Frequently Asked Questions
Why does South Pole have less than 24 hours of communications?
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 - a special orbit 22,236 miles above the equator, traveling in the direction of the Earth's rotation. At that altitude and velocity, the satellite appears to remain stationary from an observer on Earth. 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, Skynet 4C, and DSCS III B7. 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. Additionally, 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 4 hours for TDRS F6, 6 hours for Skynet 4C, and 3.5 hours for DSCS B7. 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:
The South Pole Event Schedule summarizes the actual event schedule for March 12 through March 25, 2017. The TDRS F6 pass window is broken up into scheduled events. The entire pass window is always available for Skynet and DSCS; therefore, these pass windows and events are always the same.
Each week, South Pole Station representatives submit a request for time three to four weeks in advance of when it is needed. NASA takes 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 is published, 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 (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. However, this is not a requirement for Skynet and DSCS. When the satellites are visible, South Pole can use them.
Satellite Orbit Precession
The National Aeronautics and Space Administration (NASA) owns the TDRS constellation.
The Department of Defense (DoD) owns the DSCS satellite. As the DSCS constellation reaches its end of life for DoD operations, the National Science Foundation (NSF) uses the satellites as they become more inclined in their orbits.
Airbus Defence and Space owns and operates the Skynet 4C satellite.
Iridium LLC (a private company) owns the Iridium satellite system.
South Pole Station uses a satellite system called Iridium when the other satellites are unavailable. These satellites (over 70) are in low Earth orbits, which makes many of them (often at least ten) visible at South Pole constantly.
South Pole Station personnel can use Iridium for telephone calls if they follow the station’s policy; however, Iridium cannot be used for Internet access because of bandwidth. These are low data rate connections that cannot support web browsing and multiple telephone calls.
However, Iridium satellites are suited for email traffic, up to a point. An email less than 100 KB goes out any time of day. The path it uses depends on which satellite is available (TDRS, Skynet, DSCS, or Iridium). Email messages larger than 100 KB (e.x., messages that contain photos as attachments) enter a queue for transmission on the next TDRS, Skynet, or DSCS event. Additionally, any large file transfers use TDRS / Skynet / DSCS.
The TDRS satellite has two communications links:
The TDRS S-Band and Ku-Band links operate simultaneously - unless an equipment problem occurs.
The Skynet 4C satellite link operates on X-Band at a speed of 1.544 Mbps.
The DSCS III B7 satellite currently provides us with a 30 Mbps southbound connection and 10 Mbps northbound connection over X-Band.
The DSCS satellite uses a ground station positioned at the USAP building in Christchurch, New Zealand. The Skynet satellite uses a ground station in Oakhanger, England, which is operated by Airbus Defence and Space. The TDRS satellite uses the National Aeronautics and Space Administration (NASA) White Sands Complex (WSC) near Las Cruces, New Mexico. All ground stations have dedicated private communication links to the Denver USAP office where traffic is routed to appropriate networks and systems, such as the telephone network, Internet, researcher university institutions, etc.