Return-path: X-Andrew-Authenticated-as: 7997;andrew.cmu.edu;Ted Anderson Received: from beak.andrew.cmu.edu via trymail for +dist+/afs/andrew.cmu.edu/usr11/tm2b/space/space.dl@andrew.cmu.edu (->+dist+/afs/andrew.cmu.edu/usr11/tm2b/space/space.dl) (->ota+space.digests) ID ; Sun, 20 May 90 01:25:21 -0400 (EDT) Message-ID: Reply-To: space+@Andrew.CMU.EDU From: space-request+@Andrew.CMU.EDU To: space+@Andrew.CMU.EDU Date: Sun, 20 May 90 01:24:50 -0400 (EDT) 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!henry@uunet.uu.net (Henry Spencer) Subject: archaic units In article <1990May18.063315.2601@tsa.co.uk> domo@tsa.co.uk (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 henry@zoo.toronto.edu ------------------------------ Date: 19 May 90 14:57:12 GMT From: swrinde!zaphod.mps.ohio-state.edu!samsung!b-tech!kitenet!russ@ucsd.edu (Russ Cage) Subject: Re: Mirrors in space In article <1990May15.195339.21672@helios.physics.utoronto.ca> neufeld@physics.utoronto.ca (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 (313) 662-4147 changes them into less tractable problems. Russ Cage, Robust Software Inc. russ@m-net.ann-arbor.mi.us ------------------------------ Date: 19 May 90 15:57:39 GMT From: usc!samsung!munnari.oz.au!csc!myb100@ucsd.edu Subject: Re: The Spaceport of the 21'st Century In article <2087.26556c48@csc.anu.oz>, myb100@csc.anu.oz 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 > markus@mso.anu.oz.au > ------------------------------ Date: 19 May 90 08:12:54 GMT From: mephisto!prism!ccoprmd@rutgers.edu (Matthew DeLuca) Subject: Re: The Spaceport of the 21'st Century In article <2087.26556c48@csc.anu.oz> myb100@csc.anu.oz writes: >In article <2744@hsv3.UUCP>, mvp@hsv3.UUCP (Mike Van Pelt) writes: >> In article <9437@hydra.gatech.EDU> ccoprmd@prism.gatech.EDU (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: ccoprmd@prism.gatech.edu ------------------------------ Date: Sat, 19 May 90 09:45:27 ADT From: LANG%UNB.CA@vma.cc.cmu.edu Subject: Re: Navstar GPS News On Fri, 18 May 90 08:47:09 EDT sJames Price Salsman 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: LANG@UNB.CA or SE@UNB.CA Geodetic Research Laboratory Phone: (506) 453-5142 Dept. of Surveying Engineering Telex: 014-46202 University of New Brunswick FAX: (506) 453-4943 Fredericton, N.B., Canada E3B 5A3 ================================================================================ ------------------------------ Date: 19 May 90 02:11:01 GMT From: js7a+@andrew.cmu.edu (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!henry@tut.cis.ohio-state.edu (Henry Spencer) Subject: Re: HST Solar Panels In article <1230003@hprnd.HP.COM> wes@hprnd.HP.COM (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 henry@zoo.toronto.edu ------------------------------ Date: 19 May 90 22:20:25 GMT From: clyde.concordia.ca!news-server.csri.toronto.edu!utgpu!utzoo!henry@uunet.uu.net (Henry Spencer) Subject: Re: space news from April 2 AW&ST In article <1990May18.081501.968@melba.bby.oz.au> gnb@bby.oz.au (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 henry@zoo.toronto.edu ------------------------------ Date: Sat, 19 May 90 17:03:22 +0100 From: D.W.Merrick%durham.ac.uk@NSFnet-Relay.AC.UK 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 *******************