Thus far, planetary landers have been flown 'open loop' in terms of their horizontal targeting with respect to the surface. While feedback control is employed to regulate descent rate to achieve close-to-zero speed at zero altitude, only the horizontal speed tends to be controllable, not the location.
The Mars Exploration Rovers incorporated a camera system (DIMES - Descent Image Motion Estimation System) to sense sideways motion, and a set of rocket motors (TIRS - Transverse Impulse Rocket Subsystem) to null the motion to maximize the probability of successful airbag landing; Surveyor and other lunar landers similarly used multibeam Doppler radar and thrusters to null horizontal motion. However, the latitude and longitude co-ordinates of the landing site were simply those that happened to be under the spacecraft when its height became zero. These were within an expected delivery ellipse specified by entry conditions and uncertainties, etc., but were not controlled.
Such fatalism is unacceptable in situations where close proximity to small sites of scientific value is required, or where a heterogeneous target region may have some sites of acceptable topography mingled with dangerous hazards. So far, the prime example of precision landing on another planet is that of Apollo 12, landing within sight of Surveyor 3, permitting the retrieval of exposed equipment from the latter. This example, however, exploited the sensing and control abilities of a human crew, which are generally unavailable. Some sample-return architectures for Mars, for example, have envisaged a rover acquiring sub-surface samples and delivering them to a separate sample-return spacecraft; the latter must have the ability to land close enough to the rover to be reachable by it.
Beyond the descent control instrumentation described in Section 5.7, additional sensing is needed to control location - much of it derived from work on weapons delivery. Inertial guidance (accelerometers and gyroscopes) may, if the delivery (entry) state is sufficiently well known, be adequate, provided that the terrain has been mapped already such that the target site is known in inertial co-ordinates. For enhanced precision, or when terrain data is not registered adequately in a co-ordinate frame, terrain-matching cameras may be used, again borrowing from their application in cruise missiles. However, it is boulders on a scale (~0.5 m) much smaller than that needed for overall navigation that pose the greatest mechanical threat to a lander, and thus their distribution is either prohibitive to map directly, or must be determined indirectly, e.g. from radar back-scatter. Scanning LIDARs are thus being contemplated for making topography maps underneath a lander to identify safe landing spots.
For landers on airless bodies, rocket propulsion is of course necessary to control sideways motion and location. For Mars, at least, consideration is being given to steerable (ram-air) parachute systems.
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