Space & Astronomy/Space and Astronomy (October 1993).iso/mac/TEXT/SPACEDIG/V11_4/V11_430.TXT

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Subject: SPACE Digest V11 #430

SPACE Digest                                     Volume 11 : Issue 430

Today's Topics:
                archaic units
             Re: Mirrors in space
        Re: The Spaceport of the 21'st Century
        Re: The Spaceport of the 21'st Century
             Re: Navstar GPS News
           GPS/NAVSTAR: Disturbing News...
             Re: HST Solar Panels
          Re: space news from April 2 AW&ST
            The great orbiting Venus saga
----------------------------------------------------------------------

Date: 19 May 90 21:49:27 GMT
From: attcan!utgpu!utzoo![e-mail]  (Henry Spencer)
Subject: archaic units

In article <[e-mail]> [e-mail] (Dominic Dunlop) writes:
>... I heard that space
>shuttles are still built in inches, and presume, given its age, that
>the HST was as well.  Is such still the case for new U.S. space
>programs? ...

I'm not sure about new programs in general, but the official units for
the space station are imperial units, because of fears that astronauts
might forget how long a centimeter is in a crisis.
--
Life is too short to spend    |     Henry Spencer at U of Toronto Zoology
debugging Intel parts. -Van J.| uunet!attcan!utzoo!henry [e-mail]

------------------------------

Date: 19 May 90 14:57:12 GMT
From: swrinde!zaphod.mps.ohio-state.edu!samsung!b-tech!kitenet![e-mail]  (Russ Cage)
Subject: Re: Mirrors in space

In article <[e-mail]> [e-mail] (Christopher Neufeld) writes:
>   Well, of course for this reason the doomsayers will keep such a
>mirror from ever being built. Just think, it's perfectly possible for a
>terribly (un)likely course of events to take place. After all, if the
>mirror ballooned outward by exactly the right amount at the centre, and
>something (cause unknown) made the extremely massive mirror rotate about
>its vertical axis [...]
>well, then, somebody might
>just get hurt.

It's much tougher to hurt someone than that.  Read on.

>The amount of light which hits the mirror and
>which could get through to the ground for a 300km diameter reflector
>amounts to about 71 terawatts.

Indeed.  And the minimum dispersion of the reflected light, assuming
an optically-perfect mirror, will be the angular width of the sun as
seen from the mirror.  That's about 8.6 milliradians here at Terra.

If the mirror is at geosynchronous altitude, a 300 km parabolic mirror
will put that energy on a spot 308 km wide.  That's about 950 W/m^2
before atmospheric losses, about 70% of what we get from the sun directly.
That's something like the insolation we get at 9 AM.

In order to be dangerous in a short time, the mirror would have to have
an angular diameter much bigger than the Sun itself.  That's one BIIIIG
mirror.
--
              Oversimplification doesn't solve problems, it just
[phone]          changes them into less tractable problems.
Russ Cage, Robust Software Inc.            [e-mail]

------------------------------

Date: 19 May 90 15:57:39 GMT
From: usc!samsung!munnari.oz.au!csc![e-mail]
Subject: Re: The Spaceport of the 21'st Century

In article <[e-mail]>, [e-mail] writes:
[...]
> the NE corner of Australia. New Zealand is two+ islands to the SW of Australia
                                                                  ^
Dang, who put that 'W' key under my finger !? :-) It should be 'E' of course

> Markus Buchhorn
> Mt Stromlo and Siding Spring Observatories, Canberra, Australia
> [e-mail]
>

------------------------------

Date: 19 May 90 08:12:54 GMT
From: mephisto!prism![e-mail]  (Matthew DeLuca)
Subject: Re: The Spaceport of the 21'st Century

In article <[e-mail]> [e-mail] writes:
>In article <[e-mail]>, [e-mail] (Mike Van Pelt) writes:
>> In article <[e-mail]> [e-mail] (Matthew DeLuca) writes:

>>>But if there's nuclear material (RTG's or reactors) aboard the satellites to
>>>be launched, won't New Zealand wimp out? (Only a half-smiley, folks.)

>> No need for smilies at all.  New Zealand's policy, if applied to
>> launches from Cape York, effecively rules out any missions to the outer
>> planets.  So, if any outer-planets or deep-space missions (the TAU
>> mission?) are contemplated, we'll need another launch site anyway.

