Using Garmin GPS for APRS
About GPSThe Global Positioning System (GPS) is a satellite-based radio location system operated by the US Depertment of Defence. 24 widely spaced satellites orbit the earth twice a day, continuously transmitting a timing signal, which allows any user with a low cost receiver to determine their Latitude and, Longitude to less than 10 metre accuracy.
How Does it Work?Each GPS satellite carries an onboard atomic clock (Caesuim with Rubidium backup) which is used to generate a 10.23 MHz timing signal. This is transmitted using a carrier frequency of 1575.42 MHz (154 * 10.23) using 'spread spectrum' modulation.
The receiver can calculate (by correlation) the arrival times of the signal from several satellites (up to 12 in the GPSIII+) simultaneously. By knowing the time and the positions of the satellites (from an on-board almanac) and their relative distances, the receiver can calculate its position - this is performed inside the GPS receiver approximately once every second and presented to the user on a Liquid Crystal Display (LCD).
The Garmin GPSIII+ shows this position as a pointer on an internally stored map, this is also output as position, speed and bearing in NMEA 0183 format to a computer port - the transmission of this information by packet radio is the basis for APRS
Custom CablesIf you have a Garmin GPS III+, you can make a cable for portable operation with a Garmin connector at one end and a 2.5mm stereo jack at the other - the main problem being the availability of the Garmin four-pole connector. This is not possible for the Garmin GPS 12, being an 8 volt unit, it has an additional dummy pin to prevent 12 Volts being applied.
These are either available as a Garmin branded 'Power / Data Cable', complete with a flying lead from Maplin Electronics (Order Number 'KQ06G'), to which a 2.5mm stereo jack can be soldered. Alternatively, a 'cloned' Garmin connector can be obtained over the internet from www.pfranc.com and soldered to the supplied Kenwood cable.
When communication is successful, the TH-D7 will bleep once, pressing the '
Incoming positions from the Kenwood TH-D7 are marked as a 'WAYPOINT'. Although the APRS symbols may appear similar to those of the GPS III+, they are not transferred. (The numbers following the '
Example GPS III+ DisplaysOn first screen, no waypoints are displayed. Upon receiving the APRS packet from M1BLR, the position is plotted as a 'WAYPOINT' in the top left hand corner of the screen.
A pseudo-random error was introduced into the timing signals transmitted from each satellite, military users (and those authorised by the US Department of Defence) were able to remove the these effects by using an additional unit, attached to the GPS receiver that removed the error.
These effects were observable by zooming in the map display to the smallest scale. The position of a stationary GPS would then 'wander' around the screen. An example of this is shown in the screenshot below (taken over a 30 minute period), where the changes in position can be seen.
What is Differential GPSDifferential GPS was devised as a method for civilian users to increase their GPS accuracy, by removing the effects of Selective Availability. It works by measuring the errors from each satellite and calculating corrections, which are transmitted from a fixed ground station in a format known as 'RTCM 104'.
The corrections are fed into the GPS and applied to the position calculations - using this method, an accuracy of up to 1m 2D RMS can be acheived. Differential GPS is still useful to those requiring such accuracy, as it removes naturally induced atmospheric effects.
The US Coast Guard operates a network of over 50 transmitters. Similarly in the UK, Trinity House operates a network of lighthouse based beacons:
A commercial (subscription based) implementation of Differential GPS in the UK uses the RDS data channel of the broadcast station 'Classic FM', this is used for an Automatic Vehicle Location application - probably similar to APRS!
A Differential GPS 'Reference Station Interface Board' kit is available from TAPR. However, the Motorola GPS module used in this kit has been discontinued.