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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, such as firefighters, and military personnel operating in urban settings. The 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.

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 become available, they can be used to augment the TOA-based calculations.

The system is based on relatively inexpensive commercial-off-the-shelf (COTS) hardware and software. At the present state of development, each SDR is a test-bed unit that includes both a GPS receiver and a transceiver that operate at a frequency 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.

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