GPS specialists race the clock to resolve concerns about harmful interference from a new U.S. wireless broadband network.
By Wayne Rosenkrans
AeroSafety World, April 2011
By June 15, U.S. aviation industry leaders, manufacturers of global positioning system (GPS) receivers,1 a mobile satellite service (MSS) company and the federal government expect a joint work group’s final report to break through a 5-month-old legal and technical impasse. The unresolved question is whether the wireless broadband network now being built by LightSquared Subsidiary2 — the first of its kind to blend satellite-based mobile communication with terrestrial base stations sharing satellite frequencies — will cause any harmful interference to GPS receivers.
LightSquared has its new satellite ready for full operation in geostationary orbit and base stations under construction to launch this network, possibly within a few months, offering nationwide digital voice, video and data at broadband Internet speeds. Designed to be sold on a wholesale basis to partner companies, these services will accommodate smartphones, tablet computers and other portable devices. The system provides the option to users to communicate only via 40,000 cellular-like base stations while their mobile devices are in range, only via satellite while anywhere in the country, or both ways, with one mobile device and telephone number.
In LightSquared’s MSS ancillary terrestrial component (ATC) design, its satellite operating in the L-band3 can be configured with a large number of high-gain multiple-beam antenna patterns, with each beam providing coverage to a specific circular area on the ground. Beams on separate frequencies can overlap, or more than one satellite can transmit on the same frequency if there is sufficient geographic separation in the beams on the ground.
Among other advanced technologies to be deployed, the U.S. Federal Communications Commission (FCC) said, are the satellite’s ability to perform real-time measurements to monitor the level of interference caused by its terrestrial communications services to its own satellite system, and to monitor/limit aggregate terrestrial uplink signal power received at its satellite from portable devices (such as handsets) operating in terrestrial mode.
LightSquared has provided MSS since 1996 but never before offered terrestrial service using its MSS ATC authority. The company has committed to the FCC to initially cover 100 percent of the U.S. population with its satellite and, in phases, at least 260 million people in the United States by the end of 2015 with LTE (long term evolution), the name of a fourth-generation (4G) radio technology for mobile telecommunications networks.
The company’s timetable, contingent upon the FCC’s next ruling, anticipates launch of LightSquared commercial services and its partners’ satellite, terrestrial and hybrid services in the second half of 2011. “By 2012, LightSquared’s service will expand to include smartphones and other innovative next-generation de-vices,” the company said.
Only six months ago, company officials considered external concerns about their system’s effect on GPS receivers important to acknowledge but basically outdated because of protections built into the design of the network, FCC public records show. But by the end of April, Sanjiv Ahuja, chairman and CEO of LightSquared, reframed this perspective to an FCC commissioner, saying that “the company’s goal [is] to work for the coexistence of a new, competitive wireless network and a robust GPS system” during a meeting about progress toward implementing the new network and cooperation with the GPS industry.
The FCC — which on Jan. 26 granted authority to LightSquared to operate this network through a conditional waiver of one element of FCC rules — has the responsibility to decide how effectively GPS-related concerns have been addressed.
While the FCC conducts the current LightSquared proceeding in its role as regulator, it also leads implementation of the federal government’s 10-year National Broadband Plan to reallocate many portions of the U.S. radio frequency spectrum, including some of those long dedicated to MSS. A key goal is to create affordable Internet access nationwide through wireless broadband technology. As a regulator responsible for public safety, the FCC — with advice from the National Telecommunications and Information Administration (NTIA) — has noted since 2003 that emissions from MSS ATC would have to be “carefully controlled to avoid interference with GPS receivers.”
The U.S. GPS Industry Council, a trade association working with the Air Transport Association of America (ATA) and other aviation organizations, worked to persuade the FCC to require further study of potential interference.
LightSquared expects to use its allocated MSS L-band frequencies for ATC base stations and for mobile devices. These MSS frequency bands “bracket” the band used for the L1 GPS signal. Many experts have urged caution, predicting a grave risk of overloading and/or desensitizing safety-critical receivers that turn GPS signals into useful positioning, navigation and timing data.
The L-band of the spectrum is one of only three MSS frequency bands also capable of supporting broadband service, said the FCC.4 The portion of the L-band allocated to LightSquared comprises 1525–1559 MHz and 1626.5–1660.5 MHz. GPS receivers operate in the adjacent 1559–1610 MHz band.
One example of a system-level concern came from Lockheed Martin, which operates two regional positioning service satellites integral to the U.S. Federal Aviation Administration’s (FAA’s) wide area augmentation system (WAAS). On Feb. 25, the company urged the FCC to withhold all authority for LightSquared to begin operating MSS ATC service “until the FCC is able to determine that the new service can be provided compatibly with radio navigation satellite services in the L1 band and under what specific conditions.”
