirfDemo. This program displayed all the info from binary mode and I was able to reorient the receiver to pick up a WAAS satellite. It now reports that it is in 3D+DGPS mode although I don't see any specific correction data, but the SiRF rep said this is a change in the rev 3.2 software, right?
musky Posted - 10 mai 2007 : 23:41:03
Unfortunately, since WAAS doesn't work on SirfIII, I can't choose if I have it or not!

I have been trying to evaluate WAAS vs no-WAAS with my MTK 32 channel, but unfortunately MTK has static navigation, which really messes up any kind of precision study you are trying to do!

I just think Carl's post about "you don't need WAAS" is a copout. There is something else going on here and I wonder what it is. Maybe it's just a case of WAAS not really helping that much for "low end" units (not Trimble for instance). Maybe there is some problem with WAAS kicking in and out and causing jumps in position when it does so. When you turn WAAS on, it requires more power, but that should not be an issue with USB "mouse" devices. Maybe the vendors are all stuck selling the "WAAS is better fantasy" and are afraid to say they don't have it.

I started another thread here, simply asking if ANYBODY ON THE PLANET can direct me to a USB mouse SirfIII unit that really has working WAAS, and still I haven't got an answer. Somebody, give me the model and I will go out and buy a dang 3rd SirfIII unit in my lifetime search for this elusive item.
nemogps Posted - 10 mai 2007 : 20:29:55
In response to Carl's post about the amount of improvement you will see with SBAS, that does not fit my personal experience. I have used handheld GPS receivers and found them to be much less accurate if DGPS is not operating. When I initially turn on my Magellan, it shows an EPE of about 30 feet (10 meters) after about 30 seconds. It only drops to 10 to 15 feet once the WAAS satellite has been received and incorporated in the calculations.

On the other hand, I have checked the data sheets for GPS chip sets from other manufacturers (I can't check the SiRF chip sets since they don't provide them to the public). They provide numbers similar to what Carl indicated.

Even so, the choice of using or not using WAAS is up to the end user. Right now there are a lot of GPS receivers being advertised as using the SiRF III chipsets and supporting WAAS DGPS, but no one seems to be able to get WAAS to work. That is not at all the same as finding out that WAAS DGPS only provides a small improvement.

BTW, how does DGPS help with "integrity monitoring"? I am not sure I see how these "small" corrections would provide a faster detection of a bad satellite.

musky Posted - 13 avr. 2007 : 14:25:41
SirfIII units are being marketed left and right with WAAS support. If you go to any of the retailers, they all advertise in big letters WAAS ENABLED. However, it is a crap shoot if you will actually get WAAS support. In my opinion this is called false advertising. The explanation that the manufacturer can just "compile" the code out seems really lame to me.

So let's do a little thought experiment here, OK? I have a GPS company and I get SirfIII chips for my unit, and the firmware source code, with the option to "compile out" WAAS functionality. I go, "gee, hmmm, I can turn WAAS on or off, I think I'd like it better with WAAS off, BUT, I'm going to lie to my marketing department and say that WAAS is really on". Or how about this, I go, "gee, hmmm, I can turn WAAS on or off, I think I'd like it better with WAAS off, BUT, I'm not going to tell my marketing department". Or I turn WAAS off and tell the marketing dept, but they intentionally lie in the ads. You pick the scenario, they are all absurd in my opinion.

Then when people find out WAAS isn't really enabled, I say, "Well you really don't need WAAS anyway!"

How about an analogy here. I agree to buy a car that is advertised with power windows. I pay for the car, take delivery, and realize the car doesn't have power windows, it has hand cranks. So I go back to dealer and say, "Hey, you told me I had power windows, and I don't have them". Dealer tells me, "yeah, but you don't need them, hand crank works just fine". And so I go, "Oh cool, thanks for the explanation!"
saimhe Posted - 29 mars 2006 : 23:30:20
Carl, I believe you are the one who at last can give a sound confirmation whether SBAS PRNs are reported in NMEA GSV message, and under which conditions. For example, does it [accidentally] depend on hardware, on firmware (except obvious cases like XTrac), must SBAS be enabled or not, etc.
beechfarm Posted - 29 mars 2006 : 12:26:29
Thank you, Carl, for your excelent explanation. You are absolutely right. When my GPS receiver receiver is fitted with an antenna and hauled to the top of the flagpole the 'wander' pattern is much smaller and all the points logged during the battery life lie within a 10m dia circle. (I have not done the analysis but I guess that 95% of the points lie in 5m dia circle.)

I have been looking for a copy of SirfView to help me with the analysis. Is it available for download or is it restricted to EVK subscribers?

Thanks also to GPSPassion
gpspassion Posted - 21 mars 2006 : 14:39:43
Yes indeed and I will link it from the current write-up here and I'm planning a summary article with real case scenarios.

Just to close the loop on the original question, any reason why SiRFstarIII is not outputting the corrections or would a new version of SiRFdemo be required to view them ?

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