Would you need just gigantic antennas and very high power transmitters? I get worse cell reception in buildings under bridges etc. how can these radio wave travel hundreds of millions of miles through asteroids and various space debri?
I’m assuming you’re thinking about New Horizons (even if you’re asking in a more general sense)?
This page here gives some good information on the comms system it uses. Essentially, the craft has a 2 metre dish antenna to send data back to Earth, which is picked up by huge (70 metre) antenna back here.
As for why that’s better than your cell reception:
- Your cell reception is mixed in with thousands of other signals - other phones, wifi connections, etc.
- The New Horizons antenna is highly directional - this means it’s pushing data (and concentrating its power) along a single point, rather than scattering it about in every direction, like a cell antenna does.
- The space between the probe and Earth is literally that - space. There’s little in the way of atmospheric disruption (at least until it gets to Earth), the chances of asteroids or debris getting in the way are very slim. Even if you account for a few bits of dust and whatnot, there’s still a lot less material between the two antennas than you’d get standing under a bridge.
- This isn’t exactly broadband speeds - about 1kb/sec. That’s about 1000 times slower than even a crappy 3G signal.
Space is very empty. Space is so empty that it’s very rare to see a star being obscured by an asteroid or space debris.
It’s the distance that makes it difficult to communicate - because radio signals spread out over distance. The ways to mitigate this are:
[ul]
[li]Use as high power transmitter as possible.[/li][li]Use a large antenna on the probe, to direct as much of the radio wave towards Earth as possible. The antenna is the biggest thing on the New Horizons probe.[/li][li]Use a large antenna on the ground, to collect as much of the radio wave as possible. NASA uses the Deep Space Network, which consists of large (up to 230 ft diameter) antennas at multiple locations on earth.[/li][li]Low data rate. The New Horizons probe only transmits at 600 bits per second. (It’s akin to using long exposures on a camera - the weaker the signal, the longer you need to average the signal to detect each bit.[/li][/ul]
Would using a high frequency not also make the beamwidth smaller for a given antenna size, thereby increasing antenna gain? (this would increase the directionality Badger mentions, I think)
You’d still need to get through the atmosphere, and only certain frequencies will do that, specifically radio waves and visible light. Anything else would just be absorbed and not reach the receiver on Earth.
scr4 has done a good job of succinctly summarizing how the deep space communication using the DSN works. By large measure, communication works through low data rate (the bits are basically stretched out to reduce the influence of statistically random noise) and the use of parity bits and other error checking to confirm signal integrity. However, it is important to note that there are several significant limitations to the DSN. First, of course it can’t communicate with spacecraft when they’re in or near opposition due to the Sun. This hasn’t typically been a problem because for the most part missions are planned such that major activies that have to be directed by mission control are scheduled when the Earth and target can see one another, but would be an issue for a crewed mission or remote spacecraft intended to operate on the far side of the Sun. Another is that the limited bandrate means it can only support a certain amount of data at any given time. Depending on the direction of various missions it may be possible to schedule usage to optimize use of all facilities, but it is easy to imagine a scenario with a crewed mission or multiple robotic missions that would overwhelm the very limited data rate. And the equipment itself is largely obsolescent, with highly specialized electronic amplifiers that are purpose built for this application. This means the system is costly to maintain or repair. It also isn’t really suitable to support smaller, less powerful transmitters that will be found in future, lower cost smallsat class probes. (There are a number of proposals for using 6U and 12U CubeSats for lunar and even interplanetary missions; such systems will obviously be power-limited, not only by the small possible size of a stowable solar collector but also by the ability to radiate waste heat, and so will have to have much lower powered communications systems.)
