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Development of future geo-location systems requires an understanding of the fundamental limits imposed by system parameters such as bandwidth, SNR, number of sensors, etc. Additionally, the system designer can choose among several existing location techniques, such as time-of-arrival (TOA), time-difference-of-arrival (TDOA), and round-trip time-of-arrival (RT-TOA). This paper presents new Cramer-Rao bounds for the RT-TOA case and provides fair numerical comparisons of the bounds for these three time-based location methodologies. Our comparisons assume independent, flat Rayleigh fading on each sensor link, different average link SNRs based on path length, and a uniform user position distribution. We find that TOA outperforms TDOA and RT-TOA, while the relative ordering of the latter two depends on the sensor geometry.