News Analysis: LightSquared is offering to reduce the power output of its transmitters and shift to the lower range of its permitted spectrum to resolve problems that were confirmed in the GPS Technical Working Group report released on June 30.
The long-awaited 750-page report of the GPS
Interference Technical Working Group
released June 30 confirmed what
multiple government agencies and private industry interests have been saying
since the group began its work: that LightSquared's plan for a
wireless broadband network
would prevent GPS devices from working properly
anywhere in the United States.
But even before the official release of the Technical Working Group Report,
LightSquared went public with a proposed workaround to the GPS interference
LightSquared is proposing that it use the lower 10MHz of the L-band spectrum
the company was given for its 4G LTE service and that it reduce the power to 10
percent of what the Federal Communications Commission originally authorized.
According to LightSquared Executive Vice President Martin Harriman, this would
mean that the LightSquared transmitters would only produce about 1,500 watts of
radio power, versus the 15,000 watts the FCC originally authorized.
It's worth noting that the test transmitters that the TWG used in its study
were running at about 1,500 watts of power, but they were using the upper 10MHz
of the frequency allocation as well as the lower 10MHz section. There has been
little formal testing of GPS interference using just that lower frequency
slice. Because of this, many of the Working Group
tests would have to be rerun
to see if the interference problem has been
The problem is one of basic physics and radio design. The GPS frequencies
span 20MHz of the L-band immediately adjacent to the upper 10MHz segment
granted to LightSquared. A strong signal in an adjacent channel can cause the
receiver's RF amplifiers to become saturated, even when the strong signal isn't
on the same frequency.
This can cause receiver desensitization, and it can create spurious signals
inside the receiver. The result is that radio reception in the affected
receiver may be impossible, degraded, distorted or some combination of the
Compounding the problem is that you can only do so much about it. GPS
receivers have to be very sensitive to pick up the faint signals from the GPS
satellites. Because they are so sensitive, they are also prone to interference.
You can design in what's called a band-pass filter to block some of this
interference, but a band-pass filter introduces its own problems, including
reducing the signal strength of the receiver being protected. When that
happens, the whole issue of high sensitivity comes to the fore again, and you're
basically stuck with diminishing returns.