Radar history and developments
“InSAR” is a nested acronym: Radio detection and ranging (radar), Synthetic Aperture Radar (SAR), and interferometric SAR (InSAR). A short review of the development of the subsequent techniques follows the acronyms in chronological order.
Radio detection and ranging (radar) refers to a technique as well as an instrument. The radar instrument emits electromagnetic pulses in the radio and microwave regime and detects the reflections of these pulses from objects in its line of sight.The radar technique uses the two-way travel time of the pulse to determine the range to the detected object and its backscatter intensity to infer physical quantities such as size or surface roughness. A monostatic radar uses only one antenna, both for transmitting and receiving, whereas in a so-called bistatic radar, the transmitting and receiving antennas are physically separated (Skolnik, 1962) Two landmark discoveries in the development of radar were Maxwell’s equations of
electromagnetism in 1873, in “Treatise on Electricity and Magnetism,” and Hertz’s experiments in 1886. Hertz generated and detected the first known radio waves, and discovered that these waves were subject to reflection or scattering (Buderi, 1996). Although first radar systems were developed as early as 1903 for ship tracking and collision avoidance (the “telemobiloskop” of Hulsmeyer (1904)), pre-WW II military considerations boosted the development of radar in the 1930s and 40s (Curlander
and McDonough, 1991).
Synthetic Aperture Radar
A specific class of radar systems are the imaging radars, such as Side-Looking (Airborne) Radar (SLR or SLAR) and later Synthetic Aperture Radar (SAR). The side-looking geometry of a radar mounted on an aircraft or satellite provided range
sensitivity, while avoiding ambiguous reflections. The first SLAR’s were incoherent radars—the phase information of the emitted and received waveforms was not retained. The resolution in the flight direction was obtained by using a physically long antenna, hence the name Real Aperture Radar (RAR). The practical restrictions on the antenna length resulted in a very coarse resolution in the flight direction, degrading with higher flight altitudes, even though high-frequency (K-band: 0.75–2.40 cm) radars were used, see eq. (2.3.4) on page 26.