Belitopia
Belitopia
Feb 18, 2020
S1E9 – Lunar Communications Network
29 min

We landed on the moon. We built a habitat on the moon.

We are living in earth orbit, and we are living in lunar orbit.

We are living on the far side of the moon, with no visibility to earth...ever.

With four space stations, two lunar bases, and over 35 crewed trips between the earth and the moon, how can we possibly communicate with each other over the long term?

How can we keep all these missions in communications with earth?

The answer, is a communications network that grows and becomes more sophisticated as time goes on. By the end of the Apollo-era, we can communicate over a half million miles without the requirement that we be line of sight with an earth based antenna. This required a sophisticated network of communications satellites and technologies…and a bit of luck.

This is the Apollo Lunar Communications Network. In the world of Belitopia.

Links

As we near the end of season one, we’re going to try a slightly new format for this episode. We aren’t going to use the future documentary format, rather we are going to stay in a current day conversation. In this episode, we’re going to be talking about the fledgling communications network being built in the 1960s, 1970s, and 1980s to support the various Apollo missions we have previously discussed in season 1. We’ve talked about part of this network briefly in episode 7, when we talked about the lunar base on the far side of the moon...the BLA base. But there’s a lot more to that network than you imagine, and a lot more to lunar communications in general than you might think. This network was the first such extra-earthly communications network, and it was developed during the early days of the space race.

Global Earth Dish Network

During the early Apollo days, during our first missions to the moon, one of the initial communications problems that had to be solved was how do you keep the moon-bound Apollo space craft in communications with earth, when the earth keeps rotating. That means, mission control, in Houston, Texas, was only in line of site of the Apollo space craft for relatively short periods of time every day — a few hours at most.

In order for Houston to maintain a 24 hour a day communications with the moon-bound space craft, a series of satellite communications stations were built around the globe. As the earth rotated, different stations around the globe were in line of sight communications with the Apollo spacecraft at different times during the day. These stations were in direct communications with Houston via landline communications channels...essentially phone calls. Each station, when it was their turn, would relay signals between the Apollo space craft and mission control. The result was a virtual 24 hour a day continuous connection between Houston and the Apollo spacecraft.

This was a great start. But as the 1960s moved into the 1970s more and more spacecraft were put into space between the earth and moon. This put a drain on this earth bound satellite network. Plus, the earth bound network required the United States to have facilities at foreign bases around the world, meaning there was a political aspect to maintaining the communications network.

Something better was needed.

Earth Orbit Communications Satellites

The decision was made to invest in a satellite based communications network for lunar communications. This started out as a series of low earth orbit satellites that could communicate with spacecraft on the way to the moon. By utilizing several of these satellites, at any given point in time, at least one satellite and/or a ground station was in line of site of the moon at any given point in time. These satellites relieved pressure on the ground base network, but did not eliminate the need for the ground stations to exist.

Eventually, as satellite technology improved, larger and more complex geosynchronous orbit satellites were put into space. This required a fewer number of such satellites in order to maintain communications with the moon, and it eliminated the need for the ground based network. At least one geosynchronous orbit satellite was in line of site with the moon at all times. They would communicate between each other, and one of them would send a signal back and forth to Houston. The result was a continuous communications network between earth and any moon bound craft, 24 hours a day, without the need for earth based relay stations around the globe.

Near vs Far Side of the Moon

Back in episode 7, which was part 2 of the Lunar Base series, we learned that we built a base, named the BLA base, on the far side of the moon...the side of the moon opposite that of earth.

As you are most likely aware, the moon rotates at the exact same speed as the rate it revolves around the earth. While this may seem like a big coincidence, it actually is pretty common in our solar system...that is to have a moon rotate around its axis at the same rate as the moon rotates around the planet. It’s a phenomenon called tidal lock.

Our moon is in tidal lock with earth. Tidal lock is the reason why we always see the same side of the moon from earth. The same side of the moon is always facing the earth. Therefore, until the space program, no human had ever seen the back side of the moon. While the Russians first took pictures of the back side of the moon from their Luna 3 probe in 1959, the first humans to see it in person were the crew of Apollo 8 as they orbited around the moon in Christmas of 1968. This is the Apollo crew that took the famous picture of the “rising earth” over the lunar horizon. This amazingly popular picture is believed by some to be a major contributor to the start of the global conservation movement.

The BLA base, named after these astronauts from Apollo 8, was the first human establishment built on the far side of the moon.

But being on the far side of the moon, it meant it was not in line of sight with the earth, ever. Hence radio signals could not get from earth to the base, which made it impossible to communicate with the base directly. This generated a huge problem that would have to be solved.

