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The estimation of transmitter location based on radio frequency reception at a single site has long been a challenge. The standard technique for location estimation has typically used an active local vertical transmitter (ionosonde) to characterize the local ionosphere for propagation path modeling. A technique that is totally passive and yields a more comprehensive estimate of the propagation path is under development at Applied Research Laboratories, The University of Texas at Austin. This technique uses tomography to describe the two-dimensional state of the ionosphere over wide ionospheric swaths. ARL:UT deployed an array of Transit satellite receivers in nine locations along a 2000-km baseline from Pharr, Texas to Pierre, South Dakota. Transit signals received at these nine locations were analyzed using tomography algorithms to estimate the electron density distribution in the plane defined by a "best-fit" arc through the receiver array and best-fit arc along the satellite orbital path. Tomographic results were then compared with conventional propagation path estimates obtained using the local ionosonde. Initial results indicate that this completely passive system will provide ionospheric propagation path estimates better than the local ionosonde method.

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