J

Transmitter

Transponder

USO h-ยป. Transmitter

Data system i

Frequency standard

Trajectory &

telemetry systems

Ground station

Receiver

Probe

Figure 10.1. Relay communication system.

and aerodynamic motion of the probe. At some point plasma forming around the probe may block the signal and the reception might be interrupted.

After re-emerging from plasma blockage the probe will continue to decelerate, experiencing often rapid and not very predictable trajectory variations resulting from wobbling, wind and turbulence conditions, active propulsion and, finally, landing on the surface. At this, the most critical phase that will often determine the mission success, the reception of the probe signal is highly important for data analysis. After the landing-probe motion ceases or becomes very slow, the signal behaviour generally becomes smooth and predictable again. In some cases, however, interference of the direct signal to the Earth or relay spacecraft with a reflected ray (i.e. multipath) can cause sharp nulls in the pattern of radiation detected in the far field. As planetary rotation, or the motion of a relay spacecraft, causes the receiver to fly through this pattern, sharp drops in signal strength can occur; such drops were observed for example on the Huygens probe.

Antenna pointing for the DTE link becomes the main issue especially for rovers. For the lighter-than-air atmospheric probes (balloons and airships) the motion after deployment is usually smooth and does not restrict link performance. By contrast, communication with fast-manoeuvring airplanes may encounter significant challenges.

Entry probes: communication basics Table 10.1. Definitions of radio-frequency bands

Frequency Band

Notes

8 to 12

12 to 18

26 to 40

Previous, since early World War 2 British radar used this band but later switched to higher frequencies. These frequencies, and sometimes up to 3 GHz are commonly termed UHF (Ultra High Frequency) - e.g. Viking Lander (381 MHz) and MER relay Long wave. Used in spaceborne radar Short wave. Common for interplanetary missions, especially low-gain e.g. Pioneer Venus (2.3 GHz), Huygens, Galileo probes

Compromise between S and X. Used mainly for terrestrial satellite communications and radars - not common on planetary missions Used in World War 2 for fire control, X for cross (as in crosshair). Common as the main up/downlink for interplanetary missions e.g. Voyager (8.4 GHz). Now becoming progressively supplanted by Ka band Kurz-under (i.e. lower in frequency than the main water absorption in the Earth's atmosphere). Used for e.g. Huygens radar altimeter, Cassini Radar. Kurz-above (higher in frequency than the main water absorption). Becoming more widely used due to the high gains achievable with small aperture. Cassini downlink

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