>Ahem.....a small pointer to life in the Sou'west Pacific: Cape York is in
>the NE corner of Australia. New Zealand is two+ islands to the SW of Australia
>and is a separate country. NZ policies are not reflected in Oz policies.
>Therefore: RTG based missions can be launched from the Cape York facility.

The original poster, whose message has been dropped from this in the name
of brevity, was suggesting a Canadian-Australian-New Zealand-US (CANZUS)
space alliance.  The last time we got into something like that (the ANZUS
military alliance) the New Zealanders decided they didn't like nuclear
thingies on ships and wouldn't let U.S. ships dock.  I have this vision
of a RTG-powered probe on the pad, and NZ deciding to pack up and go
home...

Pesonally, I would like to see a Pacific-Rim space organization, but I'm
just not terribly thrilled with New Zealand...

[I suppose, though, I had best shut up, before someone starts throwing
 the space station project in my face as an example of how not to run
 an international program. :-(  ]


--
Matthew DeLuca
Georgia Institute of Technology      Do not meddle in the affairs of wizards,
Office of Computing Services         for they are subtle, and quick to anger.
ARPA: [e-mail]

------------------------------

Date:        Sat, 19 May 90 09:45:27 ADT
From: [e-mail]
Subject:     Re: Navstar GPS News

On  Fri, 18 May 90 08:47:09 EDT  sJames Price Salsman <[e-mail].
  edu> writes:

> GPS is a group of 13 satellites now in operation and 27 more to be
> launched by 1994 [I believe this is in error, as there are to be
> 27 total satellites in the constellation --jps], each of which
> produces and encrypted P code that the military uses to guide missles,
> and another signal, called the Clear/Acquisition signal, that has
> been available for commercial uses like surveying and timing
> communications networks.
>
> But the DoD has decided that even the C/A signal is too accurate
> to be generally available, so it has begun a practice it calls
> "selective availability."  That delicious piece of bureaucratese
> means that the DoD will introduce random noise on the C/A signal,
> known in some circles as "dithering," to make it dificult or even
> impossible to use.

From the U.S. Naval Observatory's BBS:

ON MARCH 29, 1988, THE SECRETARY OF THE AIR FORCE SIGNED A LETTER
ADDRESSED TO THE CHAIRMAN, JOINT REQUIREMENTS OVERSIGHT COUNCIL, WHICH
OFFICIALLY EXPANDED THE GPS OPERATIONAL SATELLITE CONSTELLATION FROM
21 (18 PLUS 3 ACTIVE SPARES) TO 24 (21 PLUS THREE ACTIVE SPARES)
SATELLITES.  THESE 24 SATELLITES ARE EXPECTED TO BE CONFIGURED INTO
SIX PLANES CONTAINING FOUR VEHICLES EACH. AS LONG AS THEY REMAIN
USABLE, THE PRESENT BLOCK I SATELLITES WILL REMAIN IN SERVICE AND WILL
BE INCORPORATED INTO THE GROWING OPERATIONAL CONSTELLATION.

A couple of other facts:

Selective Availability consists of two components: randomly  dithering
the basic clock frequency in the satellite and transmitting degraded
satellite orbit information.  The dithering affects the P-code as well
as the C/A-code and the carriers.  Authorized uses have access to a
"key" to undo the effect of S/A.

Surveyors (and other civilians) do have access to the P-code (it's not
secret).  Only when Anti-spoofing is activated (which, reportedly, will
be done only for testing and in times of "national emergency") and the
P-code is encrypted, will its use be denied to civilians.

================================================================================
 Richard B. Langley                            BITnet: [e-mail] or [e-mail]
 Geodetic Research Laboratory                  Phone:  [phone]
 Dept. of Surveying Engineering                Telex:  014-46202
 University of New Brunswick                   FAX:    [phone]
 Fredericton, N.B., Canada  E3B 5A3
================================================================================

------------------------------

Date: 19 May 90 02:11:01 GMT
From: [e-mail]  (James Price Salsman)
Subject: GPS/NAVSTAR: Disturbing News...




From _Data_Communications_ (ISSN 0363-6399) vol. 19, no. 6, May 1990, page 56
(c) 1990 McGraw-Hill Inc, reproduced without permission.

