Geoscience Reference
In-Depth Information
The latest generation of GPS receivers is based on dedicated-channel architecture, where every
channel tracks one satellite either on the L1 or L2 frequency. Consequently, even though multichan-
nel receivers are more accurate and less sensitive to loss of signal lock, they can suffer from the
interchannel bias. Fortunately, in the modern receiver this bias can be effectively calibrated by the
receiver's microprocessor. Any remaining bias can be removed by the differencing techniques (see
Section 9.7.1.2).
All transmitted signals are clocked in coincidence with the PRN transitions for the P-signal and
occur at the P-signal transition speed. On the L1 channel the data transitions of the two modulat-
ing signals (i.e., that containing the P(Y)-code and that containing the C/A-code) is such that the
average time difference between the transitions does not exceed 10 nanoseconds (two sigma) (ICD-
GPS-200, 1993). Because the GPS
Control Segment
uses GPS P-code measurements to compute
the broadcast GPS orbits and clock corrections, there is a possible lack of synchronization between
the epochs of the orbits and the C/A code. These instrumental biases (differential code biases,
DCBs) are estimated for each day for all GPS satellites as part of the clock estimation procedure
with an accuracy of about 0.1 nanoseconds (see, for example, www.aiub.unibe.ch/download/papers/
codar_00.pdf; ftp://gage.upc.es/pub/gps_data/GPS_IONO). The P1-C1 DCB is generally on the
order of a nanosecond, and P1-P2 can reach up to a few nanoseconds. Similar biases (~1 to 15 nano-
seconds in range) exist in GPS receivers; these can be calibrated by the users. If not accounted for in
the point positioning procedure, the DCBs may result in a pseudorange error of up to 5 m.
9.5.5 a
n t e n n a
P
h a s e
c
e n t e R
l
o c a t i o in
The physical (geometric) center of the antenna usually does not coincide with the phase center
(the electrical center) of the antenna—a point to which radio signal measurements are referred.
Moreover, the phase centers for L1 and L2 signals generally do not coincide, and the location of the
electrical phase center can vary with variable azimuth and elevation of the satellites and the inten-
sity of the incoming signal. This effect should not, in general, exceed 1 to 2 cm, and for modern
antennas, it reaches only a few millimeters. Using the correct phase center offsets becomes very
important when different antenna types are used in a survey. Detailed antenna calibration tables that
account for the average spatial relationship between the Antenna Reference Point (ARP), such as
Bottom of Antenna Mount (BA) and electrical phase centers, are provided by the National Geodetic
Survey (NGS) (www.ngs.noaa.gov/ANTCAL/) and are normally included in any commercial GPS
software.
9.5.6 R
e f e R e n c e
s
t a t i o n
e
R R o R
This error affects relative positioning in case the reference (base) receiver location has incorrect
coordinates in the WGS84 (ITRF2005) reference system. The ITRF (International Terrestrial Ref-
erence Frame) is an alternative to WGS84 realization of the global terrestrial reference system that is
produced by the International Earth Rotation Service (IERS) based in Paris, France (http://itrf.ensg.
ign.fr/). It includes many more stations than the original WGS84. Note that the current implementa-
tion of ITRF, labeled ITRF2000, and WGS84 are generally considered to be equivalent. For more
information on ITRF, the reader is referred to http://itrf.ensg.ign.fr/ITRF_solutions/index.php.
9.5.7 n
u M b e R
of f
v
i is i b l e
s
a t e l l i t e s
a n d
t
h e i R
g
e o M e t R y
GPS is a line-of-sight system, where the path of the signal between the satellite and the ground
receiver must be clear and unobstructed. Thus, GPS positioning accuracy and reliability strongly
depend on the surrounding environment. A minimum of four satellites in view is required to obtain
the position fix; however, redundant observations will significantly improve the accuracy. The satel-
lite geometry is also very important, as weak geometry even with a sufficient number of satellites
in view may provide a low-accuracy solution. Satellite geometry refers to the relative position of
