IP Source

Wednesday 7 March 2012

GPS Spectrum Wars

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.

New York-based Harbinger Capital has been buying stock in wireless companies for several years, including a start-up venture called SkyTerra that planned to develop a hybrid 4G wireless network that would primarily use geostationary satellites with 40-watt transmitters augmented by very-low-power ground stations called ancillary terrestrial components, or ATCs for short. The ATCs are similar to the ground booster stations used by Sirius satellite radio to fill in dead spots
in urban areas within its broadcast coverage area. The FCC granted SkyTerra the authority to use ATCs to boost signal strength in 2005.

In 2010, Harbinger bought all the stock in SkyTerra and renamed it LightSquared. The business plan called for LightSquared to use to frequency bands, 1526 to 1536 MHz and 1454.2 to 1555 MHz, for space-to-earth downlinks. SkyTerra’s downlinks would have to share spectrum with Inmarsat, also licensed to use frequencies in the 1525- to 1559-MHz band. The firm paid millions of dollars to Inmarsat to reach an agreement to shift some of its satcom frequencies to make
room for LightSquared’s two downlink channels.

Few eyebrows in the GPS community were raised while all this was going on because engineers saw virtually no threat from LightSquared’s 40-watt satellite transmitters. Just as importantly, GPS engineers knew that the ATCs, used to fill in coverage gaps, would have to be limited to very low power so as not to jam reception from LightSquared’s own satellite transmissions. A side benefit was that the low-power ATCs would not cause GPS interference.

All that changed in January 2011 when LightSquared applied to the FCC for a waiver to offer terrestrial-only service for its 4G network. Freed of the need to limit ATC ground station signal strength to a whisper to safeguard 4G reception from satellites, the terrestrial-only service would have allowed transmissions that would be millions of times stronger — a roar that could have deafened GPS receivers in the neighboring band, as well as other L-band devices with sensitive receivers.

How strong would the signal from the ATCs be? LightSquared’s waiver potentially allowed it to crank up power to nearly 16 kilowatts per station. But the FCC required LightSquared to prove that there would be no significant interference with GPS. It quickly became apparent that such strong transmitters would be problematic, so LightSquared soon dialed back the peak power of the ATCs to just under 1,600 watts. It subsequently offered to use temporarily only the lower of its two L-band downlink channels in order to move farther away from the satnav band.

However, even using much lower power, the new ground station plan quickly got the attention of the GPS community, which united and created an uproar of its own. LightSquared planned to create a network of up to 40,000 ATCs, each of which could produce more than a billion times the signal strength of GPS depending upon receiver distance and elevation angle relative
to the ground station antenna. Even the most-robust aviation-grade GPS shadow mask filters operating nearby would go deaf in the presence of such a roar in a neighboring sector of L-band.

American farmers joined the protest against LightSquared, expressing concern that LightSquared’s L-band ground stations would interfere with agricultural GPS receivers they use to guide the application of fertilizers and pesticides. The charge was led by John Deere, a high-profile manufacturer of agricultural GPS devices.

International opposition to the LightSquared plan also mounted. The European Commission expressed opposition because it feared the high-powered ground stations would interfere with reception of its Galileo satnav system signals aboard aircraft fitted with that satnav system operating in U.S. airspace.

LightSquared responded by claiming that GPS receiver manufacturers design and build defective equipment, making it susceptible to interference outside of the protected 1559- to 1610-MHz frequency band. The firm said that “properly designed” GPS systems would suffer no such performance loss.

GPS manufacturers responded that the entire L-band sector from 1525 to 1559 MHz below the 1559- to 1610-MHz satellite navigation band, as well as the 1610 to 1660.5 MHz above the band, always was intended to be a “quiet” or low signal strength spectrum neighborhood. There was consensus among legacy L-band licensees that all users would whisper signals, not roar them.

As an aside, Inmarsat was noticeably absent from the LightSquared versus GPS battles even though it would share L-band spectrum with LightSquared if its system becomes operational. Inmarsat already had reaped sizable revenues from its agreements with LightSquared to shift frequencies to avoid interference from its ground station network. Because Inmarsat primarily serves users who operate in overwater and remote areas, LightSquared’s proposed network of higher-powered ground stations in the U.S. posed no threat of interference.

United Defense, Uncertain Outcome

GPS interference tests were conducted in 2010 by a technical working group, commissioned by the National Telecommunications
and Information Administration (NTIA). The group included RTCA GPS experts and LightSquared representatives. It concluded that LightSquared’s high-powered ATCs indeed could pose a threat to GPS. Further research would be needed to quantify the

In January 2012, the National Executive Committee for Space-Based Positioning, Navigation and Timing (PNT), wrote a letter to the FCC and NTIA stating that the nine federal departments and agencies it represents had reached a “unanimous conclusion” that LightSquared’s original and subsequently scaled-back plans for ATCs “would cause harmful interference to many GPS receivers.” Moreover, the letter also stated that the FAA concluded that the ATCs would interfere with GPS-based TAWS boxes as well as GPS navigation systems.

