Newsgroups: sci.space.shuttle
From: henry%spenford@zoo.toronto.edu (Henry Spencer)
Subject: Re: navigation in space
Organization: SP Systems, Toronto
Date: Tue, 19 Dec 1995 19:14:28 GMT
Lines: 24

In article <4b5nas$bd@sneezy.icinet.net> warthog@GOLDRUSH.COM (Warren Benson) writes:
>I'd like to read a detailed description of how navigation in space was
>done by  Apollo astronauts in trans-lunar flight.  How would it be done 
>by a Mars mission?

The best detailed description of Apollo navigation -- be warned that it's
technical -- is in the epilog of R.H. Battin's book "An Introduction to
the Mathematics and Methods of Astrodynamics" (AIAA 1987).  The original
Apollo requirement was for the spacecraft to be able to find its way to
the Moon and back with no help from Earth -- none, zero -- and that's
what Battin describes.  (The spec was set at a time when the Cold War was
particular intense, and there seem to have been fears of deliberate
interference with any system using ground-based help.)

Apollo 8 did nearly all of its own navigation, with ground-based backup,
but later lunar missions moved progressively toward more and more help
from the ground to improve accuracy and make things easier. 

As for how the autonomous navigation was done, basically it was star
sightings for attitude, Earth and Moon sightings for position, and
inertial guidance for minute-to-minute operations between sightings.
-- 
Look, look, see Windows 95.  Buy, lemmings, buy!   |       Henry Spencer
Pay no attention to that cliff ahead...            |   henry@zoo.toronto.edu

From: Mary Shafer <shafer@orville.dfrc.nasa.gov>
Newsgroups: sci.space.shuttle
Subject: Re: Navigating without GPS
Date: 08 Jun 2000 17:30:55 -0700

"Jorge R. Frank" <jrfrank@ibm-pc.org> writes:

> JF Mezei wrote:

> > Also, in the case of the moon missions, (based on what I saw in the
> > movie Apollo 13), how did they ensure that the craft had the absolute
> > right attitude before firing any engines ? I can understand how, once
> > you'fe left earth (or lunar) orbit how an INS can function but how
> > much of an error would it get after 3 days of travel ?
>
> The crew had the capability to realign the INS (called an inertial
> measurement unit, or IMU) using optical star sightings.  This is done
> about once every two days for the shuttle; I don't know how often they
> had to do it for Apollo.  On Apollo 13, of course, the IMU was powered
> down for most of the trip after the explosion.

The Blackbird was using an ANS, Astrological Navigation System, to fix
position in the '50s.  Since it was a recce airplane, flying just
outside Soviet airspace, it needed to know position pretty precisely.

The ANS can see three bright stars (not including the sun) from the
ramp in broad daylight, so you know it works even better at altitude.
The biggest limit to its accuracy is the currency of the star map.

--
Mary Shafer    http://www.dfrc.nasa.gov/People/Shafer/mary.html
shafer@orville.dfrc.nasa.gov Of course I don't speak for NASA
Senior Handling Qualities Research Engineer
NASA Dryden Flight Research Center, Edwards, CA
For non-aerospace mail, use shafer@spdcc.com please

From: Mary Shafer <shafer@orville.dfrc.nasa.gov>
Newsgroups: sci.space.shuttle
Subject: Re: Navigating without GPS
Date: 09 Jun 2000 12:29:10 -0700

Mary Shafer <shafer@orville.dfrc.nasa.gov> writes:

> The Blackbird was using an ANS, Astrological Navigation System, to fix
> position in the '50s.

Honestly, the most ignorant people in the world are posting on
sci.space.shuttle these days.  It's not an "Astrological" Navigation
System but an "Astroinertial" Navigation System.

The Dash-1 describes it as "an inertial navigation system employing a
star tracker to eliminate gyro drift and to limit position error.  The
system provides a steering signal to the autopilot for guiding the
aircraft automatically along a predetermined flight path.  It provides
heading, attitude, and position information to cockpit displays.  The
ANS can control the CAPRE side-looking radar and technical objective
cameras for imaging operations.  The ANS supplies navigational data to
the electromagnetic-reconnaissance sensor and mission data to the
mission recorder system."

I'm not going to study the thirty or forty pages describing its
operation, the star tracker search algorithm, the status lights and
messages, and notes, cautions, and warnings, but it may interest
readers to know that taking off without a working chronometer is
strongly discouraged as time errors greater than 2 seconds may produce
erroneous ANS updates, even though start tracking is possible, and the
ANS can be updated with tacan.

