Radar range resolution calculator software#

In its quadratic mode, mapdrift begins by dividing the signal data into two halves (or subapertures) in azimuth, each of length T a/2. This error has the form exp ( j2π k qx 2) where x is the azimuth coordinate and k q is the quadratic phase coefficient being measured. 16, we illustrate use of the mapdrift concept to detect and estimate an azimuth quadratic phase error with center-to-edge phase of Q over an aperture of duration T a. While mapdrift estimates quadratic and cubic phase errors best, it also extends to higher frequency phase error. Originating at Hughes Aircraft Corporation in the mid 1970s, the mapdrift algorithm became the first robust autofocus procedure to see widespread use in operational SAR systems. High-frequency phase error primarily degrades image contrast. PGA is an elegant algorithm designed to detect both low-frequency phase error and high-frequency phase error that varies rapidly over the aperture. By low frequency, we mean phase error that varies slowly (for example, a quadratic or cubic variation) over the aperture. The mapdrift algorithm is ideal for detecting and removing low-frequency phase error that causes image defocus. We describe two common autofocus algorithms in this chapter, the mapdrift algorithm and phase gradient autofocus (PGA). We refer to the process that automatically estimates and compensates for phase error as autofocus. The high probability of significant phase error in the azimuth channel of a SAR system operating at fine resolution (typically better than 1-m azimuth resolution) necessitates the use of algorithms during or following image formation to measure and remove this phase error.

Radar range resolution calculator software#

Other hardware and software sources of phase error also are likely to be present even in a well-designed system. Without this location accuracy, phase errors will exist across the azimuth signal aperture and cause image distortion, defocus, and loss of contrast. Location accuracy on the order of a fraction of a wavelength is necessary, perhaps to a few millimeters in the case of X-band operation at 10-GHz center frequency. A major source of uncertainty in the relative phase among these signals is the exact location of the radar antenna at the time of transmission and reception of each pulse. The synthetic aperture achieves fine cross-range resolution by adjusting the relative phase among signals received from various pulses and coherently summing them to achieve a focused image. Ron Goodman, Walter Carrara, in Handbook of Image and Video Processing (Second Edition), 2005 3.1 Autofocus Algorithms