Geoscience Reference
In-Depth Information
Downlink data
Coded ranging signals
Position information
Almanac
Space
segment
Uplink data
Satellite ephemeris
Clock corrections
Almanac
Control
segment
User
segment
Master control station
fIGURe 9.3
Primary segments of the Global Positioning System (GPS). (From Rizos, C., in
Manual of
Geospatial Science and Technology
, pp. 75-94, Taylor & Francis, London, 2002. With permission.)
tAble 9.1
basic Components of the GpS Satellite Signal
frequency
(Mhz)
Ratio of fundamental
frequency (f
o
)
Wavelength
(cm)
Component
Fundamental frequency f
o
10.23
1
2,932.6
L1 carrier
1,575.42
154
19.04
L2 carrier
1,227.60
120
24.45
L5 carrier
a
1176.45
115
25.50
P-code
10.23
1
2,932.6
C/A code
1.023
1/10
29,326
W-code
0.5115
1/20
58,651
Navigation message
50 × 10
-6
1/204,600
N/A
a
New frequency planned under GPS Modernization Plan; not available as of June
2005 (see Section 9.11).
than the C/A-code-based measurement. Ephemeris data, which are continuously transmitted by
each satellite, provide information regarding the satellite position in space, the status of the satellite
(healthy or unhealthy), and current date and time. In addition to the broadcast ephemeris, each satel-
lite transmits the almanac data showing the orbital information for that satellite and for every other
satellite in the system. The almanac is used by the GPS receiver to search the sky for the satellites
to acquire the signals. After the codes and data removal by the receiver's processing electronics,
the signal's carrier phase can be measured; it provides GPS measurement of the highest accuracy.
In addition, a Doppler shift can be observed, providing a measure of the range rate to the satellite
between the consecutive epochs of observation. In general, dual-frequency receivers can provide
carrier phase, pseudorange, and Doppler on L1 and L2. Single-frequency receivers measure pseudo-
range, carrier phase, and Doppler on L1 only.
