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Joined: Feb 23, 2005 Posts: 376 Location: Catford, London, UK
Posted: Sun Jun 11, 2006 12:48 pm Post subject:
Hi Quantum,
If you haven't got your WAAS birds back yet, maybe you should try a(nother) factory reset (e.g. remove battery overnight). I switched my Holux236 to WAAS using MMSiRFSetup, with no other changes, put it at the window (which faces south) and soon saw our EGNOS sat 33 (elevation just under 30 degrees).
VisualGPSce reports "Differential GPS" but I've never seen the "33" signal strength bar "blue". But maybe this is correct if blue means that the *path length* is being used for navigation (i.e. contributing to the DOP), whereas the WAAS is only providing *correction* data? It's a pity that VisualGPSce doesn't indicate which satellites are actually being corrected (which my Garmin does, with a little 'D' in each relevant bar).
I'm with Lutz, that the position errors are generally related to atmospherics and satellite quality/geometry and may vary over 12/24 hours or more. However, you shouldn't need to set up camp at the calibrated reference point (as you have one "good" reference measurement). If you can now get consistent measurements at *any* fixed location over several days then you'll "prove" that the WAAS is working well.
I also doubt the usefulness of WAAS for on-the-road use. To get some sort of GPS fix, the receiver generally only needs to see any 3 of about 10 birds reasonably above the horizon. For WAAS it also needs to see the very specific geostationary position at a relatively low elevation to the south. And if the basic fix is "bad" (high DOP) then corrections from WAAS aren't going to help much. If you're having problems with accuracy in a car, I suggest you try an external, roof-mounted antenna. But bear in mind that most factory-fit GPS systems use extra sensors (e.g. wheel-rotation and/or intertial/compass) to overcome GPS limitations.
The Paris measurements you linked to are interesting, but I'd like to see some details of the experimental method. I presume the tracks were all taken on the *same* journey, or the comparison is almost meaningless. But anyway, surely SiRF3's claim to fame is its incredible sensitivity, not necessarily its absolute accuracy? It's been pointed out elsewhere that using measurents from "difficult" satellites (e.g. at very low elevation, though lots of atmosphere, or bounced off buildings, etc.) can degrade the accuracy. The Nemerix receiver may not be "seeing" some of these satellites, since on paper (although I always take such things with a large pinch of salt) it seems to have 6-8dB less sensitivity than SiRF3.
This quite old link is not directly relevant, but the experimental method looks good and it's interesting (but not surprising) how two similar GPSs "see" very different numbers of satellites at the same time. And I can confirm that Garmins do have a habit of "pretending" to be receiving a position when they're not (Fig.6), which can make geocaching in wooded surroundings very frustrating.
Still no WAAS. I'll try removing the battery overnight, but I believe that in the Holux 236 there's also a coin cell, which retains the almanacs and ephemerals. I'm not willing to unsolder nor short it out.
AllyCat wrote:
VisualGPSce reports "Differential GPS" but I've never seen the "33" signal strength bar "blue". But maybe this is correct if blue means that the *path length* is being used for navigation (i.e. contributing to the DOP), whereas the WAAS is only providing *correction* data? It's a pity that VisualGPSce doesn't indicate which satellites are actually being corrected (which my Garmin does, with a little 'D' in each relevant bar).
Right, you will never see a blue bar on SBAS birds. I don't know whether DOP plays a role in this. And I should note for others present that you will never see SBAS birds at all, until you turn on SBAS and configure it. They are satellites numbered higher than NMEA 30.
I agree that it would be very nice if the VisualGPSce bars showed whether DGPS correction; didn't know that was possible. Might suggest it to the author.
AllyCat wrote:
I'm with Lutz, that the position errors are generally related to atmospherics and satellite quality/geometry and may vary over 12/24 hours or more.
Rest assured, I won't be camping out at the baseline.
The largest source of error is atmospheric. SBAS corrections are made for both troposphere and ionosphere, as reference stations are ground-based. Atmospheric error will mostly screw with altitude measurements. Another reason to take measurements over time is the movement of the GPS birds, but I am not a fanatic (although it may look like it).
AllyCat wrote:
For WAAS it also needs to see the very specific geostationary position at a relatively low elevation to the south. And if the basic fix is "bad" (high DOP) then corrections from WAAS aren't going to help much.
