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Before the complex can successfully send or receive information
to or from a spacecraft, it must first locate the spacecraft
and track its movement across the sky. JPL assists in locating
the spacecraft by sending it’s predicted location to the
CDSCC’s Pointing Computer.
This predicted positioning
information is known as ‘Radiometric Predicts’. The
predicts are used by the Pointing Computer to generate commands
that are fed into the drive system of the antenna, and tune the
receivers to the expected signals. Differences between the predicts
and the actual position of the spacecraft are returned to the
navigation team at JPL to help improve future estimates.
The CDSCC uses several radio frequency bands to communicate with
spacecraft. These are:
S band at approximately 2,000 MHz
X band at approximately 8,000 MHz
Ku band at approximately 15,000 MHz
Ka band at approximately 32,000 MHz
Two types of transmissions
are sent to a spacecraft, these are called ‘ranging’
and ‘command’.
Ranging transmissions are used
to determine the distance a spacecraft is from Earth. Command
transmissions are used to control spacecraft functions, activate
experiments, and alter direction.
The 70-metre antenna has the
capability to transmit a signal to a spacecraft at 400 kilowatts.
However 20 kilowatts, which is the capacity of the 34-metre antennas,
is more common. The higher transmitter power is needed for distant
spacecraft or for emergencies such as when on-board systems fail
to manoeuvre a spacecraft into a proper receiving position.
Commands need to be sent well
ahead of time. This is to ensure that the signal has enough time
to travel to the spacecraft. Even though radio signals travel
at the speed of light, approximately 300,000km per second, it
would take more than half a day for a signal to reach a spacecraft
beyond the edge of the planetary solar system. |
