Deap Space Navigation

Hmm... can they do the same thing with the dishes as they do with
radar and make them electronically steered instead, so you don't have
to build the big huge 70m dishes for DSN work? Or is the real problem
the lack of transmitter power from the craft in mars and they need the
big dishes to amplify as much of the signal as possible for clean
communitcations?

NASA/JPL did something like that, when the Voyagers encountered various
planets, when they borrowed the use of the "Very Large Array" radio
telescope system somewheres in the southwest. This array is something
like 27 movable dishes in a Y pattern. In exchange for a month's
exclusive use, NASA/JPL upgraded the dishes, receivers and computers and
fixed various software issues the obseveratory used to live with. So to
have no outages when a Voyager was deep in at Saturn. Of course you
have periods of time when Saturn was below the horizon there, so you
have to manage the timing of various data download speeds.

In article ,
Pat Flannery wrote:
It would also be very helpful to have a second southern-hemisphere DSN
site, perhaps in South America. Having only one can become a severe
bottleneck when busy parts of the solar system are in the southern sky.

Due to the plane of the ecliptic, all the planets well stay at pretty
much the same inclination in the sky all of the time...

Nope, sorry Pat, you flunk. :-) Due to the inclination of Earth's equator
to the plane of the ecliptic, the maximum elevation of (say) Jupiter in
our northern sky varies about 47deg from one side of Jupiter's orbit to
the other. (Where *Earth* is in its orbit is of little importance.) It
spends roughly half its time in the northern sky, half in the southern.

The effect is magnified by the fact that some objects of interest spend
part of their time well outside the plane of the ecliptic. Ulysses is an
especially bad case, with its orbit tipped nearly 90deg to the ecliptic.

Generally a northern-hemisphere site will still get *some* view of a
near-ecliptic object each day, even when it's in the southern sky, but
when you discount for local terrain and the need for a certain minimum
elevation above the horizon to minimize noise, the coverage windows can
get pretty short.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |

In article ,
John Stoffel wrote:
Henry DSN has been thinking about optical communications (most likely
Henry in the near IR rather than the visible) for a long time...
Henry ...it would be a big change.

I can see it being a complete change in philosophy, but wouldn't the
bandwidth gains be worth it?

There's little doubt that DSN is headed that way in the long run, but it
won't happen soon unless some surprise solutions to the major technical
issues are found.

Hmm, how about using near UV instead of IR for the frequency? Would
that help since it could punch through cloud cover better?

IR penetrates clouds a bit better than visible or UV, but nothing even
vaguely optical gets through serious cloud cover very well.

The really-long-term solution, of course, is to put the receivers in
space. But that won't really work well until we have much more routine
access to Earth orbit.

Or would you want a microwave wavelength?

Microwaves are what DSN is using now -- the transition from S-band
(wavelengths around 12cm) to X-band (around 3.5cm) is pretty much
complete, and there have been experiments with going farther.

...microwaves probably wouldn't be too good either in rainy weather...

This is definitely an issue as microwave wavelengths get shorter. Mostly
it doesn't so much kill the signal altogether, as impose higher minimum
antenna elevation, so a probe down near the horizon can't be heard.

So the real benefit of going with an IR comm link is not the frequency
change, but the narrowing of the beam so you nead less energy on the
Mars transmitter for a given data rate, but you now need to point
within a degree or two of your receiver? What kind of pointing
requirements do the radio transmitters have? 5-10 degrees?

Much tighter than that -- X-band requires about 0.5deg pointing already.
Optical requires doing *much* better: a 1um IR beam from 10cm optics is
less than a thousandth of a degree wide. It's to the point where the beam
may not cover the whole Earth, so it has to be pointed at a specific
region of Earth. (Usually the pointing would be done with the aid of a
beacon beam sent out by the relevant Earth station; also, typically you'd
be pointing the final optics, not the whole spacecraft.)

Disregarding a whole bunch of details and speaking rather loosely, you
need a certain amount of energy captured by the receiver to reliably
receive a bit. Optical takes an initial hit because practical optical
collectors won't be as big as DSN's microwave dishes, but the beams are
so much narrower that even so, a much larger fraction of the transmitter
energy ends up in the receiver. So bit rate can go up, or transmitter
power can go down, or both.

Henry The #1 way to upgrade DSN is just to build more antenna dishes
Henry to roughly the existing designs...

Hmm... can they do the same thing with the dishes as they do with
radar and make them electronically steered instead, so you don't have
to build the big huge 70m dishes for DSN work?

It's possible in principle, but as I understand it, there are still some
technical difficulties in making electronically-steered receivers work as
well as dishes for really weak signals. It's coming but it's not what
you'd build today.

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.

Or is the real problem
the lack of transmitter power from the craft in mars and they need the
big dishes to amplify as much of the signal as possible for clean
communitcations?

See above -- collecting area is generally the crucial factor.

Henry It would also be very helpful to have a second
Henry southern-hemisphere DSN site, perhaps in South America...

