Communications, Future Needs in

Space programs, whether unpiloted space probes or human spaceflight missions, must be able to send large amounts of data to and from space. In the past, data might consist of navigational and spacecraft control information, radio conversations, and data collected by onboard experiments. But with today's permanent human presence in space, and for most future missions, the amount of data is much larger. For example, video transmissions are now common, and many spacecraft that conduct experiments are collecting richer sets of data over longer periods, owing in part to greater onboard data storage capacity. Hence, the major challenges in space communications of the future are handling the larger quantities of transmitted data and extending the Internet into space.

New Generation Satellites

The need to support more data transmissions has spawned the development of a new generation of space communications satellites. The mainstay of space communications since the early 1980s has been the Tracking and Data Relay Satellite System (TDRSS). TDRSS consists of an array of five operational satellites parked in geosynchronous orbit over the Earth's equator. Rather than direct communications between a spacecraft and the ground, spacecraft communicate with TDRSS satellites, which in turn communicate with ground stations. As the name implies, these satellites act as a relay point for any communication between the ground and a spacecraft.

Besides forming the main communications link between the space shuttle and National Aeronautics and Space Administration (NASA) ground stations, TDRSS is used by many other NASA and government spacecraft. These include the Hubble Space Telescope, the Upper Atmosphere Research Satellite, the Earth Resources Budget Satellite, Landsat, the Ocean Topography Experiment, the Earth Observing System, and the International Space Station.

Recognizing it will need more capacity in the near future, NASA has recently embarked on a TDRSS modernization program. In June 2000, NASA launched TDRS-H, the first of its new generation of communications relay satellites. By the end of 2002 it planned to have two more in place: the TDRS-I and TDRS-J. The new satellites will offer the same S-band and Ku-band communications of the original TDRSS satellites. However, the newer generation satellites will also support higher bandwidth links that are necessary for transmitting data such as high-quality video and highresolution images.

The new generation satellites, like the older satellites, will support S-band communications, which operate at frequencies of between 2.0 and 2.3 GHz (gigahertz). Within the S-band communications there exists single access in which there is one back-and-forth link between the ground and spacecraft via the TDRSS satellite. This S-band single access communication channel can support data transmission rates of 300 Kbps (kilobits per second) in the forward direction (from the ground to the spacecraft via the TDRSS satellite) and up to 6 Mbps (Megabits per second) in the opposite direction. Typically, the forward transmission consists of command and control data being sent to the spacecraft, and the return transmission can include data and images.

TDRSS also supports another S-band mode of operation called multiple access, in which the TDRSS satellite receives data from more than one spacecraft source simultaneously and sends these data to an Earth station. In this multiple access mode of operation, a forward data rate of 10 Kbps and five return data streams of up to 100 Kbps can be supported.

For higher speed transmissions, TDRSS supports Ku-band communications, which transmits at frequencies between 13.7 and 15.0 GHz. Ku-band communications supports forward data rates of 25 Mbps and return rates of up to 300 Mbps. To put this into perspective, this is about 50 times faster than a 56 Kbps dial-up modem, which is commonly used to connect to the Internet.

The new satellites will also support even higher transmission rates such as Ka-band transmissions, which operate at frequencies of between 22.5 and 27.5 GHz. The Ka-band systems will allow forward data transmission rates of 25 Mbps and return rates of up to 800 Mbps. The three new satellites will be phased in as replacements for the originals, some of which have been in space for over ten years.

Extending the Internet

The new generation TDRSS will handle the larger amounts of data being sent between spacecraft and researchers on Earth. Another effort will try to extend the Internet into space. The Interplanetary Internet Project (IPN), launched in 1998, began to explore the technical challenges to pushing the boundaries of the Internet into outer space. At one end of the spectrum are straightforward matters, such as the top-level domain (TLD) name extensions to be approved for use in space. On Earth, we use country TLD designations such as .uk or .ca (for the United Kingdom and Canada, respectively). In space, the naming structure might be similar including TLD designations for each planet or spacecraft. Other issues that are being investigated are how to handle the basic transmission of data. Existing Internet technology will not work in space applications, largely because of the great distances data must travel. Specifically, many of the underlying communication protocols used to carry Internet traffic, to surf the web, and to access information will not work efficiently over the vast reaches of space.

The downfall of using existing communications technology for an interplanetary Internet is the delay encountered when packets must traverse interplanetary distances. For that reason, the IPN is looking into new protocols and technologies to carry Internet traffic in space. For instance, proposed Interplanetary Gateways could serve regions of space. Combined with perhaps new Internet communications protocols, this potential technology could avoid the problems created by the long distances and transmission times in space. For example, if a person on Earth were communicating with someone on Mars, rather than sending individual communications packets and acknowledgements back and forth between the two, an Earth-based gateway would send the acknowledgement and then pass the packet between Earth and Mars to a similar Martian gateway.

Once such technologies are developed, the next thing needed would be an interplanetary Internet backbone to carry the traffic. NASA is already studying an idea for a Mars network of multiple orbiting satellites. These satellites would be launched over several years, possibly starting in 2005. This system would create high-speed connections between Mars and Earth that could be used as the basis of an interplanetary Internet backbone.

see also Communications for Human Spaceflight (volume 3); Guidance and Control Systems (volume 3); Interplanetary Internet (volume 4); Satellites, Future Designs (volume 4).

Salvatore Salamone

Bibliography

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Gedney, Richard T., Ronald Schertler, and Frank Gargione. The Advanced Communications Technology Satellite: An Insider's Account of the Emergence of Interactive Broadband Services in Space. Mendham, NJ: SciTech Publishing, 2000.

Heck, André, ed. Information Handling in Astronomy. Boston: Kluwer Academic Publishers, 2000.

Kadish, Jules E., and Thomas W. R. East. Satellite Communications Fundamentals. Boston: Artech House, 2000.