Deap Space Navigation

Dr J R Stockton writes:

AIUI, Arecibo has a shallowish upwards-facing dish and over it is
suspended an aerial cabin in a fixed (?) position.

How about (in another valley) having a near-hemispherical upwards-facing
dish with an aerial assembly on motorised cables so that it could
routinely be moved wherever required in the bowl, so steering the beam

Arecibo has you beat on this - the reciever can be moved to steer
up to 20 degrees from straight up. It can track an object for up
to two hours. They use motor drives on a suspended stationary
platform.

over the entire upwards hemisphere? There would of course be a loss of
effective area away from the vertical; but ISTM that, while not too near
the horizon, the loss would not be great. The aerials would be dockable
at the centre of the dish, for access.

The big problem is not collecting area, but noise. If you go off center,
the feed illuminates the (warm) ground, which is terrible for noise.
Arecibo has a 15 meter fence around it to minimize it, but if you try to get
far from the zenith it's a still worse.

For access, Arecibo has a catwalk and cable car. Since the suspended part
is quite heavy (many 10s of tons, at least) a person is not much extra.

The dish could be actively shaped to be a paraboloid of appropriate axis
orientation, or some equivalent of a correcting lens could perhaps be
used.

Arecibo beat you to this, too. It now has a 3 mirror system that
focuses correctly with a spherical primary.

Or the assembly on the cables could itself be a smaller mirror, shaped
to redirect the waves onto a hut-with-aerials at the centre of the bowl
(Tip : beware flash floods!).

Arecibo has 2 mirrors in the suspended sphere, and the focal point is
there too.

Lou Scheffer

Dr J R Stockton wrote:

AIUI, Arecibo has a shallowish upwards-facing dish and over it is
suspended an aerial cabin in a fixed (?) position.

Nope, it's moveable.

In brief. Arecibo is a spherical reflector that uses a moveable line
feed (as opposed to the point feed used with parabolic reflectors) to
steer the direction of the beam.

Why a line feed? Because unlike a parabolic reflector which focuses an
incoming plane wave to a point, a spherical reflector focuses an
incoming plane wave to a line. Thus, one needs to use a distributed
antenna rather than a point antenna as the feed.

Why a spherical reflector? By physically moving the line feed, one can
steer the beam without steering or deforming the reflector. This isn't
possible with a parabolic reflector.

How far can one steer the beam? Not too much. The usual limitation in
any reflector antenna used to receive signals from space is spillover
which allows the feed to see signals from the warm ground rather than
just cold space.

The dish could be actively shaped to be a paraboloid of appropriate
axis orientation, or some equivalent of a correcting lens could
perhaps be used.

Easier said than done, I fear. That's why Arecibo ended up in its
current configuration.

Or phase-shifting might be employed between the aerial assembly and
the rest of the electronics.

You're confusing the role of the primary and secondary. Steering the
beam from the primary (feed) can distort the antenna pattern of the
secondary (reflector), but it won't steer the antenna pattern. The only
way to steer the beam is to impart a phase shift across the aperture of
the secondary.

--
Dave Michelson

In article id,
Dr J R Stockton wrote:
Valleys with vaguely suitable shapes, you can find. The big problem is
that an Arecibo-type dish is not very steerable. It can only look at
things in a limited range of celestial latitudes, and even if your target
is in that strip of sky, you only get one short look each day...

AIUI, Arecibo has a shallowish upwards-facing dish and over it is
suspended an aerial cabin in a fixed (?) position...
How about (in another valley) having a near-hemispherical upwards-facing
dish with an aerial assembly on motorised cables so that it could
routinely be moved wherever required in the bowl, so steering the beam
over the entire upwards hemisphere?

As others have already noted, this *is* done at Arecibo (or was; I haven't
kept track of the recent changes). In practice, it doesn't give you
anywhere near a full hemisphere of steering; instead of 180deg, the useful
steering range is 20-30deg (if I'm remembering correctly). There is only
so much you can do to steer a fixed dish by playing games with the feed.

You might be able to do a bit better if you worked really hard at it, but
do note that Arecibo's limited steering range is a big handicap to it, and
so some smart people have put considerable thought into widening that
range, with very limited results.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |

Well, everything that is in Orbit around the Earth with a greater
inclination than the latitude of Arecibo would be with the narrow range
that the big dish has. At some point. Tracking known stuff when the time
is right, and just scanning in between, eventually would would have them
all.

I would the the benifits of size would be a big plus.

--
Craig Fink
Courtesy E-Mail Welcome @
--

On Sat, 23 Dec 2006 11:31:25+0000, Henry Spencer wrote:

In article id,
Dr J R Stockton wrote:
Valleys with vaguely suitable shapes, you can find. The big problem is
that an Arecibo-type dish is not very steerable. It can only look at
things in a limited range of celestial latitudes, and even if your target
is in that strip of sky, you only get one short look each day...

