roberts@CMR.NCSL.NIST.GOV (John Roberts) (03/21/91)
>From: amichiel@rodan.acs.syr.edu (Allen J Michielsen) >Subject: Re: Computer Controlled Landing? >Organization: Syracuse University, Syracuse, NY >In article <> henry@zoo.toronto.edu (Henry Spencer) writes: >>In theory, I think the only thing the computer can't do is lower the >>landing gear. ... In practice, I believe >>it is still the case that all landings to date have been manual. >Part of the arguable difference is the EXACT terminology being refered to. >When NASA & Henry say that every landing has been manual so far, they really >mean that the pilot controlled the stick and put the gear on the ground. >What is being left out, is that the pilot only took control away from the >computer a very short time before this event. If my memory serves, the first >flights, used the computer to fly the shuttle down to about 100 feet. That's the impression I had. So getting dumped in the ocean would be pretty difficult. (Anyone know the distance from the KSC runway to the ocean?) I *think* that maneuvering in extremely thin atmosphere (where control surfaces don't work, so rocket thrusts must be used) is considered sufficiently tricky and time-critical that the computer can generally do a better job than the pilot. (Anyone know whether the Shuttle is unstable under those conditions?) Close to the ground, the years of pilot experience and direct feedback allow the pilot to do a better job. >Al. Michielsen, Mechanical & Aerospace Engineering, Syracuse University > InterNet: amichiel@rodan.acs.syr.edu amichiel@sunrise.acs.syr.edu > Bitnet: AMICHIEL@SUNRISE John Roberts roberts@cmr.ncsl.nist.gov
jng@sli.com (Mike Gilbert) (03/27/91)
Regarding the computer-controlled landing discussion, and the observation that
the astronauts take over at 100 feet AGL:
I worked at Intermetrics (a NASA subcontractor for the fifth "backup" flight
computer's software) back when the shuttle was being developed. I had no
personal involvement in the shuttle program, but another engineer who did
explained the landing problem to me as follows:
The AP-101 shuttle flight computer has only single-precision integer
arithmetic. Using single precision, the Autoland software can't compute its
location with the precision required to do a landing.
For example, it might only know the shuttle's height AGL to, say, +/- 20 feet,
since the least significant bit of the AGL value would represent a 20-foot
difference. This tolerance isn't a problem for most of the descent, but a
touchdown 20 feet BELOW the surface of a runway would be a big problem. This
would explain why the astronaut has to take over, but only at the last minute.
Again according to this engineer, the algorithm was re-coded as (software)
double precision, but it then ran too slowly to be real time ("five seconds
after the crash, it would know exactly how it should have landed...").
I cannot personally vouch for this explanation, but the engineer who explained
it to me (back in 1979 or so) should have been in a position to know.
I recall seeing that the new shuttle computers will have hardware double
precision integer arithmetic, so perhaps Autoland will become more useful
when the new computers become operational.
Mike Gilbert
Software Leverage, Inc.