RTCA Recommends LightSquared Upper Channel Operations "Should Not Be Allowed"

Testing conducted by the Radio Technical Commission of Aeronautics (RTCA) at the request of the FAA shows that a proposed wireless broadband service from LightSquared will interfere with airborne GPS reception.

In a report presented to the National Space-Based PNT (Positioning, Navigation, and Timing) Advisory Board on the issue, the RTCA concluded that " LightSquared terrestrial authorization would be incompatible with the current aviation use of GPS, however modifications could be made to allow the LightSquared system to co-exist with aviation use of GPS." In the executive summary dated May 26, 2011, the report addresses the issues analytically based on existing domestic and international standards and includes results of tests of four certified aircraft GPS receivers.

The study’s conclusions and recommendations are strictly based on an assumed set of operational parameters for the LightSquared system and identified source mitigations. These operational parameters would produce less Radio Frequency Interference (RFI) than if LightSquared were to operate at its fully-authorized limits. In addition, the analysis is based upon the assumption that all equipment is minimally compliant with the interference rejection requirements in harmonized domestic and international standards. Additionally since GPS is an aviation safety service, the analysis includes 6 dB safety margin as is standard practice. Results from the four receivers tested show that these receiver models are significantly more resilient to interference from the LightSquared terrestrial base stations than limits derived from the standards.

The impact of a LightSquared upper channel spectrum deployment is expected to be complete loss of GPS receiver function. Because of the size of the single-city station deployment, GPS-based operations below about 2000 feet will be unavailable over a large radius from the metro deployment center (assuming no other metro deployments are nearby). Given the situation in the high altitude U.S. East Coast scenario, GPS-based operations will likely be unavailable over a whole region at any normal aircraft altitude.

The results of this study indicate that terrestrial base station operation at the lower 5 MHz wide channel (1526.3 – 1531.3  MHz) is compatible with aviation GPS operations for all the representative scenarios (including both signal tracking and initial acquisition). The study indicates that for terrestrial base stations using only the lower 10 MHz channel at 1526 -1536 MHz, there is a small positive margin for GPS tracking (but not necessarily initial acquisition) in the presence of mean aggregate terrestrial network interference. As noted above, these conclusions are based upon specific assumptions about LightSquared operation.

The main recommendations from this aviation GPS receiver operational assessment are that from an aviation perspective, LightSquared upper channel operation should not be allowed. The RTCA says that further study is recommended to more carefully determine a refined terrestrial base station power versus frequency limit considering the determination of the lowest path loss for the low altitude enroute scenario, confirmation of acceptable receiver susceptibility for GPS initial acquisition and signal tracking in the presence of the 10 MHz bandwidth terrestrial network interference, and computation of the cumulative probability distribution function for the aggregate path loss.

FMI: www.pnt.gov, www.rtca.org