One thing I did notice was it's a sensitive little unit and likes a clear view to the sky. Putting it inside your helmet may reduce the reception. Also it's consistent with the VACC readings, which are calculated differently to SDOP, as they are 3D measurements and not 2D so there is an extra directional estimation.

Andrew I think the settings on the 10 hz unit may be different to mine. I seem to have slightly better VACC values than you are getting. Here's a comparison between GT-31 and G-Rays in the same pouch on my arm. Both units were showing 9 to 10 sats and HDOP 0.8 to 1.04

I notice a significant time difference between the two units, is this an almanac thing or something else?

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sailquik said..
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Anyhow, a few meters off satellite position makes a big difference to positioning accuracy, but should not affect Doppler much at all.

Thinking about this, I assume for doppler accuracy you need to know the angle of the satellite, relative to the direction you're traveling. If you're heading directly towards or away from a satellite then one sat should be enough. I guess that's why sats directly overhead aren't a lot of good, and sats on the horizon have too much atmospheric interference. Because we're not going to be traveling directly towards a sat for very long, we need several to work out our direction and relative speed to all of them to calculate speed over ground.

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decrepit said..

sailquik said..
<<<<<<

Anyhow, a few meters off satellite position makes a big difference to positioning accuracy, but should not affect Doppler much at all.

Thinking about this, I assume for doppler accuracy you need to know the angle of the satellite, relative to the direction you're traveling. If you're heading directly towards or away from a satellite then one sat should be enough. I guess that's why sats directly overhead aren't a lot of good, and sats on the horizon have too much atmospheric interference. Because we're not going to be traveling directly towards a sat for very long, we need several to work out our direction and relative speed to all of them to calculate speed over ground.

All true, but the actual satellite position variance of a few meters or less will effect the angle in an incredibly small increment.

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sailquik said..


All true, but the actual satellite position variance of a few meters or less will effect the angle in an incredibly small increment.

Yes, I guess I thought that was obvious.

Those error figures from Roos G-rays1 are very similar to those I got. (see my results in this thread). IMHO, they are too high. See also the comparison with the GW-52 above.

Of concern is the error values you have from your GT-31 in this example. It is rare for me to see values for 10 second runs much higher than 0.25kt and they are commonly below 0.20kt.

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sailquik said..
Those error figures from Roos G-rays1 are very similar to those I got. (see my results in this thread). IMHO, they are too high. See also the comparison with the GW-52 above.

Of concern is the error values you have from your GT-31 in this example. It is rare for me to see values for 10 second runs much higher than 0.25kt and they are commonly below 0.20kt.

To illustrate I grabbed a random recent result for a GT-31. In this example, there are a few just in the 0.3's but most around 0.25kt. I have seen plenty better than this, and very few worse. As illustrated earlier, I commonly see an nearly order of magnitude better from the 5Hz GW-52 10 second results