AIUI, Arecibo has a shallowish upwards-facing dish and over it is
suspended an aerial cabin in a fixed (?) position...
How about (in another valley) having a near-hemispherical upwards-facing
dish with an aerial assembly on motorised cables so that it could
routinely be moved wherever required in the bowl, so steering the beam
over the entire upwards hemisphere?

As others have already noted, this *is* done at Arecibo (or was; I haven't
kept track of the recent changes). In practice, it doesn't give you
anywhere near a full hemisphere of steering; instead of 180deg, the useful
steering range is 20-30deg (if I'm remembering correctly). There is only
so much you can do to steer a fixed dish by playing games with the feed.

You might be able to do a bit better if you worked really hard at it, but
do note that Arecibo's limited steering range is a big handicap to it, and
so some smart people have put considerable thought into widening that
range, with very limited results.

Dave Michelson wrote:
Dr J R Stockton wrote:

AIUI, Arecibo has a shallowish upwards-facing dish and over it is
suspended an aerial cabin in a fixed (?) position.

Nope, it's moveable.

In brief. Arecibo is a spherical reflector that uses a moveable line
feed (as opposed to the point feed used with parabolic reflectors) to
steer the direction of the beam.

Why a line feed? Because unlike a parabolic reflector which focuses an
incoming plane wave to a point, a spherical reflector focuses an
incoming plane wave to a line. Thus, one needs to use a distributed
antenna rather than a point antenna as the feed.

This was true for a number of years, but they changed (in about 1998)
to a 3 mirror system that focuses to a point. The secondary and
tertiary mirrors are in the dome that hangs where the line feeds were
(and I think some line feeds may still be in use)

See:
http://www.news.cornell.edu/releases...o/project.html

Lou Scheffer

In article ,
Craig Fink wrote:
Well, everything that is in Orbit around the Earth with a greater
inclination than the latitude of Arecibo would be with the narrow range
that the big dish has. At some point. Tracking known stuff when the time
is right, and just scanning in between, eventually would would have them
all.

Yes, but the subject under discussion was deep-space tracking and
communication, where it's a grave handicap to have such a limited field
of view and such limited contact windows.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |

wrote:

This was true for a number of years, but they changed (in about 1998)
to a 3 mirror system that focuses to a point. The secondary and
tertiary mirrors are in the dome that hangs where the line feeds were
(and I think some line feeds may still be in use)

See:
http://www.news.cornell.edu/releases...o/project.html

Thanks for the link. Very much appreciated!

--
Dave Michelson

Jim Kingdon wrote:
A friend visited Greenbank national observatory recently. All but one
dish is mothballed stored pointing straight up. Seems no one wants to
buy user time on any but the largest dish there.

Well, the Howard E. Tatel telescope, for example, is 26 meter.

That's pretty small by Deep Space Network standards. I couldn't give
numbers, but this isn't the only unused radio telescope in this size
range (I remember one in Colorado that people wanted to use for an
amateur Mars missions for example).


There are two. They are about a mile from where I am sitting right
now. They need a bit of refurbishing to get back into good condition.

Henry Spencer wrote:


DSN already routinely combines signals from several dishes, and there is
interest in the idea of building large arrays of small dishes rather than
a few big ones -- sort of a halfway step, since the small dishes would
still be mechanically steered -- but that's still an experimental idea.
Again, not what you'd build today.


Actually, this one is very close. If a 70 meter dish failed today, I'd
bet they'd do it. A new 70 meter class dish costs $100M, and an array
of smaller dishes has lots of advantages. It can be quite a bit
cheaper, depending on the details. It can be split to point at many
targets, it has fewer points of failure, it can be maintained a little
at a time, the size can be increased in smaller increments, it can be
spread out geographically to add weather and disaster diversity, and so
on....

The days of big dishes are numbered.

Lou Scheffer

wrote:

Actually, this one is very close. If a 70 meter dish failed today,
I'd bet they'd do it. A new 70 meter class dish costs $100M, and an
array of smaller dishes has lots of advantages. It can be quite a
bit cheaper, depending on the details. It can be split to point at
many targets, it has fewer points of failure, it can be maintained a
little at a time, the size can be increased in smaller increments, it
can be spread out geographically to add weather and disaster
diversity, and so on....

The days of big dishes are numbered.

I agree with all of the above, but am curious about the cost.
It would take at least eight or ten 25-metre dishes to equal the
collecting area of a single 70-metre dish. (25-metres being a fairly
standard diameter reflector for use in radio astronomy arrays :-)

The pattern of a widely spaced array of large antennas would be a lot
more complicated than that of a single large antenna. In particular,
the grating lobes would be very noticeable. (Almost all conventional
arrays are closely spaced arrays of small antennas.) Having said, that,
this has been done before with the VLA and suitable use of feedback
control and RF-over-fibre links would solve lots of immediate problems.

The good news, of course, is that much technical experience and
expertise has developed over the past decade (in particular) from work
on multi-antenna radio astronomy projects ranging from the VLA to the
SKA (Square Kilometre Array). (Yes, there are significance differences
between aperture synthesis and conventional arrays, but by and large the
people involved could likely handle both or either as required.)

--
Dave Michelson