On Earth, an aircraft's altitude is conventionally determined by measuring the outside air pressure; making use of the fact that the atmosphere gradually thins in a well-understood manner as altitude is gained. On the Moon there is essentially no atmosphere so another method had to be devised to determine how high the LM was above the surface. This was particularly important given the fact that there are few clues a pilot can use to determine speed or altitude by eye. There are no trees, roads or houses; no haze to give a sense of depth or distance. Most remarkably, there is little variation of topography as one descends from high to low altitudes. Just a pock-marked landscape of large craters overlaid with small craters, peppered with even smaller craters.
The lunar module made sense of its altitude above this landscape by firing radio pulses at it from the antenna of the landing radar mounted on the underside of the descent stage. The four microwave beams from the antenna did two things: (1) they used the time taken for pulses to reflect off the surface to determine current altitude using conventional radar techniques; and (2) the frequency of the returning signal was measured for any shift due to the Doppler effect, yielding their rate of change of altitude, or vertical speed, as the LM descended towards landing. For the later missions, which approached their landing sites over mountainous landscapes, a simplified model of the terrain profile was added to the computer as part of its programming to compensate for the natural changes in height that were encountered by the LM on its planned ground track. The computer also took account of the antenna's slant angle; that is, its angle away from true vertical in which it was pointing at any moment. The height and change of height data derived from the radar's sensing was used by the computer in its control of the descent. It also drove the tapemeter display for the crew.
The antenna operated in one of two positions, depending on the flight mode of the LM. Throughout most of the descent, starting from the time they began their powered descent to leave orbit, the LM was flying on its back, with the crew looking up into a black sky. In this mode, the landing radar antenna was in its 'descent' position, angled 24 degrees from the LM's vertical axis. For the final phase of the landing after pitch-over, when the LM adopted a vertical attitude, the landing radar moved to its 'hover' position to aim in a direction parallel to the spacecraft's x axis and therefore pointing straight down.
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