5 ms·

I can’t imagine it taking 15 minutes. I used to play with serial connected dumb dongles back in 2000-ish. These only communicated with my software on the other

I can’t imagine it taking 15 minutes. I used to play with serial connected dumb dongles back in 2000-ish. These only communicated with my software on the other end of the serial cable and a pure cold-start was spec’d at 45 seconds. I imagine maybe that figure has improved.

tjoff 6y ago
Lots of devices take a really long time to do this. Most devices have internal battery backup that keep the almanac between sessions, at which point it can be really fast. But the (admittedly somewhat old) watches I've had get a really long time to lock if the almanac is out of date or you have traveled a lot since last it was used. It takes 12.5 minutes for the almanac to get propagated. Not sure how much of the almanac you really need or if you can download multiple at the same time. But from practical experience 12.5 minutes seems about right with the devices I've used.
worewood 6y ago
It's not dependent on the device. The satellites themselves send their position data but on a very low data rate (I think its just a couple bits per second) so if the device doesn't have the data it does take 15 or so minutes. Maybe the "cold-start" you are referring to wasn't so cold, it may have the almanac data saved from a previous utilization.
- eknkc 6y ago
  Yeah, there are 3 possibilities. If there is absolutely no data cached in the device, it requires the almanac. Basically a map of all satellites in the constellation. Takes roughly 15 minutes because satellites transmit data really slow. This data is valid for a couple of months so a device can cache it a long time. If the almanac is present, it provices the coarse information. The device still requires precise status of the satellites it is receiving signal from. Each satellite transmits that within something like 30 seconds. So it would take 30 seconds for a precise fix. If the device knows everything about the constellation, can process the satellite signal immediately and pinpoint the location. If you are using a mobile phone, it should always be in this state because the required data is provided fast via internet. In case of other devices like offline watches, the almanac might be synchronised from time to time when you connect it to your phone or computer. And the fix would take up to 30 seconds.
reaperducer 6y ago
I built a GPS rig for my car from a Palm III, a bunch of serial adapters, and a GPS antenna liberated from a minivan back in 1999. It took every bit of 15 minutes to get usable locks and data. It probably took longer. Sometime I'd be at work before the map started moving. I have a little clip-on geotracker I bought in 2010 that I use for cross-country train trips. It takes at least 10 minutes to lock on, and since it can only be either "on" or "charging," but not both, there are long gaps in the tracks on the maps.
makomk 6y ago
I don't think it generally should for modern devices. They generally just throw compute at the problem, attempt to acquire every possible satellite at every possible location in parallel, and then download all their ephemeris data in parallel (which is more precise than the almanac, but only available once you've actually acquired the satellite). Makes the almanac kind of redundant, and typical acquisition time is anout 45 seconds doing it that way since the ephemeris data is broadcast much more often.
sokoloff 6y ago
If I cold start a Raspberry Pi attached GPS in an airplane moving at 175 knots, it can easily be 20-30 minutes before it will determine a fix.
londons_explore 6y ago
I used some mid 90's GPS receivers in the military, and they could take up to an hour for an initial fix. They required an antenna on a pole and cryptographic keys to be loaded too (for the unjammable encrypted signal)... They didn't have maps - you had to use coordinates on the screen to look up your location on a paper map... Everyone thought they were kinda pointless, because why not just triangulate off some hilltops which would take 5 mins rather than waiting an hour...
jcrawfordor 6y ago
You'd be surprised how many GPS devices never cold start. I've worked with multiple serial GPS receivers that stored the last solution in nonvolatile memory as a start point. A quick way to check: buy a brand new one, plug it in, and see if it starts offering (low-quality) fixes for Shenzhen or wherever it was QAd. Unfortunately some receivers won't actually provide NMEA sentences until the quality flag goes on, so it's not quite this obvious, but I just recently bought an industrial cellular modem with onboard GPS and experienced a 30 minute or so wait before the quality flag came on and I started getting NMEA sentences. Presumably it was busy discovering it wasn't in Kansas any more.

tjoff 6y ago

Lots of devices take a really long time to do this. Most devices have internal battery backup that keep the almanac between sessions, at which point it can be really fast. But the (admittedly somewhat old) watches I've had get a really long time to lock if the almanac is out of date or you have traveled a lot since last it was used. It takes 12.5 minutes for the almanac to get propagated. Not sure how much of the almanac you really need or if you can download multiple at the same time. But from practical experience 12.5 minutes seems about right with the devices I've used.

worewood 6y ago

It's not dependent on the device. The satellites themselves send their position data but on a very low data rate (I think its just a couple bits per second) so if the device doesn't have the data it does take 15 or so minutes. Maybe the "cold-start" you are referring to wasn't so cold, it may have the almanac data saved from a previous utilization.

eknkc 6y ago
Yeah, there are 3 possibilities. If there is absolutely no data cached in the device, it requires the almanac. Basically a map of all satellites in the constellation. Takes roughly 15 minutes because satellites transmit data really slow. This data is valid for a couple of months so a device can cache it a long time. If the almanac is present, it provices the coarse information. The device still requires precise status of the satellites it is receiving signal from. Each satellite transmits that within something like 30 seconds. So it would take 30 seconds for a precise fix. If the device knows everything about the constellation, can process the satellite signal immediately and pinpoint the location. If you are using a mobile phone, it should always be in this state because the required data is provided fast via internet. In case of other devices like offline watches, the almanac might be synchronised from time to time when you connect it to your phone or computer. And the fix would take up to 30 seconds.

reaperducer 6y ago

I built a GPS rig for my car from a Palm III, a bunch of serial adapters, and a GPS antenna liberated from a minivan back in 1999. It took every bit of 15 minutes to get usable locks and data. It probably took longer. Sometime I'd be at work before the map started moving. I have a little clip-on geotracker I bought in 2010 that I use for cross-country train trips. It takes at least 10 minutes to lock on, and since it can only be either "on" or "charging," but not both, there are long gaps in the tracks on the maps.

makomk 6y ago

I don't think it generally should for modern devices. They generally just throw compute at the problem, attempt to acquire every possible satellite at every possible location in parallel, and then download all their ephemeris data in parallel (which is more precise than the almanac, but only available once you've actually acquired the satellite). Makes the almanac kind of redundant, and typical acquisition time is anout 45 seconds doing it that way since the ephemeris data is broadcast much more often.

sokoloff 6y ago

If I cold start a Raspberry Pi attached GPS in an airplane moving at 175 knots, it can easily be 20-30 minutes before it will determine a fix.

londons_explore 6y ago

I used some mid 90's GPS receivers in the military, and they could take up to an hour for an initial fix. They required an antenna on a pole and cryptographic keys to be loaded too (for the unjammable encrypted signal)... They didn't have maps - you had to use coordinates on the screen to look up your location on a paper map... Everyone thought they were kinda pointless, because why not just triangulate off some hilltops which would take 5 mins rather than waiting an hour...

jcrawfordor 6y ago

You'd be surprised how many GPS devices never cold start. I've worked with multiple serial GPS receivers that stored the last solution in nonvolatile memory as a start point. A quick way to check: buy a brand new one, plug it in, and see if it starts offering (low-quality) fixes for Shenzhen or wherever it was QAd. Unfortunately some receivers won't actually provide NMEA sentences until the quality flag goes on, so it's not quite this obvious, but I just recently bought an industrial cellular modem with onboard GPS and experienced a 30 minute or so wait before the quality flag came on and I started getting NMEA sentences. Presumably it was busy discovering it wasn't in Kansas any more.