PNT without GNSS: Exclusive interviews

Photo: Safran Federal Systems

GNSS — delivering up to millimeter accuracy from 20,200 km in space with a received signal of one tenth of one millionth of one billionth of a Watt — is, in Arthur C. Clarke’s famous definition, “indistinguishable from magic.” Yet, in addition to the inherent errors in the transmission, propagation, and reception of their signals, GNSS are increasingly challenged by jamming and spoofing attacks, especially in and near conflict zones.

For that reason, as any regular reader of this magazine knows, combating jamming and spoofing and building resilience in positioning, navigation, and timing (PNT) systems has been a constant theme of many of our articles and industry news items for years.

The U.S. National Space-Based Positioning, Navigation and Timing Advisory Board has been focusing on how to “protect, toughen and augment” GPS, with the third word referring both to enhancements to GPS and to the “provision and use of alternate sources of PNT that complement, back up, or replace (partly or entirely) use of GPS.”^(*)

For this cover story, I discussed complementary sources of PNT with executives from four companies that design, produce, and/or operate them. They cover a wide range of complementary PNT technologies. Read the exclusive interviews below: 

• Iridium owns and operates a constellation of satellites in low-Earth orbit (LEO) and has global rights for L-band spectrum. This enables it to operate the Satellite Time and Location (STL) system developed by Satelles before it recently became part of Iridium. STL protects critical infrastructure by providing a timing signal that is independent of GNSS constellations and 1,000 times stronger than the GPS signal.
• Spirent Communications’ latest simulation system brings together GNSS and a wide range of other PNT systems. It simulates L-band, S-band, alternative navigation signals, signals of opportunity and emulated inertial outputs. It focuses particularly on the new and emerging LEO constellations, including Xona Space Systems’ PULSAR signals, and enables users to inject new signals via I/Q data files.
• SpacePNT has developed an FPGA-based hardware/software/firmware spaceborne GNSS receiver technology specifically targeting the fast-growing New Space satellite market. The company’s innovations include a precise orbit determination algorithm that can process signals from the Galileo High Accuracy Service and from geostationary orbit (GEO) satellites.
• VIAVI Solutions has developed a system that aggregates signals of opportunity, as well as Iridium LEO and Inmarsat GEO sources; weighs and cross-verifies them; then converts the output to the legacy GPS L1 signal and feeds it to a GPS receiver. It can also aggregate a stand-alone cesium clock.

(*) From Dr. John Betz’s presentation on “Augmenting GPS for Critical Infrastructure” at the April 24, 2024, meeting of the PNT Advisory Board.