For near Earth satellites, much of the communication throughput has gone from ground-based receiving stations to the use of the orbiting NASA Tracking and Data Relay Satellite System (TRDSS or “T-Dress”), or for commercial and some small government satellites, Globalstar and ORBCOMM. It has been informally proposed that a similar type of system but in solar orbit be developed to support both communications and high precision navigation for future space exploration efforts; sort of a combination TDRSS and GPS for interplanetary spacecraft. In an informal proposal I worked on we called it the Planetary Telemetry and Positioning System (PlaTePoS…sound it out) and it would have had a constellation of five satellites orbiting at a perihelion of about 0.85 AU and aphelion of about 1.3 AU, with moderate inclinations to the plane of the solar system, staggered to assure that at least two and typically three satellites were accessible to any any spacecraft with a ~6W low rate X-band transmitter out to the orbit of Saturn (by comparison, the Apollo S-band trasmitter took 20W to transmit from Luna to Earth, although to be fair it was transmitting analog video and audio as well as various telemetry channels). The system could have also served as part of a Near Earth Object observatory for locating potentially hazardous asteroids and ancillery space weather early warning system. The showstopper was that the power requirements would have ether necessitated a solar array roughly the same size as the arrays currently supporting the ISS or an onboard nuclear reactor (not an radioisotope thermoelectric generator; an actual fission reactor). Size, mass, and space treaty constraints pretty much doomed the concept, though I feel that with coming and recent advances in membrane structures for both the antenna and solar arrays, combined with the electrostatic ion propulsion system developed for the now cancelled Jupiter Icy Moons Orbiter, it could still be feasible in a Delta IV-H class payload. An optical system operating at lower throughput power, although beyond the current state of the art at planetary distances, is also a future possibility for really deep space or extremely low power missions.
Anyway, we’ll need something like this for future planetary exploration because the DSN just isn’t sustainable, and the limitations of a ground-based system to support a higher volume of missions will scale unfavorably versus just building more ground stations or increasing transmitting power of spacecraft.
That’s a good point as well; orbiting systems are limited to the bands that are mostly transparent in the atmosphere, so both spectrum and the natural interference of other signals are mostly alieviated by solar orbiting receivers.
Stranger
I’m sure they are also transmitting the data using sophisticated error correcting codes to enable them to operate very close to the Shannon limit (the theoretical limit on transmission rate over a specified bandwidth with a given signal to noise ratio.)
Yep. New Horizons uses Turbo Codes, which is one of the classes of near-Shannon codes that you mentioned. It has the interesting property that incoming bits are not classified as strictly 0 or 1, but rather a range based on the received signal. If the received amplitude is 0.55, for instance, a typical code would round that to 1. However, it would lose information in doing so–the reality is that the value of the bit was uncertain. Turbo Codes make use of this uncertainty, using the more-certain bits to fill in for the less-certain ones.
This is fantastic information. I need to collect my thoughts and have several questions I will post after work tomorrow. Thanks again!
Indeed.
Would it then be too much bother to use high freq to transmit to another space-based satellite which is closer to the earth and then have that satellite use longer waves to transmit to earth?
ETA: Ah, I see Stranger addressed this.
You guys really need to get out more…
You should have seen our logo. Even by the standards of most program/mission patches it was pretty awful. It is just impossible to make a platypus look anything but absurd.
Stranger
Would there be any benefit if there were a repeater floating around between the probe and Earth?
That is essentially what this concept of a solar orbiting constellation would do. The constellation would transmit back to a Earth-orbiting satellite (could be TDRSS S-band but more likely a dedicated satellite as TDRSS really wasn’t designed for interplanetary use) through the closest member and from there downlink to a ground station.
The problem is one of necessity. If we were doing missions like JIMO or were truly invested in a crewed mission to Mars (as inadvisable as that would be at this juncture) the need for this would be obvious. With the scarce manifest of interplanetary missions that NASA is currently funded to support, a several billion dollar interplanetary navigation and relay system just doesn’t seem like a justifiable expense. It is worth noting, however, that there was absolutely no cost justification for commercial applications of GPS when it was conceived; it was purely a system for high precision military navigation, and the current multi-billion dollar commercial GPS market is in effect subsidized by DoD investment in and maintenance of the GPS constellation.
Stranger
The data is sent via Dish Network. That’s why it takes so damn long to get here.