July 2004
Waiting for Cassini's "Safe
Arrival" Call
By Diane K. Fisher
The evening of June 30, 2004, was nail-biting time at Cassini Mission Control.
After a seven-year journey that included gravity assist flybys of Venus, Earth,
and Jupiter, Cassini had finally arrived at Saturn. A 96-minute burn of its main
engine would slow it down enough to be captured into orbit by Saturn's powerful
gravitational field. Too short a burn and Cassini would keep going toward the
outer reaches of the solar system. Too long a burn and the orbit would be too
close and fuel reserves exhausted.
According to Dave Doody, a Cassini Mission Controller at the Jet Propulsion
Laboratory (JPL) in Pasadena, California, there was a good chance the
Earth-bound Cassini crew would have to wait hours to learn whether or not the
burn was successful. Of the three spacecraft-tracking Deep Space Network (DSN)
complexes around the globe, the complex in Canberra, Australia, was in line to
receive Cassini's signal shortly after the beginning of the burn. However, winds
of up to 90 kilometers per hour had been forecast. In such winds, the DSN's huge
dish antennas must be locked into position pointed straight up and cannot be
used to track a tiny spacecraft a billion miles away as Earth turns on its axis.
"The winds never came," notes Doody.
The DSN complex at Goldstone, California, was tracking the carrier signal from
Cassini's low-gain antenna (LGA) when the telltale Doppler shift in the LGA
signal was seen, indicating the sudden deceleration of the spacecraft from the
successful ignition of the main engine. Soon thereafter, however, Goldstone
rotated out of range and Canberra took the watch.
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Right after entering Saturn orbit, Cassini
sent this image of the part of the Encke Gap in Saturn's rings. Image credit NASA/JPL/Space Science Institute. |
After completion of the burn, Cassini was
programmed to make a 20-second "call home" using its high-gain antenna (HGA).
Although this HGA signal would contain detailed data on the health of the
spacecraft, mission controllers would consider it a bonus if any of that data
were actually captured. Mostly, they just wanted to see the increase in signal
strength to show the HGA was pointed toward Earth and be able to determine the
spacecraft's speed from the Doppler data. If possible, they also wanted to try
to lock onto the signal with DSN's closed-loop receiver, a necessary step for
extracting engineering data.
Normally it takes around one minute to establish a lock on the HGA signal once a
DSN station rotates into range. Having only 20 second's worth of signal to work
with, the DSN not only established a lock within just a few seconds, but
extracted a considerable amount of telemetry during the remaining seconds.
"The DSN people bent over backwards to get a lock on that telemetry signal. And
they weren't just depending on the technology. They really know how to get
flawless performance out of it. They were awesome," remarks Doody.
Find out more about the DSN from JPL's popular training document for mission
controllers, Basics of Space Flight
http://www.jpl.nasa.gov/basics and the DSN website at
http://deepspace.jpl.nasa.gov/dsn.
For details of the Cassini Saturn orbit insertion, see
http://www.jpl.nasa.gov/basics/soi.
Kids can check out The Space Place at
http://spaceplace.nasa.gov/en/kids/dsn_fact1.shtml to learn about the
amazing ability of the DSN antennas to detect the tiniest spacecraft signals.
This article was provided by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.