With no “practical solutions or mitigations” available, PNT said that “no additional testing is warranted at this time.” It also added that PNT fully supports the White House’s plan to open up 500 MHz of spectrum for wireless broadband. The letter was signed by deputy secretaries of Defense and Transportation Ashton Carter and John Porcari, respectively, who are executive committee co-chairmen.

LightSquared immediately complained that the GPS interference tests were rigged to put its proposed system at a disadvantage. But the firm now isn’t just battling Trimble, Garmin, Deere and other GPS manufacturers. It’s going up against the federal government.

If the FCC and NTIA disapprove LightSquared’s plan, the company financially plunges into very hot water. Billions have been invested in the technology by Harbinger Capital Partners and time is running out before marketing partners lose patience and possibly back out of agreements. Sprint, for instance, set a Jan. 31, 2012, deadline for LightSquared to gain necessary approvals to proceed with network development.

With so much at stake on both sides, the outcome of the battle between LightSquared and the GPS community is uncertain. The FCC and the White House want to make broadband as universally available to homes in the 21st century as electricity became available in the 20th century during the FDR administration.

“Politically, there’s a high degree of design to put Internet into every home,” comments one GPS expert. And LightSquared stands to reap billions in profits from wholesaling wireless broadband connectivity using the L-band spectrum.

GPS users, particularly the aviation community, have staked their future on L-band satnav. The standards for aviation grade WAAS receivers have been in place for more than a decade and thousands of boxes are in service today. Aviation industry groups claim it would take 10 to 15 years to develop and certify new GPS avionics that would be immune to interference from
LightSquared’s ground stations. Retrofitting the fleet with new gear would cost several billion dollars.

“The FCC jumped on approving LightSquared’s request before it considered the implications,” says another source. “You can’t just throw away safety services and the FAA’s next generation air traffic management system.”

“Effectively, the FCC is directing that a quiet-spectrum neighborhood be rezoned for concert rock bands at the threshold of pain. It is being suggested that its current neighbors should simply add more insulation and foot-thick windows to their houses,” says Bradford Parkinson, GPS pioneer and emeritus professor at Stanford University.

“Radio frequency spectrum is a limited natural resource. You can’t make more of it,” adds another source. L-band is appealing for wireless broadband because it suffers less signal loss in precipitation than Ku or Ka band. The demand for L-band could make the LightSquared versus GPS battle a zero sum game. One side has to lose for the other side to win.

But the battle over GPS is just one of many frequency spectrum struggles facing the aviation community. Civil users, for instance, are quietly gaining L-band spectrum near the 1435- to 1525-MHz band used for flight-test telemetry.

The military also has plenty of history on spectrum wars. Years ago, the FCC had to sort out a frequency use battle between DirectTV and Hughes Radar Systems, developer of the APQ-181 radar aboard the B-2 bomber. Later the FCC accidentally sold off that military-use frequency band to a multinational organization, requiring the U.S. Air Force to retrofit all 20 B-2
aircraft in service with a new radar. The loss of the B-2 radar band license is emblematic of problems the military is having with holding onto spectrum it needs for new technology systems while civilian users are demanding more capacity for video-capable PDAs and other high-speed wireless broadband applications.

Modern military aircraft, operating with airborne data links such as the Joint Tactical Information Distribution System, have need for large amounts of bandwidth. JTIDS, for instance, operates on 51 channels in the 969- to 1206-MHz range in the bottom of L-band. Neighboring L-band frequency spectrum being bought by civil users has the military scrambling for additional spectrum in other bands, potentially costing the Pentagon billions of dollars for new data link systems.

There also is a threat to aviation users from bandwidth hunters looking for spectrum above the 3.7-MHz upper limit specified in the National Broadband Plan. Now the spectrum neighborhood around the 4.2- to 4.4-GHz radio altimeter band is being eyed for other uses. Precise and reliable radio altimetry is vital to the functioning of GPWS boxes. It also is used by some digital flight control systems to trigger certain changes in control laws during landing.

But potential L-band encroachment remains the main concern for civil aviation users. The next generation of civil GPS receivers will use both the L-1 channel centered at 1575.42 MHz and the L-2 channel at 1227.6 MHz to increase position fixing precision.

The dual frequency system will enable receivers to detect and correct for ionospheric signal distortion. Eventually a third GPS channel, L-5 centered at 1176.45 MHz, will become available for safety services. Each channel will require 20 to 30 MHz bandwidth to be available to assure proper GPS receiver performance. That means there will be triple the number of L-band GPS frequencies that must be protected from encroachment from other users, particularly wireless broadband network service providers.

The message is clear. The LightSquared versus GPS battle was a harbinger of huge spectrum wars ahead. The aviation community initially was slow to respond to this latest threat to GPS operability, but it cannot afford to relax even as it, along with other GPS stakeholders, prevails on this single non-compatible frequency use issue.

Aviation users better be vigilant so that they can detect future frequency invaders in the early stages. They must mount a vigorous defense to prevent encroachment by the wireless broadband industry, among other non-aviation interests, if they are serious about building a 21st century air traffic management system.


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