DOD DITHERS DIGITAL DATA

Telephone network synchronization is an unlikely topic for heated
controversey, but that is what the U.S. Department of Defense has
provoked by tampering with the Navstar Global Positioning Satellite
(GPS) system that AT&T plans to use as a network clock.

GPS is a group of 13 satellites now in operation and 27 more to be
launched by 1994 [I believe this is in error, as there are to be
27 total satellites in the constellation --jps], each of which
produces and encrypted P code that the military uses to guide missles,
and another signal, called the Clear/Acquisition signal, that has
been available for commercial uses like surveying and timing
communications networks.

But the DoD has decided that even the C/A signal is too accurate
to be generally available, so it has begun a practice it calls
"selective availability."  That delicious piece of bureaucratese
means that the DoD will introduce random noise on the C/A signal,
known in some circles as "dithering," to make it dificult or even
impossible to use.

Meanwhile, some commercial equiptment manufacturers and users, such
as land surveyors, are already relying on the signal and now are
angry that the DoD is changing the rules.  "There is a big
controversey about why the government is doing this," says Jim
Jespersen, a staff member of the time and frequency division of
the National Institute of Standards and Technology (Boulder, Colo.),
"especially since the threat from the Russians is not so severe."
[The "Russians" have a very accurate GPS system of their own,
called GLONASS, so someone is confused here... --jps]

GPS is run by the U.S. Air Force Systems Command's Space Division
in Los Angeles.  The officer in charge of the project, Col. Marty
Runkle of the Joint Program Office, could not be reached for comment.

As for AT&T, George Zampetti, a Bell Laboratories scientist who
is in charge of developing AT&T's synchronization scheme, says
that the company plans to use the C/A signal even if it is
ditthered.

Zampetti and John Abate, another Bell Labs scientist, say AT&T
will use 3B2 computers to filter out the noise to get close to
the true signal.  Filtering will slow down but not eliminate the
use of GPS, Abate says.

"We could go a month and still maintain" on error in 100 billion
events, Zampetti says.

The key to the system is Rubidium clocks that actually pass timing
to AT&T's switches and transmission network.  Those Rubidium clocks
can maintain network timing to meet requirements of ANSI and CCITT
standards, Zampetti says.  AT&T would use GPS to calibrate and monitor
the rubidium clocks.                          -John T. Mulqueen

[The main article (of which that was a sidebar) talks about MCI
and Sprint's use of Loran, atomic clocks, and describes GPS.
The ANSI standard in question is T1.101, by committee T1X1.3,
which describes syncronization for high-bandwidth long-haul
digital transmission. --jps]

------------------------------

Date: 19 May 90 22:16:13 GMT
From: zaphod.mps.ohio-state.edu!usc!cs.utexas.edu!news-server.csri.toronto.edu!utgpu!utzoo![e-mail]  (Henry Spencer)
Subject: Re: HST Solar Panels

In article <[e-mail]> [e-mail] (Wesley Cole) writes:
>How different are space craft solar panels from panels that can be purchased
>commercially?  ...  I'm sure that NASA panels must be space flight
>certified (very resistant to vibration, heat, etc) but do they use the
>same process to convert sunlight to electricity at the same efficiency as
>earth bound panels?

The underlying process is definitely the same.  Space solar arrays are
generally different from ground ones in being optimized for light weight
and radiation resistance, subject to constraints requiring them to be
resistant to vibration, extreme temperatures, etc.  I don't know how the
efficiencies compare offhand -- the extra constraints are balanced, to
some extent, by cost-is-no-object engineering.

>Do solar panels on spacecraft somehow track the sun to maximize their energy
>output?  ...

Depends on the spacecraft.  The bigger birds tend to have their solar arrays
mounted on rotating joints so they can track the Sun on at least one axis of
rotation (with overall spacecraft attitude constrained to keep that axis
more or less perpendicular to the sunlight much of the time).  Smaller ones
sometimes just have solar panels on all faces and accept that half of them
will be in shadow at any time.  Spin-stabilized satellites often are halfway
in between, with a cylindrical solar array around the body and the spin axis
kept at right angles to the sunlight.

>How does this
>effect the HST's operation?  When they point the telescope at a particular
>object do they have to move the solar panels at the same time...

