Slashdot Mirror
Busy Signals for Deep Space Experiments
lionchild writes "Just when you hated getting those 'Network Busy' signals on your Cell Phones..imagine what it's like to deal with communications in deep space after all these years of putting satellites and probes out there into the space lanes. Check the article out on space.com
" The saddest part is the poor state that the deep space network of dishes is in, with some of the 70 meter antennae approaching their fifth decade with no repair funds on the horizon.
Someone ought to tell those folks over at Space.com that the word antennae applies only to the sensory projections of an insect.
When you're talking about radio receivers, the plural of antenna is antennas.
It's in the dictionary if you don't believe me.
Funny how a bunk statement is bunked by another bunk statement
(a) Photons are massless, so you can't use newtonian gravity F=GmM/r^2 to compute gravitation effects on it.
(b) There is no such thing as "stuff" out there to pull light beam away from us. That is even more completely bunk. According to your logic, the planets will be pulled away from us too.
(c) There is a light horizon from the solar system. It is but the future light cone of the event called the 'solar system' now in a space-time diagram. The word you want to use is "event horizon".
The more correct answer is that the curvature of space-time caused by the sun's mass is not enough to curve space upon itself, i.e. start with a photon, and the
photon will follow the curvature back onto the source. (Roughly speaking of course.)
Mode (3) smart-aleck mode. Press * to return to main menu.
The number of options for contacting a deep-space spacecraft (which includes anything beyond geo-synchronous orbit) are surprisingly limited - basically NASA's Deep Space Network, NASA's Tracking and Data Relay satellites, and then whatever time can be purchased on the various radio telescopes and ground stations around the world. For a spacecraft at Mars the signal is weakened by an inverse square factor of billions relative to near-Earth satellites, so you need highly directed large and sensitive receivers to hear anything. NASA has been upgrading the TDRS satellites but they aren't much use for really deep space missions because of their limited size. Except for commandeering Arecibo, the 70-meter DSN antennas are about all that's available right now...
Energy: time to change the picture.
You asked how often these antennas break down. While I don't know any statistics, I can give you an idea of what sorts of things make up one of these installations.
First of all, the antenna has to be pointed at the satellite that it's tracking. This is not an easy task, since the satellite is moving and it's a long way away. To make matters worse, the beam width of the antenna is damn small, so you have to point it very accurately. Doing this involves some tricky work, when you consider how much a 70 meter antenna weighs. And you can't ignore the wind in the desert, though to be fair they do stow the antenna when the wind gets above 20 or 30 miles per hour, as I recall. So in addition to having to move a lot of steel, you have to position it very accurately. Take a complex polynomial to describe the trajectory in azimuth and elevation (or is it right ascension and declination? I don't remember) and servo all that steel to track.
OK, that's the mechanics.
Now let's talk about the transmitter. The satellites you're talking to are rather far away. Remember our friend one-over-r-squared? Well, when r is large, the energy falls of a hell of a lot. So when transmitting you try to have as much power as possible. Think megawatts. Megawatts at microwave frequencies. How do you generate such power at those frequencies? You take a vacuum tube that stands about six feet high and you put a lot of current into it. You cool that sucker with a lot of water. Did I mention that the antenna is out in the middle of the desert?
OK, now you have an idea about the transmitter.
Ah, yes, the receiver. Well, the satellite can't afford to transmit a megawatt of power. If it's lucky, it can muster ten or twenty watts. At planetary distances, the energy arriving at the antenna is comparable to the amount of energy that arrives at the moon when you hold up a lit match. Not much!
How do you make a receiver that will detect a signal that weak? This is a very complicated topic, but let me summarize by noting that the key component is a superconducting maser. This is basically a chunk of copper hollowed out, evacuated, and then cooled down to a very cold temperature. In the desert.
To coordinate all of this you have to have some computers. When I last visited the control room at Goldstone it seemed pretty darn big to me. It had something like six or seven rows of 19" racks, each row with something like thirty or forty racks. That was over 20 years ago, so I might be off by a factor of two or more.
Oh, yes, how about some facilities for the people who support the installation and the project people from JPL.
And it's out in the desert. The control electronics at the big antenna are in one building and the people who drive the antenna sit in another. There's a long tunnel between the two. You're out in the desert, so keep an eye out for scorpions when you go into the tunnel. I never saw a live one, though I did see several dead ones.
So in answer to your question about maintenance, yes, there is some stuff that needs maintenance.
azimuth and elevation (or is it right ascension and declination? I don't remember)
It's azimuth and elevation. At least those're the only terms I've ever heard used to locate something in the night sky.
I once read an article about how much of this was going to be solved by the use of a real deep-space communications network. The idea was to have them launch some relay satellites at some stable orbital points in the solar system, and instead of having ground stations here dedicated to communicating over these great distances, you'd ideally have a relay near your probe relay its transmissions back to earth. Once you get into deep space, you can start using more efficient optical methods for communicating between relays, and communication from earth basically just relies on your ability to get the message to the nearest relay satellite and let it route your message appropriately.
:/
All of this has the added benefit of allowing all of the various probes and interplanetary craft to be in communication at the same time.
Unfortunately, aside from the original paper I read, I haven't heard of anything more about these ideas. It's possible they've been tabled as too expensive for now..
It's a shame because I think this project would be really fascinating and could probably save a ton of money in the long run.
RA and dec are also mounted on what is commonly called an equatorial platform, meaning that the platform is offset so it can turn in synch with the rotation of the earth via a single drive while pointing at the same object. I'm not sure, but I think it's the declination axis that does this. Altitude (elevation) and azimuth must both alter at varying rates to track a celestial object. That being said, equatorial mounts are much larger and more expensive than alt/az ones, so many big scopes nowadays use the simpler mount with computer control to do the tracking.
In short, both alt/az and equatorial (RA/dec) are pointing strategies, but RA/dec is a coordinate system.