GPS question

[KARL]

Someone has suggested that GPS navigators emit some form of transmission signal - presumably to satellites (what else ?). Others say this is not so. What is the truth in this ?

[sifellis]

mainly, if not only, they will transmit a bluetooth signal.

I have a BT surf star gps reciever......reciever being the operative word, that links with my phone for my sat nav. I can't imagine that the little matchbox size gps unit has neither the power nor the need to transmit to a satellite.

[NORMANTHEDOG]

surely that is exactly what it does,otherwise how would your sat nav know where you are, to tell you the route,it has to know your position,to tell your system the way.otherwise it cant work.if i go out to my car now and put in i want to go to london,the system will give me a route,it does this by finding out where i am,and the only way that is done is by the satellites picking up a signal from my sat nav antenna.

[jake-the-peg]

Sat nav systems work by measuring their distance to satellites based on time signals transmitted by the satellite not by handset.

http://newsimg.bbc.co.uk/media/images/41159000 /gif/_41159560_galileo_gps416.gif

Your position can however be determined by your mobile phone as the phone companies can determine which cell your phone is registerred with

[heathfield]

There is no transmission from a GPS receiver to satellites.

Apart from the display, the only output from a receiver will be in the form of positional and quality control data passed to a computer or data storage device, either by cable or, as Sifellis says, some wireless system such as bluetooth or a radio link.
(My job was in marine charting and surveying, and have worked with GPS positioning systems since they were first invented).

[heathfield]

Actually, as of yesterday, there were 30 functioning GPS satellites. Occasionally a satellite will be made unavailable due to testing, or re-positioning, by Ground Control. Two will be made unavailable in the next few days. The requirement for the GPS system is to have a minimum of 24 functioning satellites at any one time.

The better quality receivers have multiple simultaneous channels, (12 channels is common). Cheaper receivers sometimes claim to be multichannel, but operate using fewer channels, perhaps only four. These work by having each channel receive signals from different satellites in a sequence, one satellite after another. This makes the computed position less accurate due to the time lag between received sets of data.

[mibn2cweus]

GPS satellites compute (triangulate) their current position from data they receive from fixed ground reference points. The satellites then broadcast their position to GPS receivers that then compute their own position based on this data.

Once a GPS receiver has computed its own position some have an additional capability to pass that information on to ground based receivers that can relay that infromation to map data base providers or other interested parties.