Integrated GPS/TOA Navigation using a Positioning and Communication Software Defined Radio

Abstract

While GPS is the natural choice for providing navigation in an outdoors environment, the urban environment places a significant challenge for positioning using GPS. The GPS signals can be significantly attenuated, and often completely blocked, inside buildings or in urban canyons. In this paper a networked radionavigation approach is described that augments the GPS signals with Time of Arrival (TOA) observations using a Software Defined Radio (SDR). The Software Defined Radio developed by NAVSYS includes the capability to operate both as a GPS receiver and also as a 900 MHz transceiver operating within the ISM band. Since both the GPS and communications functions reside within common radio hardware, they can be linked to provide a positioning capability that leverages both the GPS derived pseudo-range and carrier phase observations and also TOA observations derived from the communications channel. The design of the Positioning and Communication (POSCOMM) Software Defined Radio is described in this paper with the networked architecture used to provide augmented navigation in an urban environment. The POSCOMM network design uses multiple “Master” units, which are operating in locations where they have access to the GPS signals. These can be either ad hoc networked participants or units pre-positioned to provide coverage in areas where GPS has difficulty operating. The Master units transmit a TOA message, which includes a pseudo-random sequence from which the time of arrival at the “Slave” unit can be precisely determined. A message is also sent including the precise time of transmission of the TOA message and the precise location of the Master unit based on the GPS observations. The time-of-arrival differenced with the time-oftransmission provides the Slave unit with a pseudo-range observation from each of the Master unit locations. This can be used to solve for the position of the Slave either using the TOA updates alone or using a combination of both the GPS and TOA observations. In this paper, simulation and test results are included to demonstrate the accuracy to which the POSCOMM network aided positioning solution can be derived. The POSCOMM combined GPS/TOA solution accuracy is a function of the following components. • Geometry provided by the Master-Slave relative locations. An analysis is shown of the typical geometry experienced by networked participants in a First Responder application. • Accuracy of the TOA observations. This is determined by the choice of the communication waveform and the ability to precisely measure the time of reception of the TOA message. Test results are shown presenting the accuracy of measurements when operating inside buildings. • Accuracy of the GPS time and position mark at the Master unit. Test results are presented showing the ability to precisely position and time sync the SDRs used to broadcast the TOA augmentation signal. The POSCOMM units are being developed to provide a robust urban navigation solution that can provide precise positioning inside buildings where the GPS signals cannot be received. Applications for this technology include firefighters and other First Responders, also military operations in urban terrain (MOUT).