Motion compensation is a key factor in SAR imaging.Generally speaking

there are two kinds of motion compensation.One is based on measured data and the other is based on echo data.In this paper we focus on the latter one.The Doppler rate is an essential parameter in the process of the motion compensation;the instability in both forward velocity and acceleration of aircraft is regarded as vibrations of the Doppler rate.In the high-frequency band SAR

synthetic aperture is narrow and the hypothesis is tenable that the slant range is longer than the length of synthetic aperture.Basing on the hypothesis

the Doppler rate in the high-frequency band SAR has nothing to do with the position of the object.Thus the formation of the Doppler rate is comparatively simple

and the motion compensation is easy to operate.However

in Ultra Wide Band Synthetic Aperture Radar（UWB SAR）

the length of synthetic aperture is relatively long

so that the hypothesis that the slant range is longer than the length of synthetic aperture is unreasonable.In the meantime

the Doppler rate in the high-frequency band SAR matters with the position of the object.Due to the above reasons

we need to re-analyze the formation of the Doppler rate in UWB SAR and study new motion compensation.Out of this aim

the paper tentatively studies motion compensation of UWB SAR in wide swath.First

We obtain the Doppler rate of different range from linear fitting

and then compensate the phase error directly instead of separating the motion parameters from Doppler rate.In the section of experiment

in order to verify the theoretical part

according to the simulated data

a quantity analysis was made on the difference of the Doppler rate in the short and long pole of mapping band respectively.Then based on UWB SAR’s echo data of Aircraft Y7 and following the theoretical instruction stated in the former part

we make motion compensation successfully.Using the method of direct average we get the result images.Finally

a comparison is made between the results obtained from two kinds of motion compensation.It turns out that the method used in this paper is effective.