Studies released at the National Space-Based Positioning, Navigation and Timing (PNT) Advisory Board meeting last week in Crystal City, Va. revealed that all GPS receiver applications were "impacted" by the proposed LightSquared communications network. The studies that led to this conclusion were based on both measurements and calculations.
Deane Bunce, co-chair of the National PNT Systems Engineering Forum (NPEF) was responsible for the presentation LightSquared Effects on GPS – Test and Analysis Results.
The NPEF conducted emission measurements, anechoic chamber studies and outdoor tests with a single LightSquared ancillary terrestrial component (ATC) base station. It was reported that LightSquared engineers examined and concurred with the test setup. All three phases of LightSquared operations (single-channel, dual-channel 5 MHz, and dual-channel 10 MHz) were examined. (Refer to the 21 page presentation for complete details.)
In summary, the measurements and analysis showed that for FAA (air navigation) operations, all receivers lost the GPS "solution" and survey receivers were affected as well. John Deere receivers (used in agriculture) lost "solution." U.S. Coast Guard coastal differential GPS service was impacted. All of NASA's high precision receivers for sciences were impacted, and the Garmin chipset used in personal navigation devices lost the GPS "solution."
Also, most GM/OnStar receivers tested lost GPS near LightSquared base stations and suffered significant degradation of service. (Not all GPS receivers and applications were tested and the test was limited to a single LightSquared transmit antenna. Aggregate effects were modeled.)
The study includes maps showing large portions of the country where GPS service would be impacted for different services. For example, when the initial LightSquared deployment was modeled, one of the aviation receivers tested had no position output above Washington, D.C. Another provided no position output in a large area extending past Frederick, Md. to the west, well north of Baltimore (almost to the Pennsylvania border), as far east as the Chesapeake Bay, and south to Stafford, Va.
The study found that out-of-band emissions from LightSquared's test transmitter appeared to be satisfactory. However, creating filters sharp enough to eliminate the adjacent band interference may be physically impossible in small hand-held devices. Furthermore, even if such a filter is designed, converting all existing GPS receivers now used will involve considerable expense, as well as a possible reduction in receiver performance, and a large amount of time to complete. The NPEF report said "Additional filtering may be more beneficial for future receivers."
The NPEF presentation said "No universal mitigation approach [was] identified."
The first option listed was to relocate LightSquared transmission to a different frequency band, observing that the FCC and NTIA should identify a spectrum location that is more suited for high-power terrestrial transmissions."
The impact on aviation was outlined in a 19 page presentation, LightSquared ATCt RFI Assessment: Aviation Scenario Aggregate RFI, Analysis Summary (RTCA DO-327), Briefing to PNT Advisory Board by R.A. Frazier.
That study showed that terrestrial base station operations involving only the lower 5 MHz channel planned for use (and centered at 1528.8 MHz) was completely compatible with aviation GPS operations.
In a wide base channel scenario, the presentation stated:
"Due to the size of the single-city base station deployment, GPS-based operations below about 2,000 feet will be unavailable over a large radius from the metro deployment center."
It added that "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."
Additional studies are proposed to test the impact on timing applications, such as those used by cellular networks and other synchronized transmission systems, including a distributed transmission system for broadcasting.
LightSquared's presentation, LightSquared and GPS Overload was short on technical detail, and focused instead on the company's investment in the system and its experience in avoiding GPS interference from its satellite-based operations. The presentation stated that LightSquared "has not taken any options for mitigation off the table."
As noted, these studies did not address the impact of LightSquared terrestrial operations on precision timing. Directions Magazine discusses this in the article GPS Vulnerabilities and LightSquared – Symmetricom Weights In where editor Joe Francica interviewed Symmetricom's vice president of marketing and business development, Manish Gupta. The article discusses options for obtaining a Stratum 1 time reference without requiring a GPS antenna by using a packet-based primary reference source synchronization solution.
Doug Lung is one of America's foremost authorities on broadcast RF technology. He has been with NBC since 1985 and is currently vice president of broadcast technology for NBC/Telemundo stations.
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