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Posted: Mon May 22, 2006 6:54 pm Post subject: Differential/WAAS/EGNOS, TomTom, & GPSSlim?
I am new in Seattle, and went for a Sunday drive yesterday to learn the area. When finished I set TTN5 to go home, but I ended up in a completely unfamiliar forested neighborhood.
I live in a forested neighborhood, but did not recognize anything around me, yet TT insisted I was home. I had prior set Home by lat/lon at my garage door, so I knew that was right. I hunted around a bit, and found my house was right around the next street corner, but this freaked me out. I had been at least 75 feet away, and on another planet.
Does anyone know whether TTN5 is compatible with differential GPS? And the current state of diffGPS in the US and what it's even called? Also, is it possible to turn on diffGPS in the Holux GPSSlim? I believe TT (inexplicably) cannot work with SiRF, if memory serves.
Joined: Apr 04, 2006 Posts: 295 Location: South East Northumberland, UK
Posted: Wed May 24, 2006 1:00 pm Post subject:
Possibly a daft question but is your house actually on a road on the map? Since you set your home position by latitude / longitude (instead of current GPS position) there are several possibilities.
Since TomTom will always snap onto the road it thinks you are following up to a fairly wide margin to allow for GPS variations if it thinks you live "off the map" it is liable to navigate you to the nearest point on the map. Also if you took a wrong turn and the roads are close together it can assume that you are on the correct road but in fact are following a parallel road but eventually it should work it out.
Surely you should have set your home location by a location on the map (ie on the road) so that TomTom will always try to get you to that location that it will have converted to an "exact" latitude / longitude rather to one off the road?
If, as you say, you live in a forested neighbourhood how good is your GPS reception?
Not aware that TomTom is compatible with differential GPS - it shouldnt be necessary for the sort of accuracy it normally needs. Dont see why TomTom shouldnt shouldnt work with SIRF especially as they sell the TomTom Bluetooth SIRF Star lll GPS unit (in the UK at least).
Sorry, I should have said that I'd set Home by 'current GPS position'.
I'd prior set it by address, but:
a. This ain't very accurate; and
b. I have a long driveway.
I've noticed this 'snap to' roads, and it causes a good deal of inaccuracy when I pull of to a service road (alongside a freeway) and TT thinks I'm still on the freeway. I do not like this snap to road feature. Here in the US (anyway), it is often very difficult to know exactly which exit it wants me to take when they're close together, but the voice command is always spot-on. When Jane says Exit, I'd better exit, if I know what's good for me. Can the snap-to road put me ~75' off, FCS?
GPS reception is quite good, usually no less than five bars. Have the Holux 236 with the new chipset. No complaints there, except when I go into Seattle's massive underpasses and TT goes nuts. Has me swirling all around and flying up to upper-level roads or skidding out across the grass... until it is able to get a solid fix when I hit clear sky again.
I seem to remember reading here that TT simply does not work in SiRF mode, but no one here has been able to confirm. I would much rather run in SiRF if I could. Seems likely that the TT pod in the UK can do SiRF mode (as that's the chipset just about everyone uses), but in practice TT uses standard (ANSI?) mode.
I am also unclear whether SiRF is necessary to run WAAS/EGNOS, but suspicion that it is. And I do not know whether modifications to the TT app are necessary in order to run WAAS/EGNOS, and apparently no one else here does either.
I wish that at least I could turn off this wishy-washy, dishonest, lock onto roads.
Well, pretty much all the satellite navigation systems have some degree of "snap to road". Basically, GPS isn't all that accurate, and if your satnav program showed you the exact reported position on screen all the time, your screen would be jumping about the map like crazy. Try running a program that records the exact reported GPS position while you drive, then overlay it on a map, and it'll be a messy zig zag, especially in areas without a clear view of the sky.
It may be jumpy because of consecutive readings every second, without WAAS.
I have in mind WAAS, but have no idea whether it will work in NEMA mode. Nor whether TomTom can benefit from it at all.
I have turned on WAAS, but it never locks 3D. May be b/c I'm in woods. Haven't tried TT yet. I don't know how I'd determine whether it benefits from WAAS, with this snap to roads mularky.
(Hm, I am beginning to suspect that there aren't any experts here)
Technically, SiRF mode is superior, and yet no one actually uses it. In fact, with the Globalsat and Holux pods it seems to not work at all! Seems SiRF requires baud of 57,600 and bluetooth can only work at a max of 38,400. But this is not common knowledge.
