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NPSTC Adds Public Safety's Voice to Working Group Assessing Interference Potential to GPS

LightSquared, which has spectrum in the 1.5 GHz band adjacent to the spectrum used by all global positioning system (GPS) receivers, plans to build an LTE terrestrial network with approximately 40,000 sites that would provide wholesale capacity to commercial broadband providers. This could radically change and degrade the spectrum environment in which adjacent GPS signals are received, as strong signals from the LightSquared facilities could block GPS reception by public-safety communications facilities and devices.

In January, NPSTC wrote to the FCC to raise concerns about this potential for interference. Accurate GPS information is very important to the public-safety community: GPS signals are used for wireless 9-1-1 location, support of dispatch operations that select the closest responder based on GPS location, mapping/response directions to responders and synchronization of simulcast systems across the country.

LightSquared committed to the FCC that it would test for interference and would submit a report by June 15 to document the potential for interference to GPS and recommend mitigation procedures. NPSTC has volunteered and been selected to participate in the technical working group, which is cochaired by LightSquared and the U.S. GPS Industry Council, to assess the potential for interference.

Members of NPSTC’s GPS interference working group submitted representative operational scenarios for public-safety utilization of GPS infrastructure and devices. These will be incorporated into the reports and test plans the technical working group will use to evaluate and measure the potential interference from the proposed LightSquared broadband network.

The development of test procedures and acquisition of devices from manufacturers for testing has followed a very aggressive timeline. Actual testing of the devices began the week of May 16 and will end on June 3. Public-safety participants in the GPS interference working group will have an opportunity to provide feedback on the degradation data, how it is interpreted and what acceptance rates for degradation would work for public safety.

NPSTC specifically wants to ensure public safety’s use of GPS is protected from potential interference that could result if the FCC grants LightSquared’s request.

Public safety uses wireless 911 location and GPS to support:

  • Dispatch of closest responder based on GPS location
  • Mapping/response directions to responders based on GPS
  • Synchronization of thousands of simulcast systems across the country based on GPS time signals and a myriad of other mission-critical functions, which requires an interference study requested by the National Telecommunications and Information Administration to be conducted by qualified technical specialists who understand the GPS, including its classified roles/uses

The FCC and public safety already have extensive experience on the problems created when interference must be resolved after the fact. Public safety has been involved in the multi-year 800 MHz rebanding initiative in which the time originally predicted for conclusion continues to expand.

Public safety simply doesn’t have the resources needed to fix another major interference problem after the fact. Fortunately, in the case of LightSquared’s application, there’s recognition of a potential problem, so it should be resolved at the outset before harmful interference has a chance to occur.

NPSTC’s letter to the FCC made these recommendations:

  • Efforts to determine the degree and impact of interference to GPS from the LightSquared system must include public safety, not just other commercial services
  • NPSTC and its designated technical representatives should be included in any testing
  • It’s premature to cap LightSquared’s financial responsibility at the $20 million offered to support testing, pending the additional analysis to determine the extent and degree of impact to the reception of GPS, as well as the full cost of any viable remedies needed to prevent interference.

The FCC should require thorough interference analysis and testing to GPS use by public safety and others before granting LightSquared authority to provide service.

However, should the FCC still move forward with a conditional grant prior to testing, NPSTC strongly recommends that any grant be conditioned on not causing interference to GPS use by public safety and other critical entities in addition to any protection provided for commercial systems.

Marilyn Ward is executive director of the National Public Safety Telecommunications Council.

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