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Despite their potential ability to produce highly resolved images of the seabed, synthetic aperture sonars are not widely used. The primary reason for this restricted use is that most synthetic aperture systems are based on the radiation and detection of short-duration modulated pulses. Due to the low speed of sound in water, the pulse repetition frequency is low and so it has been difficult to maintain the required pulse-to-pulse phase coherence. This paper describes a new approach to synthetic aperture sonars based on continuous transmission with some form of frequency modulation. That is, a sonar that transmits and receives continuously but uses some form of frequency coding (in this case a linear frequency sweep) to determine range. Using a continuous transmission frequency modulated sonar it is possible to make a synthetic aperture sonar that can produce coherent apertures many wavelengths long. In addition to the combination of synthetic apertures and continuous transmission frequency modulation, further modifications are suggested to reduce the effect of lateral towfish movement and the effects of medium turbulence resulting in random path-length variations.