Instrumentation

To land from orbit requires the controlled change (and therefore the measurement) of the spacecraft's dynamical state. During atmospheric entry, deceleration measurements (or during descent, pressure measurements) can provide

- predictive data

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25 30 35 40 45

Figure 5.3. Slant range versus time during NEAR descent to Eros. The altitude at the start was about 5 km (from Dunham et al., 2002).

- predictive data

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Minutes after brake-1

Figure 5.3. Slant range versus time during NEAR descent to Eros. The altitude at the start was about 5 km (from Dunham et al., 2002).

convenient information on altitude, but on an airless body these cues will be absent.

Usually radar altimeters are used; these can also measure descent rate via the Doppler shift. With three beams making Doppler measurements, it is possible to deduce all three orthogonal velocity components. A usual strategy is first to null any sideways component of velocity, then to descend vertically.

If the entry conditions are such that the vehicle will be braked completely, it may be adequate to assume vertical descent (this may be the case on Mars). However, it may still be necessary to monitor and control the vehicle attitude during a later powered descent phase. Gyros may therefore be necessary to provide an attitude reference (usually descent is short enough that no attitude updates, as are needed on orbital platforms to correct gyro drift, are necessary). If suitable illumination exists on airless bodies (e.g. dayside landings on the Moon) it may be that optical horizon sensing can replace or augment gyroscopic determination.

The Viking landers included complete inertial guidance (i.e. a gyro-stabilized platform with accelerometers) in addition to a radar altimeter. Pathfinder was equipped with gyros: it used a radar altimeter to time the ignition of a braking rocket.

The Surveyor lander used a pulsed radar altimeter to generate an altitude reference at 100 km. At 80 km altitude, a separate RADVS (Radar Altimeter and

Figure 5.4. Surveyor RADVS (Parks, 1966).

Doppler Velocity Sensor) turned on, using a four-beam Frequency-Modulated Continuous Wave technique (Figure 5.4). A central beam at 12.9 GHz was used to measure the altitude, while the three outer beams at 13.3 GHz were used to determine the three components of the velocity. To accommodate the large variation in range (and therefore signal strength) amplifiers provided 40, 65 or 90 dB of gain. The lander also used gyroscopes as inertial references during the burn, and a star tracker for attitude determination prior to the burn.

Laser ranging is now also widely anticipated for Mars and small-body landers -most likely as a LIDAR (light detection and ranging), which is able to generate a range map of the region under the lander in order to assess its topography, and thus its suitability as a landing site, before committing to a possibly hazardous site. The asteroid mission Hayabusa employed a combination of the LIDAR instrument, a short-range LRF laser rangefinder, the ONC optical navigation camera and an FBS fan-beam sensor to inform the autonomous navigation system (Kubota et al., 2003; Hashimoto et al., 2003).

A scientific consideration for powered descent is the effect that impingement of the exhaust plume from the retro rockets might have on the surface material: the descent engines of the Viking spacecraft and the Mars Polar Lander were designed with flared or multiple nozzles to minimize these effects. Luna 16

similarly had two smaller engines for terminal descent. It is usual in any case for the last metre or two of descent to be made as a free-fall.

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