how does quickGPS work?

[mvl]

Just a nagging curiosity:

Can someone explain the science behind why tomtom takes forever to find a GPS (if it has been off for a while), and why quickGPS helps in some way?

I've seen tomtom take 15-20 minutes indoors without quickGPS, then lock onto a strong signal. What can it possibly be doing to miss a strong signal for 15 minutes of trying?

FYI, I also noticed my sirius satellite radio do the same thing if its been off for a while.

 

[Gilbert]

While I could answer your question, I suggest you do a search on ?Quickfix? and ?ephemeris? as this has been explained many times in many messages here, even in the past couple of weeks or so. It also is in your user manual.

 

[Bama Rambler]

I too suggest you search because you can read more than I can tell you here but here's a quick synapsis of what you're asking.

Quickfix contains the location (ephermis data) of the satellites for seven days. Your TT uses this data to figure your position once it locks onto the signals. Without the file the GPS has to download the data from the satellites and this can take a while (Mine usually takes about 2 minutes with a good strong signal). If you're inside a building or if you're moving it usually takes longer because of the errors introduced during the download. The satellites push the ephemeris data out at regular intervals so if the GPS senses an error in the data it has to wait till next time to re-download it.

Even if the GPS has a strong signal it won't show you it has until it has the ephemeris data. That data can either be downloaded from the satellites or be supplied in the form of a Quickfix file.

 

[rcacs]

You dont state which model of TT you have (should put it in your signature line).

Having said that, not all TT devices even use the QuickFix, for example my Rider2.

My Rider rarely takes more than 15 seconds to pick up enough sat's to start functioning, no matter how long its been turned off. Using it indoors of course is part of your problem.

http://uk.support.tomtom.com/cgi-bi...3RleHQ9Z3BzIHF1aWNrIGZpeA**&p_li=&p_topview=1

Cheers!

 

[sageuvagony]

Like rcacs, mine takes no more than 15 seconds as well to pick up a signal. When I power it on from inside the house it takes a bit longer. The device is not meant to be used in-doors.

 

[Rick F.]

Here's a good post to a thread on another forum that explains how the QuickGPSfix works:

http://www.pocketgpsworld.com/modules.php?name=Forums&file=viewtopic&p=412446#412446

And if that's not enough, a very good explanation on how GPS units work. It explains the difference between 'Cold Starts' & 'Warm Starts'. If you use your GPS unit everyday, it will find the satellites much faster.

http://www.pocketgpsworld.com/howgpsworks.php

Hope this helps.

 

[RobertB]

I think the "how GPS works" link may answer the question I was about to ask: Why did it take my unit so long to get a GPS fix when I turned it off in Dallas and turned it back on in Austin? The short answer is probably, "because it didn't find the satellites it expected and went into 'Factory Start' mode, even worse than 'Cold Start'." Thanks!

 

[Rick F.]

Since you turned your GPS on in a different city (Lat/Lon different than where you turned it off), some of the ephermis data was old. Your unit needed to download new data... which according to the information in those links above requires a stronger signal strength (28dbHZ or >) from at least 4 satellites - and for at least 1 full minute without interruption.

This is why TomTom suggests even though we may know the rest of a route we're traveling, to leave the unit on until we arrive our destination and THEN turn it off. This way the TomTom will acquire satellites more quickly when we turn it back on.

Hope this helps.

 

[sab]

So next month when I leave Montreal and fly into Miami at 1 am with 2 kids how long might my TT take to lock in so I can start the drive to my hotel.

 

[dhn]

It could be 10 or 15 minutes but hopefully it'll be less.

The time is reduced if: a) the gps is set up in a clear area not obstructed by tall buildings, trees, etc. (in other words, if the rental car is in a parking garage, drive out of the garage first) and 2) if the gps is in a stationary position till the signal is received.

I can imagine that in the middle of the night with kids in tow, the last thing you want to do is, essentially, nothing but it will reduce the signal capture time.

May I also suggest prior to travelling, that you get printed directions from Miami airport to your hotel using Mapquest or Google Maps. Just in case.

 

[sab]

It could be 10 or 15 minutes but hopefully it'll be less.

The time is reduced if: a) the gps is set up in a clear area not obstructed by tall buildings, trees, etc. (in other words, if the rental car is in a parking garage, drive out of the garage first) and 2) if the gps is in a stationary position till the signal is received.

I can imagine that in the middle of the night with kids in tow, the last thing you want to do is, essentially, nothing but it will reduce the signal capture time.

