Position, Navigation, and Timing Technologies in the 21st Century

When GNSS is not available due to blockage or jamming, mixed SOOP can augment GNSS for indoor positioning [18, 19] and complement GNSS to maintain aiding to an integrated inertial solution [27, 28]. When GNSS is challenged, the internal clock starts to drift even under aiding from other non‐GNSS sensors. To maintain a stable timing source and enable fast reacquisition, it is possible to perform time – and in particular frequency – transfer with SOOP [8, 99]. Early studies showed such a possibility with DTV and CDMA signals to enable fast reacquisition when GNSS becomes available again.

SOOP can also be used for information assurance against spoofing. Cellular networks are synchronized to GPS, and new ATSC standards recommend the same for DTV transmitters. Because spoofing has only a local effect, widely distributed cell towers and DTV transmitters and their GPS timing sources are unlikely to be affected simultaneously by a spoofing attack. As a result, the time and frequency information derived from SOOP can be used as an independent source to mount countermeasures against spoofing.

For public broadcasting stations, information about the transmission characteristics and transmitter location can be found from the regulatory agency’s database. However, there is a need to determine the locations of private and commercial radio and TV broadcasting transmitters, which may not be known to the public, for opportunistic use as a reference for PNT. Such approaches as simultaneous localization and mapping of emitting radio sources (SLAMERS) [16] are valuable for solving the problem. However, from the PNT perspective, the most desirable approach is for broadcasting sources to encode their transmitter location and in particular clock error parameters onto their data stream in a way similar to radio beacons [100]. Transmission of the transmitter’s location and clock data offers an add‐on PNT service to the main business of broadcasting at the expense of intermittent overhead. As part of the accelerated convergence of broadcasting, communications, and Internet, it serves as an enabler of ubiquitous positioning and seamless transition of outdoor and indoor positioning for the burgeoning LBS market.

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