It has to do with geography. South Pole is as far south as you can go on the planet. The Earth’s curvature blocks South Pole from seeing most satellites in what is called geosynchronous orbit. That’s a special circle around the earth on a plane that cuts through the equator where satellites appear to remain over one location. These satellites are similar to those that broadcast satellite TV to the satellite dishes mounted on houses.
The South Pole Station uses high inclination geosynchronous satellites. Specifically, TDRSS and GOES. Inclination is an orbital parameter that describes the amount of north-south movement a satellite makes in its orbit over 24 hours. When the value is great enough (roughly 8.7 degrees), the geosynchronous satellites are visible at South Pole.
All satellites are in elliptical orbits with an inclination parameter. Most are kept very small (less than 0.5 degrees) with periodic station keeping maneuvers. This makes terrestrial ground station design simpler. It also keeps them from being visible at South Pole. Also, over 24 hours all geosynchronous satellites have a ground trace that looks like a figure eight (8). Satellites with large inclinations have bigger figure eights. When the lower lobe of the 8 extends below 8.7 degrees south, the satellite is visible at South Pole.
The daily satellite visibility ranges from roughly 3.5 hours each for TDRS F4, F5, and F6 to roughly 7 hours for GOES-3. If you look at the Satellite Elevation Angles Graphic , you can see the elevation angles are very low. There are few ground stations in the world that look at satellites with the low elevation angles used at South Pole.
There are several things that affect the connection time:
See the Satellite Pass Windows Graphic . The plot and table show when satellites are visible at South Pole for December 5 through 6, 2009. Notice the GOES/TDRS F4 and TDRS F5/F6 overlap. South Pole can use only one satellite at a time during an overlap. Using GOES and all of the TDRS F5 pass windows represents a best case for event schedule for South Pole Station. That would give 11 hours and 14 minutes of communications time.
The South Pole Event Schedule Graphic summarizes the actual event schedule for December 5 through 6, 2009. The F5 pass window is broken up into three events. For GOES the entire pass window is always available; therefore, GOES pass windows and events are always the same. The data table shows South Pole has a total of 9 hours and 39 minutes of communications time between the two satellites compared to the theoretical maximum duration of 11 hours and 14 minutes.
Each week, South Pole Station representatives must submit a request for time three to four weeks in advance of when it’s needed. NASA takes all the requests from all of the organizations that use the satellites and builds a schedule that makes the most efficient use of the network. Many things go into the schedule, including mission priority, emergencies, spacecraft status, mission status and needs, ground station status, scheduled maintenance, etc. After the schedule comes out, NASA issues a schedule of TDRS Unused Time (TUT) that is available on a first come, first serve basis. South Pole Station can add TUT to our custom NASA built Confirmed Event Schedule to get even more time.
Putting the schedule together, managing and updating requests, making sure scheduled events occur, checking TUT (it varies hourly), trouble shooting problems, and working with NASA personnel requires the effort of a person dedicated to that task in the Denver USAP office. GOES on the other hand, does not have this requirement. When the satellite is visible South Pole can use it.
Satellite Orbit Precession
The National Aeronautics and Space Administration (NASA) owns the TDRS constellation.
The National Science Foundation (NSF) owns the GOES satellite, which was turned over to the NSF by the National Oceanic and Atmospheric Administration (NOAA) under a Memorandum of Agreement in the 1990s.
Iridium LLC (a private company) owns the Iridium satellite system and contracts with Boeing to operate it.
South Pole Station uses a satellite system called Iridium when the other satellites are unavailable. These satellites (over 70) are in low Earth orbits that make many of them (often at least ten) visible at South Pole all the time.
South Pole Station personnel can use Iridium for telephone calls if they follow the station’s policy on making such calls; however, Iridium cannot be used for Internet access because of bandwidth. These are low data rate connections that cannot support Internet surfing and multiple telephone calls like the TDRS and GOES satellites can.
However, Iridium satellites are suited for email traffic, up to a point. An email less than 100 KB goes out any time of the day. The path it uses depends on which satellite is available (TDRS, GOES, or Iridium). Email messages larger than 100 KB (for example, those messages that contain photos as attachments) enter a queue for transmission on the next TDRS or GOES event. Also, any large file transfers use TDRS and GOES.
The GOES satellite operates at 1.544 Mbps into South Pole Station and 1.024 Mbps out of the Station.
The TDRS satellite actually has two communications links:
The TDRS S-Band and Ku-Band links operate simultaneously (unless there is an equipment problem).
The GOES satellite uses a ground station at the University of Miami operating under contract to the National Science Foundation (NSF) . The TDRS satellite uses the National Aeronautics and Space Administration (NASA) White Sands Complex (WSC) near Las Cruces, NM. Both ground stations have dedicated private communication links to the Denver USAP office where traffic is routed to appropriate networks and systems, like the telephone network, Internet, researcher university institutions, etc.