May I also suggest prior to travelling, that you get printed directions from Miami airport to your hotel using Mapquest or Google Maps. Just in case.

When I purchased my XL for $540 (2 years ago) I swore I would never use Mapquest again . Oh well.

All kidding aside, thanks for the advice.

 

[mvl]

So if I understand correctly:

Tomtom "hears" he satellites pretty quickly, and then quickly knows how far it is from each one, but not where the satellites themselves are.

The delay is due to waiting for each satellite to broadcast its position? (which they don't do that frequently)

And quickgps is a daily update on where the satellites are (or are expected to be) for the next 24 hours?

 

[canderson]

So if I understand correctly:

Tomtom "hears" he satellites pretty quickly, and then quickly knows how far it is from each one, but not where the satellites themselves are.

The delay is due to waiting for each satellite to broadcast its position? (which they don't do that frequently)

And quickgps is a daily update on where the satellites are (or are expected to be) for the next 24 hours?

Actually, any one can "speak" for the rest. When you first start to look for them, one of them will need to provide the current data for the lot. Takes a bit for that data to come across, at which time you start seeing locks on the satellites within view.

 

[sageuvagony]

This is still a bit confusing to me. All I know is that I had my TomTom powered on here in Florida on the way to the airport. When I arrived at the airport, I shut it off. I boarded the plane, and a few hours later I landed in Texas, went to pick up my car rental, turned on my TomTom, and got a sat lock within 2 minutes. I traveled half-way across the Country. I'm not seeing where these 15 minute holds are coming from. I know that if I had to wait 15 minutes for a signal lock, I'd be pretty pissed off. Why have I never had to wait so long for a lock?

Same thing for when I went to Pennsylvania from Florida which is about 1,000 miles. I also had to do a cold boot at the Pennsylvania airport because I had switched SD cards to watch movies on the airplane. Why did I also get a quick lock after having had flown over 1,000 miles, AND a cold boot?

 

[dhn]

The newer models of TomTom (x20's, x30's and now 740) have a chip that is more robust (SirfIII?) than in previous models. So, signal capture is probably faster in the newer models than, say, a TT One.

 

[mvl]

This is still a bit confusing to me. All I know is that I had my TomTom powered on here in Florida on the way to the airport. When I arrived at the airport, I shut it off. I boarded the plane, and a few hours later I landed in Texas, went to pick up my car rental, turned on my TomTom, and got a sat lock within 2 minutes. I traveled half-way across the Country. I'm not seeing where these 15 minute holds are coming from. I know that if I had to wait 15 minutes for a signal lock, I'd be pretty pissed off. Why have I never had to wait so long for a lock?

Same thing for when I went to Pennsylvania from Florida which is about 1,000 miles. I also had to do a cold boot at the Pennsylvania airport because I had switched SD cards to watch movies on the airplane. Why did I also get a quick lock after having had flown over 1,000 miles, AND a cold boot?

Based on what others are saying, I suspect the tomtom still knows where the satellites are from before your flight. So if it immediately knows how far away satellites are, and remembers the satellite position from before takeoff, then maybe that's why IDs its location quickly.

I'm assuming that satellite locations are written to the SD/flash memory since the "installing QuickGPS" step in Home appears to copy files into the tomtom. So maybe that's why it survived a cold boot.

So I would deduce that flights wouldn't be a problem, as long as you don't leave tomtom off too long.

 

[JeanLuc_]

It is simpler than that. In short, a GPS only sends a signal that contains a "time" value. There is no distance nor position sent, just "time". All GPS sattelite clocks are in sync. Since the signal takes time (a few ms) to go from the SAT to the receiver, the receiver derives a distance from the time, that is the difference in time between the "time" value sent, and the actual time it is received.

Now, GPS sats are turning around earth and are prone to variation in path, and de-sync in clocks. The ground stations send signals to the sats to re-sync and re-align. This is what is "compiled" in the ephemeris, which is sent in addition to the "time" value once in a while from the sats.

Having this ephemeris already loaded before receiving it from the sat permits the receiver to "know" where the sat is supposed to be when receiving its "time" value, hence, makes it quicker to know the position of the receiver.

Here is a good resource to understand the principles behind all this:

http://www.go.ednet.ns.ca/~larry/gps/gps_talk.html

Here are the official almanachs (ephemeris data)

http://www.navcen.uscg.gov/gps/almanacs.htm

and here is another more "in-depth" description:

http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html

Hope this helps!