And it is hard to believe that no one knows the recipe to set best accuracy. Even whether WAAS/EGNOS works in NEMA. I've just spent a day plowing through threads on settings such as Static Navigation, GGA/VTG, SBAS, etc, but it is still not clear the advisable complete settings for accuracy in a SiRF3 BT pod, nor an app to make them which won't disable my Holux 236.
I should have thought that accuracy which is better than 45 meters would be desirable --and a priority here-- after so many years, but apparently I am venturing into New Science.
Joined: 24/06/2003 00:22:12 Posts: 2946 Location: Escaped to the Antipodies! 36.83°S 174.75°E
Posted: Thu May 25, 2006 9:53 pm Post subject:
Quantum wrote:
(Hm, I am beginning to suspect that there aren't any experts here)
Nope, but there are lots of us here who will try to be though.
Quantum wrote:
Technically, SiRF mode is superior, and yet no one actually uses it.
SiRF mode may output more information than NMEA, but all you are interested in is your position lat/long, speed, height, heading and how accurate the positioning information is (the accuracy varies according to a lot of factors).
The NMEA protocol has provision for all this information in an easy to understand protocol.
SiRF mode is only available from SiRF GPS chipsets. NMEA mode is a de-facto standard that pretty much all GPS receivers output. Guess which one TomTom (et al) prefers to support?
Quantum wrote:
And it is hard to believe that no one knows the recipe to set best accuracy. Even whether WAAS/EGNOS works in NEMA.
Holux have a program called GPS Viewer that will switch WAAS/EGNOS mode on.
NMEA does support WAAS information, the NMEA $GPGGA string will indicate the type of fix (3D-Differential for WAAS). You will need a good view of the sky for 5 to 30 minutes to get WAAS working.
WAAS/EGNOS is nice to have and improves accuracy, but it isn't absolutely essential for navigation. The mapping program fudges things to lock you onto the nearest road and compensate for the positioning inaccuracies in the GPS receiver.
Yes, I had tried GPSView, but when I check WAAS, the following issues:
- I never have fewer than 7 satellites with blue bars, and I have had it out in open sky for 20 minutes (albeit overcast, sort of drizzly), but it still only ever says "Acquiring 3D". If DGPS 3D might take an hour, will it always take an hour to lock 3D, or is it just initially gathering almanacs (or whatever)?
- My heading -always- reads NW in GPSView. The number after that does change with direction, but if that is supposed to be Degrees, it is not correct.
- When I was parked, the altitude drifted from 50m up to 75m and back again, for some reason. I figured multipath, so I changed to another location, which was better drifting 101-104m. But geez...
- However, GPSView does give my traveling speed correctly when I am moving.
When -not- in WAAS mode it does get a 3D lock, but that seems less accurate. The 236's GPS reading was 25 meters in error the other day, which is the reason I am investigating this. I was expecting 3m error at the most, for this unit.
I tried to get the firmware version from GPSView, but could not find it. I tried to determine whether it is in fact in DGPS mode, and I believe the sixth number would be 2 in WAAS, but unfortunately the window is too narrow to read it, even in Landscape mode. Is there another utility which works well with the Holux 236? Afraid to try SiRFDemo, after the horror stories with the 236.
Wish I didn't have to learn all this. Accuracy is alot more painful than I'd expected. Thank goodness TomTom has worked about as expected, or I'd shoot myself in the head.
Joined: 24/06/2003 00:22:12 Posts: 2946 Location: Escaped to the Antipodies! 36.83°S 174.75°E
Posted: Fri May 26, 2006 2:12 pm Post subject:
Quantum wrote:
it still only ever says "Acquiring 3D". If DGPS 3D might take an hour, will it always take an hour to lock 3D, or is it just initially gathering almanacs (or whatever)?
The GPS has to get the Almanac data for WAAS the first time it locks on. This may take a while, I don't know how long may 5 minutes, maybe an hour.
Can you see the WAAS satellite in your view of the sats? I think it is 35 or 47 for your part of the world. I used my Garmin GPS in New Jersey about a year ago and it got a 3D-Differential lock in a few minutes. The WAAS satellites are over the equator so you need a good view of the southern sky. If you are a long way north then getting a lock may be harder. On top of a hill is a good place to try.
Quantum wrote:
When I was parked, the altitude drifted from 50m up to 75m and back again
Yeah, it does that. The altitlude is about 1.5 times less accurate than the lat/long position. As you have noticed, the position reported will drift around by a few meters, even when you are sitting still. TomTom (et al) compensate for this so you generally won't notice it.
