The Federal Communications Commission (FCC) announced in mid February that it would ask for public comment on its intention to revoke its conditional waiver granted to LightSquared in January 2011 to build a network of L-band ground stations to augment broadband data communications satellites. LightSquared's proposed ground-to-ground and satellite-to-ground network would have supported a new generation of mobile broadband communication devices, such as smart-phones, laptops and PDAs.
Several federal agencies objected to LightSquared's plan on the grounds that it had the potential to degrade or deny GPS signal reception. So, they recently petitioned FCC to pull the plug on LightSquared.
FCC's intended revocation of the LightSquared waiver is a major setback to the Commission because it wants to open up as much as 500 MHz of bandwidth for broadband connectivity by 2020 according to its“Connecting America: The National Broadband Plan”. The strategic plan was published in March 2010 at the behest of the U.S. Congress and the Obama administration with a prime goal of ensuring every American has access to broadband connectivity at competitive prices. In June 2010, the White House released a memorandum directing the heads of executive branch departments and agencies to work with the FCC to make possible this goal.
The Plan states that wireless broadband is critical to achieving this objective and that the federal government must ensure “efficient allocation and management of . . . spectrum . . . to encourage network upgrades and competitive entry.”
This requires identifying and freeing up radio frequency spectrum. At present, the FCC only has 50 MHz in free spectrum inventory, a tiny slice of what will be needed to provide coast-to-coast high-speed wireless connectivity.
The Plan, though, potentially collides with several civil aviation radio frequency uses within the 225- to 3700-MHz band, including emergency locator transmitters, ILS glidepath transmitters and ADS-B universal access transceivers for light air–craft, plus flight test telemetry, ATCRBS and Mode S transponders and Inmarsat satcoms, along with GPS and other global navigation satellite system receivers.
The FCC intends to license up to 90 MHz in the L-band Mobile Satellite Spectrum for wireless ground stations and that has the aircraft industry deeply concerned. This is an ultra-quiet niche of the L-band spectrum shared by global navigation satellite systems, along with Inmarsat and Iridium satcom systems. Satcom and satnav systems rely upon ultra-sensitive receivers to detect extremely weak signals transmitted from satellites orbiting 420 nm to 19,300 nm above the earth's surface.
GPS space vehicles, for instance, only transmit about 20 watts in the 1575.42-MHz L1 frequency band and the satellites rotate in mid-earth orbit just under 11,000 nm away. By the time the signal reaches GPS receivers, it can be as weak as 10-16 watt. That's roughly comparative to sitting across a stadium from someone who is whispering to you and yet you're able to hear the message in spite of background noise from flags waving, flies buzzing and birds chirping. That requires very sensitive and selective hearing. Your ears have to be able to filter out background noise that's at least as strong as the whisper, if not stronger.
If crowds in the stadium start cheering, then it drowns out the whisper. Similarly, if strong signals are broadcast in L-band frequencies near the GPS frequencies, then they drown out the faint signal received from the navigation satellites.
High-precision GPS receivers use 20.46 MHz of bandwidth centered on 1575.42 MHz in the 1559- to 1610-MHz band reserved for most satellite navigation systems. Current generation, civil aviation TSO-C145/C146 WAAS receivers are designed and certified to have peak sensitivity from 1585.19 MHz to 1585.65 MHz. The next generation of GPS receivers, which will be compatible with Galileo among other satnav systems, will need at least 24 MHz of bandwidth centered on the same L1 frequency because they will provide higher precision position fixing and other advanced features. They will have peak sensitivity from 1560.42 MHz to 1590.42 MHz, thus using 50% more bandwidth than today's aviation spec GPS systems.
The highest precision GPS systems, such as those used for survey work, actually use wider bandwidth receivers. In essence, higher precision GPS receivers need greater bandwidth. These systems look at signals in adjacent bands outside of the satnav spectrum, but they have filters that prevent weak broadcast signals in neighboring bands from causing interference.
Aviation grade GPS receivers are tuned to reduce receiver sensitivity above and below the 1559- to 1610-MHz band to minimize interference. Receiver sensitivity is linearly and gradually decreased outside of 1575.42 MHz ±10.23 MHz with so-called “shadow mask” filters in order to earn TSO C145/C146 certification. The shadow mask filter is carefully engineered to enable the GPS receiver to use a broad range of L-band frequencies in neighboring bands to maximum position fixing precision and yet reject noise in those bands that would degrade or disrupt performance. Below and above the 1559- to 1619-MHz band, for instance, an aviation-grade GPS receiver is able to filter out signals from low-power L-band Inmarsat and Iridium satcom systems. Well-designed civil GPS receivers actually use 30 to 40 MHz of bandwidth centered on 1575.42 MHz.
But the shadow mask filters for aviation GPS receivers were not designed to provide immunity from extremely strong signal sources in neighboring L-band sectors, such as those that might come from high-power ground stations rather than satellites. It's as though they're trying to listen to a whisper from across a jammed stadium during a Super Bowl touchdown.