Geoscience Reference
In-Depth Information
shift scaled to range rate is given by
D
=
λ
˙
Φ= ˙
+
c δ,
(5-5)
where the derivatives with respect to time are indicated by a dot. The raw
Doppler shift is less accurate than integrated Doppler.
The Doppler shift is measured in the carrier tracking loop of a GPS re-
ceiver (Misra and Enge 2001: Sect. 9.6). Assuming a known satellite velocity,
the Doppler shift can be used to estimate the velocity of the user.
5.3.4
System capabilities and accuracies
Two operational capabilities are distinguished: firstly, the initial operational
capability (IOC) and, secondly, the full operational capability (FOC).
IOC was attained in July 1993, when 24 (Block I/II/IIA) GPS satellites
were operating and were available for navigation. Ocially, IOC was declared
by the DOD on December 8, 1993.
FOC was achieved when 24 Block II/IIA satellites were operational in
their assigned orbits and the constellation was tested for operational military
performance. Even though 24 Block II and Block IIA satellites were available
since March 1994, FOC was not declared before July 17, 1995 which indicates
an extensive testing phase.
The selection of the GPS observation technique depends upon the par-
ticular requirements of the project; especially the desired accuracy plays a
dominant role.
Point positioning
When using a single receiver, usually point positioning with code pseudo-
ranges is performed. The concept of point positioning is simple (Fig. 5.2).
Without clock errors, trilateration in space (i.e., using three ranges) solves
the task to determine the point coordinates. Using pseudoranges, four ob-
servations are necessary to account for the three coordinate components and
the receiver clock error. For point positioning, GPS provides two levels of
service: the standard positioning service (SPS) with access for civilian users
and the precise positioning service (PPS) with access for authorized users.
SPS performance standards are based on signal-in-space performance.
Contributions of ionosphere, troposphere, receiver, multipath, topography, or
interference are not included. Furthermore, SPS is provided on the L1 signal
only; the L2 signal is not part of the SPS (Department of Defense 2001).
The global average positioning domain accuracy amounts to 13 m horizontal
error (95% probability level) and 22 m vertical error (95% probability level).
