Residual motion estimation for UAVSAR: Implications of an electronically scanned array

The UAVSAR instrument, employing an L-band actively electronically scanned antenna, had its genesis in the ESTO Instrument Incubator Program and after 3 years of development has begun collecting engineering and science data. System design was motivated by solid Earth applications where repeat pass radar interferometry can be used to measure subtle deformation of the surface, however flexibility and extensibility to support other applications were also major design drivers. In order to make geophysically useful repeat pass interferometric measurements it is necessary to reconstruct the repeat pass baseline with millimeter accuracy, however onboard motion metrology systems only achieve 5-15 cm accuracy. Thus it is necessary to recover the residual motion from the data itself. Algorithms for recovering the motion based on along-track offsets between the repeat pass interferometric pair of images were described in [3], [1] and [4]. Later these techniques were extended to use azimuth subbanded differential interferograms in [5]. This paper provides a derivation for the formula for the along-track offsets (or corresponding the subbanded differential phase), i.e. the relative displacement between two SAR images in a interferometric pair in the along track direction, as a function of baseline for systems employing an electronically scanned antenna. The standard formula for systems not employing electronically scanned antenna for the along-track offsets, Deltas, has the form in given equation where b_c is the cross-track baseline, b_h is the vertical baseline, thetas_lscr is the look angle, thetas_az is the azimuth or squint angle, rho is the range and d = 1 for left looking systems and d = -1 for right looking systems. A key feature of this formula is the along-track offsets only range dependency is from the derivatives of the baseline with respect to along-track position. In the electronically scanned case this in no longer true and - an additional range dependency arises that is a function of the electronic steering angle.