LightSquared Claims New Antenna Solves GPS Interference Issue
By J. Mac McClellan, Director of Publications, EAA 747337
October 31, 2011 – LightSquared, the company that plans to build hundreds of broadcast stations to cover the U.S. with 4G broadband service, has renewed its claims that antenna design can prevent its signals from interfering with GPS reception.
The frequencies LightSquared plans to use are adjacent to the GPS frequencies. The GPS signal that travels from satellites orbiting thousands of miles above the earth are weak and testing has shown the powerful LightSquared signal can interfere with reception, particularly for aviation GPS receivers and for precision devices used in agriculture, public safety, survey, and other applications.
LightSquared now claims that at least three antenna makers have designed replacement GPS antennas that will prevent possible interference. LightSquared says these new antennas could replace existing GPS antennas quickly and easily and for as little as $6.
It is a fact that antennas can be designed to reject certain frequencies while not reducing sensitivity to the desired frequency. An antenna can also be designed to reject signals from one direction - say the horizontal or below - without harming reception from signals in other directions.
However, none of these antenna designs have been demonstrated to prevent possible interference in the real world of aviation. And to determine if the antennas can prevent interference will take thousands of hours of flight testing in a variety of airplanes operating at many locations across the LightSquared coverage area.
Perhaps even more important for GPS aviation users is that an antenna is a part of the aircraft structure. The antenna must be strong enough to withstand the air loads of flight and this is a very important issue in high-speed airplanes. The antenna profile must not change the airflow in such a way that diminishes airplane performance or alters an aircraft’s flying characteristics. And the antenna must demonstrate how and if it collects ice in flight, and how any ice accumulation will change the loads on the antenna and the wake it creates.
Any antenna must also earn specific approval for installation on each type of airplane. The low-profile GPS antennas now in use have been approved on a wide range of piston airplanes because of the small size of the antenna and the small loads it places on the aircraft structure. But antenna approval and specific location of the antenna on the airframe is a much more complicated issue at jet speeds. Typically antennas earn approval only after analysis on each specific type of airplane.
The bottom line is that even if an eventual antenna design change can prevent GPS interference from LightSquared signals it would require many years to design such antennas for aircraft, gain approval for each antenna installation, and then install the new antennas on the entire fleet. Since every category of aircraft, including military, airline, and general aviation, relies on GPS, and since the FAA NextGen air traffic control system is built around GPS navigation, possible interference must be prevented unless every airplane relying on GPS can be upgraded to a new antenna.
Newly designed GPS antennas may be able to demonstrate rejection of LightSquared interference in a laboratory setting, or perhaps even in flight testing, but that still doesn’t make an antenna change a viable solution to the interference problem in aviation.