Earth stations that uplink the signal to these satellites depend on an extremely sensitive GPS/WAAS receiver with a much higher-gain antenna than those common in aviation GPS receivers, said Jennifer Warren, vice president, technology policy and regulation, Lockheed Martin. “If signal reception is disrupted, these antennas will be unable to perform a safety-critical function to uplink the proper [WAAS] signal for broadcast from the regional positioning service satellites’ L1 signals.” This erroneous broadcast would not be detected immediately by normal methods but quickly would trigger a WAAS shutdown if there were no WAAS backup, she added.
The FCC’s waiver conditions have shifted the adversarial interactions of this proceeding into a cooperative and constructive mode. This mode was facilitated by LightSquared’s agreement to convene an expert technical team, called the LightSquared Work Group, “to study fully the potential for overload interference/desensitization to GPS receivers, systems and networks.”
LightSquared had considered its waiver request as “a minor modification to its license” to operate an MSS ATC network, said Jeffrey Carlisle, the company’s vice president, regulatory affairs and public policy.
A series of interactions and agreements since 2001 with the GPS and aviation industries also persuaded LightSquared that mitigation of harmful interference to GPS receivers was a settled issue by 2010, he said. Moreover, plans for a wireless broadband network based on MSS ATC — including its scale and frequency re-use plan — had been widely known since 2003, yet further concerns were not raised by representatives of the GPS or aviation communities during other FCC proceedings between 2003 and late 2010, Carlisle added.
“No party objected [previously to FCC] approval of LightSquared’s business plan either initially or on re-consideration,” he recalled. In November 2010, he had said that “concerns raised by some parties regarding the coordination with GPS operations are irrelevant to this proceeding and should be resolved through collaborative processes among the interested parties that are already in place” in light of protective measures already required by the FCC.
The following month, Fred Campbell, president and CEO of the Wireless Communications Association International, said that many industry groups and the FCC were unprepared for the full implications and scope of the LightSquared network. “Until LightSquared’s recent proposal, the deployment in the L-band of 40,000 terrestrial base stations using the LTE air interface was not contemplated by the [FCC],” Campbell said. Even the prior FCC decisions did not “expect an MSS ATC licensee to deploy 40 million mobile devices,” he said.
This year, Kris Hutchison, president of Aviation Spectrum Resources, a communications company serving the air transport industry, noted on March 29 that LightSquared may have misinterpreted the aviation and GPS industries’ silence on MSS ATC between 2003 and 2010.
“Participation in proceedings that occurred years ago and addressed interference arising from a markedly different deployment scenario … does not resolve concerns that arise from the current interference environment between more sophisticated and extensive GPS and ancillary terrestrial component operations,” Hutchison said.
In December 2010, Charles Trimble, chairman of the GPS Industry Council who now co-chairs the Working Group with LightSquared’s Carlisle, similarly had told the NTIA that introduction of LightSquared MSS ATC service “based on densely populated, strong signal adjacent-band transmitters is orders of magnitude more significant than under the original  MSS ATC mode of operation.”
For example, existing FCC regulations on MSS ATC — such as separating base stations from airport runways, taxiways, aprons and takeoff and landing flight paths by at least 190 m (623 ft) — originally resulted from concerns about interference to the satellite communication transceivers aboard aircraft, an issue raised in 2003 by The Boeing Co. Many of the recent public responses about LightSquared to the FCC reflect concern about whether such existing regulations are adequate for today’s environment.
In explaining its conditions for the waiver, the FCC noted that in addition to concerns renewed by the private sector, the federal government’s NTIA had submitted concerns about “the potential for adverse impact of mobile satellite service/ancillary terrestrial component operations in the L-band on GPS and other global navigation satellite system receivers.”
Work Group Testing
Analysis of interference to GPS receivers involves consideration of factors such as the number of GPS satellites available, the received signal strength of the GPS signal, whether GPS receivers have an obstructed or clear view of the sky, LightSquared’s terrestrial broadband signal strength, and distance of the GPS receiver from the terrestrial wireless broadband transmitter, either a base station tower or handset, according to the FCC.
The concept behind most testing is to provide an interfering set of simulated signals at the LightSquared downlink and uplink frequencies in the presence of a controlled set of simulated GPS L1 and L2 signals, with varying signal-power levels and varying numbers of satellites, including WAAS signals for some tests. Unlike earlier preliminary tests by individual companies, the emulated LightSquared signals are amplified and filtered using proprietary transmit filters provided by LightSquared.
To support this testing and analysis, LightSquared also has been providing technical details of its equipment, channelization plan, output power, out-of-band emission characteristics and emissions mask for its MSS ATC network.
The Work Group’s April 15 report to the FCC details progress so far, and tables included in the magazine version of this article focus on its Aviation Sub-Team, which is studying the risk of harmful interference to GPS receivers common in commercial aviation. Six other sub-teams also are conducting tests and analysis on other categories of GPS receivers.
“LightSquared plans in all three phases [of network deployment] to operate base stations at least 4 MHz away from the GPS band at 1559 MHz,” the report said, offering a hint about types of mitigations that may be in the works.
