In the early days through at least Apollo 11, ARIA used S-Band downlink for telemetry and voice, uplink for voice. Contact with Houston was through HF SSB both upper and lower sideband. Sometime around Apollo 12, I can’t remember before or after, we started working with the military Tactical Satellite (TACSAT). By Apollo 14, we were using TACSAT communications to drive the biomed computers at Houston. I seem to remember that the data loss was something on the order of 1 bit in 1 billion. Pretty pure. Without GPS, the exact location of and ARIA was never precise enough to relay ground track positioning data. I don’t remember where the TACSAT signal was brought to ground and fed to Houston.
Acquiring the Apollo spacecraft was probably quite a bit easier for ARIA than the ground stations. As I remember it, the ground station antennas had a 1/2 degree S-Band beam width. The ARIA had a 1 degree beam width but we also had an additional distinct advantage. With only a 7 1/2 ft antenna, we could sweep the horizon over a 80 degree horizontal arc (40 degrees either side of center) for the signal.
In addition to the S-Band antenna our dish had four VHF antennas, one in each quadrant, used for signal acquisition. When the antenna was initially unstowed for tracking, the antenna operator would control it manually to acquire the spacecraft. As soon as a VHF signal was acquired the dish would automatically seek to equalize the signal strength from each of the VHF antennas thereby “homing in” on the S-Band signal (either 2272.5 or 2287.5 MHz). The antenna operator would put the antenna in AutoTrack Mode starting the gyroscopes that would allow the antenna to continuously track the spacecraft even if there were momentary signal losses due to aircraft motion, atmospheric interference or other causes.
The antenna with the 40 degree VHF beam width sweeping capability was put to good use a couple of times allowing us to obtain telemetry data that we might not have otherwise been able to receive.
Each aircraft had the equivalent of a small telephone book of computer generated “look angles”, i.e., antenna azimuth and elevation at 10-second intervals, for each orbital pass it was supposed to cover based on a nominal flight plan (a neat trick given the ground tracks and altitudes of the aircraft and spacecraft at a given point in time over a specific lat/long on the globe.) They had to have the nominal look angles on board the aircraft in the event they had no contact with ARIA Control given the vagaries of HF communications.
Under normal circumstances we, in ARIA Control, would start receiving a real-time Look Angle update from the Houston tracking computers for a specific ARIA at H-46 (H- “Horizon Time”, AOS (Acquisition of Signal) – remember that VHF and S-Band are both line-of-sight). We would have the message complete by H-42 minutes, then it would be reviewed by ARIA Control support staff and put on the 100 word/min teletype to the ARIA by H-36. The reason I remember the times so well is that in addition to monitoring ARIA status I was also responsible to for maintaining a checklist for each aircraft for each pass to insure that their pre-pass checklist was complete prior to turning them over to NASA (the term we used was “Go Remote”) and the MSFN for the duration of the pass – at 30,000 feet, generally 10 minutes.
As a matter of interest, my checklist for each ARIA had about twelve items beginning at H-50 (when we requested the look angles from NASA) to H+14 when the aircraft, after LOS, would give us a “Quick Look” as to their estimate of the quality of the telemetry data received during the pass; 64 minutes of a 90 minute orbit. Now imagine maintaining individual checklists on four, five or six or more aircraft spaced 5, 8, 10, 15 or 30 minutes apart around the globe – all at the same time – particularly when more than one aircraft would be supporting two successive orbits. To be politically incorrect, a one-armed paper hanger had nothing on me. It took a lot of concentration and a lot of focus. That is why we would spend about 100 hours of sim time between each mission.
An example of ARIA’s VHF sweep capability saving the day, and NASA’s butt in the process, occurred on Apollo 5 –
The mission profile called for the LM to fire the Descent Propulsion engine over Carnarvon on Rev 3. Well, the engine didn’t fire. So all of the pre-planned look angles for tracking the LM over the Atlantic Ocean went to hell in a handbasket.
We had two aircraft, ARIA 4 flying from Dakar, Senegal and ARIA 5 flying out of Johannesburg, South Africa over the South Atlantic. So the immediate focus was to get whatever telemetry they could from the LM on Rev 4. But where to point the antenna? The last position data to the tracking computers had come from Carnarvon. Because of the 1/2 degree beam width on the NASA ground antennas, they never saw the LM again.
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Chris Kraft and Robert Gilruth watch procedings from the back row of the Mission Operations Control Room in Houston during Apollo 5, 22nd January 1968.
NASA image S68-18733.
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Finally, Stan Davis, our ARIA Liaison at Houston for the mission called ARIA Control and gave us some “best guestimate” coordinates for a possible ground track. It required ARIA 4 flying easterly off the West Coast of Africa to do a 135 degree turn and fly to the Northwest. Even then he would be almost 1,000 miles off the ground track (instead of the normal 100 mi.). But, thanks to the VHF acquisition, he still managed to obtain about 6 minutes of S-Band telemetry. Coupled with the 10 minutes of data from ARIA 5 downrange, NASA had 16 minutes of LM telemetry to figure out what went wrong. Interestingly, ARIA 5, being at the most southerly portion of the orbit, had almost no course corrections to make to acquire the LM.
Prophetically, we had simulated almost the identical scenario with “ARIA 4” prior to the mission, and the ARIA 4 crew was in the back of ARIA Control watching the sim at the time! So rather that simply following blind directions, they could actually visualize what the situation was and knew exactly how to respond.
Arguably, without the 16 minutes of ARIA data, the LM descent propulsion system test would have either had to be flown again (jeopardizing the timeline for all future launches) or crammed into a later mission. So, yes, ARIA’s VHF sweep capability proved to be worth more than its weight in gold.
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Click the image to download a 950kb PDF file.
“The ARIA personnel are to be congratulated for a job well done for their normal support and in acquiring the spacecraft when nominal pointing data was not available.”
“At approximately 4 hours in the mission, the first DPS ignition occurred. An automatic shutdown occurred prior to a large thrust build up. This no-burn contingency was passed to ARIA control for information and action. ARIA 3 was released from further support at 0254Z.
ARIA Control plotted the approximate trajectory by hand from which the TSP, look angles and times were determined for ARIA 4 and 5. ARIA 4 acquired at 04/56/59 with an LOS at 05/03/54.
Two-way lock was achieved even though the maximum elevation angle during the pass was only two degrees above the horizon. ARIA 5 acquired at 05/03/41 with an LOS at 05/11/29. Two-way lock and solid track was achieved.”
Excerpt from the Network Controller’s Mission Report for Apollo 5 written by Network Controller Captain George Ojalehto.
Scan by Bill Wood.
Relevant pages extracted by Stan Anderson.
(Full 3MB Mission Report here.)
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