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10 OUR ANCESTORS CAME FROM OUTER SPACE
The navigation system was not complicated either. A platform
was stabilized by three gyroscopes supporting a sextant and a
telescope and was connected to an electric computer in permanent
contact with earth.
It was enough to turn either the telescope
or the sextant and take aim of certain points on the moon s
surface or some star—Canopus, for example—and the computer
would transmit the exact angles of the sightings with the three
axes of the stabilized platform to earth with all the necessary
information.
The distance from earth or moon was measured simply by taking
the angular reading of the moon s disk or the two sides of the
earth. To make these readings, the capsule had to be moved on
all three principal axes, and this was achieved by firing small
rockets placed all around the service module. To avoid overheating,
Apollo had to be rotated constantly so that one side would
not be exposed all the time to the sun.
What were the means of communication between Apollo and
the earth? At close distances the exact position
of Apollo was
measured by tracking radar from earth in the C band between
5,715 and 5,815 MHz (megahertz, a unit of frequency). The
radar signal was received and amplified by a transponder and
retransmitted by Apollo back to earth. The coded messages from
Houston to Apollo were transmitted in ultrahigh frequencies
(UHF) on the 450-MHz band, in one direction only. Voice and
telemetry were carried on very high frequencies (VHF) on the
259- and 296-MHz bands.
When Apollo arrived within proximity of the moon, the commimications
carriers previously used could not reach that far so
all communications went through one single, very powerful,
transmitter with a directional antenna in the S band, between
2,106 and 2,287 MHz, with a great niunber of channels, each
transmitting several signals at the same time through multiplexing.
For instance, there were seven channels to feed medical information
about the physical condition of the astronauts, nine to
restransmit the stored telemetry data from the passage behind the
moon that could not be beamed directly. The communication
systems were improved from one Apollo mission to the next,
especially the TV quality.
Today the Apollo program has long been terminated and