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How NASA connects with its latest moon orbiters
- — 02 July, 2009 10:14
Flying to the moon is not easy feat but communicating with as well as relaying information to and from rocketing space probes is the most critical part of the mission.
NASA's Lunar Reconnaissance Orbiter (LRO) mission launched June 18 is using modified General Dynamics Advanced Multi-Mode Transceivers that support data rates of up to 512 kbps to send health and status updates and receive command instructions from NASA.
LRO will communicate at S-Band frequencies through the NASA Ground Network and Deep Space Network (DSN). The S band ranges between 2 GHz and 4 GHz and is the usual frequency for many satellites.
NASA's DSN is made up of myriad systems. It includes an international network of antennas that supports interplanetary spacecraft missions and radio and radar astronomy observations for the exploration of the solar system and the universe.
The DSN currently includes of three deep-space communications facilities placed approximately 120 degrees apart around the world: at Goldstone, in California's Mojave Desert; near Madrid, Spain; and near Canberra, Australia. This placement permits constant observation of spacecraft as the Earth rotates, and helps to make the DSN the largest and most sensitive scientific telecommunications system in the world, NASA said.
In a white paper issued last Fall, NASA said the DSN will be modified to meet new performance and interoperability requirements for its planned moon shots. NASA stated: A small constellation of Lunar Relay Satellites (LRS) will be placed into orbits with long term stability that provide periodic coverage of the entire surface of the Moon as well as Low Lunar Orbit (LLO).
Two LRSs provide periodic coverage of the entire Moon for sortie support. Medium and high rate links will be provided between the LRS and Lunar Communication Terminals (LCT) at the Outpost. Lunar surface communications will use commercial IP network technologies running for interplanetary communications based on an open, standards-based architecture. S- and Ka-bands are employed for both the Earth-Moon long haul links and the lunar orbit-to-surface links. S-, K- and Ka- bands are used for primary surface-to-surface links while S-band is used for contingency voice channels. Standards will be coordinated with other national space agencies to ensure international interoperability.
NASA said the Interplanetary Internet must be tough enough to withstand delays, disruptions and disconnections in space. Glitches can happen when a spacecraft moves behind a planet, or when solar storms and long communication delays occur. The delay in sending or receiving data from Mars takes between three-and-a-half to 20 minutes at the speed of light, NASA said.
Unlike TCP/IP on Earth, the DTN does not assume a continuous end-to-end connection, NASA said. In its design, if a destination path cannot be found, the data packets are not discarded. Instead, each network node keeps the information as long as necessary until it can communicate safely with another node. This store-and-forward method means information does not get lost when no immediate path to the destination exists. Eventually, the information is delivered to the end station.
For its primary mission, LRO will orbit above the moon at about 31 miles, or 50 kilometers, for one year. The spacecraft's instruments will help scientists compile high resolution, three-dimensional maps of the lunar surface and also survey it at many spectral wavelengths. A series of four engine burns through June 27 will finalize LRO's initial orbit. During this phase, each of its seven instruments is checked out and brought online.
The satellite will explore the moon's deepest craters, examining permanently sunlit and shadowed regions, and provide understanding of the effects of lunar radiation on humans. LRO will return more data about the moon than any previous mission.
The LRO was launched in tandem with the Lunar Crater Observation and Sensing Satellite, or LCROSS which will ultimately slam into the moon on Oct. 9. The satellite will search for water ice in a permanently shadowed crater at the moon's south pole.
LCROSS and its attached Centaur upper stage rocket are now in a long, looping polar orbit around Earth and the moon. Each orbit will be roughly perpendicular to the moon's orbit around Earth and take about 37 days to complete. Before impact, the spacecraft and Centaur will make approximately three orbits.