 The small tracking station in Titusville. Dick Flagg (left) and Wes Greenman. |
 Dick Flagg and radios in the back of the car |
 I open a soda pop can - it's a hot day! |
|
On
November 29, 1972, the day of my visit the moon rocket Apollo 17 was on
its launcher a few hundred meters from the beach of the Atlantic. It
had
been there since August and in the launch control center at the VAB
preparations
were going on for picking up the final countdown the following morning.
I visited a telemetry station where data from the different stages of
the
booster was received and recorded. The station was a large room with
ceiling-high
rack sections for each rocket stage. These racks contained TM-receivers
and discriminators. It was somewhat surprising that rather low
frequencies
were used for this telemetry. Instead of microwave equipment, which one
could have expected, all telemetry frequencies were in the P-band, i.e.
between 200 and 300 MHz.
The receivers were crystal-controlled and had wide-band FM discriminators, The noise figure was often high (10 dB), but this was compensated by low noise antenna pre-amps. From the TM-station data was passed on to meters and indicators in the control center (firing room). In this room a lot of engineers were placed at consoles to read off instruments and watch the condition of the space vehicle. In the future, i.e. when the space shuttle becomes operational, the idea was to reduce this work force by a factor 10 by the use of a computer system that fully monitors and supervises the checkout, launch and flight of a space vehicle.
In the nearby VAB work
was
on going on around the clock the with the next big space project,
Skylab.
On top of the large Saturn-5 booster the work and living section of the
space station was mounted. The airlock and the astronomical observatory
were still missing. At my visit a so called "refrigeration test" was
going
on, i.e. the system that keeps the sunny side of the station cool and
the
shadowy side warm was being tested. Despite the fact that the launch
date
was more than five months distant, intensive activity was going on
around
the cylinder-shaped can that was part of the space station when it was
orbited in May 1973. Skylab used frequencies in the P-band just as the
Saturn 5 booster did. Because Skylab orbited between 50oS
and
50oN its
signals were
audible in Sweden.
I arrived there at
about
11 a.m. with my friends from the Physics and Astronomy Department of
the
University of Florida (Gainesville) Dick Flagg and Wes
Greenman.
We set up a tracking station near a fruit-stand on the beach (see
pictures
below) just south of Titusville which was equipped to receive Apollo 17
on 258.5 MHz (TM from last stage of booster), 259.7 MHz and 296.8 MHz
(up-
and downlink AM voice signals from the spacecraft). This tracking
station
was-- situated about 20 km due west of the launch pad and consisted of
a variety of homemade and surplus units (See
block diagram). Using this equipment we planned to pick up signals
during the launch phase and during two subsequent transits over Florida
before the spacecraft would be propelled out of its parking orbit and
into
a moon-bound trajectory.
|
The small tracking station in Titusville. Dick Flagg (left) and Wes Greenman. |
Dick Flagg and radios in the back of the car |
I open a soda pop can - it's a hot day! |
During the course of the evening, the countdown proceeded smoothly and the spectators in the Space Center viewing area spent the time eating sandwiches and making music in various ways. Others watched the big rocket with large telescopes in which you could almost see the rivets in the tank walls of the huge "bird". As the countdown approached zero everybody's attention was focused on brightly illuminated ivory rocket which stood out starkly against the background of a tropical thunderstorm far away over the Atlantic.
At T~30 seconds (2152 local time) the countdown was halted by the terminal countdown sequencer that controls the final 3 minutes of the countdown. The error was quickly localized to the sequencer itself, and after having simulated the error and its fix in a laboratory in Huntsville, Alabama the problem was solved by applying "jumper wires"!!
The lift-off finally took place 2 hours and 40 minutes late at 00.33 local time (0533 UT) December 7, 1972. The flame of the missile changed the Florida night into day and 15 seconds after lift-off the excited voices of the astronauts were heard on the loudspeakers in the viewing area. (The lift-off picture was taken by Dick Flagg.) Gradually the rocket faded into a little green speck of light that disappeared behind the thunderstorm 6 minutes after launch. Our little Titusville station picked up strong voice signals up to 8 minutes after launch on 259.7 MHz. Thanks to the clear weather the ignition of the second stage was clearly visible from the ground some three minutes after lift-off.
On the next transit the
spacecraft
passed very close to the local horizon and telemetry and voice signals
were picked up weakly between 0343 and 0346 local time (0843-0846 UT).
