Nixon Apollo 11 phone call

External media

Nixon's Apollo 11 phone call

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Wikimedia Commons • Video (1:12) - President Nixon speaks to Apollo 11 crew

 Apollo 11 Moonwalk Part 2 of 4 • Call starts at 8:37

Apollo Lunar Surface Journal • Audio clip

V • T • E

The Nixon Apollo 11 phone call was the conversation on the evening of 20 July 1969 between President Richard Nixon, in the Oval Office, and Neil Armstrong and Buzz Aldrin at Tranquility Base. Houston time for the start of Nixon's first sentence is 21 July 1969, 03:48:30 UTC, ground elapsed time 110:16:30. He called the room the Oval Room. CapCom Bruce McCandless II in Houston put the phone onto the air-to-ground loop. Nixon's own voice went up. McCandless did not read the speech for him.

A cut of the CBS split-screen broadcast times four silences with an on-screen clock. The second clock reads 7.20 seconds. That clock starts when Nixon stops on the word "Earth" and stops when Armstrong starts "Thank you, Mr. President."

CBS split screen during the call. The second clock, 7.20 seconds, is the pause after Nixon's speech.

The four clocks

The clocks are burned into the picture and left on screen after each pause ends. In order:

Clock	Seconds	Who had just stopped	Who starts next
1	6.28	McCandless, ending "Neil and Buzz, the President of the United States is in his office now and would like to say a few words to you. Over."	Armstrong: "That would be an honor."
2	7.20	Nixon, ending "one in our prayers that you will return safely to Earth."	Armstrong: "Thank you, Mr. President."
3	3.44	McCandless, calling the command module: "Columbia. Columbia. This is Houston. Over."	Michael Collins: "Loud and clear, Houston."
4	9.68	McCandless: "Roger. I got a P22 Auto optics PAD for you."	Collins: "Roger. Go ahead."

Clocks 1 and 2 are the surface crew. Clocks 3 and 4 are Collins in lunar orbit, after the presidential call, on the same Earth-Moon distance. The Apollo Lunar Surface Journal gives the same four handoffs at about 110:16:16, 110:17:36, 110:18:34, and 110:18:44.

All four clocks at the end of the cut: 6.28 s, 7.20 s, 3.44 s, 9.68 s.

On the Houston Public Affairs recording of the same night, the Nixon pause measured from the last sound of "Earth" to the first sound of "Thank you" is 7.55 seconds. The on-screen clock reads 7.20. Same hole. The clock was started a third of a second later.

What the silence is

On the Houston mix, and on the CBS program that took that mix, Nixon and McCandless are local. Their voices have not been to the Moon. The crew is heard only when the reply has come back. A silence after an Earth speaker, before a lunar speaker, is one round trip plus however long the man on the Moon waited before he talked.

A silence after the crew, before Houston or Nixon talks again, has already paid the downlink. Nixon can answer as fast as a person in the Oval Office can draw breath. A quick line from Nixon is a man responding. It says nothing about the distance.

The 16 mm film frame of Aldrin's salute is at about 110:17:54, during Armstrong's thank-you, after the 7.20 second pause had already ended.

The circuit

The voice path that night, each way:

1. White House Communications Agency phone in the Oval Office.
2. Bell System long distance to the Manned Spacecraft Center, Houston.
3. A phone patch onto the air-ground voice loop. McCandless says "Go ahead, Mr. President. This is Houston. Out." Then Nixon talks.
4. NASA Communications Network (NASCOM) from Houston to Goddard Space Flight Center, then on to Goldstone, California. Voice-bandwidth landline and microwave. No lunar hop on this leg, and no Pacific satellite.
5. Goldstone unified S-band uplink to the lunar module. Uplink voice rides a 30 kHz subcarrier on the phase-modulated carrier. The voice channel is low-pass limited to 3 kHz.
6. The lunar module relays the voice on VHF to the backpack radios. That hop is meters.

