Houston we have a�hello? hello?
Satellites around the Moon and mobile basestations on its surface may be necessary to provide communications for missions to the Moon and Mars; E&T explores.
Of the many engineering disciplines that mankind had to master before Neil Armstrong could take his famous "one small step" on to the Moon 40 years ago, few were as critical as radio communications.
Mercury and Gemini, two human space-flight programmes that NASA conceived as the precursors of Apollo, had left the agency with a communications network that could track and communicate with spacecraft operating in low Earth orbits.
The Manned Space Flight Network (MSFN), as it was known, consisted of a series of ground stations (radars and telemetry antennas) around the world, managed from the Mission Control Center in Houston, Texas. Up to 15 of these ground stations were needed, because each could only have line of sight to an orbiting Mercury or Gemini spacecraft for a maximum of 10 minutes.
With 45 years' experience designing communications systems for NASA's manned space missions, Oron Schmidt, a communications engineer based at the Johnson Space Center in Houston, has seen it all - from those early spacewalks just a few kilometres outside the atmosphere, to the Moon landings, the space shuttle and the International Space Station (ISS).
"When I first came to work here in June of 1964 we were in the middle of the Gemini programme," he recalls. "At that time, we were still using some of the basic communications techniques that we had established for Mercury. This consisted of communicating voice, telemetry, biomed, and suchlike through primarily VHF [very high frequency] radios. For Gemini we added some digital techniques, especially for the telemetry; we were still at VHF but we were moving into UHF [ultra high frequency]."
When the time came to design the communications architecture for the more complex Apollo programme, it was clear that the MSFN was going to need major re-engineering to cope with lunar distances. At 384,000km away from Earth, it needed more powerful transmitters, operating at higher frequencies and using larger parabolic antennas to establish and maintain a reliable connection.
The up side is that you don't need so many ground stations: three spaced equally around the planet are enough to compensate for the Earth's rotation and maintain near-constant line of sight (especially to the Moon's equator, which is why all the Apollo landings took place in equatorial sites).
Upgrading the Deep Space Network
NASA already had experience in this type of configuration. By 1969, the Deep Space Network run by the Jet Propulsion Laboratory had been operational for a few years, mainly to acquire data from, track and send command sequences to space probes, such as Mariner 4, on their interplanetary journeys.
The Deep Space Network's three main sites (in Australia, Spain and California) were each equipped with a large, 26m dish antenna. Close to each of these sites, NASA built a second set of 26m antennas - only in this case they were attached to the MSFN.
While the conventional, 9m dish antennas of the MSFN were still useful to provide communications during the Earth-orbital and post-injection phases of Apollo missions, the larger antennas provided the radio links between Houston and both the Lunar Lander and the lunar-orbiting spacecraft.
As for the required increase in radio frequency, "we decided to use S-band communications," says Schmidt. "We developed the 'Unified S-Band System', where we had everything modulated on S-band [around 2.2GHz], either on the baseband or on the subcarriers."
The system allowed for voice, television, command, tracking and ranging to be transmitted from a single antenna.
VHF and UHF frequencies still had a role to play on the surface of the Moon, providing communications between the Lunar Lander and the astronauts whenever they ventured out for a moonwalk.
MSFN gets replaced
Where to next, then? This might have seemed like a perfectly reasonable question back in 1969. Surely a Mars landing was now only a question of a couple of decades at most away?
Four decades and one Space Race later, we know the answer. Not only have human beings still been unable to land on Mars, they haven't ventured more than a few hundred kilometres above sea level since 1972.
That leaves the engineers responsible for supplying the current crop of astronauts with adequate communications support with only the near-Earth environment to worry about - just like in the Mercury and Gemini times.
This time around, though, the MSFN is gone. In its place, NASA has deployed a constellation of nine geostationary communication satellites called the Tracking and Data Relay Satellite System (TDRSS). Apart from increasing the amount of time during which low-Earth orbiting spacecraft can communicate with the ground, TDRSS has been designed to handle much greater bandwidths.
"On the space station we started out at 50Mbit/s, which was about the same data rate that the space shuttle could sustain," says Schmidt. "Then, four or five years ago, we went up to 150Mbit/s, and we have plans to go to 300Mbit/s."
Such generous data rates explain the surprisingly high quality of those frequent live TV interviews with the station's crew members. "Most of the available bandwidth is currently gobbled up by television," says Schmidt. "That's because our video processing equipment currently on the station is extremely inefficient. It can't compress video so, for good quality, it can easily consume 30 or 40 Mbit/s."
