Differential/WAAS/EGNOS, TomTom, & GPSSlim?

[Quantum]

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.

Ipaq 4700, Brodit cradle, Holux 236.

[Greenglide]

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).

[Quantum]

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.

[elyl]

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.

[Quantum]

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.

[Quantum]

(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.

Tsk, how do mere civilians get along?

[Skippy]

[Quantum]

Thanks Skippy.

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.

[Skippy]

[Quantum]

I'd thought that I should see the WAAS birds in the GPSView display, but had no idea what they'd be referenced as. Thanks again, Skippy.

Here's a useful site, for WAAS anyway. (now that I know this)

Indeed 35 and 47 cover my area. Although 35 is on the move, it is still transmitting all but range data.

After leaving the Holux on all night, I find that it still is "Acquiring 3D", but in examining the satellites it sees:

[Trevor1234]

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.
Navcore 9.510
Central and Western Europe v855.2884
GPS World Traffic cameras
Home 2.7
iPhone 3G

[Quantum]

Hm, yeah, thanks for the useful tip, Trevor.

[Skippy]

[Quantum]

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.

[Skippy]

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