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FYI, I went to the Seattle Calibration Baseline at WGS84 47° 40.51629N 122° 15.12894W -50.21m elipt., and took many measurements, in different modes for the chipset.
My average latitude is 47° 40.51610'N with a standard deviation of 1.41 on the last two digits, and my average longitude is 122° 15.12857'W with a standard deviation of 1.7 on the last two digits. The bottom line is that with my preferred settings, my static measurements are 1.71' too far south, and 0.95' too far east. That is in feet. I haven't calculated what the widest wandering was physically, nor altitude metrics, as my curiosity is satisfied.
Lutz remember now, as I said above, there is a setting in the SiRF chipset called, "DOP Selection". Its default is "Do not use".
I have not been able to find a reference anywhere on the internet explaining what this does --and this is why I asked for your input and was hoping for kinder words-- but doesn't it make sense that it engages an algorithm which does something like weight those birds that are most spacially-separated, heavier?
Remember, I am not saying that this is what it actually does, because nobody seems to know, but somesuch.
Clearly though, my investigations are not welcomed here, as no input or questions. I can't say as I understand why, but so be it.
Joined: 02/11/2002 22:41:59 Posts: 11878 Location: Massachusetts, USA
Posted: Sun Jun 04, 2006 2:21 am Post subject:
Your ponderings (investigations) are pushing far beyond what the majority of the users here is needing/looking for (think consumer grade accuracy). This may explain the reactions you are getting, and the lack of expertise (myself included). I am not sure but gpsinformation.net or other more specialized sites may have better answers.
as for "kinder words" - I kinda have to maintain my bad reputation somehow, it's definitely nothing personal :-) _________________ Lutz
Joined: Feb 23, 2005 Posts: 376 Location: Catford, London, UK
Posted: Thu Jun 08, 2006 12:18 am Post subject:
Hi Quantum,
To get the WAAS working, did you make any changes to the standard Holux236 settings (except of course turning WAAS on)?
Also, did you resolve your concerns about DOP? As Lutz said, it is just a measure of the satellite geometry (i.e. relative positions). The best simple description I found (via Google) of what the SiRF DOP mask does is this:
"SiRF receivers can be configured to reject fixes for which the value of DOP is too high. This function will cause the receiver to report an invalid solution if the DOP exceeds the value in the parameters".
Back to one of your earlier points; if you have a hardware background, it's quite possible to make a cable to upgrade your Holux firmware. But you'll need Holux to give you access to the upgrade files, of course. The connections on the 236's mini "USB" connector use simple "RS232" protocol, *but* inverted and with "TTL" levels. The "lash-up" I used was the supplied Holux charging lead, plugged into a USB type A female socket ("borrowed" from a spare USB hub) and connected via a MAX232 chip (on a micro prototype board) to COM1 on my PC. You do this entirely at your own risk of course!
Ya, thanks AllyCat. I do have the background to set that up, and in fact the input on the pod is CMOS levels, but am not willing to go to that trouble for SiRF, given the high accuracy of the newest Nemerix chipset as I'd noted above.
I'd also found that DOP reference, and it is replicated by many other companies; I guess by lazy engineers who need to draw up specs. It doesn't really explain though, Auto PDOP/HDOP, nor the algos. I can tell you though that Auto PDOP/HDOP increases wander. I have not tested HDOP alone, which may reduce it.
I didn't mess with any settings other than those which are DOP and SBAS-related. If you cinch down power settings, communication with the pod will get difficult. SBAS greatly improves altitude accuracy, although it still drifts about a meter. Use the mean average. Dennis Groning, the author of SiRFTech, has been very responsive and open to suggestions. He has a new release coming out in the next few days which will have enhancements, including more DGPS monitoring stuff. The reason tabs are divided the way they are in SiRFTech is because each represents a different command.
As I mentioned above, I'd gone to the Seattle Calibration Baseline and taken ten measurements, one each minute, for each of SBAS+DOP/SBAS/DOP/NoCorrection modes. I put the results in an OpenOffice spreadsheet, did averages and standard deviations, and then made a scatter chart of coordinates and tracking line chart of altitude measurements. My chart page was very instructive.
But I wanted more data by this point, so went back to take 4x as many measurements (every 30 seconds, for 20 minutes each), and include HDOP alone. On my first page of observations, suddenly all the bars in VisualGPSce went away. It claimed it still had 10 birds, and DGPS 3D, but I didn't believe it. I hadn't made any changes to the pod yet, but the bars just spontaneously disappeared.
I cold-reset and power-cycled everything, set and reset my settings, and the signal bars ultimately came back, but I never saw my WAAS birds again, except for about 2 seconds a day. That's been several days. I believe the pod freaked out, from some bug in the firmware. (GSW3.1.0-SDK_3.1.00.07-C24P1.00)
Be cautioned that DGPS may give the most benefit with dynamic measurements, not static.
Joined: 02/11/2002 22:41:59 Posts: 11878 Location: Massachusetts, USA
Posted: Thu Jun 08, 2006 3:45 pm Post subject:
As I said before, to be statistically significant you need much longer timeframes. The birds cycle the earth every half day, so you need a good multiple of that. Then you need stable weather etc.
I wouldn't blame the firmware. Have you checked the maintenance pages for the birds?
DGPS is ironically targeted at static measurements, and has very limited benefit for dynamic scenarios (ok, with the exception of aviation altitude measurements) _________________ Lutz
I doubt that sitting at the Calibration Baseline for 48 hours would give much different data than I'd gotten. I think 40 observations per mode would answer the questions I have.
BTW, the second time I went, I trimmed the tall grass around the brass disk to reduce multipath, and my new observations appeared to be even more stable than before, just eyeballing it. But I didn't chart it.
I don't know where the maintenance pages for the birds are, but they wouldn't be down for days; the WAAS birds are in commercial use. Also, I see an occasional peep out of one or the other.
I base my static-vs-dynamic suggestion on the last page of Passion's SBAS evaluation (above). It is worth testing. There are other reasons I say this.
However this does not include WAAS birds, which are run by the FAA. (above PRN 30) I looked around the FAA site, but couldn't find anything. The Coast Guard does operate ground reference stations, but not the birds.
BTW, you being in my home state, would get the WAAS birds much easier than I, as they are equatorial.
Leif, those are the two new WAAS birds, which I believe will improve coverage for Canada & Alaska. (NMEA 48 & 51) They're not operational yet.
The two operational WAAS birds are NMEA 35 & 47. It's weird, because there are several designating schemes, and you have to analyze them in order to understand what they're talking about. The PRN number is used by the FAA, and in this case the birds are PRN 122 and 134 for the US. But these are known by their NMEA designators in most of our software. To convert PRN to NMEA, subtract 87. This meant is to confuse things, and to keep out the shoe salesmen. These two birds are primary for the US, and are also known respectively as AOR-W and POR. AOR-W (opposed to AOR-E for Europe) is presently being moved. I have never found a current status webpage on 35 or 47; I've got to think it's out there though.
I should have mentioned that, another reason I believe this loss of the WAAS birds is due to a firmware freakout in my Holux 236, is a local friend still sees both birds just fine on his expensive gear.
Now, almost a week later, I still do not see either 35 nor 47. It's ironic that I, being the only one who eschews the prevailing orthodoxy by wanting DGPS, am the only one who can't get it. God? Did you do this?
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