Understanding GPS performance for data logging in car cameras

jokiin

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It's important to understand how the GPS functions as to why it behaves the way it does, with car cameras the priority is always video and processing power is allocated more for video than anything else, you'll notice car cameras typically boot and start recording very quickly, GPS is a lower priority process and it will hunt for a signal after a recording has commenced.

There are three distinct modes for the GPS, cold start, warm start and hot start, most people would be aware that it takes three satellites to get a lock, the GPS will increase accuracy as more satellites are locked and will sometimes lock 7 or 8 satellites at once, each satellite provides its own position information and instructions to the receiver known as Ephemeris data, the download of this information takes 12.5 minutes (if it all comes down error free, otherwise it starts again), this data provides accurate forecast position information valid for the following few hours but only for that particular satellite, there is also Almanac data which is much less accurate but valid for much longer periods and for all satellites, obviously you typically get a signal much faster than 12.5 minutes and some basic triangulation and positioning info can be provided relatively quickly in some circumstances depending in the state of the GPS, cold, warm or hot.

A cold start is when the GPS has been off for more than 4 hours and this process must begin from scratch each time based on Almanac data of roughly where the Satellites might be.

A warm start (for most solutions) is around 30 minutes, if the GPS had signal within 30 minutes prior to a restart then the Ephemeris data collected is still valid and the GPS has an idea which Satellites to look for and reasonably close to where they should be at the time, the warm start is generally quite quick in comparison to a cold start, GPS solutions that have an internal RTC battery can often offer warm start times that are longer than 30 minutes.

A hot start is when you have a signal already and you drive out of coverage, through a tunnel, under a bridge or similar, when you come back into a coverage area the GPS knows what Satellites you were connected to and will look straight back to those same ones and will pick the signal back up very quickly.

It’s typical behaviour for the GPS to be slower to catch the signal initially when starting up undercover and with no signal, when there is no signal and a cold start or warm start happens if you don't get access to open sky within a relatively short time the GPS effectively gives up looking where it thinks it should be looking and starts looking elsewhere for signal which might not always be the appropriate location so it will take a bit to get back to where it needs to look.

GPS has got smarter over the last few years and there's things like forward error correction to improve cold start performance and there’s options like dropping invalid data for situations like the GPS starting up when there's no signal to be had to try and improve the cold start hunt time, it's quite an involved process though and something that is still being improved.
 

mariomart

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Thanks Jokiin :)

No matter how long it takes a GPS to get a fix it sure beats the hell out of having the missus try and read a map ;)

Have a very Merry Christmas

Ho, Ho, Ho.............
 

kamkar1

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My cam usioally idicate GPS speed within 30 seconds or so, but yesterday it was 2-3 minutes about it. ( Cold start )

But it is fine with me :) the GPS stuff is just icing on the cake.
 

fieldofview

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In terms of accuracy, I'm impressed with the GPS and software in my Mini 0801. At steady state speed, the speed reading in the video matches the reading on road side speed boards. I don't know how well calibrated the road side speed boards are but they are put up by our Ministry of Transportation and there seems to be consistency between the boards I encounter. The dashcam's speed reading matches my cellphone's GPS speed and the speed reading on the GPS I use for field work. However be aware that if speed is changing such as on acceleration or deceleration, that there is a lag time on any GPS speed reading. However my car's speedometer reads about 7% higher than actual speed (drive 107 km/hr to achieve 100 km/hr true velocity) so I tend to use a GPS for a speedometer.
 

REyewitness

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Excellent...excellent write up! Love it
 
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