It is true, said Javad Ashjaee, founder of Javad GNSS Inc.: The proposed LightSquared broadband network causes interference with high-precision GPS receivers. However, “the root of the problem is us, the GPS manufacturers,” he added. “We designed our receivers without paying reasonable attention to other systems that may come close to the GPS bands.”
Ashjaee recently issued a statement outlining his views on the technical side of the debate. “The problem manifests itself in two ways,” he said. “1) Most GPS receivers do not have proper protective filters in the antenna section, and 2) The receivers don’t have a means to indicate if there is any interference in the area of operation.
“The first problem of inadequate filters causes our GPS receivers to ‘invite’ LightSquared signals to arrive to our GPS receiver. This can block and/or damage the GPS signals and cause the receiver to not function properly. We solved the first problem by adding a set of ceramic and SAW filters in the signal entry to our GNSS receivers. The filters were all existing off-the-shelf components. The filter system works fine and does not impose any noticeable negative effect on the quality of GNSS signals and solution results. In particular in has no noticeable effect on the multipath mitigation capabilities of receivers that need intact and undisturbed GNSS signals. The cost of the additional filters is negligible.”
The second problem, he said, is inadequate test and warning features inside GPS receivers: Most receivers do not give any information regarding the existence of interference. He noted that such interference not only comes from LightSquared but also from other sources, including radio station signals. He said his company has solved the second problem in its receivers by adding an “interference analysis” feature that works even better than specified by National Telecommunications and Information Association (NTIA) test procedures and in a much shorter time. “It does it in 30 seconds, rather than 30 days,” he said, “and it does it by a click of a button which any novice user can do easily in the field.”
Ashjaee said he believes erroneous letters sent to the FCC and testimonies given before Congress “have tilted the opinions of many people and journalists in the wrong direction.” But he is encouraged that the problem is technical in nature. “The good thing about technical issues is that they can readily be demonstrated, as we have and will continue to demonstrate on any platform,” he said. “Electrons do not have party affiliations and do not follow the guidance of special interest groups and lobbies.”
Ashjaee said that the filter system designed by his company proves his position. ”It works well,” he said. “It does not have flat ‘group delay’ across the frequency band, and it does not affect the integrity of GPS signals. In particular, the multipath mitigation features of the GPS signals are well preserved.”
Related technology advances are underway, he said. “Our group delay compensation technique also opens the door for time transfer applications in subnanosecond level (as good as 0.1 or even 0.01 nanoseconds) without the need for temperature-controlled antennas. We plan to produce such products in a few months.”
According to Ashjaee, the benefits of the LightSquared network far outweigh any temporary challenges. “My only interest in this issue is to integrate LightSquared communication channels inside our GNSSreceivers and provide fast, reliable and inexpensive RTK,” he said. “I am absolutely sure that we are not up against any laws of physics.”
Additional information can be found on Javad’s website.
Ashjaee Summarizes Technical Issues in LightSquared Debate
October 21, 2011