The reply runs the same chain backward. Downlink voice, during the moonwalk, rides the 1.25 MHz subcarrier. Goldstone had this voice circuit for the whole EVA. The uplink handover from Goldstone to Honeysuckle Creek was at 05:42 UTC, ground elapsed time 112:10, about two hours after the call. Honeysuckle's copy of the Net 1 recording has the call on it, marked as Goldstone audio.

The television of the astronauts, by the time of the call, was the Parkes 64 m dish. Houston settled on Parkes about nine minutes into the EVA and stayed there. Those pictures were scan-converted in Sydney and sent to Houston. The voice on the program was still the Goldstone downlink. A satellite hop, had one been in the voice path, would have added about a quarter of a second. The Honeysuckle Creek and Pacific-satellite description is the television route.

NASCOM, as described in the 1965 Apollo Unified S-Band conference (NASA SP-87, Dickinson), is voice-bandwidth channels on landline, microwave, submarine cable, and HF, routed site to Goddard to Houston. The 600, 1200, and 2400 bit/s figures in that paper are data circuits. They are not a multi-second delay on the voice channel. The station clocks were kept to about a millisecond (Granata, in the same volume). That is how well the clocks agreed. It is not time added to the conversation.

Distance, and the round trip

The number people reach for is "about 2.6 seconds." This is that number, from the distance at this hour, with the station and the landing site put back in.

1. The time. Launch was 16 July 1969, 13:32:00 UTC. Ground elapsed time 110:16:30 is 4 days, 14 hours, 16 minutes, and 30 seconds later, which is 21 July 1969, 03:48:30 UTC. The 7.20 second pause is about one minute after that. The Moon was approaching Earth at 58 m/s, so that minute shortens the one-way light time by 0.01 milliseconds. Dropped from here on.

2. Center to center. JPL DE441, through Horizons, geocentric range of the Moon at that instant:

0.00259342948790 astronomical units.

The astronomical unit used by Horizons is 149,597,870.700 km, so

0.00259342948790 × 149,597,870.700 = 387,971.5 km.

The speed of light is 299,792.458 km/s, so the one-way light time from the center of the Earth to the center of the Moon is

387,971.5 / 299,792.458 = 1.29413 seconds.

The round trip on that same line is 2.58827 seconds. Horizons reports the one-way downlink light time as 0.02156890 minutes, which is the same 1.29413 seconds.

3. The lunar module is on the surface, not at the center. The sub-Earth point at 03:48:30 UTC was longitude 352.71° (7.29° west), latitude 2.14° north. Tranquility Base is 0.674° north, 23.473° east. The angle between them is 30.79°. With a lunar radius of 1,737.4 km, the distance from the Earth's center to the landing site is

sqrt(387,971.5² + 1,737.4² − 2 × 387,971.5 × 1,737.4 × cos 30.79°) = 386,480 km.

The site is 1,491 km closer than the Moon's center. That is 4.97 milliseconds each way, 9.95 milliseconds on the round trip.

4. Goldstone is on the surface, not at the center. At 03:48:30 UTC, from the Goldstone complex (35.426° N, 116.890° W), the Moon was at azimuth 237.9°, elevation 27.3°. The range from that point to the Moon's center was

0.00257357275460 × 149,597,870.700 = 385,001.0 km,

which is 1.28423 seconds one way. The dish is 2,971 km closer than the Earth's center along the line of sight, because the Moon is 27° up. A dish a few kilometers away at the same complex, which is the scale of the difference between the 26 m Apollo antenna and the coordinates used here, moves the light time by about 0.01 milliseconds.

5. Goldstone to the lunar module. Take the 1,491 km surface correction off the Goldstone range. The line of sight from Goldstone is within a degree of the geocentric line, and the error in that correction is about 15 km, or 0.05 milliseconds. The radio path is

385,001 − 1,491 = 383,510 km,

383,510 / 299,792.458 = 1.27925 seconds one way,

2 × 1.27925 = 2.5585 seconds round trip.