I would think that just having something on or near the equator would
be a good thing.

Not quite as good, but it could still be helpful.

How around working with ESA and putting in a large
set of dishes at the Arianespace launch complex? I can't imagine that
they don't have the land and the needs of their own to communicate
with spacecraft.

If I recall correctly, there's already a French 35m dish at Kourou, in
fact, and ESA now has its own deep-space dishes in Australia and Spain.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |

"Henry Spencer" wrote in message
...
Generally a northern-hemisphere site will still get *some* view of a
near-ecliptic object each day, even when it's in the southern sky, but
when you discount for local terrain and the need for a certain minimum
elevation above the horizon to minimize noise, the coverage windows can
get pretty short.

"Hello, you have reached the Deep Space Network. Please leave your message
and we'll get back to you as soon as possible." beep
"Hi! This is Ulysses. I've got a-" beep

Henry Spencer wrote:

In article ,
Pat Flannery wrote:

It
spends roughly half its time in the northern sky, half in the southern.



Up here in North Dakota, we've never had a time when any of the planets
wasn't visible in our sky, except when hidden by daylight.
The only time when you'd have a problem with telemetry is when they get
near the sun, when our GEO cable sats get jammed near the spring and
fall equinoxes for a few minutes each day.

Pat

"Pat Flannery" wrote in message
...
The only time when you'd have a problem with telemetry is when they get
near the sun, when our GEO cable sats get jammed near the spring and fall
equinoxes for a few minutes each day.

Yep. 20 years ago, when I sold satellite dishes, we'd get calls twice a
year, bitching about this. Usually the same people, every time, no matter
how often we explained it. The dish simply couldn't find the match in front
of the firestorm.

The only time when you'd have a problem with telemetry is when they get
near the sun, when our GEO cable sats get jammed near the spring and fall
equinoxes for a few minutes each day.
Yep. 20 years ago, when I sold satellite dishes, we'd get calls twice a
year, bitching about this. Usually the same people, every time, no matter
how often we explained it. The dish simply couldn't find the match in front
of the firestorm.

I see a missed business opportunity he These customers obviously were
(are?) watching TV 24 hours a day, 365.25 days a year. You need to selll
them high-endurance dishes because they are using them much more than the
usual, design-to user. The difference then pays for the hassle they cause
you twice a year.

Cum grano salis, and all that.

Jan

On Thu, 30 Nov 2006 06:40:18 -0600, Scott Hedrick wrote
(in article ):


"Pat Flannery" wrote in message
...
The only time when you'd have a problem with telemetry is when they get
near the sun, when our GEO cable sats get jammed near the spring and fall
equinoxes for a few minutes each day.

Yep. 20 years ago, when I sold satellite dishes, we'd get calls twice a
year, bitching about this. Usually the same people, every time, no matter
how often we explained it. The dish simply couldn't find the match in front
of the firestorm.



Still happens with KaKu band DBS satellite TV - fortunately, it's
during the day when most worthwhile viewers (e.g., those who pay their
bill on time) are at work and so don't call the overworked, underpaid,
under-intelligent CSRs to complain.

--
Herb Schaltegger
"You can run on for a long time . . . sooner or later, God'll cut you
down." - Johnny Cash
http://www.angryherb.net

Herb Schaltegger wrote:

Still happens with KaKu band DBS satellite TV - fortunately, it's
during the day when most worthwhile viewers (e.g., those who pay their
bill on time) are at work and so don't call the overworked, underpaid,
under-intelligent CSRs to complain.



What really torques me off is Homeland Security's constant tests of The
Emergency Broadcast System.
This used to happen once a month, now it seems to happen around once a
day, generally in the middle of a good show on The Military Channel, or
a really choice movie, like "Barbarella", where she's about to get
screwed by the hairy guy in the goofy ice sailboat, and running around
in clothes that would make a leotard look concealing.
Yesterday, Homeland Security got together with NOAA and had them send
out a emergency _weather_ statement that demanded we go to our local
emergency channel (and figure out what Osama Bin Laden had done to the
weather by hijacking HAARP apparently.).
After two minutes of that, they finally explained that it was just a
test, and that the Sun would indeed rise this morning.
Around here, emergency weather statements generally involve a tornado
heading straight at you, and this would be odd in late November under
sunny skies and with the temperature reaching a balmy 10 degrees F. :-)

Pat

On Thu, 30 Nov 2006 08:12:13 -0600, Pat Flannery wrote
(in article ):

Around here, emergency weather statements generally involve a tornado heading

straight at you, and this would be odd in late November under sunny skies and

with the temperature reaching a balmy 10 degrees F. :-)

It was partly sunny with temperatures of about 73 degrees here
yesterday, with a major cold front on the way. This is the stuff F4
and F5 tornadoes are made of . . . :-/

--
Herb Schaltegger
"You can run on for a long time . . . sooner or later, God'll cut you
down." - Johnny Cash
http://www.angryherb.net