"Gap" in GPS fixes with Racebox Mini

[vafan13]

Heyo. This past weekend is the first time I got the racebox mini to (mostly) work with LapTimer. In reviewing the GPS fixes, it appears to do the full 25hz rate most of the time but then regularly have gaps were there are no fixes for 0.5-1 second. This can then look weird on video if it happens during accel or decel as the MPH will "jump". The GPS was mounted to the windshield ~4 inches from the phone and no other BT device, except my car, is connected.

Do you know what's going on here and how to fix it?

[Harry]

In case it is 0.5 to 1s, this is a bandwidth problem. They may occur in case you have BT devices around adverting. You can look for nearby BT devices by using apps like LightBlue. In case the gap is bigger and LapTimer is falling back to the internal sensor (1 Hz rate for at least 3 or 4 seconds), there is a disconnect. Btw. LapTimer should interpolate any gap to the video frame rate. So a drop to 1 Hz is usually invisible..

[vafan13]

Thanks. I assume there's a lot of other BT devices at the track. Is there a way to *only* have it connect directly to my specific GPS?

And then looking more closely at the Fixes with gaps...it appears the ones with the jumping in video also have abnormally large jumps in the distsnce. Then a nearby but esrlier one will have an expected distance increase (~7ft) but with a ~1 second jump in time and have a "freeze" in speed. Almost like one is making up for the other. Attached is an example. In the associated video the speed is static at 117mph starting at that 1:32.2 fix and then jumps to 123mph between the 1:33.28 and 1:33.44 fix.

[Harry]

That’s probably the noise one needs to expect from a sensor not filtering data a lot. Consumer level GPSes (like those in smartphones) are heavily filtered / smoothed to show “nice” data. Devices like VBOX sport (and probably Racebox) do almost no filtering / smoothing but deliver raw data. Instead of losing information early (in the chip), they leave it to post processors to evaluate and act accordingly. In this case, both speed values may be in precise and need smoothing. But sure, when having a speed change like this within a fraction of a second, LapTimer’s by frame interpolation will not show a smooth development.

[Harry]

PS Bluetooth bandwidth is a common resource, you can’t isolate devices broadcasting. They will use bandwidth. The only option will be to turn them off. Btw. chances are good it is in your own car. Other car’s BTs will hardly interrupt your communication on track. There is a lot of shielding from the car’s bodies. Possible, but not so probable.

[vafan13]

PS Bluetooth bandwidth is a common resource, you can’t isolate devices broadcasting. They will use bandwidth. The only option will be to turn them off. Btw. chances are good it is in your own car. Other car’s BTs will hardly interrupt your communication on track. There is a lot of shielding from the car’s bodies. Possible, but not so probable.

Thanks for both replies! Do you think unpairing my phone from the car while tracking would help, or at least be a good practice?

[Harry]

Unpairing will remove any communication between car Hi-Fi and the phone but may trigger advertisements by the car stereo. So you need to test…