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Iridium Seeks More Ground Terminals, Raising GPS Interference Concerns

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January 17, 2018
Inside GNSS, January/February 2018

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GPS firms are concerned about a proposal by another L band satellite firm to build more ground terminals as part of a combined satellite-ground communication network.  

The proposal is from Iridium, a mobile satellite service (MSS) that provides global communications with a constellation of 66 non-geostationary satellites operating in the 1618.725-1626.5 MHz band. The constellation is also the backbone of Satelles, a GPS backup service that provides 20- to 50-meter unaided position accuracy worldwide and microsecond timing accuracy.  

Iridium is halfway through a $3 billion update that entails replacing all the satellites with more advanced spacecraft that enable faster data speeds. To take better advantage of this enhanced capability it asked the Federal Communications Commission (FCC) in April to modify its license to add a new class of ground stations called Certus for expanded terrestrial, maritime, and aeronautical operations. The request is being weighed in FCC Docket SES-MOD-20170413-00389 and three related dockets: SES-AMD-20170726-00812, SES-MOD-20170413-00389 and SES-AMD-20170726-00813.

Iridium said its terminals have co-existed successfully with GPS devices for two decades, and operate on ships and airplanes that have GPS devices installed. “In fact,” the company wrote in an FCC filing, “most of Iridium’s commercial, military, and civilian government customers depend on properly functioning GPS devices in addition to the critical communications capability provided by Iridium’s network.”

The GPS Innovation Alliance (GPSIA) said in FCC comments filed in September that it is actively working on an agreement with Iridium to ensure there is no interference with GPS receivers. GPSIA said it “is confident” an agreement can be reached and asked the FCC to condition approval of Iridium’s request on adherence to any such agreement.

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If an agreement cannot be reached, however, the GPSIA asked the Commission to limit Certus terminal operations to modes that offer protection equivalent to OOBE (out of band emission) levels of -95 dBW/MHz into the 1559-1610 MHz band.

Hexagon Positioning Intelligence, which encompasses Calgary, Canada-based NovAtel and Antcom, which is based in Torrance, California, said the proposed system had the potential to impact Radio Navigation Satellite System (RNSS) users. The output power involved and the amount of the Certus terminals to be deployed “cause great concern regarding the unimpeded operation of RNSS receivers.”

The firm is asking the FCC to exercise the same due diligence it has toward other applications including one from LightSquared (now Ligado) in 2010 — a proposal that became the center of a heated fight over interference and standards that is still ongoing. Asserting that the Iridium application does not include enough information to simulate the impact properly, Hexagon asked that the FCC establish a technical working group or similar process to investigate more fully and “ensure unimpeded coexistence of the modified Iridium terminals with the established RNSS systems.”

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