Collins, in orbit, was about 110 km above the surface. On the near side that adds under a millisecond to the one-way time. The same 2.56 second round trip is the radio part of his two clocks.

The phone, and the equipment

The 2.5585 seconds is the space leg, Goldstone to the crew and back. The timed silence on the Houston recording also includes the ground legs that sit between Houston and Goldstone, both ways, and the short hops inside the lunar module. It does not include the White House phone. Nixon's voice is already in Houston when the clock starts, and Armstrong is heard when his voice arrives back in Houston.

Leg	Distance	One way	Round trip, inside the timed pause
Oval Office to Houston, great circle	1,953 km	6.5 ms at the speed of light	Outside the pause. A 1969 long-distance voice circuit, microwave and switching included, is still well under 50 ms each way.
Houston to Goddard, great circle	1,972 km	6.6 ms	13 ms
Goddard to Goldstone, great circle	3,539 km	11.8 ms	24 ms
S-band voice filter, 3 kHz low pass, and the modulator		a few milliseconds	under 10 ms
Lunar module VHF relay to the backpack	meters	under 1 ms of propagation	under 10 ms, allowing the analog relay
Goldstone to the lunar module and back	383,510 km each way	1.27925 s	2.5585 s

The ground legs above are great-circle light times, which is the floor. Coax is slower than free space, and the real route is longer than the great circle. Stretch both by half and the Houston-Goddard-Goldstone round trip is still under a tenth of a second. Adding the filter and the backpack relay does not get it to a tenth of a second either.

So the path that has to finish before Armstrong can be heard is:

2.559 seconds of space + under 0.10 seconds of wire, microwave, and gear = under 2.66 seconds.

What the clocks are made of

Subtract a 2.56 second radio round trip:

Clock	On screen	After 2.559 s of radio	What the remainder is
1	6.28 s	3.72 s	Armstrong, after "Over," before "That would be an honor."
2	7.20 s	4.64 s	Armstrong, after Nixon's speech, before "Thank you, Mr. President."
3	3.44 s	0.88 s	Collins answering a call he was waiting on.
4	9.68 s	7.12 s	Collins, after the offer of the P22 pad. The surface journal marks that gap "(Pause)."

Giving the wires a full tenth of a second trims each of those remainders by 0.1 s. The Nixon clock is still four and a half seconds of a man waiting until the speech is over.

The shortest surface answer is not one of the four clocks. On the Houston Public Affairs tape, Nixon's line about the recovery ship Hornet ends and Aldrin starts 4.34 seconds later: "I look forward to that very much, sir." That is 2.56 seconds of radio and about 1.8 seconds of Aldrin answering. On the CBS aircheck Walter Cronkite talks across that reply, so the on-screen timer never marks it.

Sources

• Apollo Lunar Surface Journal, Apollo 11 mobility, the call at 110:16 to 110:19: NASA host, journal mirror. The Public Affairs audio is a11a1101216-1105300.mp3, t = 0 at 110:12:16.
• NASA SP-87, Proceedings of the Apollo Unified S-Band Technical Conference, Goddard, 1965. Varson, functional description, Figure 2 (uplink 30 kHz voice subcarrier, downlink 1.25 MHz). Dickinson, NASCOM, pp. 269-274. Granata, station time. Scan: honeysucklecreek.net.
• Colin Mackellar, Honeysuckle Creek, Apollo 11 audio: Goldstone carried Net 1 through the EVA, handover at 05:42 UTC. apollo11_audio.html.
• Parkes and the television path, including the note that Houston did not add a six-second delay on the return leg: CSIRO.
• Range and light time: JPL Horizons, DE441, target 301, 21 July 1969 03:48:30 UTC. Geocentric, and a topocentric site at 35.426° N, 116.890° W, 1.002 km.
• The four on-screen times are from the CBS cut with the delay clocks burned in. CBS broadcast of the call: the short clip, and the moonwalk aircheck Apollo 11 Moon Walk CBS News Coverage, the call at about 2:02:00.

See also

• Moon landings
• Buzz Aldrin