The Aviation Sub-Team also is focusing on base station carrier frequency configurations that “have the potential to create third-order intermodulation products [that is, spurious signals overlapping GPS signals] that may fall within the GPS L1 band,” the report said.
GPS Receiver Design
GPS receivers employ various methods of rejecting out-of-band emissions that cause harmful interference, including combinations of radio frequency and intermediate frequency filters at the antenna and receiver, and receiver processing gain. FAA performance requirements for aviation GPS receivers — subcategorized for different applications as standalone GPS, GPS/WAAS and GPS/LAAS (local area augmentation system) — define the tolerable level of interference to the GPS signal without causing malfunction or error. “For interference centered at frequencies within the range of 1553.8–1593.8 MHz, the maximum tolerable interference levels for standalone GPS, GPS/WAAS and GPS/LAAS avionics … are a function of the bandwidth of the interference,” the Work Group said.
“Current-generation civilian airborne receivers used for instrument meteorological conditions navigation all rely on the GPS coarse acquisition (C/A) code signal broadcast at 1575.42 MHz (L1), and typical receivers have 3-dB pre-correlation bandwidths ranging from 2 to 20 MHz. WAAS-capable airborne receivers additionally rely on L1 C/A-like signals that are broadcast by geostationary satellites, which provide differential corrections and integrity data to the aircraft from a ground network.”
Giving a sense of how the LightSquared proceeding and wireless broadband pressures ultimately may influence GPS, the FCC said in March: “We emphasize that responsibility for protecting services rests not only on new entrants but also on incumbent users themselves, who must use receivers that reasonably discriminate against reception of signals outside their allocated spectrum. In the case of GPS, we note that extensive terrestrial operations have been anticipated in the L-band for at least eight years. We are, of course, committed to preventing harmful interference to GPS, and we will look closely at additional measures that may be required to achieve efficient use of the spectrum, including the possibility of establishing receiver standards relative to the ability to reject interference from signals outside their allocated spectrum.”
However, Garmin International, the U.S. GPS Industry Council, Trimble Navigation and the ATA on March 29 were quick to defend the reasonableness of the designs of current GPS receivers. “GPS and radio navigation satellite services … were designed to operate (and have successfully operated for nearly 30 years) when there were no [MSS ATC] allocations in the adjacent 1525–1559 MHz band,” they said. “Even if it were possible to design GPS receivers that could accommodate a high-power, ubiquitous-coverage terrestrial neighbor … it would not have been rational to do so, given the allocation tables [in FCC rules].”
GPS Receiver Limitations
The GPS Industry Council also had described in December 2010 the limitations facing the designers of GPS receivers. “During three decades of evolving GPS receiver design, engineers have created layers of radio-frequency filtering in order to provide the selectivity needed by the GPS receiver to acquire and track GPS signals; to maintain positional accuracy; and to reacquire GPS signals following loss of lock,” the council said. “At L-band frequencies, [filter limitations] mean a significant amount of adjacent band energy makes it past the first radio frequency filter.
“GPS engineers addressed this problem by down-converting the signal [to lower frequencies] and improving the selectivity with filtering in each stage. This is because at lower frequencies it is possible to construct a filter that eliminates most of the adjacent band energy. However, for a strong adjacent band emitter, the later filtering stages in a GPS receiver never get a chance to perform their function.”
The council’s preliminary ideas for mitigation of interference risk include locating base stations as far as possible from mobile satellite application devices such as GPS receivers; establishing power limits on terrestrial transmitters in proportion to their frequency proximity to the GPS band; and establishing a power limit for wireless broadband users’ transmitters operating in MSS ATC service bands “to address the potential effects of concentrating these handsets in small, high-density geographic areas.”
Suggestions for better stakeholder cooperation, such as those conveyed to the FCC in January by Michael Swiek, executive director of the GPS Industry Council, seem to have refocused many of the interested parties toward scientific assessment and solutions. “Any jeopardy to GPS must entail careful, constructive, deliberative and transparent analysis by a party that is best suited to develop the appropriate methodology and test scenarios, analyze the resulting data, and provide an unbiased report that all parties can rely on for its impartiality and seriousness,” Swiek said.
- The term “GPS receiver” has been used generically for various types of devices under test.
- “LightSquared Subsidiary” encompasses the firm’s most recent predecessor company names, Mobile Satellite Ventures and SkyTerra Communications.
- The L-band is a general designation for frequencies from 1 GHz to 2 GHz. In the United States, the FCC has allocated L-band spectrum for mobile satellite service downlinks in the 1525–1544 MHz and 1545–1559 MHz bands and for mobile satellite service uplinks in the 1626.5–1645.5 MHz and 1646.5–1660.5 MHz bands.
- Other companies authorized by the FCC to provide MSS ATC services are Globalstar, the DBSD North America subsidiary of ICO Global Communications, and Terrestar Networks, according to the federal government’s National Broadband Plan.