6
$\begingroup$

A bit of spin on Which LEO missions are currently supported by NASA's DSN (and why)? is what I have in mind.

The Deep Space Network home-page writes to say

Each complex consists of at least four deep space stations equipped with ultrasensitive receiving systems and large parabolic dish antennas. There are:

• One 34-meter (111-foot) diameter High Efficiency antenna.

• One 34-meter Beam Waveguide antenna. (Three at the Goldstone Complex and two in Madrid)

• One 26-meter (85-foot) antenna.

• One 70-meter (230-foot) antenna.

Apparently each station is commanded remotely as mentioned on that page

All the stations are remotely operated from a centralized Signal Processing Center at each complex. The Centers house the electronic subsystems that point and control the antennas, receive and process the telemetry data, transmit commands, and generate the spacecraft navigation data.

There are any number of space-craft operating in space

  • Voyager 1
  • Voyager 2
  • Rosetta
  • Dawn
  • Spirit
  • Curiosity
  • Juno
  • Kepler
  • Herschel, to name just a few ...

What I'm curious about

  • Can a station communicate with more than one space-craft using simple time-slicing without re-orientation of the antenna/s?
  • What algorithm (is that the proper term?) is followed to orient an antenna? Surely any given antenna isn't moved from zenith to nadir then back & around at random ...
$\endgroup$
  • $\begingroup$ time-slicing without re-orientation of the antenna would require two spacecrafts in the same direction as seen from the DSN antenna. $\endgroup$ – Uwe Nov 9 '18 at 23:51
9
$\begingroup$

Yes, a station can communicate with more than one spacecraft, but not the way you suggest.

Each DSN complex consists of several antennas, each of which is called a Deep Space Station. A "station" is one antenna. Each antenna has a single transmitter, so any one station can only transmit to one spacecraft at a time.

However if more than one spacecraft is in the antenna beam, which is often the case at Mars, then a station can receive data from more than one spacecraft at a time. This capability is called Multiple Spacecraft Per Aperture (MSPA). Of course, the spacecraft have to be on different frequencies. So one station can uplink to only one spacecraft, but it can downlink from more than one spacecraft in the beam.

Below is the DSN display showing MRO and Odyssey on the same antenna (DSS-43). You will sometimes see the opposite, where there is one spacecraft on two or more antennas, like Maven on DSS-34 and DSS-45, also in the picture below. When at the same complex, that is called "arraying", where the signal from two or more antennas are combined to simulate a larger antenna for increased data rate. That also only works for receiving. Antennas cannot (today) be arrayed for transmitting. You can also see the same spacecraft on two different stations at two different complexes. In that case either the spacecraft is in the middle of a handoff from one complex to another to make for one long communication pass, or a special navigation data type is being collected using very long baseline interferometry.

DSN display showing MRO and Odyssey on one antenna

Time multiplexing is not useful, since it can take quite some time to lock on to and acquire a signal. Once you have it, you don't want to lose it and then have to find it again. It is better to have multiple receivers.

I suppose that in effect we do use time multiplexing of the antennas, but on the scale of hours. Since it takes around half an hour to lock on to a signal, you stay with it for at least a few hours, usually eight to ten hours, and then switch to another spacecraft.

As for the "algorithm" used to point an antenna, we have very accurate models of the position of Earth, the position of any given antenna on Earth, and of the target spacecraft. You just compute the vector to the target and point the antenna in that direction. The direction changes with time as the Earth rotates (and as the Earth and target move relative to each other), so the vector is continuously updated for the antenna to track the target. (I'm not clear on your question there, so if this doesn't answer it, then you need to clarify the question.)

$\endgroup$
  • $\begingroup$ Say two of three craft are located so the antenna only need be oriented along a single axis, and the third located so the antenna must be reoriented along all three axes - which craft would take preference? My gut says reorient along a single axis. For the other part of the algorithm (+: i'll drop you a note in chat $\endgroup$ – Everyone Dec 19 '13 at 17:02
  • $\begingroup$ Assuming the target space-craft is far enough for communication to be necessarily asynchronous, does the space-craft transmit in a loop until it receives an acknowledgement? How is any signal lost during lock acquisition reconstructed? $\endgroup$ – Everyone Dec 19 '13 at 17:46
  • 1
    $\begingroup$ It might be better to expand on your question or ask a new question than to have a long discussion in comments. $\endgroup$ – Mark Adler Dec 19 '13 at 18:08
  • $\begingroup$ About your final paragraph, maybe add a word or two on how much relative motion of the Earth and the spacecraft needs to be taken into account? I suspect it's not much at all on an Earth-Mars link because the target won't move a major fraction of the beamwidth in the few minutes of signal travel time, but could be somewhat significant past Jupiter or so with times measured in hour or hours rather than minutes. $\endgroup$ – a CVn Apr 26 '17 at 11:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.