Читать книгу Imagery and GIS - Kass Green - Страница 87

Radar imagery has many uses. Because of its long wavelengths and use of active sensors, radar imagery can be acquired under almost any conditions. It is not impacted by clouds or fog and can even be acquired at night. Areas that experience constant cloud cover such as the Amazon jungle and coastal Alaska, which have very little optical imagery, can be regularly imaged with radar. In addition, depending on the wavelength, radar imagery has the ability to penetrate through the leaves/canopy of the forest and image more of the forest structure or to penetrate into the soil and reveal more about the soil characteristics and wetness. Two radar images of the same area can produce an image with parallax much like a stereo pair of aerial photographs. This parallax can be used in radargrammetry (photogrammetry for radar) to produce topographic/elevation data. Another topographic mapping method called interferometry uses two radar antennas that are spaced apart and can receive a single radar signal that is out of phase. Topographic information is extracted by analyzing the phase shift.

While radar systems have been and will continue to fly on aircraft, companies that provide these services change rapidly. Space-based systems have provided greater stability and sources of radar imagery over time even though many of these missions have ended as well. The imagery and products generated from these space-based missions are more readily accessible and available for use. The very first space-based radar mission was called Seasat, launched in 1978 by the United States with the goal of monitoring ocean conditions including the polar sea ice. The radar sensor was L band using HH polarization. Unfortunately, there were technical issues with the Seasat power system and the mission lasted less than 100 days. The next radar missions were part of the US shuttle program and called the Shuttle Imaging Radar (SIR) experiments. SIR- A (1981) and SIR-B (1984) used L-band radar with HH polarization, the same as Seasat. SIR-C (1994) was designed to experiment with multiband radar and used L, C, and X bands with multiple polarizations. After this, the United States has mostly disregarded radar imagery except for the Shuttle Radar Topography Mission in 2000. The goal of that mission was to use interferometric radar imagery to create topographic data for the majority of the inhabited earth’s surface. The project was a joint one between NASA and the agency then called the National Imagery and Mapping Agency and now called the National Geospatial-Intelligence Agency (NGA). The mission was highly successful in collecting 1-arcsecond elevation data for most of the earth.

Since the early days of Seasat and the Shuttle Radar missions, many other countries have launched platforms with radar sensors into space. Of particular note are Canada, Japan, and the European Space Agency (ESA). Table 4.5 presents a summary of some of the more important radar sensors, both historically and operating today (Lillesand et al., 2015). Because of the varying look angles used to collect radar imagery, the spatial resolution of these images also varies greatly and is not recorded in this table. Almaz-1 is listed in the table because it is the first commercial radar system, however it did not last long and is not as significant a data source as the other sensors in the list.

Table 4.5. Summary of important space-based radar sensors

Soon, two new countries look to enter the radar data collection group. They are Spain, with Paz, a dual-polarization X band radar, and the United Kingdom with NovaSAR-S, a tripolarization S-band radar. In addition, Sentinel-1B was launched in April 2016. Launches are regularly postponed, and therefore it is important to check mission websites for the current status of any of these radar sensors.