There are 2 groups of applications for which you would want to use a GPS receiver: Time and position determination


Time determination

Before a GPS receiver can determine its position, it will need to know the exact time, since it needs to calculate the distance to the various orbiting GPS satellites to determine its own positions.  Because the speed of the timing pulses send by the satellite are having the speed of light (300.000km/sec) we are talking here about the need to know the time with an accuracy of about 10ns (0,000.000.01 sec) or 3 meter at the speed of light.  This is about 1 million times more accurate than a DCF77 or WWV radio clock.

So, as a 'byproduct' a GPS receiver can deliver an extremely accurate time.  With the Rockwell GPS module that we use in this project there are several signals we can use:

In the serial NMEA or Rockwell binary data the time is encoded with an accuracy of about 1 second (NMEA) or 10ms (Rockwell binary).  There is a 1 Hz TTL pulse available of which the rising edge is synchronous with the UTC second epoch with a maximum deviation of 1 us.  There is a 10kHz TTL signal which is also synchronous with UTC.

You can use these signals to lock a frequency source, or to run a computer system on a very accurate time.  The 1Hz pulse can be buffered to RS232 levels and send to the serial port handshake signals in order to create a very accurate time-lock whithin the operating system. There are several drivers that can use these signals - see the links page.

The 1Hz or 10kHz signals can be used to lock frequency generators.


Position determination

When the GPS receiver knows the exact time up until the last 1/billionst of a second, it can measure how long it took for the signals of the GPS satellites to travel to its position.  When it knows the distance to 3 satellites the receiver does some number crunching and can determine its own position on the earth surface.

The position, speed, direction etc. is offered by the GPS receiver in the form of a serial data stream, formatted according to the NMEA standard.  The is extensively documented in the data sheets of the module and on other web sites.

The NMEA data is an international standard.  There are hunderds of programs available that accept this serial data.  Some of the more well knows programs are: Route66, Microsoft Autoroute, Easy Travel Pro, TomTom etc etc.


GPS technology

There are also lots of programs to display and analyze the received data. These programs show the position of the satellites around the earth, the reception quality of the satellites etc.  You can use these programs to evaluate the quality of the reception, antenna posiotion, etc. Apart from this, the software is not very useful but a great starter and has high educational value.