Index Home About Blog
From: (Henry Spencer)
Subject: Re: Apollo Navigation
Date: Sun, 17 Oct 1999 23:26:14 GMT

In article <>,
David Grah - Mary Daniel  <> wrote:
>...How did the Apollo flights navigate to the moon (direction) and how
>did they know where they were (position)?

Originally, Apollo planned to mostly use optical navigation:  the crew
(mostly the Command Module Pilot) would align the spacecraft using star
sightings, and then sight on landmarks (on Earth or Moon) to fix its
position.  This method had already been studied in some detail, and it had
the great advantage that it could be done entirely on the spacecraft.
Remember that Apollo was being planned at the chilliest time of the Cold
War, and it was thought possible that the Soviets might attempt to jam
space communications or confuse radio navigation.  The original Apollo
requirements demanded completely self-contained navigation, with no
assistance from the ground, not even voice communications.

As the 60s proceeded, fears of deliberate interference faded, and radio
navigation got better and better as it was used extensively for unmanned
spacecraft.  By having the spacecraft essentially echo back a radio signal
from the ground, distance and direction could be determined quite
precisely.  (Determining direction was done indirectly, by measuring the
relative velocity between the ground station and the spacecraft using
Doppler shift, and then watching how that changed as the Earth rotated.)

In the end, complete on-board optical navigation was no longer thought a
high priority, and the ability to fly a full landing mission that way was
sacrificed when program memory got tight in the onboard computer.  Radio
navigation, with the computing done on the ground and the results sent up
by voice, was the normal method.  Optical navigation was retained as an
emergency backup, for aborts only, with some simplifications (notably,
sighting on Earth's horizon rather than on landmarks).

Both methods were tested on Apollo 8.  Optical-navigation accuracy started
out good, deteriorated as the spacecraft got farther from Earth, and then
improved again as it got closer to the Moon.  By the time the crew was
setting up for lunar orbit insertion, optical and radio navigation data
agreed so closely that it was not clear which one was better, and the
radio data was sent up for the maneuver only because the flight plan said
so and following it was simpler than changing it.  Results from the return
leg of the flight were similar.
The space program reminds me        |  Henry Spencer
of a government agency.  -Jim Baen  |      (aka

From: (Henry Spencer)
Subject: Re: [Q] Apollo velocity measurements
Date: Sat, 30 Dec 2000 03:52:14 GMT

In article <3a4ccc5c$>,
Mike Mckenzie <> wrote:
>I was wondering if anyone could shed some light on how velocity
>measurements were made during the Apollo missions, for instance,
>after the insertion burns, etc..  Were these made via the S-band

Yes.  Doppler shift for instantaneous velocity, out-and-back delay for
precision ranging, and Doppler measurements exploiting Earth's rotation
for precision direction.  The backup system was optical measurements made
on board, with star sightings for orientation, landmark or horizon
sightings (on Earth or Moon) for position, and velocity determined from
change in position.
When failure is not an option, success  |  Henry Spencer
can get expensive.   -- Peter Stibrany  |      (aka

Index Home About Blog