Aquiring Satellites

[+currykev]

Hi all,I recently travelled to the Southern Hemisphere and it took almost 1 hour for my gps to lock onto my new position. I think I'll now have the same problem now I am back. Is this usual?

[OedipusX]

Hi all,I recently travelled to the Southern Hemisphere and it took almost 1 hour for my gps to lock onto my new position. I think I'll now have the same problem now I am back. Is this usual?

 

That sounds unusually long to me. I believe most GPS units allow you to enter your new location from the satellite search page so that it has an idea which satellites to begin looking for. If you did that, then I would imagine it would take a modern GPS unit less than 10 minutes to get a fix. If you just turned it on at your new location and let it do it's thing, then maybe a bit longer but certainly no longer than 30-40 minutes tops.

 

My personal experience when travelling between N and S hemisphere is that it took my GPS (when entering the new location) less than 5 minutes to get a fix.

[+Team Sieni]

Hi all,I recently travelled to the Southern Hemisphere and it took almost 1 hour for my gps to lock onto my new position. I think I'll now have the same problem now I am back. Is this usual?

Yes.

 

Your GPS should have a cold-start or "new location" function. For example, my GPSr (ancient Garmin GPSMap 76S) has two options for this "autolocate" and "enter location". In the former you just let it get on with it, in the latter you enter your approximate location on the map, then let it get on with it.

 

What it is doing is downloading and thinking about all the info about where the satellites are expected to be (technically called the almanac I think, but I'm no expert). Subsequently it will assume it is roughtly where it was when it was last used and make minor adjustments.

 

This can also happen if it has not been used for a while or has been stored without batteries for a while.

 

There should be a section in your user manual about it.

 

All the same, 1 hour seems a bit steep. But then I normally just leave it somewhere safe where it can see the sky and let it get on with it.

[+The Forester]

To do anything like a warm start your GPSr needs to 'know' its position to within about 500 miles. Here's why:

 

Each satellite sends a data string called a 'navigational message'. That string of data is composed of 1,500 bits of data in 5 subframes. The transmission rate is extremely slow, only 50 bits per second which is about the same data rate as an old-fashioned telex machine. Your GPSr needs to know which satellites to 'look' for and to do this it needs to know which satellites are above its local horizon. If it doesn't have that data, it needs to read every part of every subframe of every navigation message from every satellite. As it takes 30 seconds to send the full message and as it must start at the beginning of the message in order to make any sense of it, the machine must wait until bit number 1 to start reading the rest of the message. Therefore it can easily take almost 60 seconds to read the message from a single satellite. If it misses so much as a single bit of data, then it has to start all over again. It has to do that for about 30 satellites, so you can see that it can easily take 15 to 30 minutes to orient itself within the sequence of navigation message datastreams for all the satellites that it knows about.

 

Only after it has got all that data can it start to try to figure out which of the satellites to 'listen' to and start to compute a position.

 

You can greatly speed up this process by inputting a start position that you know to be within a couple of hundred miles of where you actually are. This enables the machinme to make an intelligent guess as to which part of which navigational message it should pay attention to.

 

If you switch the machine off in Newcastle, Tyneside and then back on again in Newcastle Australia, the machine is totally confused as it is looking for the satellites it expects to see in its old location. It's just a matter of luck when it will 'realise' that its estimated position is grossly in error and it starts to look at the bigger picture and start from scratch. That can be expected to take anywhere from 15 to 30 minutes and can take 30 to 60 minutes if it's getting a noisy signal which must be rejected if a checksum error occurs.

[+Kryten]

I took my Etrex to Sydney and couldn't get a lock in among the city centre buildings until I went to park near the harbour bridge. Even then it took 40 minutes to acquire all of the data that it needed. I was forced to stay in the park and look at the scenery, tragic

[+Boardslider]

My old Etrex would actually ask if I'd moved more than 500 miles if it was having trouble getting a fix. Don't know exactly what that helped it with, but it used to take about 5 mins, when switched on in a holiday destination.

 

My Forerunner had a fit when I drove it to Scotland and had me standing around waiting to start my run for about 10 mins

 

Never seems to be a problem for the GPS60....

 

The best is Tom Tom, you can actually download quick fix data via your mobile and even if you haven't turned it on for a week it still gets a rapid fix.

[+kbootb]

The Forester beat me to it as usual.

 

But just to re-inforce, if the signal area is poor, then an hour is not unusual. We just took delivery of some shiny new Garmins and they took about 40 mins to locate.

 

Not all GPS units allow you to input an approximation. Ours ask if you are indoors, have moved over 600 miles, and then checks the date and time. I assume that telling it you have moved outside the 600 mile radius shortcuts it trying to search out the expected satellites and kick starts the cold start procedure.

 

Having said that, took my gps to Cyprus and Crete and it locked in about as quickly as it does in Epping Forest about 10 miles from home.

[+currykev]

Thanks my Learned friends.I shall have lunch in the park today and see what happens.I will let my gps know who's the boss!

[+rogerncook]

My Garmin GPSMAP76 works fine for position changes when flying aroung Europe, but when I went to Arizona, I had to tell it roughly where I was using the map. I'm going to HK, China and Oz in a few weeks and expect I will need to do the same each time. Although I also have a Garmin GPSMAP76c which gets a fix always much faster than the plain 76 wherever I am.

[+Boneychest & Catsuey]

I took my Etrex to Sydney and couldn't get a lock in among the city centre buildings until I went to park near the harbour bridge. Even then it took 40 minutes to acquire all of the data that it needed. I was forced to stay in the park and look at the scenery, tragic

 

Did you join a team to complete the World Cache (Australia) while you were there? You could have been first to be two team members at the same time