Cold Start-up Tip for GPSs

wallyworld

This is probably so basic it's probably already been covered elsewhere...but here goes.

One thing I've notice with my Garmin Nuvi 670 is that it can take a long time to acquire satellites on a cold start. This is not great when you are leaving home and need directions nearly right away (which I do!)

Apparently, the longer the GPS is off, the longer it could take to acquire the signals the next time the GPS is turned on (or so I've heard).

Anyway, something that I've found useful is to turn the GPS on at home (where I actually can acquire satellite signals in my sitting room) and let it "warm up" just before I am getting ready to leave (5 min or so).

Then, when I'm ready to go in the car, it just takes a second to acquire the satellites again.

On some occasions, it's taken the Nuvi 5 minutes or so to find the signal from a cold start, and this process seems to be a cure for this.

Hope this helps someone

Lex Luthor

don't know what the situation is with Garmin, but I check the ToMtOm site regualrly and it provides updates for GPS fixes to help find the stalleites faster.

Once you keep it up to date, it finds them within 10 seconds of power on

wallyworld

Lex:

Interesting...generally the Garmin is very good all around, but the cold start up after the unit has been off for a number of days does take a bit of time.

I was curious as to whether GPS brands had a similar problem.

Sun

robinph

The startup time will be common to all makes/ models if you have had the unit off for a long time between uses, or if you have traveled a long distance between the last time you tuned it on such as if you take it on holidays with you. The very first time you ever tuned the unit on it will have taken 20+ ish minutes probably to get the initial fix, this is because it has to wait to download data from one of the satellites about the relative locations of all the others that may be above you at the time. If you've left the unit off for a long period though then it will not have been receiving recent updates from the satellites as to their new positions so it will take time for it to get this information again, and this is also why if you take it on holidays with you its information will not be relevant to your new location so will effectively be a cold start situation again.

The updates you will be getting from TomTom will most likely be map fixes or firmware upgrades, they will not be anything to do with the actual satellite locations or to do with the speed of the initial fix. If your turning the unit on every day though then it should be getting a fix very quickly.

Alun

Here's a good description of the various types of start, and explains the type of data the GPS has to have (almanac, ephemeris, time) to achieve this.

http://www.navigadget.com/index.php/gps-knowledge/ttff-time-to-first-fix/

One thing the article doesn't refer to though is that if there's something up with the clock in your GPS, this can screw up things royally. I have an old Garmin etrex where the little backup battery or supercap that maintains the clock when it's off with no batteries in it seems to have died, and it has great difficulty getting a fix even after only being off for a week. I have to keep a set of batteries in it constantly just to keep the clock going.

Another thing that can mess things up is if you start moving around while trying to get a fix. It takes 30 seconds to load the ephemeris data for each satellite in view, and if this is interrupted for even the briefest of periods, it'll have to wait for the start of the next 30 second slot.

fat-tony

robinph wrote: »

The updates you will be getting from TomTom will most likely be map fixes or firmware upgrades, they will not be anything to do with the actual satellite locations or to do with the speed of the initial fix. If your turning the unit on every day though then it should be getting a fix very quickly.

The updates from TomTom home include an ephemeris, which updates the tables of satellite orbit/locations in the unit. This changes about once a week. The idea is to speed up the initial location of the satellites:D

311

I don't have any trouble with my garmin ,starting up. Maybe a minute or so.
Sometimes if it's cloudy ,etc. It will take a little longer.

Alun

fat-tony wrote: »

The updates from TomTom home include an ephemeris, which updates the tables of satellite orbit/locations in the unit. This changes about once a week. The idea is to speed up the initial location of the satellites:D

They call it Quickfix. It's based on 'predicted' (not sure by whom) ephemeris data. I'm a little sceptical though. I mean, if the people who designed and run the GPS system felt that ephemeris data should only have a 30 min validity, then 'forecasting' it 7 days in advance seems a little dangerous to me. OK, you can con the GPS into thinking it has valid ephemeris data so that it at least appears to get a fix of some sort more quickly, but what about the accuracy of that fix?

robinph

fat-tony wrote: »

The updates from TomTom home include an ephemeris, which updates the tables of satellite orbit/locations in the unit. This changes about once a week. The idea is to speed up the initial location of the satellites:D

Well I have to say I'm impressed they went to the bother of doing that. Clickly

But, not sure how much use it would actually be to anyone. If your using your GPS every day then it's irrelevent as you'll always have up to date information, and if your not using it at least each week then your probably quite unlikely to have figured out how to connect the unit to your PC in the first place anyway.

parsi

The O2 Orbit ships with a program which allows you to autoamtically download the ephemeris as well. It _seems_ to be based on your Cell ID (or network id)

Alun

robinph wrote: »

But, not sure how much use it would actually be to anyone. If your using your GPS every day then it's irrelevent as you'll always have up to date information, and if your not using it at least each week then your probably quite unlikely to have figured out how to connect the unit to your PC in the first place anyway.

If you're using it every day you'll have up to date almanac information, yes, but you'll still have to wait for ephemeris data to load before you get an actual fix. The idea behind Quickfix, and other similar systems, is that you'll also always have (fake or predicted, depending on who you believe) up-to-date ephemeris data as well, so will get an almost instant fix, effectively a hot start every time you switch on. It remains to be seen how accurate this predicted ephemeris data is, and how that affects the quality of the fix it results in.