Ligado Representatives Raise Questions about Harmful Interference Threshold

October 14, 2016–Representative of Ligado Networks LLC, including their engineering consultants, raised a myriad of questions today about the Department of Transportation’s use of a particular threshold for determining harmful interference to GPS receivers.

The question of whether a 1 dB increase in the noise floor is the appropriate standard for assessing harmful interference to GPS receivers prompted considerable discussion during a day-long workshop hosted by DoT, including from representatives of Ligado and their engineering consulting firm, Roberson and Associates LLC, as well as from GPS device and technology manufacturers.

The workshop was the fifth held as part of DoT’s GPS adjacent-band compatibility assessment, which is designed to determine the interference tolerance masks that are necessary for various categories of GPS and other GNSS [Global Navigation Satellite System] receivers to operate without harmful interference from operations in adjacent spectrum, including Ligado’s planned nationwide LTE network.

The initiative was launched in the wake of a 2012 decision that the proposed L-band LTE network of LightSquared (Ligado’s predecessor company) could not operate without causing harmful interference to GPS devices (TRDaily, Jan. 13, 2012). “The objective of this test is to collect data to determine Interference Tolerance Masks (ITM) for categories of GPS and GNSS receivers processing signals in the 1559-1610 MHz Radionavigation Satellite Service (RNSS) frequency band, as well as receivers that process Mobile Satellite Service (MSS) signals in the 1525-1559 MHz band to receive differential GNSS corrections,” according to the DoT test plan, which was released in March.

“DOT is neither developing standards for GPS receivers, nor is it developing standards for transmitters operating in the adjacent radiofrequency bands. These ITMs will be used to assess the adjacent band interference power levels that can be tolerated by GNSS receivers processing desired signals in the RNSS band.”

DoT conducted testing on six categories of receivers in April at the U.S. Army Research Laboratory at the White Sands Missile Range in New Mexico. A second round of testing was done in July at Zeta Associates in Fairfax, Va., and at MITRE Corp. in Bedford, Mass.

At today’s workshop, representatives of DoT, Zeta Associates, and MITRE discussed the results of the testing and fielded a host of questions, including many from Ligado and Roberson and Associates about the 1 dB noise floor increase threshold. Testing done for Ligado by Roberson and Associates focused on whether LTE signals would impact the actual performance of GPS devices and whether there would be meaningful degradation of the function of those devices from the perspective of end-users. Ligado and Roberson and Associates argue that the 1 dB standard is not a useful tool to assess that, but DoT and GPS manufacturers such as Trimble Navigation Ltd., Garmin International, Inc., and Deere & Company disagree.

Ligado says the testing it commissioned confirmed that LTE signal levels agreed to by the company will not impact consumer GPS device user performance (TRDaily, May 11).

Attendees at today’s workshop or those who listened by phone included representatives of government agencies such as DoT, the Department of Defense, the FCC, the National Telecommunications and Information Administration, the National Institute of Standards and Technology, the National Aeronautics and Space Administration, the State Department, and Innovation, Science and Economic Development Canada, as well as companies and groups such as Trimble, Garmin, Deere, Ligado, Rockwell Collins, Inc., Aviation Spectrum Resources, Inc., the Airline Pilots Association International, and the GPS Innovation Alliance. Representatives of Wall Street banks and private equity firms also listened in.

Karen Van Dyke, director-positioning, navigation, and timing (PNT) & spectrum management in the office of the assistant DoT secretary-research & technology, stressed that the purpose of the testing initiative is to develop parameters to protect existing and future GPS applications. “We are really looking at this as an engineering study to understand the power levels that can be tolerated in the adjacent bands,” she said. “Our effort is not about any one particular company.”

She and other DoT officials and the department’s contractors defended a 1 dB increase in the noise floor as a way to measure harmful interference. They said the test data was processed to produce a 1 dB interference tolerance mask.

Ligado and Roberson and Associates representatives, in particular, asked why the officials did not take into account that some of the devices appeared to have experienced interference at levels above 1 dB. Ms. Van Dyke said that “I think we’re on a slippery slope here” to modify that protection threshold.

Gerry Waldron, an attorney for Ligado, repeatedly asked whether the 1 dB standard was designed to protect the worst-performing devices. The first time he asked that question, he was told it was. Later, he was told that test results were for devices in the aggregate.

Karl Shallberg of Zeta Associates said his company’s testing showed degradation “at interference levels similar to 1 dB,” adding that exceeding that level can adversely affect receiver performance. Kenneth Zdunek, vice president and chief technology officer for Roberson and Associates, questioned Mr. Shallberg’s conclusion, saying the results appear to indicate some interference wasn’t seen at 1 dB. “Not all receivers show a degradation right at 1 dB, but some do,” Mr. Shallberg replied.

Mr. Zdunek also questioned whether the difficulty in acquiring a signal from one low-elevation satellite would impact the experience of an end-user.

Others, including representatives of Garmin and NovAtel, Inc., a provider of GPS technology, said it would.

Mr. Zdunek said that in the testing his company did for Ligado, all of the satellites were degraded and cellphone performance was not impacted. But a Garmin representative said that cellphones are more resistant to interference than other devices that rely on GPS.

Industry representatives asked DoT to release additional data to the public, including the impact on devices of higher dB readings and noise floor results during testing when no LTE devices were present.

Ms. Van Dyke said that DoT shared the raw results of the testing, including the impact on devices of greater noise floor changes, with agency partners that had signed non-disclosure agreements. She said officials would consider releasing additional aggregate data publicly.

Ms. Van Dyke said the next step in the process is to develop “representative use cases,” adding that officials want input from industry. The goal is to have some use cases completed by the end of the year and hold another workshop early next year, she said. DoT will publish a report on all of the testing, although she did not say when that would be.

Mr. Waldron asked if it was correct that DoT doesn’t plan to write “standards.” She said that is correct.

DoT also plans to finalize propagation models, antennas characterization, and tolerable power levels.

Valerie Green, Ligado’s executive vice president and chief legal officer, commended DoT for today’s presentations. “It certainly has been a long time coming,” she said. She also reiterated her request for officials to make additional data available that has been shared with “the decision-making agencies like FCC and NTIA.”

In a statement released this afternoon, Ligado said, “We’re glad that DOT’s nearly two years of testing is finally over. Now the government agencies with authority over our nation’s spectrum policy, the FCC and the NTIA, can take this information and all the other studies into account and make critical decisions. It is odd that the DOT study did not address at all the actual harm to consumers, as the other studies did. DOT also made clear that it recommends a mask based on the worst device it could find. Others will have to decide whether that is sound policy.” – Paul Kirby,

Courtesy TRDaily