kenny@m.cs.uiuc.edu (12/09/88)
/* Written 11:06 pm Dec 6, 1988 by karn@ka9q.bellcore.com in m.cs.uiuc.edu:sci.space.shuttle */ [Sun-synchronous orbit] is a major advantage since many pictures can be taken of the same spot over time with a relatively constant illumination angle. The 57 degree inclination orbit of Atlantis's payload has some nasty characteristics as far as earth illumination goes; as the orbit plane precesses, there will be long intervals during which most of the ground underneath the satellite is in darkness, which would make a visible light satellite rather useless. /* End of text from m.cs.uiuc.edu:sci.space.shuttle */ Say again? Any satellite orbit, over the long run, spends half its time with its ground track in the night hemisphere. Sun-synchronous orbit passes over any given spot at the same local time each day (modulo 12 hours, not 24), but the time is latitude-dependent. It gets consistent viewing angles, but has the same problems with darkness. (Let me qualify this; some sun-synchronous satellites follow the sunrise-sunset line, but the low illumination angle isn't terrific for photoreconnaissance, as the shadows get really long and contrasty.) About the only exception is the Earth-Sun L2 point, which always sees the sunlit hemisphere. You'd need kind of a big scope, though, to do recon work from there. 8-) A non-sun-synchronous orbit is definitely second best because of the constant illumination angle you get with a syn-synchronous orbit, but is would be far from useless. Considering, too, that the STS cannot achieve an orbital inclination of greater than 57 degrees because of range safety considerations (look at the STS-27 ground track sometime, and you'll see) the NRO might be willing to accept second best. The idea that the device is a synthetic-aperture radar still makes a lot of sense. A-T
karn@ka9q.bellcore.com (Phil Karn) (12/15/88)
>>... The 57 degree inclination orbit of Atlantis's >>payload has some nasty characteristics as far as earth illumination >>goes; as the orbit plane precesses, there will be long intervals >>during which most of the ground underneath the satellite is in >>darkness, which would make a visible light satellite rather useless. >Say again? Any satellite orbit, over the long run, spends half its >time with its ground track in the night hemisphere.... I misspoke myself. I meant to say that with a 57 degree inclination orbit, there will be long intervals in which the *desired target area* will be in darkness during every pass of the day. There will, on the other hand, also be long intervals in which every pass over the desired target area will be in sunlight. In a sun-synchronous orbit, you get a happy medium, with two sets of passes per day over any given target area; one set will be in darkness and the other in sunlight. If bounding the maximum interval between optical "peeks" at a given spot (the central USSR, say) is important, then the 57 degree orbit is far worse than the sun synchronous one. Regarding plane change maneuvers: these are extraordinarily expensive in terms of fuel, especially in low earth orbit where velocities are high. You have to subtract out a substantial component of the velocity in one direction with rockets and add it back in another; there is no "highway" up there against which you can steer... Phil
kenny@m.cs.uiuc.edu (12/16/88)
karn@jupiter.UUCP (Phil R. Karn) writes:
PK>each day's passes over Moscow will all find it in darkness;
henry@utzoo.UUCP replies:
HS>what's the equivalent "shadow period" for, say, Sakhalin Island or
HS>Vladivostok?
[...]
HS> There is therefore quite strong evidence that either (a) the
HS> satellite does not require daylight over the USSR to function,
You're both assuming that the satellite's primary mission is observing
the USSR. It's quite possible that the `hot' areas demanding up-to-
the-minute satellite observation are at lower latitudes, say, Central
America or the Middle East. Either area has higher-probability
scenarios for US military action than the USSR.
Moreover, correct me if I'm wrong, but I make the regression of the
nodal angle about 4.3 degrees per day. This gives a 69-day cycle with
respect to the Sun, as the spacecraft's passes come 20 minutes or so
earlier each day. The spacecraft will be usable for a given
high-latitude area during about half the cycle (more in summer, less
in winter) so there will be windows of only several weeks, not several
months, that the spacecraft is nonfunctional for a given target.
Any bets that ONR is depending on the existing KH-11 to last the
winter, and planning on using the new bird come spring, when the
geometry gets much better?
