Telecommunication is one of the most important functions of entry probes: it transmits to Earth all the science and engineering data that are the main goal of the mission. Tracking of the probes is another function that can help to analyse the probe dynamics during the entry and descent, providing independent science data on the atmosphere of a planet.

During entry, if communications are to be attempted at all, only status tones or very low data rates are possible. This is because the attitude during entry and descent may be very dynamic, preventing pointing of high-gain antennas. Depending on the wavelength of the communication link and the aerothermochemistry of the plasma sheath, transmissions may be completely blocked for a short period (the entry ‘blackout’).

During the highly dynamic entry phase data rates in direct-to-Earth (DTE) links are usually very small due to the great distance to the Earth and the use of low-gain antennas on probes. A relay link (Figure 10.1) uses a much shorter distance to the relay orbiter to boost the received signal strength though using a less efficient receiving antenna than on Earth. The probe data received on the orbiter is re-transmitted to the Earth using the high-gain antenna of the orbiter.

Motion of the probe affects the frequency, amplitude and phase of the signal at the receiving station. The entry process includes phases that are significantly different from the point of view of the communications link.