Precise Orbit Determination of Global Navigation Satellite System of ...

Chapter 3 Orbit Tracking System and Their Error Budgets
(1) the space segment, a small self-contained hardware unit of dimensions 400x250x180 mm with a mass of 20
kg, a power consumption of 32 W and host satellite independent communication links,
(2) the ground segment, a network of 29 small autonomously transportable operating ground tracking stations,
which can be installed at any site that is connected to a power line, for generation of tracking data,
(3) the control segment consisting of a command station (Stuttgart), which is responsible for space segment
control and data dumping; a master station (Oberpfaffenhofen) which is responsible for station network
management, user support, and data preprocessing, quality control and distribution; and a calibration station
(Potsdam), where a third generation laser system is operated to produce simultaneous laser ranging observations
for periodic system calibration.
Two PRARE signals (X and S-Bands) are emitted from space segment to earth. Both signals are modulated with
PN-codes (10 MChips/s for X band and 1 Mchips/s for S-Band) and spread-spectrum binary data. They are all
generated inside the space segment derived from its central ultra-stable oscillator and permanently disseminated
by two dipole antennas.
As PRARE has four independent receiver channels, up to four preselected stations can be handled
simultaneously. The transportable and automatically operating ground stations receive the X-band downlink
signal and demodulate the PN-code. Then the PN-sequence is remodulated on the X-band that is retransmitted to
the space segment (regenerative transponders) containing ground station measurement data used for the
preprocessing of the data. The PRARE space segment measures both two-way range and the received two-way
Doppler-shifted signals very precisely by comparing the phase of the received signal to the phase of the on-board
clock. The overall accuracy stems mainly from this two-way configuration of the system that eliminates the most
clock errors of one-way systems.
Ranging data is acquired by determination of the signal delay between outgoing and incoming signal (PN-code
correlation method, 91 averaged measurements per second), range-rate by counting of the microwave cycles and
the phase shift of the return signal (1 measurement per second). The high data precision is based on the full
coherent two-way principle of the system, the high signal frequencies, and the appropriate resolution of the space
segment receiver counters.
S(t 2 )
S(t 3 )
S 2
S 1
23
r(t 4 )
r(t 1 )
Figure 3-3 PRARE Two-Way Range Measurement
S(t 4 )
S(t 1 )
The basic observation can be written as follows
(1)Two-way range observation:
Assuming the signal is transmitted from the satellite. From Figure 3-3, two-way range observation is
L cF(
t − t ) = S + S + ε
(3-2)
= 4 1 1 2
where
c speed of light in vacuum.
F conversion factor from seconds of PRARE time to UTC units