Early Lunar Orbit Satellites

One possible solution to the problem was to put satellites into lunar orbit, just like we did in earth orbit. Eventually, this would happen, but in the early days of the moon program, it was discovered that it was difficult to maintain an orbit around the moon for any significant period of time. Irregularities in the mass of the moon causes any object in lunar orbit to naturally decay and eventually crash into the lunar surface. This was a huge problem. While a solution was found to this problem in time for the Lunar Skylab program to take advantage of a stable lunar orbit, it took awhile for this capability to be discovered, and it was of limited usefulness during the planning and creation of the BLA base.

Another solution was needed.

L4 and L5 Lagrangian Points

That solution involved satellites placed at the Earth-Moon L4 & L5 Lagrangian points.

What are the Lagrangian points? The Lagrangian points are positions relative to the Earth and Moon that provide stable orbits ... stable positions ... where satellites and other objects can exist without having their position degrade and fall into either the earth or the moon. Links to more information about the Lagrangian points and where they are located are in the shownotes.

There are five such points in the earth-moon system. However, two of them are quite useful for our communications purposes...they are the L4 and L5 Lagrangian points.

These points are in an orbit around the earth at the same distance from the earth as the moon is from the earth. They also are the same distance away from the moon as they are from the earth. The earth, moon, and Lagrangian point form an equilateral triangle...that is a triangle with each of the three sides the exact same length. There are two such points, one that is in orbit ahead of the moon, orbiting the earth in front of the moon. The other is in orbit behind the moon...that is orbiting the earth behind the moon.

Satellites could be placed at either of these two locations, and they would remain in that stationary position relative to the earth and the moon. They would be stationary relative to the moon, and would rotate around the earth at a rate equivalent to the rate the moon rotates around the earth, namely once every 27 days.

How would satellites in these orbits appear from either the earth or the moon? From the earth, the satellites in either L4 or L5 would appear to move across the ecliptic at the same speed as the moon...in other words, they would rotate through the zodiac once every 27 days. From the perspective of the moon, they would appear to be stationary in the sky, just like the earth appeared to be stationary in the sky.

Where in the lunar sky they would appear would depend on where you were on the lunar surface. But the key was that one of the two satellites, either L4 or L5, or the earth itself would always be visible from any point on the lunar surface. This made communications satellites at these two positions valuable in communicating with astronauts on the lunar surface.

In the specific case of the BLA base, the base is nearly directly on the opposite side of the moon from the earth. This position meant the satellites would appear very low in the east or western sky...L4 in the eastern sky, L5 in the western sky. These two satellites would be used to communicate between the BLA lunar base and the earth.

If L4 was visible in the eastern sky, and L5 was visible in the western sky, why were both satellites needed? Why couldn’t they have just one satellite at either L4 or L5? Why did they have to have both?

Well, for one thing, as astronauts roamed the surface of the moon, it would be nice to have a satellite at both locations, which would allow more coverage of a greater portion of the lunar surface. This wasn’t a big reason, though, because there was no plans on roaming far enough from the base to make that much difference in the position of the satellites in the sky.

But there was a bigger reason...and that was because of lunar wobble.

You see, when I said that the earth and L4/L5 points were stationary in the sky, I wasn’t quite being accurate. The moon, like most other heavenly bodies, including the earth, have a wobble to them. While they rotate around their North-South axis, their North-South axis also rotates a few degrees. Think of a spinning top. The axis of the top doesn’t stay fixed, it rotates at a slower rate than the rate the top spins, but it does move. The same thing happens with planets and moons. Their north-south axis moves over time.

Earth and Moon Precession

In the case of the earth, this wobble is very very slow. In fact, one rotation of the axis takes around 26,000 years. A very slow rotation. This is called the earth precession, and it results in the North Star...the star the North Pole points towards...changing over the course of thousands of years.

The moon also has a precession...it also wobbles. In the moon case, the wobble is caused by a complex series of gravitation pull changes caused by how the moon rotates around the earth. The earth gravitation pull varies as the moon rotates around the earth, and the moon orbit is not perfectly round. The net result is the moon wobbles rather rapidly and rather significantly.

Because of this wobble, the earth appears to move in the lunar sky in the shape of a series of ellipsis. These ellipsis’ are around 10 degrees to 15 degrees of arc across the sky (that’s about 5%-8% of the total sky from horizon to horizon). The earth would move in this ellipsis over the course of a lunar month...27 days. This is enough wobble that it can be noticed. The earth would appear in different spots relative to the background stars day after day. The wobble is significant.

Links to more information about the lunar wobble and precession in general is contained in the shownotes.

The same thing occurs with the L4 and L5 Lagrangian points. They also move in 10-15 degree arcs over the course of a lunar month. From the BLA lunar base, however, these Lagrangian points are near the horizon. As a result of this wobble, over the course of a month, the Lagrangian points will dip below the horizon, then go above the horizon, in the shape of an ellipse. When L4 is below the horizon, L5 is above the horizon. When L4 is above the horizon, L5 is below the horizon.

