An antenna array, comprising: a first array (60,300) of radiator elements (62-1,62-2...62-N;; i, k, N;); an excitation source (64A, 64B) connected to at least a first set (60A, 60B) of the first array radiator elements (62-1,62-2...62-N; i, k, N;); a receiver (64A, 64B) connected to at least a second set (60A, 60B) of the first array radiator elements (62-1,62-2...62-N; i, k, N;); a second array of radiator elements (82-1,82-2...82-Nj; j, Nj; 220; 320) spaced from the first array of radiator elements (62-1,62-2...62-N; i, k, N;), the second array comprising a variable phase shifter (84-1, 84-2...84-Nj; 222; 322) connected to each of said radiator elements (82-1,82-2...82-Nj; j, Nj; 220; 320) and a switch (88-1, 88-2... 88-Nj; 224; 324) arranged to connect the phase shifter (84-1, 84-2...84-Nj; 222; 322) to either a reflective termination (86-1, 86-2... 86-Ni; 228, 328) or a load device (90-1,90-2...90-Nj; 226; 326), so that in a first mode, the phase shifter (84-1, 84-2...84-Nj, 222; 322) is connected to the reflective termination (86-1, 86-2... 86-Nj; 228, 328) and in a second mode the phase shifter (84-1, 84-2...84-Nj, 222; 322) is connected to the load device (90-1,90-2...90-Nj; 226; 326); an array controller configured in a measurement mode to set the phase shifters (84-1, 84-2...84-Nj; 222; 322) to the first mode, to control the excitation source (64A, 64B) to radiate energy from said at least a first set (60A, 60B) of the first array radiator elements (62-1,62- 2...62-N; i, k, N;) so that some of the radiated energy is reflected from the reflective termination of each of said second array of radiator elements (82-1,82-2...82-Nj; j, Nj; 220; 320) back to at least said second set (60A, 60B) of first array radiator elements (62-1,62-2...62-N; i, k, N;), and to process signals from said receiver (64A, 64B) as a result of said excitation to measure a bistatic range (Rijk) through a signal path from one of the radiator elements (i) of the first set of the first array elements to a radiator element (j) of the second array and to a radiator element (k) of the second set of the first array elements, wherein the first array is a feed array, the second array is a reflector comprising a set of retro reflectors arranged on a reflector surface of the array in a distributed configuration, and the load device is an absorptive load. a first array (60,300) of radiator elements (62-1,62-2...62-N;; i, k, N;); an excitation source (64A, 64B) connected to at least a first set (60A, 60B) of the first array radiator elements (62-1,62-2...62-N; i, k, N;); a receiver (64A, 64B) connected to at least a second set (60A, 60B) of the first array radiator elements (62-1,62-2...62-N; i, k, N;); a second array of radiator elements (82-1,82-2...82-Nj; j, Nj; 220; 320) spaced from the first array of radiator elements (62-1,62-2...62-N; i, k, N;), the second array comprising a variable phase shifter (84-1, 84-2...84-Nj; 222; 322) connected to each of said radiator elements (82-1,82-2...82-Nj; j, Nj; 220; 320) and a switch (88-1, 88-2... 88-Nj; 224; 324) arranged to connect the phase shifter (84-1, 84-2...84-Nj; 222; 322) to either a reflective termination (86-1, 86-2... 86-Ni; 228, 328) or a load device (90-1,90-2...90-Nj; 226; 326), so that in a first mode, the phase shifter (84-1, 84-2...84-Nj, 222; 322) is connected to the reflective termination (86-1, 86-2... 86-Nj; 228, 328) and in a second mode the phase shifter (84-1, 84-2...84-Nj, 222; 322) is connected to the load device (90-1,90-2...90-Nj; 226; 326); an array controller configured in a measurement mode to set the phase shifters (84-1, 84-2...84-Nj; 222; 322) to the first mode, to control the excitation source (64A, 64B) to radiate energy from said at least a first set (60A, 60B) of the first array radiator elements (62-1,62- 2...62-N; i, k, N;) so that some of the radiated energy is reflected from the reflective termination of each of said second array of radiator elements (82-1,82-2...82-Nj; j, Nj; 220; 320) back to at least said second set (60A, 60B) of first array radiator elements (62-1,62-2...62-N; i, k, N;), and to process signals from said receiver (64A, 64B) as a result of said excitation to measure a bistatic range (Rijk) through a signal path from one of the radiator elements (i) of the first set of the first array elements to a radiator element (j) of the second array and to a radiator element (k) of the second set of the first array elements, wherein the first array is a feed array, the second array is a reflector comprising a set of retro reflectors arranged on a reflector surface of the array in a distributed configuration, and the load device is an absorptive load.