wallagh@tjalk.cs.vu.nl (Wallagh Serge R) (03/23/89)
I still don't understand why you need 3 satelites to cover all the communication with the Shuttle. Aren't there enough tracking stations on the ground? There is (at least) a couple in the US, some in Europe and in Australia. That should be enough , I think. But it isn't. Why? An other question: What are the costs for 1 Kg payload in the space shuttle, ariane and (if it's known) with the Russian satelites.
johnson@hpcilzb.HP.COM (Phil Johnson) (03/25/89)
> I still don't understand why you need 3 satelites to cover all the > communication with the Shuttle. > Aren't there enough tracking stations on the ground? The shuttle flies in a relatively low orbit--just a few hundred miles, compared to 22,300 miles for geosynchronous communications satellites. At this low altitude, the shuttle is only "visible" over small patch of the Earth--a radius of a thousand miles, or something like that. Most of the orbital path is not within the coverage area of any (friendly) ground station. If you've ever looked closely at the orbital map displayed at Mission Control, you may have seen odd-shaped "circles" at various places. These represent the coverage areas of particular ground stations (I suppose they're not round due to topographical or atmospheric effects on the tracking dish at low angles). From this map it's obvious how little of the earth's surface is within these coverage areas. On the other hand, communications satellites in high orbit can cover the better part of a hemisphere. Phil (then again, maybe I don't know what I'm talking about at all) Johnson
henry@utzoo.uucp (Henry Spencer) (03/25/89)
In article <2102@botter.cs.vu.nl> wallagh@cs.vu.nl () writes: >I still don't understand why you need 3 satelites to cover all the >communication with the Shuttle. >Aren't there enough tracking stations on the ground? Tracking stations are expensive, and NASA would like to close them down in favor of satellite relaying. >There is (at least) a couple in the US, some in Europe and in Australia. >That should be enough , I think. But it isn't. Why? Remember that tracking stations can talk to the shuttle only when it is above their horizon. It is at a fairly low altitude, so the circle of coverage of a ground station isn't all that large. You need a *lot* of stations for continuous coverage. >What are the costs for 1 Kg payload in the space shuttle, ariane and >(if it's known) with the Russian satelites. Depending on exactly which set of numbers you believe (there are several possibilities for government-run launchers), shortly after Challenger essentially all viable launchers in the West cost about $11000/kg for government launches to low orbit. This includes the shuttle; the NRC study of the shuttle program found that (contrary to certain partisan propaganda) the shuttle was not significantly more expensive in practice than expendables, although it wasn't any cheaper either. Expendable prices have come down somewhat since, for those launchers that have commercial competition. The lowest number I've seen was about $4500/kg, and I suspect that was based on very optimistic assumptions. These are numbers for fairly large launchers; the small ones are pricier. I'm afraid I don't remember the current price for commercial launches on Soviet boosters, but it's safe to assume that they're a factor of several less. The Soviet launchers are less sophisticated and are built in much larger numbers on stable, well-established production lines. -- Welcome to Mars! Your | Henry Spencer at U of Toronto Zoology passport and visa, comrade? | uunet!attcan!utzoo!henry henry@zoo.toronto.edu
petej@phred.UUCP (Pete Jarvis) (03/28/89)
In article <2102@botter.cs.vu.nl> wallagh@cs.vu.nl () writes: > >I still don't understand why you need 3 satelites to cover all the >communication with the Shuttle. >Aren't there enough tracking stations on the ground? >There is (at least) a couple in the US, some in Europe and in Australia. >That should be enough , I think. But it isn't. Why? > The TDRS satelites are designed to replace the ground tracking stations to save NASA $30 million a year. They also fill in the gaps that the ground stations could not provide. They also provide for more timely down-linking of telemetry data from the Shuttles so they don't have to "save" it and down-link it later when they are over the appropriate ground station. Last, but not least, the TDRS will provide an important link to the Hubble Space Telescope allowing image data to be transmited to its ground station (Reston, Virginia) on a continous basis. Peter Jarvis - Physio-Control -- Redmond, WA.
pat@hfsi.UUCP (Pat) (03/29/89)
In article <2516@phred.UUCP> petej@phred.UUCP (Pete Jarvis) writes: >In article <2102@botter.cs.vu.nl> wallagh@cs.vu.nl () writes: >> >>I still don't understand why you need 3 satelites to cover all the >>communication with the Shuttle. >ground station. Last, but not least, the TDRS will provide an important >link to the Hubble Space Telescope allowing image data to be transmited >to its ground station (Reston, Virginia) on a continous basis. My understanding is Freedom command is in Reston, VA. THe HST is controlled by GFSC Greenbelt MD for Flight Dynamics and satellite engineering. The STScI Space Telescope Science Institute at JHU Baltimore MD will receive all scientific data and schedule time for the HST. STScI does not have a TDRS downlink, they have a Hi-Speed line to Goddard, that is unless Dr Jackson of STScI was mis-informed. Pat Bahn
henry@utzoo.uucp (Henry Spencer) (03/29/89)
In article <2516@phred.UUCP> petej@phred.UUCP (Pete Jarvis) writes: >... Last, but not least, the TDRS will provide an important >link to the Hubble Space Telescope allowing image data to be transmited >to its ground station (Reston, Virginia) on a continous basis. This is literally correct but gives the wrong impression. The HST is not the only satellite using TDRS relaying; things like the most recent Landsats are set up for it as well. It's intended to be a general relay system for major low-orbit satellites. The HST is merely its most visible near-future customer. -- Welcome to Mars! Your | Henry Spencer at U of Toronto Zoology passport and visa, comrade? | uunet!attcan!utzoo!henry henry@zoo.toronto.edu
marcus@illusion.UUCP (Marcus Hall) (03/30/89)
In article <2516@phred.UUCP> petej@phred.UUCP (Pete Jarvis) writes: >In article <2102@botter.cs.vu.nl> wallagh@cs.vu.nl () writes: >>I still don't understand why you need 3 satelites to cover all the >>communication with the Shuttle. >>Aren't there enough tracking stations on the ground? >>There is (at least) a couple in the US, some in Europe and in Australia. >>That should be enough , I think. But it isn't. Why? > >The TDRS satelites are designed to replace the ground tracking stations >to save NASA $30 million a year. Well, at $100 million for each TDRS, plus about $250 million in launch costs (okay, so there were a few other things happening on the mission, but the TDRS was the primary payload) we'll call it $300 million per TDRS in orbit. Not counting TDRS-C at all, the 3 TDRS's come to $900 million. This would keep a $30 million per year system of ground stations operating for 30 years. Is TDRS likely to last long enough to reach economic payback? Note that NASA already is planning for the follow-on advanced communication satellite, so it doesn't seem like they're waiting for 30 years to replace the TDRSS. > They also fill in the gaps that the >ground stations could not provide. They also provide for more timely >down-linking of telemetry data from the Shuttles so they don't have to >"save" it and down-link it later when they are over the appropriate >ground station. Last, but not least, the TDRS will provide an important >link to the Hubble Space Telescope allowing image data to be transmited >to its ground station (Reston, Virginia) on a continous basis. > >Peter Jarvis - Physio-Control -- Redmond, WA. Yes, there are other advantages to the TDRSS, and the ground stations would need upgrading to get the communication capacity of the TDRSS, and even then it would not provide the constant coverage required for efficient use of the HST, etc. I think that you have to look at it as an enhancement of the current capabilities instead of a cost-saving measure. marcus hall marcus@illusion.UUCP ..!mcdchg!illusion!marcus
petej@phred.UUCP (Pete Jarvis) (03/30/89)
In article <1989Mar28.234309.2079@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes: > >This is literally correct but gives the wrong impression. The HST is >not the only satellite using TDRS relaying; things like the most recent >Landsats are set up for it as well. It's intended to be a general relay >system for major low-orbit satellites. The HST is merely its most >visible near-future customer. >-- I wasn't giving the wrong impression. I know the TDRS is used for other things, but didn't have time to list everything. What was wrong was my saying that the HST data was downlinked to Reston,VA. It's not, as pointed out by someone else. I had that confused with Freedom Space Station. Peter Jarvis...........
jbs@fenchurch.mit.edu (Jeff Siegal) (04/03/89)
>In article <1989Mar28.234309.2079@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes: >[...] >[TDRS is] intended to be a general relay >system for major low-orbit satellites. What is the bandwidth of the TDRS system? Jeff Siegal
petej@phred.UUCP (Pete Jarvis) (04/04/89)
In article <11419@eddie.MIT.EDU> jbs@fenchurch.UUCP (Jeff Siegal) writes: > >What is the bandwidth of the TDRS system? > >Jeff Siegal The TDRS system goes all the way up to Ku band frequencies. Peter Jarvis..........