--
Mary Shafer    http://www.dfrc.nasa.gov/People/Shafer/mary.html
shafer@orville.dfrc.nasa.gov Of course I don't speak for NASA
Senior Handling Qualities Research Engineer
NASA Dryden Flight Research Center, Edwards, CA
For non-aerospace mail, use shafer@spdcc.com please

From: Mary Shafer <shafer@orville.dfrc.nasa.gov>
Newsgroups: sci.space.shuttle
Subject: Re: Navigating without GPS
Date: 09 Jun 2000 10:12:45 -0700

"Greg D. Moore" <mooregr@greenms.com> writes:

> Mary Shafer wrote:

> > The ANS can see three bright stars (not including the sun) from the
> > ramp in broad daylight, so you know it works even better at altitude.
> > The biggest limit to its accuracy is the currency of the star map.

> 	I'm impressed.  Do you now which stars? Or does it vary?

The star tracker can see stars "overhead" at the time.  Of course
which stars it sees varies, but the aircrew has told me that three is
usual on the ground.  The star map has 61 stars in it and needs to be
updated every fifteen months for maximum accuracy.

With good tracking and a current map, the probably radial error for
the star tracker is 0.3 nmi using rapid alignment as part of the
preflight checklist and 1.25 nmi after two fixpoints (about 1 hr)
following airstart alignment as part of a scramble takeoff.  The very
best the INS can do is 1.28 nmi, with rapid alignment and fixpoints
every hour.

--
Mary Shafer    http://www.dfrc.nasa.gov/People/Shafer/mary.html
shafer@orville.dfrc.nasa.gov Of course I don't speak for NASA
Senior Handling Qualities Research Engineer
NASA Dryden Flight Research Center, Edwards, CA
For non-aerospace mail, use shafer@spdcc.com please

Newsgroups: sci.space.shuttle
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Navigating without GPS
Date: Fri, 9 Jun 2000 02:48:15 GMT

In article <39400B1D.3618AB0F@videotron.ca>,
JF Mezei  <jfmezei.spamnot@videotron.ca> wrote:
>Before the advent of GPS, what methods of navigation did the shuttle use ?

Actually, "does" rather than "did" -- the shuttle conversion to GPS is
only now in progress (last I heard).

It's a combination of inertial navigation, star trackers, and radio
tracking by ground stations and TDRS.  (Remember, the T in TDRS is
"tracking".)

>If it was solely innertial navigation system, how would such a system detect
>the speed/altitude changes as a result of an elliptical orbit ? (eg: no
>thrusters fired)

Inertial systems in free fall detect only non-gravitational accelerations.
This generally isn't a big problem, since the gravitational fields are
very predictable.

>Also, in the case of the moon missions, (based on what I saw in the movie
>Apollo 13), how did they ensure that the craft had the absolute right attitude
>before firing any engines ? I can understand how, once you'fe left earth (or
>lunar) orbit how an INS can function but how much of an error would it get
>after 3 days of travel ?

The Apollo inertial platforms were regularly re-aligned by star sightings;
in particular, this was done before maneuvering.  The shuttle uses the
same method, except that it's largely automated.

>Also, in the case of Apollo 13, during their return, did they truly "blindly"
>fire an engine for 10 seconds without any guidance except for the commander
>having an eyesight on the earth ? In the movie, they depict the austronauts as
>having problems maintaining a straight line during that 10 seconds. Was that
>realistic ?

The movie scene is somewhat dramatized and is a loose combination of
several real events.  Don't take it as gospel.  If memory serves -- my
references aren't handy -- all the real burns took place under control of
the LM guidance system, although various improvised methods were used to
align it and check its alignment (since debris around the spacecraft made
star sightings difficult).
--
Microsoft shouldn't be broken up.       |  Henry Spencer   henry@spsystems.net
It should be shut down.  -- Phil Agre   |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.history
From: henry@spsystems.net (Henry Spencer)
Subject: "why no stars?"
Date: Sat, 14 Oct 2000 03:51:57 GMT

Interesting tidbit relating to the "why aren't there any stars in the
lunar-surface photos?" question that the Apollo-was-faked nuts ask from
time to time...

The answer, of course, is that the stars are too faint to show up in a
correctly-exposed photo of a lunar landscape in brilliant sunlight.  The
same issue has turned up in another context.  According to the Science
papers on Eros (which I'm only just getting around to reading), this was a
minor difficulty in trying to do optical navigation of NEAR/Shoemaker near
Eros:  at close range, the asteroid is too bright to get a good image of
it with stars visible in the background.

(The solution was to get an attitude fix from the star tracker at the same
time the camera snaps the asteroid.  The one complication is that the
relationship between the two sets of optics isn't precisely fixed.  Once a
day, the camera shoots a long exposure of sky to get some star images, and
that measures the tracker-camera offset... which does vary slightly.)
--
Microsoft shouldn't be broken up.       |  Henry Spencer   henry@spsystems.net
It should be shut down.  -- Phil Agre   |      (aka henry@zoo.toronto.edu)