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I was going through the Space Shuttle Handbook, chapter Guidance and Navigation. In one of the Note boxes, it is written

Radar altimeter altitude is not used for navigation or guidance. The information is displayed for the crew to monitor

I have multiple queries:

  1. I understand there is GPS, TACAN and MLS. Do these systems provide better navigation solution than if used radar altimeter?

  2. Is there provision to use radar altimeter in case other systems fail? Or maybe in case of certain abort?

  3. If that is so, then is altimeter only used for monitoring purposes?

  4. And suppose radar altimeter does indeed show some anomaly is there any flight rule for that? What crew can actually do?

  5. If they can't do anything, isn't that just increasing the weight of the shuttle?

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The radar altimeter was intended only for landing, to provide a direct, precise measurement of the altitude of the vehicle above the runway. It locked on around 5000 feet altitude but was primarily useful only in the final stages of landing, from an altitude of 100 feet down.

It was also intended for use in the autolanding system, which was never used operationally.

There were two independent radar altimeters. The measurements from the two systems were averaged for use in the autolanding system.

Once the autolanding system was abandoned, the data from the altimeters was not used in the onboard navigation systems. It was simply displayed to the crew.

A full description is available in the 1982 Press Reference starting on page 438. That is the source of this diagram.

enter image description here

As far as flight rules go, the radar altimeters were considered a "nice to have" once autoland was abandoned. Flight Rule A8-1001 states that they were "highly desirable for night landings or low ceilings. However, there are no mission duration impacts for RA failures."

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    $\begingroup$ So pilot cab use the altimeter for manual landing ? $\endgroup$
    – zephyr0110
    Commented Oct 21, 2019 at 5:45
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    $\begingroup$ Yes, from the SCOM: "When the radar altimeter (RA) sensor selected by the commander (pilot) is locked on, an “R” appears to the right of the altitude digital, and the digital is driven by the RA data on the commander’s (pilot’s) PFD. A floating yellow triangle also appears to the right of the tape at the value of the RA altitude. If no RA is locked on, a “K,” indicating thousands of feet, or an “M,” indicating nautical miles will appear to the right of the altitude tape digital indicating the units." $\endgroup$
    – Organic Marble
    Commented Oct 21, 2019 at 10:51
  • $\begingroup$ Why was the autoland scrapped? $\endgroup$
    – Vikki
    Commented Oct 22, 2019 at 7:54
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    $\begingroup$ @Sean some of the tests were a little scary, the crew didn't want it, and one of the main rationales for it (really long shuttle missions) vanished. It worked great in the simulator. $\endgroup$
    – Organic Marble
    Commented Oct 22, 2019 at 11:55
  • $\begingroup$ Imagine the other way around ... they would have used the radar altimeter - so the space-shuttle flying around earth at ~7.8 km per second - and now would follow the exact surface of the earth ... The joy of passing the Himalayan Mountains ... over a distance of ~3000 km which you pass in a little over 6 minutes (384 seconds) ... you have height differences of around 7 km ... not pleasant $\endgroup$
    – eagle275
    Commented Feb 13, 2020 at 13:32

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