I almost never lock fewer than seven birds, amongst all these trees, with HDOP ~.8 and PDOP ~1.2 so that's not an issue. My WAAS birds are at about 15° above the horizon, yet I held a DGPS 3D lock about 80% of the time while moving (before the failure).
TomTom freaked me out, in saying I was 'Home' in unfamiliar woods. I also do not know whether DGPS corrections are assimilated and understood by TomTom. But I believe that doing something, is better than doing nothing. I've heard the bad news about EGNOS, but it turns out that's only one bird. I am confident in WAAS, as it's used commercially.
Nemerix is definitely doing something right, and I don't believe that it's because of lower sensitivity, as their older chipsets did not do nearly this well on dynamic tracking. SiRF2 wandered even more than SiRF3 as well.
I am very impressed with the new Nemerix for accuracy in difficult conditions, and especially given their much longer running time than SiRF3.
AllyCat wrote:
The Paris measurements you linked to are interesting, but I'd like to see some details of the experimental method. I presume the tracks were all taken on the *same* journey, or the comparison is almost meaningless.
I am quite sure he used all the pods at the same time, as that was his method for the static measurements, and he knows the difference it makes. Might ask in his forum. He banned me a while back for talking my progressive (liberal) politics.
Might let Passion know about my Calibration Baseline method, as he obviously doesn't know how to do this, nor how to calculate physical error.
Joined: 02/11/2002 22:41:59 Posts: 11878 Location: Massachusetts, USA
Posted: Sun Jun 11, 2006 6:52 pm Post subject:
Quantum wrote:
I also do not know whether DGPS corrections are assimilated and understood by TomTom.
Definitely not. TomTom has no idea what receiver is spitting the data out, and does not look at PDOP (as you may know TomTom also ignores the altitude completely). If the position is DGPS enhanced then TomTom will gladly use it, of course. But rather unknowingly.
We don't allow edits because this will make you think before submitting a post, and generally raises the quality of the posts. I said "generally"... _________________ Lutz
Definitely not. TomTom has no idea what receiver is spitting the data out,
...
If the position is DGPS enhanced then TomTom will gladly use it, of course. But rather unknowingly.
But see, that's the question. Are DGPS corrections assimilated in the end coordinates which TomTom uses?
If they are in a different NMEA sentence, then ya, likely ignored.
Can you fix my quoting problem above then Lutz? It makes my post confusing.
I wish I could disable snap-to-road, but since I can't, at least the coordinates it is snapping from, are more accurate.
I wish TomTom would also give altitude. I guess the reason they don't is on the assumption that it's way off. But it is much better with a DGPS 3D lock.
Joined: 02/11/2002 22:41:59 Posts: 11878 Location: Massachusetts, USA
Posted: Sun Jun 11, 2006 9:12 pm Post subject:
Altitude is simply not required for street navigation, that's why they ignore it.
There have been some discussions regarding elevated roads and highway intersections but those are very much corner cases, and the "correct" road can be guessed from other parameters too. _________________ Lutz
Well, elevation is nice info to have, especially in mountainous areas. Some of us just like to know. Maybe it could be turned on/off.
Then again, I'd like to know the name of that lake or river or park I'm passing, and when I cross into a town's city limits (cops?) as well as the town's name, but I'm never going to get those in TomTom either.
Joined: 24/06/2003 00:22:12 Posts: 2946 Location: Escaped to the Antipodies! 36.83°S 174.75°E
Posted: Mon Jun 12, 2006 1:56 am Post subject:
Quantum wrote:
I wish I could disable snap-to-road, but since I can't, at least the coordinates it is snapping from, are more accurate.
You don't want to disable snap to road for normal street navigation or your position would jump about all over the place.
The only reason to disable it is if you are in a plane or on foot.
Quantum wrote:
I wish TomTom would also give altitude. I guess the reason they don't is on the assumption that it's way off.
The altitude reported by GPS is not that bad. If you are in an unfamiliar area then it's interesting to know your height. _________________ Gone fishing!
Joined: 02/11/2002 22:41:59 Posts: 11878 Location: Massachusetts, USA
Posted: Mon Jun 12, 2006 3:28 am Post subject:
You never know what receiver is used, and in which one of the many possible wrong ways the geoid correction is calculated or ignored. _________________ Lutz
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