Iridium may serve as ADS-B backup in Alaska

 - February 1, 2007, 11:30 AM

The FAA’s Alaska Region this year will assess the suitability of a communications satellite system with an unusual history to supplement its Capstone automatic dependent surveillance-broadcast (ADS-B) project. Called Iridium, and employing 66 satellites orbiting the earth, the system’s operating company in 2000 faced bankruptcy, and only an 11th-hour intervention and buyout prevented all 66 space vehicles being deliberately destroyed by fiery reentries into the earth’s atmosphere so as to not worsen the company’s operating losses. Following acquisition of the system, Iridium’s new owners obtained significant contracts with the Department of Defense and other customers, and its future now seems secure.

But why does Capstone even need a supplementary satellite system? In ADS-B, widely dispersed ground-based transceiver (GBT) stations are key elements, receiving transmissions from equipped aircraft and retransmitting them to distant ARTCCs, and also receiving information from the ARTCCs and then rebroadcasting it to aircraft in their local areas. But flying down a valley in Alaska, or being behind an intervening mountain range, can block ADS-B’s UHF transmissions to and from the GBTs, effectively negating the system’s safety benefits.

The Iridium system is uniquely suitable to supplement ADS-B. Its 66 satellites circle the earth in polar orbits, traveling north and then south over the poles, with 11 satellites in each of six orbits spaced 60 degrees apart and ensuring that at least three satellites are always in view, with the number increasing closer to the poles. Additionally, most other communications satellites, such as the GEOs used in WAAS, orbit the earth over the equator, and above 70 degrees north or south their signals become too low on the horizon to be usable. Iridium satellites also orbit at around 480 miles high–compared with the GEOs’ 23,000 mile altitudes–and can therefore use much smaller, lower-power transceivers and simple antennas that are affordable for single-engine commercial operators.

Last year FAA and University of Alaska aviation officials, along with engineers from Iridium service provider General Dynamics, flew Iridium proof-of-concept evaluations in a Cessna 180 along routes where terrain signal blocking would occur between the aircraft and the GBTs. The onboard electronics package, which included an Iridium handheld telephone, simultaneously transmitted ADS-B data to a GBT and an Iridium satellite. Recordings of both tracks correlated closely until the GBT lost line-of-sight reception from the aircraft, but the Iridium track continued faultlessly, with the GBT track rejoining it when line-of-sight reception returned.

Last month the FAA awarded General Dynamics a $2 million contract to undertake more detailed flight evaluations. The company will now conduct exacting assessments of Iridium’s integrity, reliability, availability and continuity.

Yet if Iridium is so promising, why was its original system owner driven to the point of nearly destroying its 66 operating satellites in 2000? In advanced technology, as in other endeavors, market-entry timing is critical. When introduced in 1998, Iridium was a technical triumph, providing voice and data messaging anywhere on the face of the earth  using battery-powered, handheld units. But faced with overwhelming competition from low-cost cellphones and the Internet, it was also a commercial disaster. Forecast sales of tens of thousands of Iridium units never materialized to validate the operating company’s investment of more than $5 billion, and bankruptcy followed less than three years later, with its total assets, including the satellites–today operated for the new owners by Boeing–being sold for $25 million.

Now Iridium is back, and if it meets the FAA’s operational expectations it could provide significant benefits, not only to Alaska’s pilots but to pilots of ADS-B-equipped aircraft flying over the oceans and other remote corners of the world, unhampered by the absence of ground-based stations.

Capstone in Brief

The FAA’s Capstone program is designed to provide “near NAS quality” services to small commercial aircraft flying at low altitudes in remote areas, far beyond the reach of standard VHF communications and well below the coverage of ATC radar. Participating aircraft carry special ADS-B transceivers which, once per second, transmit their GPS position and their identification, course, speed, altitude and flight path over a common radio datalink. All other ADS-B-equipped aircraft within reception range receive this data, which then appears on the moving maps of their cockpit displays of traffic information (CDTI), thereby providing each pilot with a clear picture of all proximate aircraft.

In addition, the aircraft transmissions are received by unmanned ground-based transceiver (GBT) stations spread throughout the operations area. The GBTs then retransmit this data via land lines, or microwave or satellite links, to the distant ARTCC, where the low-altitude movements are blended into the overall traffic picture. These links also allow the ARTCC to send local traffic and flight information back to the GBTs, which then retransmit it over the ADS-B frequency as traffic information service-broadcast (TIS-B) and flight information service broadcast (FIS-B). TIS-B automatically adds the positions and movements of non-ADS-B-equipped aircraft to the Capstone pilots’ CDTIs, while FIS-B provides pilots with weather reports and forecasts, notams, pireps and other important flight data.

Capstone started in 2000 around Bethel, in western Alaska. Today, the program has more than 200 participating aircraft. A second phase will begin soon in the Juneau/Ketchikan region, involving a similar number of participants.
“Capstone is simply about safety,” said FAA program manager Jim Hallinan. “We have the most challenging environment of all 50 states, but independent studies show safety improvements up here of between 25 percent and 40 percent.” Hallinan cited the case of an aircraft that crashed shortly after departing a remote strip at night in midwinter. “We couldn’t find it with a conventional air search, so we asked Anchorage ARTCC where its recorded ADS-B track stopped. A helicopter then flew right to it and picked up the injured pilot, who the medics said would not have lasted the night out there.”