Tue, Jun 14, 2011
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.
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