Geoscience Reference
In-Depth Information
real-time broadcasting to the users. A more accurate alternative to the broadcast orbits is provided
by the International GPS Service (IGS). Predicted, rapid, and postprocessed (final) GPS orbits are
available with accuracy ranging from ~200 cm (predicted) to ~10 cm (rapid), to ~5 cm (final orbits)
(http://igscb.jpl.nasa.gov/components/prods.html).
9.5.4 R
e c e i v e R
n
o i s e
, i
in t e R c h a in in e l
b
i a s
,
a n d
o
t h e R
i
n s t R u M e n t a l
b
i a s e s
The most basic kind of noise is thermal noise
*
produced by the movement of the electrons in any
material that has temperature above 0 Kelvin (Langley, 1998a, 1998c). The commonly used mea-
sure of the received signal strength is the signal-to-noise ratio; in the case of radiofrequency (RF),
the most commonly used measure of the signal's strength is the carrier-to-noise-power-density ratio,
C/N
o
, defined as a ratio of the power level of the signal carrier to the noise power in a 1 Hz band-
width (Langley, 1998a, 1998c; Van Dierendonck, 1995). C/N
o
is considered a primary parameter in
describing the GPS receiver performance, as its value determines the precision of the pseudorange
and carrier phase observations. Typical values of C/N
o
for modern high-performance GPS receiv-
ers (L1 C/A code) range between 45 and 50 dB-Hz. For example, for a C/N
o
equal to 45 dB-Hz and
a signal bandwidth of 0.8 Hz, the root mean square (RMS) tracking error due to receiver thermal
noise for the C/A code is 1.04 m, and for high-performance GPS receivers with narrow correlators
(Van Dierendonck et al., 1992) with spacing of 0.1 chips, and the same bandwidth and C/N
o
, the
RMS is only 0.39 cm. The RMS tracking error due to noise for a carrier-tracking loop with a C/N
o
of 45 dB-Hz and a signal bandwidth of 2 Hz is only about 0.2 mm for L1 frequency (Braasch, 1994;
Langley, 1998c). In a GPS receiver, the noise translates into an error in range measurement that
ranges between 0.1 and 3 m (see Table 9.2). In summary, a good quality receiver should contribute
less than 0.5 ms error in interchannel bias and less than 0.2 m in noise.
tAble 9.2
GpS error Sources and their Approximated Magnitudes under
SA on/off
p-code
C/A-code
error Source
SA off
SA on
SA off
SA on
Satellite
Orbit prediction
Clock stability
3-5 m
1-3 m
10-40 m
10-50 m
3-5 m
1-3 m
10-40 m
10-50 m
Signal propagation
Ionosphere (two frequencies)
Ionosphere (model)
Troposphere (model)
Multipath effects
Relativistic propagation
cm-dm
2-150 m
dm
1-5 m
~2 cm
cm-dm
2-150 m
dm
1-5 m
~2 cm
cm-dm
2-150 m
dm
5-10 m
~2 cm
cm-dm
2-150 m
dm
5-10 m
~2 cm
Receiver
Receiver noise
Interchannel bias
Hardware delays
Antenna phase center
0.1-0.3 m
0.5 m
dm-m
mm-cm
0.1-0.3 m
0.5 m
dm-m
mm-cm
0.1-3 m
0.5 m
m
mm-cm
0.1-3 m
0.5 m
m
mm-cm
* An electrical current generated by the electronsí random motion.
