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Figure
9.19
General
arrangement
of
TT&C
ground
facilities
for
a
space
segment
consisting
of
two
geostationary
satellites.
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Earth Stations and Network Technology
Figure 9.20
A typical TT&C Earth station with a full-motion antenna for tracking and testing as well
as several limited motion antennas used for full-time TT&C links and communications
services. (
Source:
Sky Perfect JSAT, Ltd.)
The baseband equipment is shown at the upper center of Figure 9.19. In the
command subsystem, the actual digital commands are taken from the incoming
data stream and modulated onto the IF carrier using either FSK or PSK, depending
on the design of the spacecraft command receiver. Other special features can be
incorporated in the command subsystem, such as providing the RF pilot carrier
modulation used by a spacecraft antenna tracking system. For commanding pur-
poses, the common carrier need be on the air only when it is sending commands
and for ranging. That allows the uplink chain to be used on another satellite or
for other purposes when commanding is not being performed.
The telemetry subsystem receives the telemetry carrier at IF from the downlink
and demodulates the actual stream of telemetry data. The process is straightforward
and directly analogous to the reception of data communication in a standard Earth
station. The third function of the baseband equipment is for ranging, which is the
process outlined in Chapter 8 for measuring the position and velocity of the satellite.
The ranging subsystem generates a baseband ranging signal, which the command
subsystem modulates on the uplink carrier. After passing through the TT&C subsys-
tem of the satellite, the ranging signal is received at the TT&C station, demodulated,
and the baseband compared with what was transmitted. The measure time delay
and frequency provide the basis for computing the range and the range rate. In an
alternative ranging approach, the ranging baseband signals are modulated on an
independent carrier and uplinked to a frequency within one of the operating tran-
sponders of the communication payload. Referred to as transponder ranging, the
technique eliminates the need to switch the command link between the commanding
9.7
Major Classes of Earth Stations
317
and ranging functions but consumes some of the communications bandwidth of
the satellite repeater. In yet another approach, the ranging signal rides along with
a communication carrier, thus consuming less transponder resource.
The interconnection and switching of command, telemetry, and ranging links
between each other and among different satellites is facilitated by the IF switching
and control subsystem. For example, during transfer orbit operations, the TT&C
link to the satellite is established through the full tracking antenna. A switchover
to the assigned limited-motion antenna normally is accomplished with the IF switch-
ing and control subsystem after the satellite is located in its assigned orbit slot and
has been fully deployed and tested. The TT&C station must be capable, however,
of switching the full-motion antenna back in line for testing purposes or in the
event of an emergency situation, which could happen at any time in the operating
lifetime of the satellite.
In a global TT&C network for non-GEO operation, several TT&C stations
transfer their data to a central location, such as the SCC. Coordination and switch-
ing of signals from literally dozens of satellites at a cental facility can be a complex
task, requiring special computer equipment and software.
9.7.1.2
Satellite Control Center
The SCC is the brain of the satellite operation, providing the computing power
and human intelligence necessary to operate and control a system of several satellites
and TT&C Earth stations. Depicted in the left half of Figure 9.19 is the configura-
tion of a typical SCC. The baseband signals between the TT&C and the SCC can
interface directly with the computer system. Additional pieces of specialized digital
processing equipment may be placed between computer and baseband in the inte-
grated system. An example of such a device is the command generator used to
format and modulate command transmissions for the uplink. A complementary
device called the telemetry decommutator accepts the telemetry TDM data stream
and demultiplexes the telemetry channels. Another type of special device is the
spacecraft simulator, usually a computer workstation programmed to behave like
an operating satellite (from a bus standpoint). The simulator is used to prepare for
critical maneuvers and for training operators to deal with normal and contingency
situations, such as a satellite temporarily losing attitude pointing control.
The single box labeled ‘‘computer system’’ in Figure 9.19 contains sufficient
computing power, redundancy and storage to support all the requirements of the
SCC, including reliability. The computer system performs real-time functions of
command generation, telemetry reception and processing, and ranging. Addition-
ally, several SCC activities are handled by the computer system in the background.
Orbital dynamics deals with the determination of the satellite orbit and the planning
of orbit correction maneuvers necessary to maintain satellite position. Using ranging
data as an input to the orbital software, the orbital dynamics personnel generate
maneuver plans, which are presented to the spacecraft controllers (i.e., the people
who ‘‘fly’’ the satellites) for action by the commanad system. The orbital dynamics
personnel and spacecraft controllers access the computer system with the work-
stations shown at the upper left of Figure 9.19. In the photograph in Figure 9.21,
spacecraft controllers can enter spacecraft commands through workstations in front
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Earth Stations and Network Technology
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