NASA’s 12th Tracking and Data Relay Satellite (TDRS) will shortly begin final processing at the Kennedy Space Center, following its delivery last week to Florida aboard a U.S. Air Force C-17 transport aircraft. Scheduled for launch atop an Atlas V booster from Cape Canaveral Air Force Station in late January 2014, the TDRS-L payload—which will be the 11th member of the satellite family to actually reach orbit, following the loss of TDRS-B in the Challenger tragedy—has been unpacked and inspected to ensure that it sustained no damage during its journey from Boeing’s Space and Intelligence Systems facility in El Segundo, Calif.
The satellite is the second in a series of three members of the new “third generation” of TDRS, the inaugural contracts for which were signed between NASA and Boeing in December 2007. Under the terms of that agreement, the aerospace giant built TDRS-K—launched in January 2013—and TDRS-L, at a cost of $695 million, in order to “ensure vital operational continuity” of an on-orbit network of communications and data-relay assets which presently support dozens of spacecraft, including the International Space Station and the Hubble Space Telescope. “The launch of TDRS-L ensures continuity of services for the many missions that rely on the system every day,” said Jeffrey Gramling, TDRS project manager at NASA’s Goddard Space Flight Center in Greenbelt, Md.
The NASA-Boeing agreement is expandable to $1.2 billion, if all options are exercised, and this indeed seemed to be the case when the space agency ordered a third satellite, TDRS-M, in November 2011. Current plans anticipate its launch in December 2015. All three satellites are designed for 15-year operational lifespans, which enables continuity until the middle or even the end of the next decade.
Emplacement of this third generation of satellites coincides broadly with the retirement of the first generation, designated “A” through “G,” all of which were launched by the shuttle in the 1980s and 1990s. Of this first generation, one (TDRS-B) was lost in the Challenger disaster, whilst two others (TDRS-A and D) have already been shut down. The others are expected to follow in the near future. A second generation of three satellites—TDRS-H, -I, and -J—were launched aboard expendable rockets between June 2000 and December 2002 and remain fully functional, despite having endured a handful of technical troubles.
With last January’s successful launch of TDRS-K, a hiatus of more than a decade since the last satellite was closed. The new third generation is visually quite distinct from its cousins of the first generation. It is based upon Boeing’s 601 spacecraft bus, first introduced more than two decades ago, but heavily upgraded over the years, and can support multiple payloads and objectives, including direct TV broadcasts and the needs of private businesses and mobile communications users. The size and output of its communications payload has also expanded, and it is capable of housing up to 60 transponders and producing 10,000 watts of power.
As well as enabling all navigation, power, propulsion, and command capabilities, the bus has twin solar arrays—each measuring 15 feet (4.5 meters) in diameter—for use whilst in direct sunlight and battery packs for use whilst in the Earth’s shadow. Its “spring-back” antennas are designed with flexible membrane reflectors, which fold up for launch and spring back into their original, “cupped” circular shape after orbital insertion. The communications hardware consists of microwave equipment, a pair of gimbaled antennas, and a phased-array antenna for forward, return, and tracking services. In addition to operating at S-band and Ku-band frequencies, the second- and third-generation TDRS provide improved overall service and substantially higher bandwidth through the Ka-band.
In March 2009, Boeing selected United Launch Alliance’s 19-story Atlas V as its vehicle of choice to deliver the third-generation satellites into orbit. Like its predecessor, TDRS-L will ride an Atlas in the “401” configuration, with a 13-foot (4-meter) payload fairing, no strap-on rocket boosters, and a single-engine Centaur upper stage. Liftoff from Space Launch Complex (SLC)-41 from Cape Canaveral Air Force Station is tentatively scheduled for 23 January 2014. As the 12th satellite in NASA’s fleet, TDRS-L will continue a proud heritage of providing near-continuous tracking, voice, and data communications relay services between ground stations and more than 20 discrete users, simultaneously, including the Hubble Space Telescope and the International Space Station. Yet TDRS arose in the post-Apollo era at one of the most uncertain times for the U.S. human space program, and even its long-awaited maiden launch in April 1983 hung for a time under the shadow of abject failure.
The concept was born in the early 1970s as one of the recommendations of a study group led by Don Hearth, then-deputy director of NASA’s Langley Research Center in Hampton, Va., which charted possible post-Apollo roadmaps for America’s future in space. The group, which included astronaut Joe Allen on its panel, felt that a system of tracking and relay satellites placed into 22,000-mile (35,000-km) geosynchronous orbits by the shuttle and operated from a single ground terminal at White Sands, N.M., would provide near-continuous voice and data traffic and eliminate the older generation of ships and costly ground stations. In fact, as well as supporting low-orbiting missions, it could relay data from satellites up to 3,000 miles (4,800 km) above Earth’s surface. Since the dawn of human space flight, astronauts had been out of contact with Mission Control for up to 80 percent of every orbit; furthermore, satellites had to tape record data and transmit it when they came within range of a tracking ship or ground station. As the shuttle effort gained momentum in the mid-1970s, it was envisaged that two TDRS relays would provide astronauts with space-to-ground voice and data links for between 85–98 percent of each orbit.
