Synthetic aperture radar (SAR) gives not only the opportunity to image targets with high resolution but also to measure their position and velocity. Without acceleration (constant speed), the position and velocity both in range and azimuth can be estimated using a multichannel SAR system. This paper introduces a method to find the lower bound on variance of estimate of position and velocity for targets. The method is based on the assumptions needed for the Cram´er-Rao lower bound (CRLB). The method works for both narrowband (NB) and ultrawideband (UWB) SAR systems. For demonstration, a monostatic single-channel UWB SAR system operating at low frequency is used. Thus, only lower bounds for estimate variance of azimuth, range, and relative speed are derived.