Geoscience Reference
In-Depth Information
5.3.3
Satellite signal and observables
Components of the signal
The ocial description of the GPS signal is given in the GPS Interface
Control Document ICD-GPS-200, available at www.navcen.uscg.gov. Details
may also be found in Spilker (1996).
The (current) components of the signal are summarized in Table 5.1.
Note that the nominal fundamental frequency
f
0
is intentionally reduced by
about 0.005 Hz to compensate for relativistic effects.
The navigation message essentially contains information about the satel-
lite health status, the satellite clock, the orbit, and various correction data.
The parameters in the block of orbit information are the reference epoch,
six parameters to describe a Kepler ellipse at the reference epoch, three
secular correction terms and six periodic correction terms.
Observables
In concept, the GPS observables are ranges which are deduced from mea-
sured time or phase differences based on a comparison between received
signals and receiver-generated signals. As mentioned earlier, the ranges are
biased by satellite and receiver clock errors and, consequently, they are de-
noted as pseudoranges. Essentially, pseudoranges differ from distances by an
unknown additive constant.
Apart from the satellite and the receiver clock bias, further error sources
can be classified into three groups, i.e., satellite-related errors (e.g., orbital
errors), signal propagation medium-related errors (e.g., ionospheric and tro-
pospheric refraction), and receiver-related errors (e.g., antenna phase center
variation, multipath), but are omitted in the subsequent simplified models.
Extended models are given in Hofmann-Wellenhof et al. (2003: Sect. 10.2.2).
Table 5.1. Components of the satellite signal
Component
Frequency or code
Wavelength
chipping rate [MHz]
Fundamental frequency
f
0
=10
.
23
Carrier L1
154
f
0
= 1575
.
42
19
.
0cm
Carrier L2
120
f
0
= 1227
.
60
24
.
4cm
P-code
f
0
=10
.
23
C/A-code
f
0
/
10
= 1
.
023
10
−
6
Navigation message
f
0
/
204 600 = 50
·