TDRS was no miracle worker. In its original incarnation it could not process or adjust communications traffic in either direction. Rather, it operated as a “bent pipe” repeater, relaying signals and data between its Earth-circling users and the highly automated ground terminal. Signals processing, therefore, occurred on the ground, and the satellite’s sophistication was devoted to its very high throughput. Located in the inhospitable New Mexico desert, White Sands provided a clear line of sight with the satellites, and its limited amount of annual rainfall meant that weather conditions would not interfere with uplink or downlink capabilities.
It was envisaged that a pair of TDRSs—one stationed over the equator, just off the northeastern corner of Brazil, known as “TDRS-East,” and a second over the central Pacific Ocean, near the Phoenix Islands, known as “TDRS-West”—would fill this urgent communications and tracking need. TDRS-A was launched in April 1983, but was almost lost when its Boeing-built Inertial Upper Stage (IUS) booster failed to insert it into its proper orbit. Only by using the satellite’s own hydrazine thrusters were controllers able to gradually maneuver it into its final location, although the result was that its operational lifetime was shortened. Ongoing problems with the IUS meant that it was almost three years before the second satellite, TDRS-B, could be launched … and that was the primary payload aboard the ill-fated Challenger on 28 January 1986.
Two more TDRS satellites (C and D) were launched in September 1988 and March 1989, the former replacing the doddery TDRS-A in the west (slightly south of Hawaii) and the latter taking up position in the east, near Brazil. Unfortunately, TDRS-C also succumbed to anomalies which affected its Ku-band relay capability. A fourth satellite, TDRS-E, was launched in August 1991 and positioned at 175 degrees West longitude to become the primary provider of communications services over the Pacific from October 1991. TDRS-A and TDRS-C, meanwhile, were relegated to the status of on-orbit “spares.”
This left only TDRS-D and TDRS-E in fully-operational status … which meant that no spare existed to support them in the event of problems. The successful arrivals of TDRS-F in January 1993 and TDRS-G in July 1995 filled this backup capability. This enabled the network to be rearranged to include two fully-operational satellites in the East and West spots, plus the fully-functional TDRS-A as a spare and the partially-functional TDRS-C designated to support NASA’s Compton Gamma Ray Observatory.
A few months before the launch of the final first-generation TDRS, in February 1995, NASA’s Goddard Space Flight Center chose Boeing to build three second-generation satellites under a contract valued at $481.6 million. Based upon the 601 “bus,” the new satellites were intended to augment the Ku-band and S-band capabilities of the first generation with the higher-bandwidth Ka-band. The ground stations at White Sands were modified to accept the new satellites. TDRS-H was launched atop an Atlas booster in June 2000, followed by TDRS-I in March 2002 and TDRS-J the following December. Although TDRS-H suffered problems with its multi-access antenna and TDRS-I lost pressure in one of its four fuel tanks shortly after launch, the second generation has supported International Space Station and other operational assets for more than a decade.
On 4 April 2013, NASA marked the 30th anniversary of the TDRS-A launch. It has been a long and rocky road for a network which was born with such promise, but very soon fell on hard times, yet matured to shine throughout the heyday of the shuttle era and today’s International Space Station. It has provided the data-relay capability for the astonishing scientific returns of the Hubble Space Telescope … and even the doddery first satellite has contributed to more down-to-earth achievements. In 1998, NASA allowed scientists at the Amundsen-Scott base in Antarctica to employ TDRS-A as a relay for research data, and it supported a medical emergency at McMurdo Station, allowing scientists to conduct a telemedicine conference with doctors in the U.S. Several of the first-generation satellites are now out of service.
TDRS-A—“the queen of the fleet,” according to NASA-Goddard’s Space Network Project Manager Roger Flaherty—was deactivated in October 2009, followed by TDRS-D in November 2011. The remaining first-generation satellites (C, E, F, and G) are expected to be retired by 2015. And by the end of that year, it is expected that the entire third-generation will be complete, with TDRS-K, L, and M inserted into geosynchronous orbit. These satellites are immeasurably more powerful than their predecessors, but, like them, they will enable TDRS to evolve through the middle of the next decade as the United States’ primary tracking and data-relay service provider for its key human-exploration programs and scientific endeavors.
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