HST's solar arrays follow the big-satellite pattern, and are on a rotating
joint.  So in general, yes, they have to turn to compensate for telescope
pointing.

>... What about when the HST passes into
>the shadow of the earth?  Do all obsevations have to cease due to the sudden
>power reduction or is it in an orbit that prevents this ever happening?
>(I assume batteries wouldn't be sufficient during this period.)

For HST, batteries are sufficient, by design.  Satellites in most low
orbits spend a good fraction of their time in shadow, and have to be
designed to cope.  A suitably-chosen sun-synchronous orbit is in continuous
sunlight for about ten months of the year, and Clarke orbit gets continuous
sunlight except for short "eclipse seasons" in spring and fall, so birds
in those orbits sometimes just cut back on operations during eclipses.

Actually, there is an added complication that designers have to think
about:  the Moon gets in the way sometimes.  (In the case of lunar-orbit
satellites, the Earth gets in the way sometimes.)  The worst case is when
your satellite's orbit is more or less lined up with the track of a solar
eclipse, and the timing is right, in which case your satellite can spend
most of its time on the *sunlit* side of Earth in the Moon's shadow.  It's
not something that happens often, but it does happen.  Back before HST
was launched, when the exact position of its orbit wasn't yet certain,
there was some concern that it might have to be put into minimum-power
hibernation mode during the 1991 solar eclipse.  (I haven't heard the
final word on this.)

>Lastly, how
>long do solar panels last in space?  With no corrosion or moving parts (except
>for tracking systems?) and barring a catastrophic meteor impact, it seems as
>though they would last many decades.

Solar cells in space suffer a gradual reduction in power output due to
accumulated radiation damage in their crystal structure.  They're designed
to resist this -- one bonus of the Challenger-induced delay in HST's launch
was the chance to switch to newer, tougher solar arrays -- but it still
limits their useful life.  Solar flares can make this a lot worse.  The
intense solar activity last fall knocked several years off the lives of
some satellites.  (Satellites in all but the highest Earth orbits usually
get considerable protection from Earth's magnetosphere, but things were
fierce enough last fall that the magnetosphere was compressed a lot and
the Clarke-orbit comsats were spending parts of their orbits outside it.)
--
Life is too short to spend    |     Henry Spencer at U of Toronto Zoology
debugging Intel parts. -Van J.| uunet!attcan!utzoo!henry [e-mail]

------------------------------

Date: 19 May 90 22:20:25 GMT
From: clyde.concordia.ca!news-server.csri.toronto.edu!utgpu!utzoo![e-mail]  (Henry Spencer)
Subject: Re: space news from April 2 AW&ST

In article <[e-mail]> [e-mail] (Gregory N. Bond) writes:
>   problem is that although the spaceport would be in Australia and
>   manned by Australians, they'd be using Soviet boosters.
>
>Well, the security guards and the cleaners might be Australians.   I
>doubt thant any of the "key" personnel would be; we have no space
>industry to build experience and no uni degrees in space engineering.

The way you build up experience is to build a spaceport and start using
it, which is what Australia is doing! :-)  Most technicians and the like
will have to be trained almost from scratch anyway, since there are no
Western technicians with Soviet booster experience.  Australia has enough
of an aerospace industry -- the space side of it isn't large but isn't
quite zero -- to supply a lot of the people needed.  It might be desirable
to have experienced US or European people in the top-level jobs for the
early stages, though.
--
Life is too short to spend    |     Henry Spencer at U of Toronto Zoology
debugging Intel parts. -Van J.| uunet!attcan!utzoo!henry [e-mail]

------------------------------

Date:     Sat, 19 May 90  17:03:22 +0100
From: [e-mail]
Subject:  The great orbiting Venus saga

Concerning the idea of a venus-earth orbiting binary; I think one would have to
be careful about how far apart they were - after all, any large tidal forces I
would expect to trigger earthquakes or volcanos, San Andreas beware!
  and what about satellites trying to keep their circular orbits? (could do
a figure of eight between the two planets, I suppose!?)

Disclaimer: any disclaimers are entirely my own invention, and are not
            representative of my highers' disclaimers.

  ...and look what a mess the early venusians made of THEIR planet!

------------------------------

End of SPACE Digest V11 #430
*******************