Lost GPS and Acceleration Data

[tulloch]

I recorded video of about 75 minutes road driving and noticed that at around 63 minutes I had lost both GPS and acceleration data. The vehicle speed was more or less stuck at around 23mph, the green dot in the acceleration circle was centered and not moving and the trace of the course was lost. I was using the Dual external GPS.

I don't know if the Dual lost its fix or the Bluetooth connection was lost, is there any way to tell from the recorded data?
If it was the Bluetooth that failed would HLT fall back to the internal GPS?
It seems strange to me that the acceleration data also failed at the same time as that comes from the internal sensors. Could there be a connection?

[Harry]

There is indeed a connection between GPS and acceleration: LapTimer uses GPS fixes as the "recording points" for any other sensor data. So every time a GPS fix is stored, all other information is interpolated to this position and stored with it. I use "interpolation" as a general term here and it differs by sensor type. For OBD values, it is indeed interpolation: LT calculates something like "weighted averages" of data points ahead and behind the GPS fix (wheel speed for example). Acceleration values are sampled at a much higher frequency and get passed through a "low passing" to get rid of "nervous" values. Once this filtering is done, max acceleration in an area around the GPS fix is determined and stored with the GPS fix.

Again one of this length "Harry answers" the short form is: once you do not get GPS fixes any more, nothing is stored.

On the "fall back" scenario: in case you an accessory like the Dual is disconnected, LT falls back to the internal sensor and will continue sampling and storing. In case the accessory is not disconnected, but simply does not deliver valid data any more, LapTimer assumes you are in a tunnel or something like this and accepts that.

A final word on your scenario: that is quite a long recording. Although LapTimer is well able to record such lengthy "laps", the test cases we verify the software against are more "track like". The biggest test case is 20 minutes long reading a 20 Hz GPS stream

- Harry

[tulloch]

Thanks Harry. I've now had a look at the CSV data export for the recording and it looks like the Dual lost the fix. Here are the last few rows of the data, am I correct or is it not possible to tell? I'm thinking if it did not fall back to the internal sensor then the connection did not fail. (It's not so easy to read, I can send the data in a spreadsheet if you like)

