treeny
Posts: 270
Joined: Thu Dec 08, 2005 12:30 am

New To GPS Stupid Quesiton

Wed Aug 15, 2007 11:50 pm

All

I am looking into buying one of the Garmin GPS units for my car or that I can carry around with me when I travel.

How does the system of knowing where you are actually work? I mean does it automatically know where it is when it is switched on or do you have to load maps as to where you are / going to be? How is this done? Data Transfer, live update etc etc?

Any help and info would be appreciated.

Note:- I am not asking for specific model / make advice just an idea as to how you can make the unit know where it is. Scenario is for example could I use a unit bought in the UK, birng it to Argentina and use it?

Cheers

Mark
 
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moo
Posts: 4033
Joined: Sun May 13, 2007 2:27 am

RE: New To GPS Stupid Quesiton

Thu Aug 16, 2007 12:01 am

It gets its current position from the GPS satellites, and from that information it knows where on the world (or above it) it is via longitude and latitude. It can then use that information to see what maps it should load and checks its store for maps that match the long/lat combination.

The number of maps you can fit on a datacard depend on their level of detail, and as such you may need to load a mapset for a particular geographical area (North America, Europe et al) manually onto the datacard before you can use it.
 
SlamClick
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RE: New To GPS Stupid Quesiton

Thu Aug 16, 2007 12:12 am

There is a galaxy of about 26 GPS satellites that send and receive signals constantly. Your unit talks to them. At any one moment there should be four to six satellites "above the horizon" in a position to solve the three-dimensional location of your unit. As to exactly how this works, I'm sure someone on here will be acquainted enough with that side to take you as far down the rabbit hole as you'd like to go.

I have a Garmin eTrex, Legend C hand-held unit. I've used it to follow along with the factory unit in my wife's car and I used it onboard a ship to Europe. It is amazingly accurate. I used it to locate a USGS Benchmark on a nearby mountain. I use it hiking and trust it, when reception is good, to within a few meters. My partners and I also have a Leica unit with the antenna mounted on a two-meter pole. It also uses ground-based stations and is accurate to a few centimeters. It is too heavy and too expensive for casual use but it sure does illustrate how good this stuff is. We found out that 2nd base on a nearby ball diamond was about two inches off-spot.  Smile

A unit designed for a car might not display your elevation but it will probably have an updatable database with highway, street and services for road trips. For example the updates will reflect new road construction but they will probably be more oriented toward hotels, restaurants, gas stations and attractions like Disneyville.

You may find an occasional annoyance. In a relative's car in Florida it wanted us to turn too soon, put us in a parking lot instead of down the street we actually wanted. We drove back out and navigated ourselves down the street in dispute and it nagged us for a couple of minutes: "At the next opportunity, make a legal u-turn" It was pretty funny actually.

Trees degrade the signal to a surpising degree. So do, of course, tunnels and overcrossings, but the system is very good.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
airfoilsguy
Posts: 3485
Joined: Sun Oct 09, 2005 7:28 am

RE: New To GPS Stupid Quesiton

Thu Aug 16, 2007 12:30 am

Here is the short version. The unit knows Exactly where all the satellites are at any given time. The satellites send out signals constantly, basically saying " I am sat 001 and it is currently 0500 o'clock". The units pick up these signals and by knowing where that satellites are supposed to be, it takes the signals it receives and figures out how long it took to get them and then using this information triangulates where the unknown variable is (the unit).
It's not a near miss it's a near hit!!
 
SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

RE: New To GPS Stupid Quesiton

Thu Aug 16, 2007 12:40 am

It still amazes me that signals from a moving station thousands of miles away (GPS) are more accurate than signals from a ground-based station (VOR) only forty miles away, but there you are...
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
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moo
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Joined: Sun May 13, 2007 2:27 am

RE: New To GPS Stupid Quesiton

Thu Aug 16, 2007 12:43 am

Quoting SlamClick (Reply 2):
As to exactly how this works, I'm sure someone on here will be acquainted enough with that side to take you as far down the rabbit hole as you'd like to go.

Heres a quick rundown of what I learnt last time I looked into this:

Receivers get a signal from a satellite which contains a unique Satellite ID, time signal sent and some other technical data called a psuedo random code. The receiving unit has a set of data in it which tells it where all the GPS satellites are at any given time on any day ever. Minor positioning error corrections are sent with the GPS signal.

Receiver measures time difference between sending and receiving (travel time), calculates the distance from the satellite to itself, while keeping the receiver and sender clocks in sync is a function of the psuedo random code signal. The psuedo random code does a lot more, like allow for amplification of the signal with very small antenna, and ensure that each satellites signal is unique and doesnt block each other. The satellites have atomic clocks on board, and the receivers can reset their own clocks based on each satellites time reading, ensureing that the receiver is in sync.

This gives it a possible number of locations on the earth it can be based on the distance from that satellite, which works out to a sphere the radius of that distance. Do this with more satellites to cut down on the possible locations by intersecting the spheres calculated for each satellite, you are whereever those spheres intersect. The more satellites you can receive a signal from, the smaller that area becomes, and also the longer you spend receiving signals from the same satellite, the smaller the area becomes, which is why if you leave a GPS receiver for a period of time, it keeps increasing in accuracy over that time.

In general, it takes 3 satellites to give you two possible locations for your receiver. Why two? Because thats where the third satellites sphere intersects the other two satellites spheres based on how the measurements are done. You can wait for a fourth satellite to come into view to be able to determine 100% which of those two points you are actually at, but in reality one of those two points nearly always is 'impossible', ie its not on Earth (IE its in orbit or further out), or that points velocity based on updates from the satellite is far too high for it to be you. A fourth satellite is also needed when your GPS receivers clock is out of sync with the satellites clocks - 3 satellites give you a rough area and the fourth satellites measurement gives you enough information to reset the receivers clocks and recalculate its position with the new accurate time reading.

So in points:

1. You need distance measurements from the satellite to you
2. You need 4 satellites to guarantee a 100% accurate location
3. In general only 3 satellites are required because most of the possibilities are ridiculous or impossible
4. A 4th satellite can give you speed and altitude measurements, allowing for aircraft navigation, as well as correcting timing issues

Hope that helps!

[Edited 2007-08-15 17:44:39]
 
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nighthawk
Posts: 4766
Joined: Sun Sep 16, 2001 2:33 am

RE: New To GPS Stupid Quesiton

Thu Aug 16, 2007 1:54 am

A standard GPS device will be able to receive a signal, and work out its position anywhere in the world. Note that in some areas coverage may not be as good as others due to the number of satellites visible over the horison.

However in order to provide you with driving instructions, the device must have a map that covers that area of the globe. These are typically sold on memory cards, so while any device will work in argentina, you will need to check that it is possible to buy a map of argentina for your device. Maps tend to be designed for a specific model, so for example a map of argentina for garmin, will not work on a tomtom etc. There may also be further limitations between different models fro the same manufacturer.

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