Bernese GPS Software Version 5.0 - Bernese GNSS Software

2. Fundamentals
Clock correction [ns]
300
200
100
0
-100
-200
-300
0 3 6 9 12 15 18 21 24
Time [hours]
Figure 2.4: SA switched off on May 2, 2000 – effect on GPS satellite clocks.
Using the broadcast ephemerides the Earth-fixed geocentric coordinates of the satellites
may be computed according to the formulae given in [Dierendonck et al., 1978]. The fourth
and the fifth subframe contain data for military use, information on the ionosphere, and
so-called almanac data (low-accuracy orbits of all GPS satellites).
The GPS user may decide whether to use the broadcast ephemerides or the precise
ephemerides (produced by the IGS) for processing. The broadcast ephemerides are available
in real-time, but they have an accuracy of “only” several meters. The precise ephemerides
have an accuracy of few centimeters and they are available with a delay of about two weeks
for final products, of below one day for so-called rapid products, and of three hours for socalled
ultra-rapid products (see Section 2.2.1). The ultra-rapid products are predicted and
may, therefore, be used for real-time or near real-time applications. With an accuracy below
one decimeter in the predicted part they are considerably better than broadcast orbits.
The satellite clock corrections are required for processing. The accuracy of this information
in the broadcast message was artificially degraded (Selective Availability, SA) for nonprivileged
users until May 2, 2000, when the degradation was disabled by the U.S. Figure 2.4
illustrates the effect of disabling SA on the GPS satellite clocks. The effect of SA was fully
eliminated in geodetic applications when only relative positions of receivers were estimated.
The IGS precise orbits contain highly accurate satellite clock corrections, too.
2.1.1.3 Signal Processing
The receivers contain elements for signal reception and signal processing (antenna, preamplifier,
radio frequency (RF) section, microprocessor, storage device, control device, and
power supply). After signal input from the antenna, the signals are discriminated, i.e.,
separated into satellite-specific signals. Usually this is achieved through the C/A-codes
which are unique for each satellite. The basic elements of the RF section are oscillators to
generate a reference frequency, filters to eliminate undesired frequencies, and mixers. The
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