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Navigation - Uplink/Downlink

Navigation - Uplink/Downlink

In order to create and maintain a successful mission, Cassini must operate its instruments to make observations and carry out experiments under just the right conditions, and at just the right time.

Accomplishing this raises many important questions, such as exactly where will the spacecraft be at a particular time, and what will be its orientation in space? When, and in what direction must it turn to capture a view of the various targets of interest? How long must an instrument's shutter remain open to obtain the right exposure? What other settings will the instrument need?

Cassini Trajectory

Determining the location, or path (called trajectory) of the spacecraft is the job of a team of navigators at JPL. Navigators obtain their information from the intricate process of tracking the spacecraft. They determine where the spacecraft will be at any given time in relation to objects in Saturn's vast system. Of course, to do that, they need to know where those objects are going to be. The predicted locations of Saturn, its rings, its moons and so on, are data known as ephemerides.

Using very sophisticated computer programs, navigators take the ephemerides and the spacecraft tracking data into account in their planning processes. After much number-crunching, they provide the predictions necessary for planning how and when Cassini will be able to make observations. They also provide information on what the spacecraft must occasionally be commanded to do in order to make small adjustments in its trajectory. These small corrections, called trajectory correction maneuvers, will assure that Cassini will be exactly where it needs to be at the proper time.

Navigators use what is known as the "uplink" and the "downlink" working simultaneously to solve these problems.

In Cassini's uplink process, there are millions of possibilities to consider, and many decisions to be made. For example, the process has to determine what scientific observations Cassini is going to make and when to make them. It must schedule when to fire its rocket engine or thrusters and how the spacecraft must be oriented when it fires them, as well as knowing what data rates to use to send measurement to Earth. Cassini's uplink process begins long before sequences of commands are actually placed on the uplink.

The process starts with the scientists (also called investigators) associated with Cassini and the Huygens probe, who are located all over the world at universities, observatories and aerospace companies.

Electromagnetic Spectrum

Commanding the Cassini spacecraft is a little like commanding a television set with a remote control. The navigators command signals on the radio-frequency uplink to the spacecraft, which it receives, decodes, and acts upon. A typical remote control uses an infrared signal with its own specially coded commands. The television set receives them, its circuitry decodes them, and the television acts upon them.

Cassini's commands are sent through computer control, which vary the phase of the uplink and, like a remote control, send the information to the spacecraft in the form of pulses.

But sending the actual command signals to Cassini is the very last stage of the uplink process. Obviously, the first step is to decide exactly what commands to send. The broader definition of "uplink" embraces the whole process of deciding what the navigators want the spacecraft to do.

70m antenna at Goldstone, California

Uplink is a general term given to radio signals sent "up" from Earth to a spacecraft. The uplink usually sent to Cassini is a highly directional, microwave radio beam in a frequency range called X-band. It's around 7 to 8 billion Hertz (cycles per second), normally at power levels less than 10 kilowatts. The purposes for the uplink to Cassini are for commanding, tracking and radio science. Uplink also refers to the extensive process of planning, designing and sending command sequences.

Likewise, downlink is the general term for the radio signals sent "down" to Earth from a spacecraft. Cassini's normal downlink is in the X-band frequency range. This frequency range is used for tracking, radio science and telemetry.

Cassini can also downlink data in S-band and in Ka-band. The S-band is a lower frequency than X-band, and Ka-band is higher. However, downlinks in the S- and Ka-band are for radio science only.