Instrumentation and Communications Subsystem
This subsystem provides the communications loop between the Telescope and the Tracking and Data Relay Satellites (TDRS), receiving commands and sending data through the HGAs and LGAs. All information passes through the Data Management Subsystem (DMS).
S-Band Single Access Transmitter (SSAT). HST is equipped with two SSATs. "S-band" identifies the frequency at which the science data is transmitted and "single access" specifies the type of antenna on the TDRS satellite to which the data is sent.
High Gain Antennas. Each HGA
is a parabolic reflector (dish) mounted on a mast with a two-axis gimbal mechanism and electronics to rotate it 100 degrees in either direction (see Fig. 5-9). General Electric designed and manufactured the antenna dishes using honeycomb aluminum and graphite-epoxy facesheets.
The HGAs achieve a much higher RF signal gain than the LGAs. The higher signal gain is required, for example, when transmitting high-data-rate scientific data. Because of their characteristically narrow beam widths, the HGAs must be pointed at the TDRSs. Each antenna can be aimed with a 1-degree pointing accuracy. This accuracy is consistent with the overall antenna beam width of over 4 degrees. The antennas
K1175_509
transmit over two frequencies: 2255.5 MHz or 2287.5 MHz (plus or minus 10 MHz).
Low Gain Antennas. The LGAs are spiral cones, manufactured by Lockheed Martin, that provide spherical coverage (omnidirectional). They are set 180 degrees apart on the light shield and aft bulkhead of the spacecraft.
Fig. 5-9 High Gain Antenna
Low Gain \/ Antennas
Access ■ Transponder t|
Antennas
I Transmitter
To/From Other Telescope Subsystems *
Data
Recorders
Data, Status, Mode Control s, trol
Solar Array Solar Array
Discrete Special Serial Commands
Data
Recorders
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Key | |
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nn |
Functional Unit |
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Functional Redundancy | |
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Interface Connection | |
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L—J |
Reference Equipment |
Control Unit
Data Interface Units
Support Systems Module
Data
Management Unit
Internal System Control Signals
Control Unit i
Data, Clock, Mode Control
Optical Telescope Assembly
Command, Data Clock
Request, Replay, Clock, Synchronizing Pulses
Reg Power
Keep-Aliv Keep-Aliv i? Pnintinn anH
Advanced Computer
Pointing and Safemode Electronics Assembly
Oven Controlled Crystal Oscillator
Science
Catmamand^Wid=el^eTd7!| Planetary
Camera Camera i s
Science Data Formatter
(Science Instrument Control and Data Handling Unit)
Fig. 5-10 Data Management Subsystem functional block diagram iTjrfrate Gyr^ ^{l
E EC
Control Unit nil
Fixed-Head
Star Tracker
Data Data
Interface Unit
Optical Control Electronics
Actuator
Control
Fine
Guidance Electronics
Electrical Power Temperature Control Electronics
Operating over a frequency range from 2100 MHz to 2300 MHz, the LGAs receive ground commands and transmit engineering data. These antennas are used for all commanding of the Telescope and for low-data-rate telemetry, particularly during Telescope deployment or retrieval on orbit and during safemode operations.
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