This is the main reason why a satellite was needed at both Lagrangian points. Because over the course of a month, at some points one satellite was above the horizon and at other times the other satellite was above the horizon. Only the satellite that was above the horizon could be used to communicate with BLA Base. Hence, a satellite was needed at both locations for continuous communications.

Communications Lag

The moon is approximately 239,000 miles away from the earth. Even at the speed of light, communications messages sent from the earth to the moon would take 1.3 seconds to arrive at their destination. A round trip message from the earth to the moon and back again would take 2.6 seconds. If you were on earth talking to an astronaut on the lunar surface, and the astronaut immediately sent what you said back to you on earth again, you would hear an echo...you’d hear your voice repeat back to you 2.6 seconds after you spoke. This delay made real time communications a bit of a challenge, but it didn’t prevent it from occurring. You just had to realize you had the delay when you were talking to astronauts on the moon, and wait a longer period of time for them to reply to your query.

However, when the signals had to be relayed through the L4 or L5 satellites, there was a greater delay. The distance from the earth to L4 or L5 was 239,000 miles, but then the distance from L4 or L5 to the moon was another 239,000 miles. This meant that the one way communications path from earth to moon via one of the Lagrangian satellites would take 2.6 seconds. A round trip message from earth to the moon and back again, via L4 or L5, would take 5.2 seconds. This was a significant delay and would make real time communications much more difficult. It was a fact of life, however, and this lag would have to be always considered.

It’s the reason why, eventually, a better and more stable set of communications satellites in lunar orbit was preferred. It would remove the long transmission time to and from L4 and L5, which would bring the round trip delay back to 2.6 seconds...much better than the 5.2 seconds caused by the L4/L5 network. But, during the life of the BLA base, this was not to be available.

Lunar Transit Communications

What about communications *during* the trip from the earth to the moon and back again? There were a number of Command-Service modules...CSMs...that were sent back and forth between the earth and the moon, bringing crews back and forth to the lunar surface to the various bases and also to the Lunar Skylab. These LT lunar transportation missions also needed to communicate to both the earth and the moon.

Communications to earth was relatively easy. A high gain antenna was attached to the CSM that was used to send messages to earth. The messages at earth were picked up either by an earth based relay station, or one of the low earth orbit or geosynchronous orbit satellites orbiting around the earth. Replies were sent back to the in-transit CSM the same way.

When the CSM reached lunar orbit, when the CSM was in line of site of earth, it could communicate with the earth. Before the L4-L5 satellites were in place, when the CSM was on the back side of the moon, they were in a blackout and could not communicate with the earth. This was the case during the early Apollo missions. This was important, because the lunar insertion burns and the trans-earth injection burns...these were the burns that put the CSM into lunar orbit and took the CSM out of lunar orbit for the return trip home...both had to occur on the back side of the moon, out of radio contact with earth. This was unfortunate, but a fact of life in the early Apollo days. Creation of the L4-L5 satellite network meant that the CSM could use the L4-L5 network when on the back side of the moon and remain in communications with earth all the time, even during the back side of the moon burns.

This entire process meant that the delay in communications between earth and moon varied. On the way to the moon, the delay was based on how far the CSM was from earth... Once the CSM was in lunar orbit, the delay went back and forth from 2.6 seconds, to 5.2 seconds, depending on whether the CSM was on the front or the back side of the moon.

The same was true for the Lunar Skylab. As it orbited the moon, when it was on the front side of the moon, it’s communications lag with earth was 2.6 seconds. When the Lunar Skylab was on the back side of the moon, the delay was 5.2 seconds as messages were relayed via the L4 or L5 satellites.

All of this certainly complicated communications, but it was still highly valuable to be able to remain in constant communications with earth, even in lunar orbit.

Because of this network, all astronauts anywhere in the earth-moon system...whether they were in transit to/from the moon, in lunar orbit, or literally anywhere on the lunar surface...were always in constant communications with earth.

Venus Flyby Communications

All of this network communications was designed for communicating between earth and various missions, spaceships, and bases at the moon or on the way to the moon.

But what about the Venus Flyby mission? This was the only crewed Apollo mission that went beyond the orbit of the moon. This wasn’t a half a million mile trip, like a round trip mission to the moon involved. This was a 60 million mile round trip mission to and around the planet Venus.

At 20-30 million miles away, communications was a bit harder. Communications lag was not measured in seconds, but in minutes. When the mission was near Venus, the round trip delay was around 200 seconds, nearly 3 and 1/2 minutes. This meant that realtime interactive communications was not possible at all. Communications was more like exchanging emails, rather then using a telephone.

Additionally, signal strength was an issue. The Venus Flyby vehicle had a large and powerful antenna for communicating with earth. But still the...

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