Updates on the frequency Scan for Time-of-Echo Compression SAR Imaging Mode

Abstract

The synthetic aperture radar imaging technique described and analyzed here is known as f-STEC: frequency-Scan for Time of Echo Compression. Various use cases for f-STEC Synthetic Aperture Radar (SAR) are identified: [High Carrier Frequency]: SAR systems operated at higher carrier frequency, e.g., Ku-band and above, f-STEC is an advantage because it utilizes the large bandwidth, avoids the typical high length-to-height aspect ratio of the antenna, and by this avoids the design compromise that would otherwise lead to reduced swath width and signal-to-noise ratio. [NewSpace SAR]: Avoid the large phased array antenna and the high cost of transmit/receiver RF front-end modules. Instead, a simple passive antenna array optimized for a fixed frequency scan rate can be used when operating in the f-STEC mode. [State-of-the-Art SAR]: These multi-mode systems are equipped with transmit/receive modules incorporating different true-time-delay settings are inherently suitable for f-STEC operation as a free add on. The basic concept behind the f-STEC imaging mode is straightforward and consists of three key aspects: 1) transmit a frequency modulated pulsed signal, 2) the steering angle of the antenna's main beam is a function of frequency, and 3) the main beam scans the imaged swath from far to near range. The angular direction of the radiation pattern's main lobe of a frequency scanning antenna is a function of the signal's RF frequency. Transmitting a chirp signal will thus generate a transmit beam scanning the swath over pulse time. Consequently the receiving radiation pattern's main lobe is instantaneously pointing towards the echo direction independently of its time-of-arrival. The frequency-scanned transmit beam is basically distributing the spectrum over the swath. Therefor, the f-STEC imaging technique trades (sacrifices) range resolution for (improved) signal-to-noise ratio, azimuth resolution, and swath width. One main advantage is that the pulse duty cycle may be significantly increased over what is common for pulsed SAR systems, allowing for a lower peak-to-average transmit power ratio. The f-STEC imaging mode is also known as f-SCAN ; further work focusing on the processing and the data compression has been published in , respectively. Recent publication on f-SCAN can be found, including several EUSAR 2024 publications. A comprehensive analysis of the f-STEC imaging technique is provided, including closed form expressions describing the operational and performance parameters as well as the timing constraints.