View previous topic :: View next topic |
Author |
Message |
Throbber Occasional Visitor
Joined: Apr 16, 2005 Posts: 12
|
Posted: Wed Jul 27, 2005 1:50 pm Post subject: |
|
|
Thanks Blackdog, I'll have a look now..... |
|
Back to top |
|
|
Kavg Occasional Visitor
Joined: Jul 12, 2005 Posts: 15 Location: United Kingdom.
|
Posted: Wed Jul 27, 2005 1:52 pm Post subject: |
|
|
I thought you only needed 3 Sat's to triangulate your position, and any greater then 3 added to position accuracy.
8O
You learn something everyday. |
|
Back to top |
|
|
Eldar Pocket GPS Moderator
Joined: Sep 24, 2004 Posts: 1294 Location: London
|
Posted: Wed Jul 27, 2005 1:58 pm Post subject: |
|
|
Throbber wrote: | Thanks Eldar, that's very helpful.
If I've got this right, resetting the Go won't make any difference to the number of satellites it can see as it's always searching for new ones.
The fact that I can only see a max of 8 or 9 is down to reception rather than a reset. |
You are correct, and seeing 8 or 9 sats is normal.
On the other hand, TTG uses a processor dedicated to handling the GPS reception and sometimes it does get stuck.
Resetting the unit will clear the processor memory. I would recommend resetting the unit once a month anyway.
Re-starting (switching the unit on and off) will not clear the memory, but will force the processor to start searching for the satellites again. |
|
Back to top |
|
|
Throbber Occasional Visitor
Joined: Apr 16, 2005 Posts: 12
|
Posted: Wed Jul 27, 2005 2:04 pm Post subject: |
|
|
Thanks again Eldar, it's good to know I'm normal.
Think I'll do as you suggest and do a rest once a month. Should the unit be on or off when you push the reset button? |
|
Back to top |
|
|
Eldar Pocket GPS Moderator
Joined: Sep 24, 2004 Posts: 1294 Location: London
|
Posted: Wed Jul 27, 2005 3:10 pm Post subject: |
|
|
Kavg wrote: | I thought you only needed 3 Sat's to triangulate your position, and any greater then 3 added to position accuracy.
8O
You learn something everyday. |
The very exact clock/time you must have! But very cheap crystal in the receiver do they use, syncronise the clock you must. For that the fourth satellite will you need! ((c) Yoda) |
|
Back to top |
|
|
Eldar Pocket GPS Moderator
Joined: Sep 24, 2004 Posts: 1294 Location: London
|
Posted: Wed Jul 27, 2005 3:13 pm Post subject: |
|
|
Throbber wrote: | Thanks again Eldar, it's good to know I'm normal.
Think I'll do as you suggest and do a rest once a month. Should the unit be on or off when you push the reset button? |
Didn't say nuffing bout your good self, I was referring to the GO
For the sake of doing things properly - off and keep the reset button pressed for about 10 seconds (don't as me why - I've got no idea) |
|
Back to top |
|
|
Blackdog Occasional Visitor
Joined: Jul 26, 2005 Posts: 15 Location: South Glos
|
Posted: Wed Jul 27, 2005 5:06 pm Post subject: |
|
|
Kavg wrote: | I thought you only needed 3 Sat's to triangulate your position, and any greater then 3 added to position accuracy.
8O
You learn something everyday. |
If you have a lock on 3 Sats you get a Lat/Long fix. If you have more than 3 birds fixed you get better accuracy and also altitude. Not sure why you'd want that in the UK though...... _________________ TT700 - Memory Map 2004 - NavMan iPaq |
|
Back to top |
|
|
Eldar Pocket GPS Moderator
Joined: Sep 24, 2004 Posts: 1294 Location: London
|
Posted: Wed Jul 27, 2005 5:23 pm Post subject: |
|
|
Blackdog wrote: | Kavg wrote: | I thought you only needed 3 Sat's to triangulate your position, and any greater then 3 added to position accuracy.
8O
You learn something everyday. |
If you have a lock on 3 Sats you get a Lat/Long fix. If you have more than 3 birds fixed you get better accuracy and also altitude. Not sure why you'd want that in the UK though...... |
Firstly, we do have altitudes in the UK, thank you very much and we do want to know where they are.
Secondly, three points of reference is enough for 3D fix (so you'll get the altitude as well), you don't the fourth one.
The point in needing the fourth satellite is to get the receiver clock synced with atomic clock on board the sats. That's how you get the exact distance and can pinpoint your location with any accuracy. Three sats - x,y,z - three equations with three variables, four sats - x,y,z, time - four equations with four variables. That's exactly why your GPS receiver cost so little - cheap clock. |
|
Back to top |
|
|
Blackdog Occasional Visitor
Joined: Jul 26, 2005 Posts: 15 Location: South Glos
|
Posted: Wed Jul 27, 2005 5:38 pm Post subject: |
|
|
Eldar wrote: | Blackdog wrote: | Kavg wrote: | I thought you only needed 3 Sat's to triangulate your position, and any greater then 3 added to position accuracy.