The video equipment is not there just for TV interviews. The cameras onboard the ISS are used by NASA staff on the ground for operational purposes and situational awareness. Whenever astronauts perform a spacewalk, for example, their helmets are equipped with an NTSC camera that lets colleagues inside the spacecraft and controllers back in Houston monitor what they are looking at.
As part of the celebrations of the tenth anniversary of the ISS in orbit, NASA is currently offering a rare, 24-hour live video webcast shot onboard the platform. Some of the Earth views are spectacular. Since the station's average speed of over 27,000 km/h means it orbits the Earth once every 90 minutes, a sunrise or a sunset can be witnessed every 45 minutes. To have a look for yourself, go to www.nasa.gov/ntv and select the 'Live Space Station Video' channel.
Constellation mission demands new network
Mankind will one day return to the Moon - and then it will set foot on Mars. That at least is the ambition of Constellation, NASA's next human spaceflight programme.
While the first Constellation lunar landing is not likely to occur before 2020 - and no one at NASA dares put a date on the first Martian mission - much of the design work is already underway.
Unlike Apollo missions, in which astronauts stayed on the surface of the Moon for a maximum of three days, future lunar expeditions will last several months, and may lead to the creation of a permanent outpost at the South Pole.
The main advantage of building a lunar outpost there is that it would guarantee continuous sunlight. The flipside is that the Deep Space Network (which will act as the Earth's termination point for a lunar trunk) will only be viewable for eight or ten days at a time, before disappearing for the next eight or ten days as the Earth precesses on the horizon.
Add to that the fact that Constellation calls for a very comprehensive geographical exploration of the Moon (including its dark side, which never sees the Earth), and you can understand why Schmidt and his team are hoping that plans for what they call the Lunar Relay Satellite part of their space communications architecture are approved.
"We need at least two satellites orbiting the Moon to give us around 80 per cent coverage," explains the veteran engineer. If they get the go-ahead for these lunar satellites, they would use them to relay traffic from the lunar landers, rovers, habitats and crewmen back to Earth using powerful Ka-band channels supporting data rates of hundreds of Mbit/s.
Why might NASA balk at the deployment of such versatile satellites?
"We are quite concerned about their cost," Schmidt admits, "as to whether or not we can have them right away when we have manned lunar landings again in the 2020s. We're planning to see if we can work without them, and then have them phased in later, once we have our surface-to-surface communications well established."
The surface infrastructure used on the Moon and Mars will need to be sorted before any traffic can be sent back to Earth. "We'll use what we call 'communications terminals', which are either permanently located on the ground or carried around by the rovers," Schmidt explains.
"It is a very similar concept to the cellphone infrastructure that we have here on Earth, where we have a single tower covering an 8km radius area. We would have similar areas of coverage on the Moon. We would probably carry the towers as high as we could. There wouldn't be as much problem with wind or anything like that to worry about, and the gravity would be one-sixth of what we have here. Still, the logistics of getting a tower up 15m or 20m is pretty serious."
One of the many communications scenarios his team envisions involves fleets of up to four manned rovers (one crewman per rover) setting out on 14-day excursions to places hundreds of kilometres away from the outpost, with the astronauts bringing their habitats with them.
"Communications for those kinds of scenarios are quite challenging," he says. "The EVA crewmen are rarely more than a kilometre away from the rovers. But the rovers can be separated up to 20km or more and still communicate. There's a requirement for 100 per cent communication coverage between the rovers. This way, if one rover gets in trouble, it can always communicate its location and status to the other one. That is going to be a challenge."
The other challenge is getting used to the idea that, as mankind continues to expand its presence throughout the solar system, not even the most powerful, expensive and perfectly designed and manufactured radio systems will let astronauts enjoy real-time communications with Earth.
According to Schmidt, the travel time for radio signals to Mars, which is 54 million kilometres away, can be up to 40 minutes. And, while that figure could one day be reduced with the use of optical communications, there are physical laws that will inevitably conspire against real-time data exchange at these and longer distances.
"That's why, on the Moon, we're not following the Apollo model where everything was controlled from the Earth," he says. Just as NASA currently does with its rovers on Mars - where the robots are allowed to operate with a significant degree of autonomy - so will astronauts on the Moon, Mars and beyond, coping with whatever situations they encounter.
"This isn't a choice. Sometimes it's just a situation that occurs when you can't communicate with the Earth for a lot of reasons for a long time. We're saying: all right, we're going to design everything to accommodate loss of communications for a long time and to have no communications with the Control Center," says Schmidt.
"That means future crews will have to be autonomous, have procedures and training to operate and to perform the mission without the Control Center looking over their shoulders all the time."
Then again, that's not a prospect likely to scare a crew who will step onto a Mars-bound Orion spacecraft knowing that it will take them six months just to get there.