alb@alice.UUCP (07/16/83)
Once all three TDRS satellites are up and working, the shuttle will be able to sustain ground-space communications 100% of the time, NOT 80-90%. As for Spacelab, as long as TDRS-1 is found to be functional, Spacelab will go up in September as planned, but it will only be in contact with the ground 20% of the time, so some data will be lost. At least, this is what NASA says now. If TDRS-1 does not work, STS-9 will not carry Spacelab.
Brahms%USC-ECLC@sri-unix.UUCP (07/18/83)
From: Bradley S. Brahms <Brahms@USC-ECLC>
After much pain and anguish, NASA got the first satellite into its
final orbit. I believe there are three planned with (i believe)
a forth as a backup. Once the satellites are operation, the
shuttles will be able to be in contact with earth about 85-90%
of the time.
Added Note: Because of the problem with the first satellite,
I've heard that there is a possibility that the Space Lab inaugural
flight may be delayed. Apparently they don't want the Space
Lab out of contact for a long period of time.
-- Brad Brahms
(arpa: Brahms@USC-ECLC)
P.S. Please correct me on any of the above if I'm wrong.
-------CSvax:Pucc-H:Physics:els@pur-ee.UUCP (07/20/83)
I beg to differ, but the shuttle has a line of sight to TDRS-1 LESS than
50% of the time. I haven't worked it out yet(in the literature they call that
'exercise left for reader'), but geostationary orbit isn't very high compared
to the diameter of the earth. Common sense tells you that to be visible 50%
of the time, TDRS would have to be infinitely far away. When the TDRS system
is operational, the three satellites (separated by 120 degrees) will each be
visible from the ground to about 1/3 of the globe. From its slightly higher
vantage, the shuttle ought to be able to see any given one more than 1/3 of the
time, roughly 40% is my guess.
els{Eric Strobel}
pur-ee!pur-phy!elskarn@eagle.UUCP (Phil Karn) (07/21/83)
In the interests of reducing netnews traffic in general and flame reduction in particular, I've worked out the percentage visibility between a shuttle orbiter and a geostationary satellite. Using my orbit program and a bit of geometry, I stepped through a one day period in 10 second steps, counting the number of steps in which the shuttle can see the geostationary satellite. The answer is 54.87%. Assumptions: 1. The shuttle is in a ~300 km, 28.46 deg inclination circular orbit. (Specifically I used STS-7.) 2. Not knowing the final location of TDRS-1, I used SBS-2 which is parked at 97 deg west. The actual position won't matter when averaged over a long interval, since the TDRS will rotate once per day around the shuttle's orbit plane. 3. The earth is perfectly spherical for purposes of visibility. 4. Communications aren't cut off until the earth itself actually blocks the direct line-of-sight path. (I.e., the atmosphere is ignored.) Phil
halle1@houxz.UUCP (07/21/83)
Not true. Common sense, and a little pencil and paper sketch, says that the satellite must be infinitely high for 50% of the GROUND to see it, but for something else in orbit, it could be seen significantly more than 50% of the time without being so far away. The exact amount would depend on the altitudes of the two objects. I haven't worked out the situation for a geosynchronous orbit and the shuttle. I expect that 33% is about right. However, that does not alter the fact that one could easily set up a situation where two satellites would be sufficient for 100% contact.
moore@ucbcad.UUCP (07/22/83)
#R:sri-arpa:-301500:ucbcad:9900001:000:851 ucbcad!moore Jul 21 14:19:00 1983 Just to put a silver spike through this topic: if you have two objects orbiting at radii of R1 and R2 over a planet of radius r, then the maximum angle the two objects can be separated and still see each other is given by Theta_Max = arccos(r/R1) + arccos(r/R2) For r = 4000 miles (Earth radius?), R1 = 26300 miles (geosynchronous radius), R2 = r + 100 miles (shuttle radius?), we get Theta_Max ~= 94 degrees, so the shuttle will be within line of sight of the communication satellite 2*94/360 or ~52% of the time. BTW, a geosynchronous satellite covers 45% of the equator and 42% of the earths surface. This is what COULD be covered, according to the geometry of the problem; I don't know if the reception is at all acceptable at the fringes of the covered region. Peter Moore ...!ucbvax!moore (USENET) moore@berkeley (ARPANET)
REM@MIT-MC@sri-unix.UUCP (07/24/83)
From: Robert Elton Maas <REM @ MIT-MC> Since the link from TDRS-1 to the nearest ground station is line-of-sight 100% of the time (after all, TDRS-1 is supposed to be geoSTATIONARY), the link from the ground station to Houston is 100% of the time via various conventional means, and the link from the orbiter to TDRS-1 is line-of-sight 50% of the time, I don't see why the overall duty time is only 20% if TDRS-1 is working. Perhaps somebody else can explain the calculation for me?