Cassini-Huygens Spacecraft

How do you keep track of an object that's been hurled away from Earth to travel for years throughout the solar system? How do you know where it is, and how fast it's traveling?

The uplink and the downlink work together to solve these problems. Uplink uses powerful radio transmitters, and downlink uses sensitive receivers, both within the Deep Space Network (DSN). Cassini-Huygens can only be tracked because it carries a radio transmitter that sends signals to Earth. (This is true with all other interplanetary spacecraft as well.) The transmitter aboard Cassini is linked with its own radio receiver, so that they can both work together when needed.

The two main types of tracking data that are used with Cassini are ranging and the Doppler effect. Using these two data types, the navigators can accurately track the Cassini-Huygens spacecraft.

Ranging

Ranging determines the distance (range) from Earth to the spacecraft and back, by placing specially coded signals (called ranging tones) on the radio uplink, and recording the exact time as they go up. When the spacecraft receives them, it puts them on the downlink right away. When they come back to Earth, the exact time is noted again. So basically, the ranging computer knows what time it sent the tones, and it knows what time they came back.

Since the speed of the radio signals is known (they travel at the speed of light), the round-trip distance can then be computed.

There are other factors to consider, too. How long did it take for the ranging tones to "turn around" inside the spacecraft's electronics? That miniscule delay is calculated from pre-launch testing. How long did it take the ranging tones to travel through the cable from the computer in the Deep Space Network (DSN) signal-processing center out to the radio telescope antenna before leaving Earth? The DSN finds that value while calibrating the system prior to each tracking period. And how far did the Earth move while the ranging pulses were traveling to the spacecraft? The navigators draw upon data gathered over years and years of observations by the astronomical community.

Highly evolved computer programs within the ranging system process these data to determine the distance between Earth and the Cassini-Huygens spacecraft.

Optical Navigation

Even though the most common means to track the spacecraft use ranging and the Doppler effect, a third data type can come into play once the spacecraft arrives near or is in orbit around Saturn. Optical navigation involves having the Cassini orbiter capture images of Saturn's satellites, with the background stars visible. These images come on the downlink as what is known as telemetry data, and once received, they are analyzed by the navigators for a more precise analysis of the spacecraft's trajectory than is available through ranging and the Doppler effect alone. Using this "opnav" data, instructions can then be uplinked to the spacecraft in the form of command data to fine-tune the spacecraft's on-board schedule of science observations, or to fine-tune a direction to point its instruments.

All three of these types of data that Cassini-Huygens uses for navigation, are subject to the round-trip-light time of around three hours across the distance between Earth and Saturn.