A-Tjwm@stdb.jhuapl.edu (Jim Meritt) (12/16/88)
In article <22000011@m.cs.uiuc.edu> kenny@m.cs.uiuc.edu writes: }Say again? Any satellite orbit, over the long run, spends half its }time with its ground track in the night hemisphere. False. Visualize an orbit over the terminator... Disclaimer: "It's mine! All mine!!!" - D. Duck
karn@ka9q.bellcore.com (Phil Karn) (12/17/88)
>}Say again? Any satellite orbit, over the long run, spends half its >}time with its ground track in the night hemisphere. > >False. > >Visualize an orbit over the terminator... You're forgetting that such a situation can't last forever. The terminator rotates in inertial space once per year, and it also "wobbles" north and south through a total angle of 47 degrees. The spacecraft's orbit plane also precesses, with the "sun synchronous" orbit being a special case where the precession matches the mean rotation rate of the terminator. But the wobble due to the tilt of the earth's axis still means that in the long run, the satellite ground track will be half lit and half dark. Phil
peter@sugar.uu.net (Peter da Silva) (12/18/88)
In article <2721@aplcomm.jhuapl.edu>, jwm@stdb.jhuapl.edu (Jim Meritt) writes: > In article <22000011@m.cs.uiuc.edu> kenny@m.cs.uiuc.edu writes: > }Say again? Any satellite orbit, over the long run, spends half its > }time with its ground track in the night hemisphere. > False. > Visualize an orbit over the terminator... The terminator moves. Over the period of a year, the terminator moves 360 degrees (give or take a little depending on which "year" we're talking about). -- Peter "Have you hugged your wolf today" da Silva `-_-' peter@sugar.uu.net
dave@viper.Lynx.MN.Org (David Messer) (12/19/88)
In article <12694@bellcore.bellcore.com> karn@ka9q.bellcore.com (Phil Karn) writes: >>>Say again? Any satellite orbit, over the long run, spends half its >>>time with its ground track in the night hemisphere. >> >>False. >> >>Visualize an orbit over the terminator... > >You're forgetting that such a situation can't last forever. The terminator >rotates in inertial space once per year, and it also "wobbles" north >and south through a total angle of 47 degrees. The spacecraft's orbit >plane also precesses, with the "sun synchronous" orbit being a special >case where the precession matches the mean rotation rate of the >terminator. But the wobble due to the tilt of the earth's axis >still means that in the long run, the satellite ground track will be >half lit and half dark. The terminator wobbles? I thought it always was 90 degrees from the direction to the sun... Assuming one can choose the precession rate and angle of the satellite orbit, it should be possible to maintain an orbit in the plane of the terminator. -- _____________________________________________________________________________ __ _ _ _ David Messer - Lynx Data Systems / ) / ' ) ) ) dave@Lynx.MN.Org -or- / / __. , __o __/ / / / _ _ _ _ __ ...{amdahl,hpda}!bungia! /__/_(_/|_\\/ <__(_/_ / ' (_</_/_)_/_)_</_/ (_ viper!dave
karn@ka9q.bellcore.com (Phil Karn) (12/20/88)
>The terminator wobbles? I thought it always was 90 degrees from the >direction to the sun... Poor choice of words. Of course it is always 90 degrees from the sun vector. But in the earth-centered coordinate system, it moves due to the non-perpendicular angle between the earth's axis and its orbital plane around the sun. It is the orientation of the earth's axis that is most important for satellite orbits, since the perturbing effect of its equatorial bulge is what makes sun-synchronous orbits possible. Phil
henry@utzoo.uucp (Henry Spencer) (12/20/88)
In article <1716@viper.Lynx.MN.Org> dave@viper.Lynx.MN.Org (David Messer) writes: >The terminator wobbles? I thought it always was 90 degrees from the >direction to the sun... Yes, but the Earth (and the sun-synchronous orbits with it) "wobbles" with respect to the Earth-Sun line. Same effect seen from different frames of reference. To get a constant-sunlight orbit at low altitude, you need an orbit whose plane rotates once a year on an axis perpendicular to Earth's orbit. Unfortunately (as far as I know), there isn't one. The plane of a sun-synchronous orbit rotates once a year around the Earth's axis, which is 23 degrees off the desired axis. -- "God willing, we will return." | Henry Spencer at U of Toronto Zoology -Eugene Cernan, the Moon, 1972 | uunet!attcan!utzoo!henry henry@zoo.toronto.edu
jwm@stdc.jhuapl.edu (Jim Meritt) (12/23/88)
In article <3115@sugar.uu.net> peter@sugar.uu.net (Peter da Silva) writes: }In article <2721@aplcomm.jhuapl.edu>, jwm@stdb.jhuapl.edu (Jim Meritt) writes: }> In article <22000011@m.cs.uiuc.edu> kenny@m.cs.uiuc.edu writes: }> }Say again? Any satellite orbit, over the long run, spends half its }> }time with its ground track in the night hemisphere. }> False. }> Visualize an orbit over the terminator... }The terminator moves. }Over the period of a year, the terminator moves 360 degrees (give or take }a little depending on which "year" we're talking about). Yep. You don't think we can match it? Wouldn't want to have to do so with propellant though. Fortunately, there are other ways... (in general, though, for unsteered orbits I bow out. You are apparently, as best I can tell, correct) Disclaimer: "It's mine! All mine!!!" - D. Duck