GPS.... How does it work exactly?

[bronze84]

So, my Thinkware F750 struggles to get a GPS signal when mounted to my windscreen. Not through fault of the cam however, it's because my windscreen has this stupid laminate bit in the middle. Anyhow, when I head out for a journey the GPS will take a fair few minutes to 'lock on'. Once it does it's great! However when I then go out to my car say 30 mins later and start the car the GPS locks on as quickly as the cam starts recording!

My question is, once the GPS has found a lock the first time round and is then turned of, does the cam somehow save the last known positions of the satellites? That's the only way I can think why it locks on straight away on a second journey?

Hope that makes sense...

 

[reverend]

Yes mate it's the time to first fix:

The TTFF is commonly broken down into three more specific scenarios, as defined in the GPS equipment guide:

• Cold or Factory: The receiver is missing, or has inaccurate estimates of, its position, velocity, the time, or the visibility of any of the GPS satellites. As such, the receiver must systematically search for all possible satellites. After acquiring a satellite signal, the receiver can begin to obtain approximate information on all the other satellites, called the almanac. This almanac is transmitted repeatedly over 12.5 minutes. Almanac data can be received from any of the GPS satellites and is considered valid for up to 180 days. Manufacturers typically claim the factory TTFF to be 15 minutes.

• Warm or Normal: The receiver has estimates of the current time within 20 seconds, the current position within 100 kilometers, and its velocity within 25 m/s, and it has valid almanac data. It must acquire each satellite signal and obtain that satellite's detailed orbital information, called ephemeris data. Each satellite broadcasts its ephemeris data every 30 seconds, and is valid for up to four hours.

• Hot or standby: The receiver has valid time, position, almanac, and ephemeris data, enabling a rapid acquisition of satellite signals. The time required of a receiver in this state to calculate a position fix may also be termed Time to Subsequent fix (TTSF)

Many receivers can use as many as twelve channels simultaneously, allowing quicker fixes.[1] Many cell phones reduce the time to first fix by using assisted GPS (aGPS): they acquire almanac and ephemeris data over a fast network connection from the cell phone operator rather than over the slow radio connection from the satellites.

https://en.wikipedia.org/wiki/Time_to_first_fix

The GPS modules have a tiny battery somewhere I believe to keep the data in memory

 

[bronze84]

Thanks very much [emoji106]

 

[reverend]

You do see some folks post here every now and then where that battery starts to play up and the GPS goes tits up with it - sometimes they replace the battery or just rip it off so it does a cold start and it seems to get it going again

 

[Mandami]

it's because my windscreen has this stupid laminate bit in the middle.

Check to see if you don't have a free area on the top of the windshield (where it should be mounted the RF antenna/or IR transmitter for the gate access).
Also, make sure your FM antenna is not built into the windshield (it may interfere) and the GPS antenna is mounted correctly.

 

[jokiin]

My question is, once the GPS has found a lock the first time round and is then turned of, does the cam somehow save the last known positions of the satellites? That's the only way I can think why it locks on straight away on a second journey?
.

a warm start is what you're referring to, details here https://dashcamtalk.com/forum/threa...-data-logging-in-car-cameras.3566/#post-41179

 

[bronze84]

Cheers chaps