Design of an agile radionavigation system using SDR techniques

This paper describes the design, development, and hardware prototyping of a frequency-agile, programmable-bandwidth radionavigation system being implemented for the U. S. Army's Operational Test Command to support soldier training and combat systems testing in GPS-denied environments such as dense forest areas and in urban terrain. Employing software-defined radio (SDR) techniques, the system is intended to be highly adaptive in order to rapidly adjust to different testing scenarios by changing its frequencies, coding bandwidths, and channelization as required by the specific application. The fundamental basis for the system is a direct-sequence spread-spectrum (DSSS) signal which is launched from multiple widely-spaced, generally terrestrial transmitters. The radiolocating receiver acquires these continuous, overlapping code-division multiple-access (CDMA) transmitted signals, decodes them, and extracts the transmitter locations and times of transmission from data streams embedded in the respective DSSS signals, in a manner analogous to GPS units. The radionavigation solutions are then obtained by solving the usual systems of nonlinear pseudorange equations by linearization techniques, Kalman filtering, or other means, but with downstream corrections for the spherical-earth geometry and RF propagation factors governing the groundwave signals. However, there are several significant features of this Theater Positioning System (TPS) which differentiate it from GPS, including its operating frequency range (<30 MHz), frequency- and modulation-agile capabilities, propagation modes (principally groundwave), and signal security mechanisms. In addition, the TPS signal structure is specifically designed to provide an effective back-up navigation source to GPS in difficult reception situations and afford maximal rejection of AC power-line noise to improve reception efficiency in urban areas. A final feature of the TPS signals permits wide-area broadcasting of low-rate data for commands, DGPS corrections, status information, and the like.