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What is the difference between the DeLorne BT-20 and LT-40?
srlm_productions
Can anybody tell be the difference between the two GPS recievers? Far as I can tell, one is square, and one is rounded. Also, how do they compare to the microsoft GPS. Finally, how accurate are they? (I need to use it for a robotics project...)
Ken in Regina
I have no experience with the DeLorme receivers. I have the Pharos 500 that comes with Microsoft Streets&Trips 2008. It works quite well.

I don't know how well receivers of this type (consumer grade) will work in your application because I don't know what you mean by "How accurate are they?". That is, I don't know what measurements you need to be accurate nor how accurate they need to be.

If you need absolute positional accuracy (the difference between the GPS location reading and where you really are), you won't get it with these devices. I have a Garmin iQue 3600 (PDA with internal GPS), Pharos 500 and i.Trek M7. The best absolute positional accuracy I've seen with any of them is about 2.5 metres and that was with WAAS enabled and a really good satellite constellation in view.

The speed accuracy is usually rated in tenths of a metre per second so that is probably pretty accurate for a robotics application, if you need to know speed.

I don't know about relative positional accuracy (the difference between GPS location readings between Point A and Point B) eg. if it says I moved 0.45 metres on heading 269 degrees it's probably awfully close. Given the relatively high accuracy of speed readings I would expect that relative location accuracy (as reflected in a trip meter, for example) must also be pretty good because the reported speed is simply the distance between two readings divided by the time between the two readings.

Oh yeah, time is another measurement. Absolute and relative time are very accurate because you are getting time readings that are only one step removed from Stratum 1, which is as good as it gets.

Even with high absolute positional accuracy, if you are dealing with maps you still have the issue of how well the maps are calibrated (with respect to the real location of things on the map).

Google Maps is a really good example of how badly map calibraion can be sometimes. In some areas I've looked at with the hybrid (road map and satellite image) view enabled, you can see that the roads in the road map do not sit right on top of the same roads in the satellite image. I found one example in Calgary where the roads on the road map were 400 metres or more away from the same roads in the satellite image. So, which were calibrated wrong? The satellite photos? Or the vector data in the road maps? .... .... I wouldn't trust the calibration of either one if (absolute positional) accuracy was really important to me.

You should be able to get the absolute positional accuracy and speed accuracy specs on most devices of interest on their manufacturers' web sites. I can't recall if any of them specify time accuracy or not.

EDIT: Okay, here are the specs on the Pharos 500 as an example:

Accuracy :
..Position:
....Autonomous < 10 meters
......SBAS < 5m
Velocity: 0.1 meter/second
Time : 1 microsecond synchronized to GPS time.

I hope that has been a little bit of help.

...ken...
Marvin Hlavac
Quote:
Originally Posted by srlm_productions
how do they compare to the microsoft GPS.
Not sure about the DeLorme BT-20 Bluetooth/USB GPS receiver, as I don't see relevant specs on the website, but the new DeLorme LT-40 would be comparable to the new Microsoft USB GPS receiver GPS-500 (which has improved sensitivity over the older Microsoft GPS-360).


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