A method is proposed for estimating the target rotation rate for ISAR imaging based on the frame processing technique. The method utilises the intrinsic structures of the frame which span the Hilbert space of radar return from which the rotational information of the target is extracted. For a prominent point, the intensity of the projection is maximum when the frame component corresponding to the prominent point is built with the target rotation rate.Introduction: Knowledge of the target rotational motion is a crucial requirement for recovering the cross-range scale of an image and compensating for image defocusing in conventional ISAR imaging processing [1]. The frame processing (FP) imaging method, proposed in [2], considers radar returns in a Hilbert space where the frame of the space can be identified. The ISAR image is therefore a projection of radar returns onto the (dual) frame viewed on a range-Doppler plane.To construct the frame of radar data space, knowledge of the target rotation rate is explicitly required. Although the FP result is likely to be acceptable by using a guessed value, an accurate rotation rate of the target is a general requirement to guarantee the image from defocusing.The estimation of target rotation rate from radar data can be carried out either in image or in signal domains. For example, the minimum entropy method for refocusing an image described in [3] is an image domain processing. While the technique is practically effective and reasonably accurate, it is computational demanding. On the other hand, the target rotation rate can be estimated from ISAR data via the methodology of sensor array data processing [4,5]. In these approaches, several prominent points are required to be identified from the unfocused image. An optimisation procedure can then applied to optimise timespace correlation of the signals based on the geometry of these prominent points with respect to target rotation rate, or maximise energies of the signals constructed based on the time-space model at these prominent points with respect to target rotation rate.Following the work in [2], a target rotation rate estimation method based on the FP technique is proposed. When the dual frame is built with the correct rotation rate, the intensity of the projection of the prominent point will be the maximum. Therefore, if a prominent point can be identified, it is possible to estimated the rotation rate of a target based on the corresponding frame component under the criterion of maximising the projection value with respect to target rotation rate. After a short introduction of the FP technique for ISAR imaging, an online target rotation rate estimation procedure based on the FP method is presented.

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