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Navigation: Tracking the Spacecraft
A spacecraft can be tracked in a variety of ways, and in fact TOPEX/Poseidon
uses three different systems simultaneously for tracking.
DORIS
Onboard the spacecraft is a system called DORIS, for Doppler Orbitography and
Radiopositioning Integrated by Satellite (shown here.) Built by the French, the
complete DORIS system includes the receiver on the spacecraft and a worldwide
network of Doppler tracking stations. This system uses microwave energy to
locate and pinpoint the position of spacecraft such as TOPEX/Poseidon by
measuring the Doppler frequency shift in the microwave signals as the Earth
and the spacecraft move relative to one another. (If this sounds like the way
radar systems work...it is.) The DORIS system is accurate to within �3 mm/s
radial velocity.
LRA
Another system on the spacecraft is the NASA Laser Retro reflector Array, or
LRA (shown here.) Ground-based laser tracking stations reflect laser beams
off the retro reflector array and measure the time it takes the signals to
return to their places of origin. This allows the ground stations to plot
precisely where the satellite has been, within �cm.
GPSDR
There is also an experimental system onboard. The Global Positioning System
Demonstration Receiver (GPSDR) was included to test the accuracy of tracking
using the U.S. Defense Department's Global Positioning System constellation of
satellites. The GPS consists of three components: the space segment, the
control segment, and the user segment.
The space segment consists of a constellation of 24 satellites, orbiting in
six planes oriented at 55 degrees to the equator. Each orbital plane contains
four satellites at an elevation of 20,200 miles (32,500 kilometers) above the
Earth. The satellites have orbital periods of 12 hours, which means they
complete two orbital revolutions in 24 hours. This results in their flying over
the same points on Earth at the same time every day.
The control segment is responsible for operating the GPS. The GPS Master
Control Station is located near Colorado Springs, Colorado. There is also a
worldwide network of monitoring stations that track the GPS satellites and
feed information to the Master Control Station. The Master Control Station in
turn updates the "navigational message" and uploads it to the satellites.
The user segment consists of small radio receivers/computers that measure the
time that the radio signal takes to travel from a GPS satellite to the receiver's
GPS antenna (there is a GPS receiver and antenna on TOPEX/Poseidon).
Using the travel time multiplied by the speed of light provides a calculation
of range to each satellite in view. From this and some additional information
on the satellites' orbits and velocities, the internal GPS receiver software
calculates its position through a process of triangulation. At least four GPS
satellites are in view of TOPEX/Poseidon at any one time.
Although TOPEX/Poseidon was the first mission to use GPS for tracking,
the results have been very encouraging. Scientists are even talking about the
possibility of achieving 1-cm tracking accuracy in the near future using GPS!
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