The Truth Is Out There: Cognitive sensing and opportunistic navigation with unknown terrestrial and nonterrestrial signals [Special Issue on Signal Processing for the Integrated Sensing and Communications Revolution]

There has been significant interest over the past few years in integrated sensing and communication (ISAC) to enable applications, such as the massive Internet of Things, highly automated transportation systems, and military surveillance. Many ISAC studies in the literature considered designing, from scratch, next-generation systems that are endowed with ISAC capabilities. This article argues that "the truth is out there" and that one can sense and exploit unknown signals, whether non-ISAC "legacy" or ISAC devised. The article presents a framework termed cognitive sensing and opportunistic navigation (COSON). COSON can be thought of as an instantiation of ISAC, but, instead of having the "luxury" of designing signals with ISAC capabilities, COSON senses arbitrary, unknown communication signals and exploits them for positioning, navigation, and timing purposes. COSON is composed of four stages: 1) Blind signal acquisition, which comprises spectrum sensing and signal activity detection, blind beacon estimation, initial Doppler and Doppler rate estimation, and blind source enumeration; 2) blind signal tracking and beacon refinement; 3) interference and multipath classification; and 4) sensing and navigation. Extensive experimental results are presented, demonstrating the broad applicability of COSON to terrestrial and nonterrestrial sources, transmitting with various modulation and multiple access schemes: cellular 4G and 5G signals, GPS, and low-Earth orbit satellite (Starlink, Orbcomm, and Iridium) signals. COSON is demonstrated to localize stationary antennas and navigate unmanned aerial vehicles and a ground vehicle, to meter-level accuracy, without global navigation satellite system signals.