Quantum wrote:
However, GPSView does give my traveling speed correctly when I am moving.
Yes, this is very accurate (0.1 mph) if you are at constant velocity.
Quantum wrote:
Wish I didn't have to learn all this. Accuracy is alot more painful than I'd expected. Thank goodness TomTom has worked about as expected, or I'd shoot myself in the head.
Methinks you are worrying a bit too much. ;)
The accuracy is not necessarily as good as manufacturers would have you believe but if TomTom works as you would expect, positioning you on the correct road and showing the correct distance to the intersection then take a deep breath and forget about all the technical details, sit back follow the directions and enjoy your drive without getting lost. :D
It isn't perfect, but it is pretty damned good! _________________ Gone fishing!
... so no WAAS. I'll have to make a point of parking facing a Southern exposure for a while. Although, I guess this means that once I get the almanacs, I cannot have WAAS when I do not see a bird.
Skippy wrote:
Methinks you are worrying a bit too much. ;)
Eh, the thing that triggered this was a significant and disturbing inaccuracy, described in my first post above.
I do need better accuracy than that.
Any idea whether TomTom can digest WAAS/EGNOS? Is the WAAS correction integrated into the text string, or is it a separate string? (making it likely to be ignored)
Hi Quantum
I have heard on the grapevine that if one sits on top of Kilimanjaro, Tanzania one can get WAAS after only a few minutes. Next time you are passing give it a try. _________________ Tomtom Go 720.
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Joined: 24/06/2003 00:22:12 Posts: 2946 Location: Escaped to the Antipodies! 36.83°S 174.75°E
Posted: Sat May 27, 2006 6:09 pm Post subject:
Quantum wrote:
I'd thought that I should see the WAAS birds in the GPSView display, but had no idea what they'd be referenced as.
Indeed 35 and 47 cover my area.
OK, here's where we are up to then:
If WAAS is not enabled correctly on your GPS then you will only get the normal GPS sats displayed numbered 1 to 32.
If you have WAAS enabled successfully on your GPS then it should show one or two of the WAAS sats numbered from 33 to 51 in the satellite display.
If the GPS has the almanac then it knows which sats it will be able to see so it will only try to lock on to the sats it knows will be visible and usable from your position.
If the WAAS numbered sats appear but they are right on the horizon in the North in the satellite view or it cycles through trying different numbers then it probably doesn't have a WAAS almanac. Wait 5 to 60 minutes with a clear view of the sky to try and get it.
If the WAAS sat or sats appear in the correct part of the sky but aren't locked on then find a place (top of a hill is good) with a clear view of the sky in the direction of the sat and then wait. Should only take a few minutes.
The inaccuracy you experience sometimes (like under heavy tree cover) is because you only have three sats locked and they aren't in a good position to triangulate your position. Ideally you need to be locked on to sats in three corners of the sky to get the best triangulation of your position. When you are under trees or tall buildings you can get readings which are off by 100 meters or more. Look up, if you can't see a good part of the sky then your position is going to be suspect. WAAS only works in good conditions so it won't help here.
And forget Kilimanjaro, Everest is the place to be! _________________ Gone fishing!
Today I drove all around the area, trying to get my almanacs. I went to several high hills and fairly clear areas, although there are tall pines all around here.
I usually had blue bars on ten birds, but I never saw a satellite numbered higher than 30, so it sounds suspiciously like I am not in DGPS mode then. I used GPSView, which Holux claims works, but I see ppl on the forums recommend going into SiRF mode to set DGPS. Scary stories in doing this though, of the 236 setting a non-functioning 57600 baud.
Granted, I never could see below 30 degrees elevation today, so if the satellite is low on the horizon, I might as well give up. But surely with such a broad coverage area, it is higher than 20 degrees in Seattle.
It'll take some time for me to find a high spot that's clear to the horizon, if it even exists here. The trees are a blessing though, for quality of life.
Joined: 24/06/2003 00:22:12 Posts: 2946 Location: Escaped to the Antipodies! 36.83°S 174.75°E
Posted: Sun May 28, 2006 8:18 pm Post subject:
Quantum wrote:
I never saw a satellite numbered higher than 30, so it sounds suspiciously like I am not in DGPS mode then.
That's would I think is happening.
I would suggest that you try using SiRFDemo. Bear in mind that if you mess up the baud settings then you will have to leave the battery out of the Holux for a few days or send it away to be reset. _________________ Gone fishing!
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