u-blox (SIX:UBXN), a global provider of leading positioning and wireless communication technologies and services, cautions against negative impacts of the US FCC’s recent approval of the use of a previously protected portion of the L band for terrestrial communication on global navigation satellite systems (GNSS) such as GPS and, subsequently, on our large US customer base.
Prior to the FCC’s decision, the license, granted to the American satellite communications company Ligado, was limited to use for satellite communication terminals. Such terminals transmit radio waves directionally toward satellites and receive weak satellite signals.
Following the recent FCC order, Ligado will be free to use these frequencies, which are very close to those used by GNSS receivers, to transmit downlink signals from cell towers and uplink signals from user equipment. These signals are transmitted at dangerously high levels compared to those received by nearby GNSS receivers.
These signals threaten to disrupt the operation of GNSS receivers from any manufacturer operating in the vicinity of a Ligado base station or compatible cellular handset. In an internal study on the susceptibility of u-blox GNSS receivers to interference resulting from such activity, we confirmed that the transmissions would disrupt the operation of nearby GNSS receivers. Our findings, outlined in an application note, show that GNSS receivers would suffer performance losses anywhere near a transmitting Ligado handset and when located up to one kilometer from a Ligado base station, a greater distance than the permitted separation between base stations.
Although we have developed recommendations to mitigate these disruptions, it is important to note that implementing these on millions of deployed products would involve a considerable cost and an inconvenience to our customers and would also affect GNSS receiver performance to some extent. The same would be true for receivers from any other manufacturer.
u-blox rejects the FCC’s testing criteria
The FCC’s decision was justified based on supposed measurements of the levels of interference causing harm to GNSS receiver operation. Drawing on our extensive expertise in this area, u-blox completely rejects the criteria employed in this testing. As a wide range of other GNSS providers and technical experts have already attested, the only generally accepted criterion for harmful interference is a level of interference that results in a 1 dB reduction in the carrier-to-noise ratio measured by a GNSS receiver.
The justification of this 1 dB criterion is quite simple. GNSS signals are well below the levels of the background noise. The lower the carrier-to-noise ratio, the poorer many aspects of performance become. For adequate performance, receivers operate with a finite link margin, and any reduction in the carrier-to-noise ratio represents a reduction of this link margin that will impact performance. The 1 dB criterion represents the smallest easily measurable change in carrier-to-noise ratio.
Company: u-blox
