The purpose of the spacecraft transponder is to receive and transmit radio signals containing command and scientific information. The method is similar to the way you converse over a CB radio known as two-way communications. The satellite antenna and ground antenna station both transmit and receive radio signals at defined frequencies that each is tuned to hear.

Radio Frequencies

Electromagnetic radiation with frequencies between about 10 kilohertz (kHz) and 100 gigahertz (GHz) are referred to as radio frequencies (RF). Radio frequencies are divided into groups which have similar characteristics, called "bands," such as "S-band," "X-band," etc. The bands are further divided into small ranges of frequencies called "channels," some of which are allocated for the use of space tele-communications. TRACE uses S-band, receiving commands at 2.039 GHz and telemetering data at 2.215 GHz. Other spacecraft may use X-band frequencies which in the 10 GHz range. These frequencies are among those referred to as microwaves, because their wavelength is short, on the order of centimeters. Often, higher frequencies are used to transmit larger volumes of data over longer distances. Most Low Earth Orbit (LEO) spacecraft such as TRACE use S-band communications.

Signal Power

Your local entertainment radio broadcast station may have a radiating power of 50 kW, and the transmitter is probably no more than 100 km away. Your portable receiver probably has a simple antenna inside its case. Spacecraft have nowhere near that amount of power available for transmitting, yet they must bridge distances measured in hundreds to tens of billions of kilometers. The TRACE transmitter only uses 5 watts of power. How can that be enough? One part of the solution is to employ microwave frequencies, and concentrate all available power into a narrow beam, and then to send it in one direction instead of broadcasting in all directions. TRACE uses two omni-directional antennas which radiate two lobes of signal to the ground station. Even when these concentrated signals reach Earth, they have vanishingly small power. The ground station antennas employ large aperture parabolic dishes to concentrate the power, cryogenically-cooled low-noise amplifiers and sophisticated receivers, as well as data coding and error-correction schemes to ensure the data within the signal is not corrupted.

Uplink and Downlink

The radio signal transmitted to a spacecraft is known as uplink. The transmission from spacecraft to Earth is downlink. Uplink or downlink may consist of a pure RF tone, called a carrier, or carriers may be modulated to carry information in each direction. Commands transmitted to a spacecraft are sometimes referred to as an upload. Communications with a spacecraft involving only a downlink are called one-way. When an uplink is being received by the spacecraft at the same time a downlink is being received at Earth, the communications mode is called "two-way."

Modulation and Demodulation

Consider the carrier as a pure tone of, say, 3 GHz, for example. If you were to quickly turn this tone off and on at the rate of a thousand times a second, we could say it is being modulated with a frequency of 1 kHz. Spacecraft carrier signals are modulated, not by turning off and on, but by shifting each waveform's phase slightly at a given rate. One scheme is to modulate the carrier with a frequency, for example, near 1 MHz. This 1 MHz modulation is called a subcarrier. The subcarrier is in turn modulated to carry individual phase shifts which are designated to represent groups of binary 1's and 0's--the spacecraft's telemetry data. The amount of phase shift used in modulating data onto the subcarrier is referred to as the modulation index, and is measured in degrees. The same kind of scheme is also used on the uplink.

Demodulation is the process of detecting the subcarrier and processing it separately from the carrier, detecting the individual binary phase shifts, and decoding them into digital data for further processing. The same processes of modulation and demodulation are used commonly with Earth-based computer systems and fax machines transmitting data back and forth over a telephone line. The device used for this is called a modem, short for modulator / demodulator. Modems use a familiar audio frequency carrier which the telephone system can readily handle.

Binary digital data modulated onto the uplink is called command data. It is received by the spacecraft and either acted upon immediately or stored for future use or execution. Data modulated onto the downlink is called telemetry, and includes science data from the spacecraft's instruments and spacecraft health data from sensors within the various onboard subsystems.