8O
You learn something everyday. |
If you have a lock on 3 Sats you get a Lat/Long fix. If you have more than 3 birds fixed you get better accuracy and also altitude. Not sure why you'd want that in the UK though...... |
Firstly, we do have altitudes in the UK, thank you very much and we do want to know where they are.
Secondly, three points of reference is enough for 3D fix (so you'll get the altitude as well), you don't the fourth one.
The point in needing the fourth satellite is to get the receiver clock synced with atomic clock on board the sats. That's how you get the exact distance and can pinpoint your location with any accuracy. Three sats - x,y,z - three equations with three variables, four sats - x,y,z, time - four equations with four variables. That's exactly why your GPS receiver cost so little - cheap clock. |
Well excuse me professor!
If you think £600 is cheap then you live in a different world to me.
All three GPS devices I have work perfectly well with only 3 sat fixes to give a 2d fix on the ground. As I don't drive an aeroplane and I have a pair of eyes I don't really need altitude. Attitude yes, altitude no. _________________ TT700 - Memory Map 2004 - NavMan iPaq |
|
Back to top |
|
|
Eldar Pocket GPS Moderator
Joined: Sep 24, 2004 Posts: 1294 Location: London
|
Posted: Wed Jul 27, 2005 6:12 pm Post subject: |
|
|
Blackdog wrote: |
Well excuse me professor!
|
Just a plain "Doctor" will be sufficient :D.
Blackdog wrote: |
If you think £600 is cheap then you live in a different world to me.
|
I didn't mean the whole navigation device, I was talking about the GPS receiver itself which is about $20.
Blackdog wrote: |
All three GPS devices I have work perfectly well with only 3 sat fixes to give a 2d fix on the ground. As I don't drive an aeroplane and I have a pair of eyes I don't really need altitude. Attitude yes, altitude no. |
But you see, 2D fix is not very accurate and even for 2D fix you receiver "assumes" a value for the altitude if it doesn't know it.
If this assumption is too far off, then the accuracy is impaired. But you are quite right, a consumer GPS device will work perfectly well with 2D fix as long as you don't mind huge positioning errors. It's not for nothing they have the "lock to the road" feature permanently on. |
|
Back to top |
|
|
Blackdog Occasional Visitor
Joined: Jul 26, 2005 Posts: 15 Location: South Glos
|
Posted: Wed Jul 27, 2005 6:41 pm Post subject: |
|
|
Good Afternoon Doctor,
I agree with what you say about the accuracy of 2D fixes not being as good as 3D (in a previous existence my life depended on them), but for your average Joe, who is nowadays the typical GPS user in the UK he only wants to get to Auntie Mabel's new house or perhaps a holiday destination or business address. If he can't seewhere he wants to go from a couple of hundred metres should he really be driving?? Clearly, where the more accurate 3D fixes are required for such things as Off Road driving (very contentious in some areas) or the up and coming Geocaching pastime and indeed hiking and other uplands pastimes, then much more accurate trilateration information is required and for these, yes four or more birds are required to enable pseudoranging to be as accurate as possible. My initial post was only intended to be a basic answer to a basic question.
Cheers,
Blackdog _________________ TT700 - Memory Map 2004 - NavMan iPaq |
|
Back to top |
|
|
Eldar Pocket GPS Moderator
Joined: Sep 24, 2004 Posts: 1294 Location: London
|
Posted: Wed Jul 27, 2005 7:20 pm Post subject: |
|
|
Good evening Mr Blackdog (as if ScaredyCat wasn't more than enough for me ;-) )
I suppose you're right and I've got carried away, also I should have phrased my first response differently - the receiver will try to track four satellites to get a more accurate fix, but failing that three satellites is enough for basic positioning. |
|
Back to top |
|
|
Blackdog Occasional Visitor
Joined: Jul 26, 2005 Posts: 15 Location: South Glos
|
Posted: Wed Jul 27, 2005 8:00 pm Post subject: |
|
|
Please not Mr.......
I'm an "Oi You!", not an "excuse me Sir"
Cheers _________________ TT700 - Memory Map 2004 - NavMan iPaq |
|
Back to top |
|
|
rossb Regular Visitor
Joined: Nov 17, 2004 Posts: 79 Location: Sheffield, UK
|
Posted: Sun Jul 31, 2005 10:22 am Post subject: Fix times and weather |
|
|
Remember that cloud cover has an effect on satellite fixes. i dont use a tomtom, but I have found satellite fix times of 45 seconds on a sunny day(cold start), and 3 minutes on a very cloudy day(cold start). If you switch on the unit for a second time after using it for a drive, you can get a lock in about 20 seconds(warm start). |
|
Back to top |
|
|
Eldar Pocket GPS Moderator
Joined: Sep 24, 2004 Posts: 1294 Location: London
|
Posted: Sun Jul 31, 2005 11:40 am Post subject: |
|
|
Yep, that's Garmin for you - affected by clouds and probably light breeze :D |
|
Back to top |
|
|
|
Posted: Today Post subject: Pocket GPS Advertising |
|
|
We see you’re using an ad-blocker. We’re fine with that and won’t stop you visiting the site.
Have you considered making a donation towards website running costs?. Or you could disable your ad-blocker for this site. We think you’ll find our adverts are not overbearing!
|
|
Back to top |
|
|
|