spacecraft directional antennas

Anybody know why spacecraft (or MERs) don't use electronically-
steerable, phased-array antennas instead of motor-driven "dishes"
which are subject to problems with the mechanics and more delicate
wiring, position sensors, and etc.


I suppose there's a good reason, I just don't know what it is. I'm
guessing it's a cost issue, which might be helped by NASA developing a
design for many missions to use or derive from.

"Ian Stirling" wrote in message
...
Gary W. Swearingen wrote:
Anybody know why spacecraft (or MERs) don't use electronically-
steerable, phased-array antennas instead of motor-driven "dishes"
which are subject to problems with the mechanics and more delicate
wiring, position sensors, and etc.


I suppose there's a good reason, I just don't know what it is. I'm
guessing it's a cost issue, which might be helped by NASA developing a
design for many missions to use or derive from.

Cost/mass/simplicity.
A dish needs nothing to form the beam.
A plane antenna needs lots of complex electronics.

with lots of power

(Gary W. Swearingen) wrote in message ...
Anybody know why spacecraft (or MERs) don't use electronically-
steerable, phased-array antennas instead of motor-driven "dishes"
which are subject to problems with the mechanics and more delicate
wiring, position sensors, and etc.

Sometimes they do, as a google search for "spacecraft phased array"
will reveal.

But if you look at the tradeoffs for phased arrays vs. body-fixed
antennas vs. mechanically gimballed antennas, phased arrays win on
rapid beam steering, lack of moving parts, and forming multiple beams.
They lose to body-fixed antennas on weight, size, and cost. They
lose to mechanically gimballed antennas on weight, size, cost, and
field of regard.

Interplanetary probes generally have no need for multiple beams, since
they usually only want to talk to Earth. They also don't need rapid
beam steering for the same reason: if you're in a planetary orbit, to
talk to a celestially fixed point (like Earth) needs at most one
revolution per orbital period. They are, however, severely weight
constrained. The MERs have the additional advantage of sitting on the
ground, and thereby not caring about the vibrations caused by moving
an HGA around.

I suppose there's a good reason, I just don't know what it is. I'm
guessing it's a cost issue, which might be helped by NASA developing a
design for many missions to use or derive from.

For missions where it was a cost issue, NASA is doing just this. See
the above google search, or look up "XPAA".

-jake

In article ,
Gary W. Swearingen wrote:
Anybody know why spacecraft (or MERs) don't use electronically-
steerable, phased-array antennas instead of motor-driven "dishes"
which are subject to problems with the mechanics and more delicate
wiring, position sensors, and etc.

Phased-array antennas are very complex, and particularly in small sizes,
they have a tendency to be heavy, power-hungry, and inefficient. Their
range of pointing is also somewhat limited.

They *have* been suggested many times, and there are a few spacecraft
using electronically-steered antennas (notably the European geostationary
weather satellites, which are spin-stabilized and despin their main radio
antenna electronically rather than mechanically). But they have enough
problems that mechanical antenna pointing generally still looks better.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |