tneff@dasys1.UUCP (Tom Neff) (09/30/88)
I know the TDRS network is supposed to provide continuous communication with a lot of non-GEO satellites, and I also know that we get partial air-to-ground coverage on Shuttle flights right now from the one TDRS successfully deployed before STS-26. As I write this, it appears Discovery is deploying TDRS-C nominally. Assuming the IUS works and the second TDRS is parked and activated, will we henceforth have continuous downlink during Shuttle missions from now on? Or will the "LOS Guam, AOS Hawaii in 6" game continue until we have three birds up there? Second question - the early diagrams had an equilateral triangle of TDRS coverage taking care of satellite and spacecraft comm. But lately I hear that when TDRS-D is deployed, TDRS-A will be parked as a hot spare while C or D assumes its duties. Are we still aiming for three live birds at some point, or are we resigned to antipodal coverage? -- Tom Neff UUCP: ...!cmcl2!phri!dasys1!tneff "None of your toys CIS: 76556,2536 MCI: TNEFF will function..." GEnie: TOMNEFF BIX: t.neff (no kidding)
phil@titan.rice.edu (William LeFebvre) (09/30/88)
NASA has been officially saying that two operational TDRSs get about 85% coverage of shuttle flights. It depends partly on the inclination of the orbit. I understand that the figure is closer to 90%. Part of the problem is the "South Atlantic Anomaly": an area of magnetic disturbance that makes communication difficult. But with two TDRS satellites, LOS will be greatly reduced and almost eliminated. The flight controllers take this as a mixed blessing: they usually take their bathroom breaks during LOS. The configuration NASA is shooting for is to have C and D fully operational on opposite sides of the globe and have A lingering around as a spare. Three satellites are not needed for full coverage. And I think that using a configuration of three in a triangle will not get any additional coverage that two opposing ones would not get. That is my limited and basic understanding. William LeFebvre Department of Computer Science Rice University <phil@Rice.edu>
jetzer@studsys.mu.edu (jetzer) (10/01/88)
In article <6732@dasys1.UUCP>, tneff@dasys1.UUCP (Tom Neff) writes: > I know the TDRS network is supposed to provide continuous communication > with a lot of non-GEO satellites, and I also know that we get partial > air-to-ground coverage on Shuttle flights right now from the one TDRS > successfully deployed before STS-26. As I write this, it appears Discovery > is deploying TDRS-C nominally. Assuming the IUS works and the second > TDRS is parked and activated, will we henceforth have continuous downlink > during Shuttle missions from now on? Or will the "LOS Guam, AOS Hawaii > in 6" game continue until we have three birds up there? > Second question - the early diagrams had an equilateral triangle of > TDRS coverage taking care of satellite and spacecraft comm. But lately > I hear that when TDRS-D is deployed, TDRS-A will be parked as a hot > spare while C or D assumes its duties. Are we still aiming for three > live birds at some point, or are we resigned to antipodal coverage? The October issue of Countdown magazine has an article about TDRS in it. There is a diagram of the expected placement of TDRS-1 and TDRS-2 (called TDRS-C until it's in orbit). Imagine that there is a large square around the earth, with the earth at the center. The two TDRSs are at adjacent corners of the square. Because of this, there will still be "about 15 percent" of the shuttle's orbit will be "in the shadow" and not covered by the TDRSs. Two ground stations have already been closed, and six more will be closed "once there are two fully operations TDRS satellites on orbit. 'Operational' in this sense does not include TDRS-1." So it does seems that TDRS-1 will be a "hot spare" once TDRS-3 comes online. TDRS-D was slated for launch during STS-29 on January 19, 1989, but that date has apparently slipped. No information concerning the relative locations of TDRS-D, -E, or -F was given. -- Mike Jetzer "Hack first, ask questions later."
tneff@dasys1.UUCP (Tom Neff) (10/02/88)
My thanks to William LeFebvre for those answers on the 2-TDRS network. The other remaining interesting question is: will TDRS-C and -D be able to see each other as well as the ground? If so, would that be useful? -- Tom Neff UUCP: ...!cmcl2!phri!dasys1!tneff "None of your toys CIS: 76556,2536 MCI: TNEFF will function..." GEnie: TOMNEFF BIX: t.neff (no kidding)
phil@titan.rice.edu (William LeFebvre) (10/02/88)
In article <6761@dasys1.UUCP> tneff@dasys1.UUCP (Tom Neff) writes: >My thanks to William LeFebvre for those answers on the 2-TDRS network. You're welcome. >The other remaining interesting question is: will TDRS-C and -D be able >to see each other as well as the ground? If so, would that be useful? If I understand things correctly, the answer is "no". They will be in opposing orbits. They will both, however, be able to see TDRS-A. I can't really see much use for it anyway. In just about any orbit imaginable, you can point your antenna at one of the two and that is sufficient. An interesting question (and one that I just now thought about): what do you do when you are switching over from one TDRS to the other? You typically only have one antenna per band, so I guess there would be a small period of LOS (loss of signal) while the antenna gets realigned. William LeFebvre Department of Computer Science Rice University <phil@Rice.edu>
karn@thumper.bellcore.com (Phil R. Karn) (10/06/88)
The South Atlantic Anomaly has absolutely nothing to do with TDRS coverage. It is simply a region of the Van Allen belts that comes somewhat closer to the earth's surface (i.e., down to low earth orbit) due to the orientation of the earth's magnetic field. Most of the radiation encountered by low orbit spacecraft is in this region. (The soft memory errors seen by the UoSAT-OSCAR spacecraft have largely been in this region). The gap in TDRS coverage is due to simple geometry: for both satellites to be visible from White Sands at reasonable elevation angles, a region near India had to be left uncovered. Phil
MorsinAc@econ.vu.nl (Triple A) (10/28/88)
In article <1934@kalliope.rice.edu>, phil@titan.rice.edu (William LeFebvre) writes: > The configuration NASA is shooting for is to have C and D fully > operational on opposite sides of the globe and have A lingering around as > a spare. Three satellites are not needed for full coverage. That was about it for TDRS-C/D, but when checking the launch schedule, I discovered they planned to get some more TDRS's up there (TDRS-E/F). So my question is, what are they supposed to do if we already got two fully operational ones (and a spare) up there? Are they to replace the old ones or what? A. Morsink - Vrije Universiteit/Free University AMSTERDAM (yes it's in Holland)
johnl@gronk.UUCP (John Limpert) (10/31/88)
In article <233@vuecon.econ.vu.nl> MorsinAc@econ.vu.nl (Triple A) writes: >In article <1934@kalliope.rice.edu>, phil@titan.rice.edu (William LeFebvre) writes: >That was about it for TDRS-C/D, but when checking the launch schedule, >I discovered they planned to get some more TDRS's up there (TDRS-E/F). >So my question is, what are they supposed to do if we already got two >fully operational ones (and a spare) up there? Are they to replace the >old ones or what? We don't have two fully operational spacecraft and a spare. TDRS-A is not "fully" operational. I'm not sure what the problems are, but it is definitely not 100% operational. TDRS-B was lost in the Challenger accident. TDRS-C is in the process of being moved to its operational location and is in a test phase. So that adds up to one semi-crippled TDRS and one soon to be operational TDRS. Two more 100% operational satellites are needed for the desired configuration of TDRS East, TDRS West and an in-orbit spare. I believe the original plan was to have 4 spacecraft in orbit, 3 operational and 1 spare. -- John Limpert johnl@gronk.UUCP uunet!n3dmc!gronk!johnl
henry@utzoo.uucp (Henry Spencer) (11/01/88)
In article <233@vuecon.econ.vu.nl> MorsinAc@econ.vu.nl (Triple A) writes: >That was about it for TDRS-C/D, but when checking the launch schedule, >I discovered they planned to get some more TDRS's up there (TDRS-E/F). >So my question is, what are they supposed to do if we already got two >fully operational ones (and a spare) up there? ... Well, for one thing, TDRS-A doesn't make a really great spare, it's not in terribly good shape. When you're relying on these things heavily, you may want a fully-functional on-orbit spare. Then too, the 2+1 configuration doesn't really give quite 100% coverage, the way they've planned to set it up. And finally, when you get really high data rates going, the capacity of any one TDRS is limited; I think one TDRS can only hack two high-speed customers at a time. Between the shuttle, the Great Observatories line (starting with Hubble), and the hypothetical space station, that's not a trivial limitation. -- The dream *IS* alive... | Henry Spencer at U of Toronto Zoology but not at NASA. |uunet!attcan!utzoo!henry henry@zoo.toronto.edu
phil@titan.rice.edu (William LeFebvre) (11/01/88)
In article <233@vuecon.econ.vu.nl> MorsinAc@econ.vu.nl (Triple A) writes: >That was about it for TDRS-C/D, but when checking the launch schedule, >I discovered they planned to get some more TDRS's up there (TDRS-E/F). >So my question is, what are they supposed to do if we already got two >fully operational ones (and a spare) up there? Are they to replace the >old ones or what? Well, TDRS is used by more than just the shuttle. It is a (more or less) general purpose data relay satellite for equipment that needs such relaying. My understanding is that E and F are going to be put up for several reasons, including extra throughput (for users other than the shuttle) and extra redundancy. A is pretty much shot at this point. I have heard that in at least some of the hardware all redundancy has been lost. I don't know what they'll do with it in the long term, especially if it loses all functionality. I also don't know where they are going to put E and F. William LeFebvre Department of Computer Science Rice University <phil@Rice.edu>