Software-defined radios (SDRs) are being developed to provide positioning and communication (POSCOMM) services inside buildings and tunnels, under tree canopies, and in other environments in which local radio- communication systems are accessible but Global Positioning System (GPS) signals are not present or are degraded. These SDRs could be especially helpful for teams of first responders (e.g., firefighters), and military personnel operating in urban settings. These SDRs are capable of navigating by use of GPS signals and can switch over to time-of-arrival (TOA) pseudolite navigation when GPS signals become unavailable, provided that signals from suitably placed TOA pseudolite transmitter nodes are available.

In a POSCOMM SDR Network, the position of a slave unit can be determined from times of arrival of signals from master units, the positions of which are known from GPS measurements.
These POSCOMM SDRs are designed to operate as parts of a system having a network architecture (see figure). Each SDR is designated as either a master or a slave unit. The master units are the aforementioned TOA pseudolite transmitter nodes: They transmit TOA messages that include pseudorandom sequences from which times of arrival of signals at the slave units can be precisely determined. They also transmit messages stating the precise times of transmission of the TOA messages and the precise locations of the master units based on GPS observations. The differences between times of arrival and times of transmission are used to calculate pseudoranges between slave and master locations. The pseudoranges are then used to solve for the slave positions. When GPS signals are available, they can be used to augment the TOA-based calculations.

The POSCOMM SDR system is based on relatively inexpensive commercial-off-the-shelf (COTS) hardware and software. At the present state of development, each POSCOMM SDR is a test-bed unit that includes both a GPS receiver and a transceiver that operates in a frequency band near 900 MHz. The hardware aspect of the design of the unit features a modular PC/104 configuration to facilitate rapid construction of prototypes and testing of SDR software to support advanced positioning and communication functions. [PC/104 is an industry standard for compact, stackable modules that are fully compatible (in architecture, hardware, and software) with personal-computer data- and power-bus circuitry.] The software is portable and designed for use in real-time flavors of Windows and Linux operating systems.

Signal processing is performed by a field-programmable gate array (FPGA) card and a Pentium-class central processing unit. Received GPS and/or TOA radio signals are converted to digital signals by use of digital antenna elements (DAEs), which are compact and can be easily modified to operate at alternative signal frequencies and sampling rates. Each DAE is responsible for radio-frequency down-conversion and up-conversion as well as high-speed analog-to-digital and digital-to-analog conversion. Each DAE uses a common sample clock and phase-locked reference local oscillator, thereby maintaining a coherent sampling environment for all transmitted and received signals.

In a test of an early prototype POSCOMM SDR system, four master units were set up around a building, and slave units were operated at various inside and outside positions. Distances between slave positions as determined by TOA and those determined by GPS were found to be no more than about 5 m in most cases.

This work was done by Alison K. Brown, Yan Lu, and Janet Nordlie of NAVSYS Corp.


This Brief includes a Technical Support Package (TSP).
Software-Defined Radios for Urban and Indoor Navigation

(reference NAVSYS-0001) is currently available for download from the TSP library.

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