Analysis of the compensation error of the effect of platform movement on the quality of radar imaging in stripmap mode

Master's thesis in the field of electricity-telecommunications

Abstract

Analysis of the compensation error of platform movement in the quality of radar imaging in stripmap mode

SAR is one of the most important branches of "remote sensing" science, which has been the focus of researchers for many years due to its wide applications. In Iran, in recent years, research has been conducted in the field of SAR signal processing algorithms and its different working modes. A major problem in most airborne SAR sensors is to compensate for motion errors caused by atmospheric disturbances. In particular, the main effects produced by platform motion errors in SAR images include geometric and radiometric resolution degradation, directional ambiguity, and geometric and phase distortion. Aircraft deviations from the straight path and constant speed can be calculated by inertial navigation system [1] (INS), global positioning system (GPS) or inertial measurement unit [2] (IMU).

In this thesis, the concepts of SAR and how it works are first described. Then the theory of RDA and CSA algorithms and the reasons for these methods are discussed in detail. Then, while describing the processing steps of each of the algorithms, simulations were performed for a common scenario with a low loci angle and their results were compared.

The main goal of this thesis is to investigate the platform movement compensation methods using the information obtained from IMU (or INS). These methods have been compared by performing calculations and simulations. 

Finally, the acceptable error value of the platform movement compared to the ideal movement path has been proposed. Using these values, the specifications of IMU sensors can be determined in terms of error.

[1] Inertial Navigation System

[2] Inertial Measurement Unit

1-1-History of SAR

Radar was initially developed for military purposes during World War II. Its primary purpose was to track aircraft and ships under adverse weather conditions and darkness. Radar has experienced steady growth with advances in radio frequency (RF) technology, antennas, and recently digital technology [1].

Primary radar systems measured the distance to a target through time delay and the direction of a target through the direction of the antenna. It wasn't long before Doppler shift was used to measure target speed. After that, it was discovered that by processing the Doppler shift, a suitable resolution limit can be obtained in the direction perpendicular to the beam or the direction of the beam. From this latter rule, which is usually attributed to Carl Wiley in 1951, it was discovered that two-dimensional images of targets and the surface of the earth can be formed using radar. This method was called the idea of ??artificial aperture radar (SAR), which actually refers to the idea of ??creating the effect of a very long antenna by analyzing the signal received from a short but moving antenna [2]. In this regard, in aerial imaging systems, digital scanners that use several optical frequency bands were installed on airplanes and satellites, which led to the development of the applications of detailed images obtained from large areas of the earth's surface. Military SAR technology entered the field of civilian applications in the 1970s, and remote sensing researchers found SAR images to be a useful complement to their optical sensors [3].

Most of the original SAR technology was developed on aircraft. But the first SAR was a satellite that seriously drew the attention of the remote sensing community to this type of sensor [2]. In 1978, the NASA satellite called "SEASAT" showed the world that detailed images can be obtained from the Earth's surface. This program caused a lot of technical development in the remote sensing community. For example, we can mention the work on SAR digital processors and applications such as measuring the length, height and direction of ocean waves [2].

Figure 1-1: SEASAT satellite [4]

SEASAT worked in the L band, frequency 1.27 GHz, at an altitude of 800 km, radiation angle 23 degrees and a bandwidth of 100 km. This satellite was able to obtain images with a resolution of 25 m in the direction of range and direction.

After the SEASAT mission, NASA approved the launch of the SIR series. The program began with the SIR-A test, which was put into orbit in 1981. After that SIR-B and SIR-C were launched in 1984 and 1994 respectively.. The European Space Agency (ESA) also participated in the development of SAR technology by launching two C-band sensors named ERS-1 and ERS-2. The first was successfully launched in 1991 and the second in 1995 [5]. In recent years, many remote sensing satellites have been put into orbit.

1-2-Radar in remote sensing

The increasing use of SAR in the remote sensing community is due to 3 basic features:

The radar carries its own light, so it works well in the dark.

Electromagnetic waves in common radar frequencies from the cloud and fog pass without distortion.

Radar energy from different materials spreads differently than optical energy, so they are a good complement to optical sensors and sometimes provide better detection of surface features than optical sensors.

An overview of common applications of SAR in remote sensing can be found in [6] and also many websites. These applications include: agriculture, soil moisture measurement, forestry, geology, hydrology, flood and sea ice observation, oceanography, ship and oil level detection, snow and ice studies, land cover mapping, height mapping and change detection (land settlement, polar ice movement, volcanic activity). Even some subsurface features have been imaged by SAR, as radar signals can penetrate some materials such as sand and dry sand. Additionally, researchers have found applications in bathymetry (sea and ocean floor mapping). [2]

1-3- The basis of SAR work

A SAR system forms an image of the earth's surface from an air-based platform or space station. It does this by radiating a radar beam almost perpendicular to the sensor's motion vector. SAR sends out phase-coded pulses and records radar echoes as they bounce off the ground. [2]

To form a two-dimensional image, the intensity of echoes should be measured in two perpendicular directions. One dimension is parallel to the radar beam (x dimension). Because the time delay of the received echo is proportional to the distance (range) to the target. By measuring the time delay, the radar places the echoes at a correct distance from the sensor along the x-axis of the image.

The second dimension of the image (y dimension) is formed by the movement of the sensor itself. As the sensor moves along a straight line above the Earth's surface, the radar beam sweeps the Earth's surface at roughly the same speed. The radar system sends pulses of electromagnetic energy and processes the echoes received from the sent pulses and places them along the y-axis of the image according to the current position of the sensor. This will produce an image with correct geometric coordinates. The y dimension is called the side (or along-track) dimension. [2]

Different modes of SAR work

A SAR system can be set up in different ways, the difference being sometimes in the type of system and sometimes in the system's working mode. The most important SAR modes are:

Stripmap SAR: In this mode, according to Figure 1-2, the direction of the antenna beam is kept constant during the movement of the radar platform. The beam sweeps the Earth's surface at a nearly uniform rate. And a strip of the ground surface is imaged.

Figure 1-2: Stripmap SAR

Spotlight SAR: The resolution limit of the stripmap mode can be improved by increasing the size of the radiation angle on the desired surface. This can be done by gradually changing the direction of the beam backwards as the sensor moves across the stage. But the antenna must be re-directed to the front and a part of the surface under imaging is lost. Therefore, only a limited area of ??the earth's surface is imaged at any time. (According to Figure 1-3)

Figure 1-3: Spotlight SAR

Scan SAR: In this mode, according to Figure 1-4, the antenna is scanned several times in range during an artificial aperture by changing the elevation angle. In this way, a much wider band is obtained, but the resolution limit is worse in the side direction.

Figure 1-4: Scan SAR

Interferometric SAR (InSAR): It is a method of SAR in which the background height or displacement is extracted from mixed images with post-processing. Two different SAR images obtained from the same or slightly different spatial positions of the same area are multiplied together as a complex conjugate. The result is an interferogram with the same displacement or height contours.

Figure 5-1: Interferometric SAR

Bistatic SAR: In this mode, the receiver and transmitter are located in different places.