I agree with everything Manfred says here, but...
If there is a lower bandwidth post-processing filter applied for low speeds (based on either trackpoint or doppler calulations, or both) the real errors will be masked in static tests.
Manfred's 2Hz bandwidth figure came from a test I suggested to the tech group some time ago to identify the true bandwidth of the filters, and to prove that aliasing has a strong influence on the output data. That was when most of the members of the tech group were at least highly skeptical of it being an influence, or dismissing it as irrelevant. It involved placing a GPS on the end of a rotating stick at a controlled (known) speed. So the effect on aliasing was clear, since the frequency of the directional component is determined by the angular velocity of the stick. The 2 Hz figure comes from the aliased component folding down, and coming through on the output at just above 2 revs per second.
Now that doesnt apply to a stationary unit if the GPS has a "not moving" mode which suppresses rapid changes more readily. So its a simple question, can we be sure that the post filtering is the same for static tests as moving? This is impossible to tell with only a 1Hz sample rate. The best we can do with this residual noise is go with the nyquist frequency (0.5Hz) which is 1/4 of the moving bandwidth.
Back to the effect of the mobile phone, Kean uses a CDMA phone, which is more powerful than a standard one. This could be the reason for his interference observations not being replicated in Australia, where the CDMA service was shut down a while ago.
In proving something you have to continually check and double check that you have covered all the bases, not cover up the bases if they dont suit the story.
As part of that original discussion, there is evidence from the SiRF protocol document in message ID 28, that the unit is capable of 100 millisecond measurements. This could be construed to imply that the sample rate is 10Hz. As to what it can do is still a subject for debate.
Figures just in from Mal who has got the data from the Sailrocket guys:
Doppler accumulated
GT11 511.39 47.338
Trimble claimed speed
501.92 47.366
Remember, this is from GT-11.
GT-31 is more accurate and more consistent.
Peak speeds from Trimble at 10hz over one second!
49.97612875
49.8890902
50.17646373
51.3446404
51.16735287
52.2344133
51.07210761
51.82737534
50.24160154
49.9614261
49.1151579
Peak Speeds from GT11 at 1hz over 1 second
49.393
50.035
OK, so the peak speed reported from the Trimble trackpoints was 52.23 knots for 1 TENTH of a second.
Does anyone really believe that in a tenth of a second the speed accelerated over one whole knot and back again for the next one tenth of a second?
This is the typical type of noise we see in high hz trackpoints (and Doppler speeds) and must be averaged out over a slightly longer period to get the true picture, which is just what the GT-11 does, hence the lower top speed reported. Dare I say that the GT-11 top speed is more realistic?!
I see some places are selling 10Hz gps for $60
www.semsons.com/
GPS Data Logger (SiRF III, Download via Bluetooth and 60,000 trackpoint) for $70
www.semsons.com/glbtbldalo.html
others with 200,000 points ($89).
etc etc.
Locosystems should be able to give us a fully sealed gps with huge memory, bluetooth download, more Hz and induction charging.
wow! - from a simple post this got pretty technical, I dig it!
It would be interesting to install a triaxial accelerometer (probably use one component though) on a board/craft and actually compare/verify the accelerations reported by the GPS.
I dont think anyone is suggesting the GT11 or GT31 is more accurate than the Trimble, but we are seeing some really good agreement between the GT11 and the Trimble on this run. Which is another piece of evidence that the data from the SIRF chips is accurate.
Kean, as you know the doppler speeds are exported from the GPS to 0.01 m/s in the SIRF binary message but we are reporting it in knots which requires a conversion that can make the result contain many more decimal digits. That is why the results are shown to three decimal places. The reference to the GT31 being more accurate is about the SIRF GPS engine being able to pick up more satellites and process the data better, not in how many digits are output.
On the subject of satellites, I believe the doppler result is improved by using satellites low to the horizon, whereas using low satellites can be less accurate for track point location calculations due to greater atmospheric inteference. The SIRF III chips greater sensitivity should help pick these up.
0.01 m/s = 0.019386 Knots
To round it up to 0.02 knots reduces precision.
Using the full conversion does not increase precision of the original data, it only preserves it. We should probably be expressing the conversion to 6 decimal places and I guess that is what happens in the software but it would be unwieldy to fully express it in the interface all the time.
Using 3 decimal places in Knots may give the impression that we can distinguish changes in speeds with a resolution of .001 knots. This is of course not the case.
The next step up in speed will be 0.02m/s which will be 0,038772 knots (not 0.04 knots!)
I know you get this Chris and your maths is one helluva lot better than mine.
