Apollo Inertial Navigation

Can anyone help with following list of questions on the Apollo inertial
navigation systems?
I am fascinated by the challenges of inertial navigation faced by Apollo -
I've done some reading but so far haven't been able to find the answers to
these particular questions.

1. How much of a role did the inertial platform and guidance computer play
in navigation to the moon? How much of the navigation was done by ground
control and tracking?
2. Was the inertial system powered up continuously from launch through to
re-entry? Or was it only powered up to monitor and track the various engine
burns during the flight? Presumably once you are on the "right" trajectory,
there is nothing to disturb you until you get close to the moon/earth, so
you can power the system down until you need it again?
3. Was it necessary to re-set or correct platform alignment during the
flight, and if so, how often and how was this done - star shots?
4. With specific reference to the gyro's, how does the accuracy/drift
specification of the Apollo gyros compare to that of cheap fibre optic gyros
that one can purchase for about US$5k these days?
5. How does the accuracy of the shuttle system compare? If you are not going
to the moon, can you get by with lower accuracy?

Steve

In article ,
SRG wrote:
1. How much of a role did the inertial platform and guidance computer play
in navigation to the moon? How much of the navigation was done by ground
control and tracking?

The onboard navigation system was originally intended as the primary
navigation reference, but in the end it was generally relegated to a
backup role, with radio tracking from the ground as primary. The two were
systematically tested against each other on Apollo 8: for major maneuvers,
the two agreed so closely that it wasn't clear which one was better.

2. Was the inertial system powered up continuously from launch through to
re-entry? Or was it only powered up to monitor and track the various engine
burns during the flight? Presumably once you are on the "right" trajectory,
there is nothing to disturb you until you get close to the moon/earth, so
you can power the system down until you need it again?

Original plans envisioned powering it down except for maneuvers and
navigation updates, but in the end it was left up all the time. One
reason for that was that the wide-field "finder" telescope was plagued by
scattered-light problems, so acquiring a good attitude fix starting from
scratch was usually difficult and time-consuming.

Getting onto the "right" trajectory is harder than you might think. Very
small errors in the initial setup grow as time goes by, so mid-course
corrections are mandatory.

3. Was it necessary to re-set or correct platform alignment during the
flight, and if so, how often and how was this done - star shots?

Yes, it was necessary to calibrate out drift, once or twice a day I think,
with star sightings. With the platform left running, the accumulated
error generally wasn't large and the narrow-field telescope usually
sufficed to find the desired star.

4. With specific reference to the gyro's, how does the accuracy/drift
specification of the Apollo gyros compare to that of cheap fibre optic gyros
that one can purchase for about US$5k these days?

Lots better, although I don't have numbers on hand. The cheap fiber gyros
are for applications which don't need long-term stability. (They came out
of work directed at things like air-to-air missiles, with flight times of
a few minutes at the most.)

5. How does the accuracy of the shuttle system compare? If you are not going
to the moon, can you get by with lower accuracy?

Not really, given applications like astronomical observations (there have
been shuttle-based astronomy missions). Again, numbers are not on hand,
but my impression is that it's better but not dramatically so.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |

Thanks to rk and Henry Spencer for the useful replies to my posting..
Steve

If you have not read the various entries you will find
that the Apollo Flight Journal contains detailed info
on practical navigation.

http://www.hq.nasa.gov/office/pao/History/ap15fj/

If you have not read the various entries you will find
that the Apollo Flight Journal contains detailed info
on practical navigation.

http://www.hq.nasa.gov/office/pao/History/ap15fj/