Hi OutdoorDad,

The GPS almanac takes around 12-13 minutes to download no matter the GPS reciever chipset. The almanac data is valid for 2-3 months so if your GPS has been powered down for more than 2-3 months it will take at least 13 minutes to get a fix.

That's how long it took to find the satellites and download the ephemeris for each one. The ephemeris is the data it needs to actually get a location fix. It's a very precise description of the satellites movements and so expires after a few hours. Once you've got one, switching off and on gives a "warm start" which takes a second or so.

The almanac is different to the ephemeris. It's a less precise description and so doesn't expire for after several months. It takes longer to download because it covers all of the satellites, rather than just the one broadcasting it, and it is broadcast more slowly. However, you don't need the almanac to get a fix; you need the ephemeris for that. The purpose of the almanac is to give the unit a good initial guess for where the satellites are, so it can find them and download the ephemeris quicker.

If the unit has been switched off for a long time, then the almanac will be out of date and it will take longer to get an initial fix. Once you have that fix, you have the ephemeris so you can switch off and on again for a "warm start". But you won't have the almanac for another 15 minutes or so. If you switch off for about 6 hours and then switch on again, it will again be slow to get the initial fix. If you leave it on for half an hour, then switch it off for 6 hours, you should get a quicker initial fix using almanac data even though the ephemeris is expired.

Getting a fix when you have an almanac already downloaded should take under a minute. If you don't have the almanac it's partly a matter of luck whether the unit looks for the satellite in the right places, and that's when it takes several minutes. However, it doesn't necessarily take 15 minutes to get the first fix because you can get a fix without the almanac, if the unit gets lucky.

I hope that makes sense.

RAS: the almanac is a description of the satellites in the GPS constellation and has nothing to do with your location. If you're going to hike to the South Pole it would make no difference to let it download the almanac from your back yard.

Outdoordad: about 15 minutes to download an entire almanac, technically 30 seconds per satellite so it changes a little from time to time. This doesn't mean you're going to have to wait that long, but rather that you *might*. If the satellites you need info about are at the front of list it may only take a couple of minutes.

If your GPS is turned on when plugged into the car's cigarette lighter then it's downloading an almanac already. In your case the stored Almanac may never get stale and you may never see that long cold start delay – the delay you do see might be only a warm start, doing the 30-second-max ephemeris downloads for each visible satellite. I was thinking of a handheld GPS used only on vacations that might be off all winter, long enough to have the almanac go stale.

Frank2135: yes, that sounds like it's waiting on a fresh almanac. If it's used weekly all spring it shouldn't have that long wait any more since it's always refreshing the almanac when it's on, and it takes a couple of months for the last download to become stale.

Brangdon: yeah, but I wasn't that brave with the details. I think the almanac is the only place to get drift and error rates, not to mention poisoning info, so an ephemeris-only fix may shift a little once that satellite's current almanac entry is available.

I didn't mean to blow this out of proportion. I was just thinking, once I get in the country on vacation why wait on almanac when turning it on ahead of time is painless? And if you want to avoid unnecessary battery consumption in the field, then do the almanac download ahead of time, before putting in fresh batteries. No big deal, it's going to work either way.

I don't understand why this is important. Even a cold start when the GPS hasn't been used in a long time never takes more than a couple minutes to get a fix. It boots up, satellites start appearing in their status window and next thing I've got a <20' fix.

I've never taken out a stopwatch and timed it, but next time I will. Turn it on from a cold start, let it get to <20' and then give it enough time to download the almanac. From what you're saying the next time I boot it up I should get to that <20' fix quicker (minutes/seconds?). We'll see.

I have been told, but don't know myself, that some newer GPS receivers will give a fix using only ephemeris data if the almanac is stale. That will seem like a warm start.

The problem is that once a current almanac is available the fix may mysteriously shift once current drift and error are available a few minutes later, and the fix may shift a lot if it turns out that satellite was in testing or broken and not really part of the constellation at all.

What make & model GPS are you discussing? The manual may be online and indicate how it handles cold starts. Remember that it needs to be left off for *months* to force a cold start.

I'm using a Garmin Geko 301 for this exercise. My older Garmin GPS V is powered from the car, it's turned on stays on so cold starts are really irrelevant.