hopf 4490G10, Technical Manual, Page: 7 / 34

H
OW
GPS
WORKS
GPS General System Information - V01.03 7 / 34
hopf
Elektronik GmbH
Nottebohmstr. 41 • D-58511 Lüdenscheid • Tel.: +49 (0)2351 9386-86 • Fax: +49 (0)2351 9386-93 • Internet: http://www.hopf.com • E-Mail: info@hopf.com
1 How GPS works
At a height of about 20,000km satellites circle around the earth twice a day on 6 different
orbits and angles (see picture below).
The GPS system
1
was developed on the basis of 18 satellites with 3 in reserve. In the
course of time this number was increased to 24 plus 3 in reserve to cover possible short-
term breaks in coverage. Therefore 6 to 11 satellite are visible from any point on earth at all
times. On board each satellite there is a highly accurate atomic clock (accuracy at least 1 x
10
-12
).
A basic frequency of 10.23 MHz is derived from the frequency of the atomic clocks. The two
carrier frequencies L1 and L2 are produced from this basic frequency.
•
transmission frequency L1 = 154 * basic frequency = 1575.42 MHz
•
transmission frequency L2 = 120 * basic frequency = 1227.60 MHz
Every satellite broadcasts by modulation all the important navigation and system data on
these two carrier frequencies. The data of the frequency L1 may be decoded for civil
purposes. The exact time can be calculated from these data and the position defined via the
antenna.
The GPS antenna receives signals from all the satellites visible above the horizon and
passes them on to the GPS receiver through a coaxial cable. Four satellites are required for
continuous time decoding.
The GPS world-time UTC
2
is calculated by subtracting the leap seconds from the GPS
world-time (GPS-UTC). At present (JAN 1999) world-time lags 13 seconds behind GPS-
UTC. The difference is not constant, but changes with every insertion of a leap second.
1
GPS = G lobal P ositioning S ystem
2
UTC = U niversal T ime C oordinated