Doppler radar trap

The measurement of radial velocity relied on the Doppler effect, whereby the movement of a transmitter altered the received pitch or frequency of whatever wave was transmitting. This is familiar to most people as the change in pitch of a constant note, perhaps from an engine or a horn, as it passes the listener. We hear its pitch drop as it passes by and departs. The same effect applies to radio transmissions where the frequency of the received signal changes slightly according to whether the transmitter is approaching or receding from the receiver. This frequency can be accurately measured and, by knowing the precise frequency with which it was transmitted, the velocity of the transmitter can be calculated. The system is similar in some respects to radar speed traps used to catch speeding car drivers.

To use the Doppler effect well, the frequency of the transmitted signal from the spacecraft had to be known with great precision. The equipment to generate a signal of sufficient accuracy - one whose frequency was precise and stable despite the thermal extremes of space, would have been excessively heavy and power-hungry. However, an elegant solution existed that kept the heavy equipment on the ground, yet could yield a measurement that was more accurate than that attainable by having the generating equipment on board. Instead, engineers simply arranged for the signal that carried voice and data from Earth to be modified in the spacecraft in a known way, and retransmitted back to the ground, this time carrying information from the spacecraft. If the frequency of the signal from Earth was precisely known, then so was that from the spacecraft.

For Apollo, the ground station transmitted data and voice signals from mission control to the spacecraft on a carrier signal called the uplink. This carrier was synthesized from a very accurate frequency standard installed at the station. Ground stations supporting the Apollo programme had some of the most accurate frequency standards available at the time. For the CSM, the carrier had a frequency of 2,105.40625 MHz while that for the LM was 2,101.802 MHz. On reception by the spacecraft antenna, an onboard transponder took this signal, multiplied its frequency by the ratio of 240/221 (about 1.086) and sent it back to Earth, using this new signal as the carrier for the downlink.

When received by the ground station, the precise frequency of the downlink was measured and compared to the uplink. If the precise 240/221 relationship was maintained, the spacecraft was neither approaching nor moving away from the ground station, such as when moving across the face of the Moon. A higher received frequency meant that the spacecraft was approaching; a lower frequency indicated receding motion. This was a very powerful system because it measured Doppler shift over both the up and down legs of the signal's journey, doubling the sensitivity of the system to the point where it could even detect the velocity change caused by the minuscule thrust that was generated when the crew dumped their urine overboard.

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