A few minutes later (0354 local time, 0854 UT)) Apollo 17 was fired out
of earth orbit and changed from VHF transmissions to microwaves
(2287.5MHz).
The attempts to tune Apollo 17 was the first test of the 9 meter dish after its installation at Biven's Arm. The aluminum dish is Air Force surplus equipment. In the focus of the dish a feed antenna is placed to "illuminate" the surface of the parabola. The feed is a 30 cm by 10 cm cylinder and is only sensitive to left hand circular polarized radio waves. Its SWR is 1.2 over a 200 MHz range around 2300 MHz (S-band).
The signal from the feed was supplied to a tunnel diode pre-amp for S-band. This unit, the most critical item of the whole receiving system, had a 3 dB noise figure and enough gain to overcome the cable loss of the inch-thick coax cable that brought the signal from the dish to the listening post in a shed 25 meters away. In this shed the rest of the equipment was located.
The receiver used for tuning the Apollo frequency 2287.5 MHz was a Motorola unit - surplus from the Apollo program itself. Only the local oscillator and the first mixer was used. The IF of the first mixer at 50 MHz was fed to a Vanguard Electronics Labs converted which brought this frequency down to 30 MHz. The final demodulation was done with a Collins R-390 shortwave receiver tuned to 30 MHz.
Voice signals from Apollo are frequency modulated on 1.25 MHz subcarrier, and by sidetuning the Collins receiver 1.25 MHz the voice signals were made available for demodulation. This could be done in two ways. The simplest method was of course slope detection with a 16 kHz bandwidth in the 455 kHz IF of the R-390. The other variant involved taking the 455 kHz IF out of the R-390 and feeding it into a narrow-band FM detector.
The dish can be steered with a 1o accuracy and the beamwidth at 2300 MHz is about 2o. But the Moon subtends only an angle of 0.5o in the sky. Pointing the antenna was therefore somewhat of a problem. Of course one could calculate where the Moon is supposed to be, but fortunately the weather was fine so the Moon was fully visible even during daytime. One person placed himself behind the dish and directed the operator in the shed with hand signals until the antenna was pointing at the Moon. After the signal was acquired the position of the dish had to be corrected towards maximum signal strength every 4 minutes.
On December 10, 1972 we picked up our first signals on S-band. The main carrier was 45 dB over noise and the voice subcarrier was 25 dB over noise. Apollo 17 passed. over the lunar disc between 1722 and 1819.10 local time (2222-2319 UT), and we measured a total Doppler frequency shift of 43 kHz. The next day the lunar module landed on the Moon and at 1518 local time we picked up main carrier and telemetry from the surface of the moon some 80 minutes after touchdown. Unfortunately the astronauts soon changed to low power which prevented us from getting voice signals because of the too low signal-to-noise ratio. The lunar module transmitted on 2282.5 MHz, but we decided to shift back to the frequency of the command module in lunar orbit, i.e. 2287.5 MHz. The lone astronaut Evans was not very talkative except when he just appeared in front of the Moon or just before he disappeared behind it. At such times he changed to high power and on December 11 we could pick up our first voice signals from the Moon. At 1722.00 local time (2222 UT) Ron Evans said: "'Standby three zero" and at l722.30, i.e. 30 seconds later, we abruptly lost the signal as the spacecraft swung, around the edge of the Moon.
The following day, December12, we concentrated on the command module and received strong voice signals on several occasions (Here [10 kB, mp3] you can hear Ron Evans say: "The barber pole it is grey", referring to a barber pole-shaped control panel indicator). We also had an opportunity to test different FM detectors. To our great surprise slope detection worked amazingly well. When using this mode the receiver is slightly detuned so that the FM signal is placed on one of the slopes of the a selectivity curve of the receiver. In this way FM is converted to AM which is heard in the loudspeaker. When using slope detection the R-390 was operated with 16 kHz bandwidth to avoid resetting the main tuning too often because of Doppler shift.
On December 14, 1972 I left Gainesville for Sweden. At Kennedy Airport I watched the astronauts leave the moon on a little pay-TV which you could find on many seats in the air terminal (see video clip below). So-the Apollo program was all over.
It seems so far away, yet so close in time. Thank you Dick for convincing me to come to Florida to see the launch and track the spacecraft!
