GPS (GLOBAL POSITIONING SYSTEM)
WHAT IS GPS?
GPS stands for Global Positioning System, and is common nowadays. Following is a brief introduction.
Although the American Defense Department originally developed GPS for military operations, the system is available for use by the
general public. Anybody can use GPS in association with modern vehicle navigation systems.
Position precision for public use is approximately 15 meters. Precision can be improved when a vehicle is used for navigation and
aboard ships through use of map matching technology and differential GPS techniques.
A total of 24 or more GPS satellites are at a high-level altitude of approximately 20,000 km on six circular tracks with an orbital radius
of approximately 26,000 km. Therefore, there are four or more satellites located in each orbital track. For civilian use, the RF carrier
frequency of the satellites is 1575.42 MHz. Spread spectrum (SS) technology is used over a bandwidth of 2.046 MHz to prevent
interference among all the satellite signals on a common carrier frequency.
POSITION DETERMINATION PRINCIPLE
The GPS receiver receives radio signals transmitted from GPS satellites. The receiver measures the time duration between when
the signal leaves the satellite and when the signal arrives at the GPS receiver. Knowing this time allows calculation of the distance
that the signal traveled from that particular satellite. By receiving multiple signals from different satellites and performing this distance
calculation multiple times, the intersection of the spherical surfaces that represent the respective radius distances to the various
satellites will determine a single point.
The precision of the determination of the intersecting point relies on the satellites and the data collected.
DATUM (GEODETIC SURVEY SYSTEM)
The latitude and longitude information required by the GPS receiver does not allow for accurate representation of the unevenness of
the Earth’s surface. For use with GPS, the current standard that defi nes the precise shape of the Earth is called WGS-84. By using this
standard in conjunction with mapping standards for each country, devices can create accurate position plotting on a map.
GPS AND APRS POSITION FORMAT
Although position can be described in many different formats, such as degrees, degrees and minutes, and degrees, minutes and
seconds, the GPS system and APRS have standardized on the degrees and decimal minutes format. Just like it is important to use
proper international phonetics when spelling on the air, it is important in APRS as in any communications system to have a default
standard for position. All operators should be trained to use the DDD MM.mm format by default in voice communications just like in
data formats. Using these standards on the air will prevent communication errors, confusion, repeats, and conversions.
Interval and Total Distance
Geographic Coordinates Format Example
Latitude DD MM.mm
(degrees and minutes)
39 49.31N
+39 49.31
Longitude DDD MM.mm
(degrees and minutes)
84 15.39W
-84 15.39