23-Feb-13 12:11:14 3804.5 55.653944 -4.510338 59.5 36.971586 109 357.611549 357.2 1 2 11 0.85 2.5 59.1278 36.740311 1 0.09 0 55.653942 -4.510409
23-Feb-13 12:11:14 3804.75 55.653981 -4.510341 59.7 37.09586 109.2 358.267717 357.1 1 2 11 0.85 2.5 59.1318 36.742797 1 0.08 -0.01 55.653979 -4.510404
23-Feb-13 12:11:15 3805 55.654017 -4.510344 59.5 36.971586 109.3 358.595801 357.2 1 2 12 0.85 2.5 59.1358 36.745282 1 0.07 0.01 55.654016 -4.510399
23-Feb-13 12:11:15 3805.25 55.654054 -4.510346 59.1 36.723037 109.5 359.251969 357.3 1 2 12 0.85 2.5 59.1398 36.747768 1 0.06 0 55.654052 -4.510394
23-Feb-13 12:11:15 3805.5 55.65409 -4.510349 59.4 36.909449 109.7 359.908137 357.1 2 2 12 0.85 1.4 59.1438 36.750253 1 0.06 0.03 55.654089 -4.510397
23-Feb-13 12:11:15 3805.75 55.654126 -4.510352 59.8 37.157997 109.8 360.236221 357.5 2 2 12 0.85 1.4 59.1478 36.752739 1 0.08 0.03 55.654125 -4.510415
23-Feb-13 12:11:15 3806 55.654163 -4.510354 60 37.282271 110 360.892389 357.5 2 2 12 0.85 1.4 59.1518 36.755224 1 0.08 0.04 55.654161 -4.510418
23-Feb-13 12:11:16 3806.25 55.654199 -4.510357 60.3 37.468683 110.2 361.548557 357.3 2 2 12 0.85 1.4 59.1558 36.75771 1 0.07 0.04 55.654198 -4.510412
23-Feb-13 12:11:17 3807.25 55.654348 -4.510367 62.2 38.649288 110.7 363.188977 357.7 2 3 12 0.85 1.4 59.1723 36.767962 1 0.02 -0.02 55.654347 -4.510383
23-Feb-13 12:24:24 4594.75 55.709844 -4.584034 0 0 96.4 316.272966 357.7 2 3 12 0.84 1.4 66.8795 41.556995 1 0.02 -0.02 55.709843 -4.58405
23-Feb-13 12:24:24 4595 55.709843 -4.584034 0 0 96.3 315.944882 357.7 2 3 12 0.84 1.4 66.8795 41.556995 1 0.02 -0.02 55.709843 -4.58405
23-Feb-13 12:24:25 4595.25 55.709843 -4.584034 0 0 96.3 315.944882 357.7 2 2 12 0.84 1.4 66.8795 41.556995 1 0.02 -0.01 55.709843 -4.584049
23-Feb-13 12:24:25 4595.5 55.709843 -4.584033 0 0 96.2 315.616798 357.7 2 2 12 0.84 1.4 66.8795 41.556995 1 0.99 0.01 55.709826 -4.584823
23-Feb-13 12:24:25 4595.75 55.709843 -4.584033 0 0 96.2 315.616798 357.7 2 2 12 0.84 1.4 66.8795 41.556995 1 0.99 0.01 55.709825 -4.584822
23-Feb-13 12:24:25 4596 55.709843 -4.584033 0 0 96.2 315.616798 357.7 2 2 12 0.84 1.4 66.8795 41.556995 1 1 0.01 55.709825 -4.58483
23-Feb-13 12:24:26 4596.25 55.709843 -4.584032 0.1 0.062137 96.2 315.616798 357.7 2 2 12 0.84 1.4 66.8795 41.556995 1 0.99 0.01 55.709825 -4.584822
23-Feb-13 12:24:26 4596.5 55.709843 -4.584032 0.1 0.062137 96.2 315.616798 357.7 2 2 12 0.84 1.4 66.8795 41.556995 1 0.99 0.01 55.709825 -4.584821
23-Feb-13 12:24:26 4596.75 55.709843 -4.584032 0 0 96.2 315.616798 357.7 2 2 12 0.84 1.4 66.8795 41.556995 1 1 0.01 55.709825 -4.584829
23-Feb-13 12:24:26 4597 55.709843 -4.584031 0 0 96.3 315.944882 357.7 2 2 12 0.84 1.4 66.8795 41.556995 1 1 0.01 55.709825 -4.584829
23-Feb-13 12:24:27 4597.25 55.709843 -4.584031 0 0 96.3 315.944882 357.7 0 3 12 0.84 1.4 66.8795 41.556995 0 1 0.01 55.709825 -4.584829

[Harry]

Please just send the original CSV to Harry@gps-laptimer.de, thanks.

- Harry

[Harry]

Had a look into the data sent. You are right, although the data continued to come in at a regular update rate, it did not change any more. No idea why, but GPS chip firmware can probably hang just like any software can

- Harry

[tulloch]

Indeed, I've seen it fail to start up properly (get a very rapid flashing green light) as well and had to restart it. Perhaps I'll treat myself to a Vbox sport.

[tulloch]

Obviously the Vbox brings us much faster update rates but how does this translate to practical benefits? I can envisage a more accurate track with the additional plotted points but does it bring for example more accurate speed readings? Would speed values fluctuate more in line withto real time speed? It's a fair investment so I'd like to understand fully the benefits before ordering one. Thanks.

[Harry]

Keep in mind VBOX is the only device built to target racers. My personal experience is limited so far (only some test recordings, it is still too cold for track days here) but speed is shown by far more instantly compared to all other GPS-s I have seen. Racelogic has done a lot of work on accurate speed evaluation, so I assume besides the 20 Hz, they will deliver better data. Still needs to be proved, I will post once I have more data / data comparisons.

- Harry

[tulloch]

I'll hold off for now and look forward to that. Thanks.