Earth Observation/Remote Sensing Satellites
Many remote sensing satellites have dual military and civil or commercial purposes. India launched its indigenously developed radar imaging satellite, RISAT-1, in April 2012. The satellite will join RISAT-2 and will provide India with the ability to image features on the ground even if covered by clouds or foliage due to its use of a C-band microwave synthetic aperture radar. RISAT-2, a radar-based reconnaissance satellite, was purchased from Israel and deployed before RISAT-1 to help track and prevent terrorist activity, such as the 2008 Mumbai terrorist attacks.
Weather satellites form another major segment of remote sensing satellites, typically operating in GEO or polar LEO orbits. These systems are primarily operated by national governments for forecasting near-term weather patterns. Delays and funding issues could endanger the robustness of some weather satellites programs, as several existing polar satellites are operating near or beyond their design life and are in need of replacement. One program in particular, the Joint Polar Satellite System (JPSS), is set to replace the aging Polar-orbiting Operational Environmental Satellite (POES) fleet.
Land imaging satellite performance is described using a variety of characteristics, including differences in spatial resolution (as measured by how many pixels compose an object), positional accuracies (as measured by the extent to which objects are represented accurately), and spectral capabilities (as measured by wavelengths of light captured, including visible and beyond-visible spectra). High-resolution land imaging satellites have resolutions below 1 meter (3 feet) per pixel, allowing users to distinguish cars from trucks, for example.
Satellites can also carry advanced payloads that can observe, measure, and produce valuable scientific data regarding land, sea, and air. Such satellites can provide detailed images of the Earth and collect a wide variety of measurements from space, such as ocean temperature, vegetation coverage, or pollution levels. These remote sensing satellites have civil, scientific, and military applications, such as providing aerial views on Google Earth, forecasting potential hurricane paths, or tracking enemy movements on a battlefield.
Military-specific observation satellites can be used for reconnaissance or other intelligence-gathering purposes. The capabilities of remote sensing satellites can sometimes obscure whether or not they are being used for civilian or military applications.
Weather satellites are a major segment of remote sensing satellites. Most weather satellites are in GEO or polar LEO orbits and have traditionally been operated by national governments for near-term weather forecasting. However, austere government budgets have caused weather-focused agencies to struggle to sustain existing programs under reduced funding. Maintaining existing programs with old, albeit reliable, data acquisition capabilities potentially inhibits the development and introduction of new, higher quality instrumentation that could increase accuracy.
While communications satellites retransmit artificial signals from one part of the Earth to another, and PNT satellites simply transmit precise artificial signals to terrestrial receivers, remote sensing satellites collect naturally generated signals from the Earth’s surface, convert them to data, and send them to Earth-bound observers. Remote sensing satellites provide detailed images of the Earth and collect a wide variety of measurements from space, such as ocean temperature, vegetation coverage, or pollution levels.
While many remote-sensing and Earth observation satellites can be used for reconnaissance or other types of intelligence-gathering, military-specific and government-run satellites and sensor payloads are guided by very different mission requirements and laws than their commercial counterparts. There are several intelligence disciplines, or INTs, in which reconnaissance satellites are used to gain information: imagery intelligence (IMINT), signals intelligence (SIGINT), and measurement and signature intelligence (MASINT).
Weather satellites are a major segment of remote sensing satellites, using a mix of electro-optical, atmospheric, gravimetric, SAR, and other sensor payloads to detect fully formed weather systems as well as precursor conditions. Most weather satellites are in GEO or polar LEO orbits and have traditionally been operated by national governments for near-term weather forecasting and long-term climate modeling.
Collectively, land-imaging satellites are systems used to observe, monitor, and track changes and developments on the Earth’s surface using a variety of optical or electronic imaging capabilities. Earth observation satellites may be distinguished from each other on the basis of spatial resolution—the level of detail their images are capable of recording. Another distinction is the sensor type, such as optical cameras, synthetic aperture radar (SAR), or various types of infrared and electronic imaging.