A bird’s eye view
Satellite remote-sensing is the collection of information in one or more spectral bands from orbiting satellites. Examples of remote-sensing platforms include weather satellites, photographic satellites and infrared land-use satellites. Public-safety personnel regularly use remote-sensing products for a variety of applications, including weather forecasts, snow-depth measurements, sea-conditions verification, forest-fire monitoring, oil-spill tracking and topography mapping.
History. From 1960 until the early 1990s, only federal governments had the resources or the authority to launch and operate satellites for remote-sensing applications. In the early days, these systems were used for national security and were highly classified. America’s first operational reconnaissance satellite was jointly operated by the CIA and the U.S. Air Force under the code name CORONA. CORONA satellites flew from 1960 through 1972. Unlike modern digital satellites that transmit images to the ground via high-speed radio links, CORONA orbited the earth with 9,600 meters of special 70mm film. Early CORONA satellites employed a single panoramic camera with 7.5-m resolution, but later versions used two stereographic cameras with 1.8-m resolution. After re-entry, exposed film was parachuted to earth with the objective of catching the capsule in midair using a specially equipped C-119 aircraft. Recovery was tricky and some canisters were recovered on the ground or not at all. Of the 144 CORONA satellites launched, 102 returned usable imagery.
Starting in the 1960s, the U.S. government also launched unclassified satellites for weather forecasting and earth science. Of these, Landsat is the best known and most widely used. In fact, Landsat is the longest running program for earth-imaging from space. The first Landsat satellite was launched in 1972 and the most recent, Landsat 7, was launched in 1999. Landsat operates at an altitude of 705 km in a nearly polar orbit. The spatial resolution varies between 15 and 60 meters and the revisit rate for a spot on the earth’s surface is every 16 days. Landsat operates in the blue, green, red, near-infrared, mid-infrared and thermal-infrared bands. According to NASA, Landsat data has been used to monitor water quality, glacier melting, sea ice, land-use changes, deforestation rates and population growth. Landsat images also have been used to assess damage from natural disasters including the 2004 Indian Ocean tsunami and the 2010 Haiti earthquake.
The National Oceanic and Atmospheric Administration (NOAA) operates several weather satellites under the Geostationary Operational Environmental Satellite (GOES) program. As the name suggests, these satellites operate from geostationary orbits and therefore can provide images in near real-time. GOES satellites are a primary forecasting tool for the National Weather Service. In addition to the weather payload, GOES satellites carry Emergency Position-Indicating Radio Beacon (EPIRB) and Emergency Locator Transmitter (ELT) receivers, which are used for search-and-rescue operations.
In 1992, the Congress passed the Land Remote Sensing Policy Act, a law that for the first time allowed private companies to build, launch and operate remote-sensing satellites. Two U.S. companies were formed in the early 1990s to exploit this opportunity, GeoEye and Digital Globe. Today, these companies provide imagery to Google Earth, Microsoft, Yahoo and other Internet search engines. GeoEye and Digital Globe compete with a French company called Spot Image and with several foreign governments for the global satellite imaging market.
Sensor types. Sensors come in two types, active and passive. Most satellite sensors are passive and operate in the infrared and visible light regions. One notable exception is the Synthetic Aperture Radar (SAR), which is an active sensor operating in the GHz range. RADARSAT is a SAR operated by the Canadian government. It transmits at 5.405 GHz and has a spatial resolution of 15 meters (post processed).
Physical limits. Imaging satellites typically operate at altitudes of a few hundred kilometers, which results in orbital periods of about 90 minutes. Depending on the orbit’s inclination and eccentricity, it may take several days for a single satellite to revisit the same spot on the earth’s surface. One might ask why these satellites are not in a geostationary orbit, which is 35,800 km above the earth? The answer is that the physical limits of angular resolution result in unacceptable blurring of two closely-spaced objects into one. The spatial resolution for a telescope (or camera lens) is calculated easily using Equation 1. Consider an especially large lens or mirror with a diameter of 2.5 meters. The wavelength of visible light is 570 nm and geostationary altitude is 35,800 km. If we plug these values into Equation 1, we get a resolution of 8.2 m (27 feet). This is terrible resolution! Two traffic signs 25 feet apart would be indistinguishable from a tractor-trailer rig. Such a system would be OK for weather forecasting, but not for spying. In contrast, the same telescope at an altitude of 400 km would have a resolution of .09 m, or 3.5 inches. With such a system, you can identify people and even measure height from the length of their shadow. You still would not be able to read license plates, however, despite what you might see on television.
Orbits. Some weather satellites operate from geostationary orbits, i.e., a geosynchronous orbit directly above the equator, at an altitude of 35,786 km. Many other remote-sensing satellites operate in a sun-synchronous orbit, which combines inclination and orbital altitude so that the satellite crosses a given point on the Earth’s surface at the same local solar time each pass. This consistent lighting is a useful characteristic for weather and spy satellites. Typical sun-synchronous orbits are 600-800 km altitude with orbital periods between 96 and 100 minutes, and orbital inclination of about 98° (i.e., slightly retrograde with respect to the Earth’s rotation).
Commercial remote-sensing satellites. The three largest private companies operating remote-sensing satellites are GeoEye, Digital Globe and Spot Image. Images from these companies are familiar to users of Google Earth. GeoEye was founded in 1992 as a division of Orbital Sciences Corp. The division was spun off in 1997 and renamed GeoEye in 2006 after acquiring Space Imaging, owner of the IKONOS satellite. GeoEye’s newest satellite is capable of 25 cm resolution, but it appears that U.S. law will limit commercial products to 50-cm resolution.
Like GeoEye, Digital Globe was created in anticipation of the 1992 Land Remote Sensing Policy Act. Its most recent satellite is capable of 46-cm resolution. Digital Globe has been the major contributor of images to Google Earth.
Spot Image is a French company that is 81% owned by EADS, a European aerospace corporation that also manufactures the Airbus aircraft. The two Spot satellites currently in orbit offer a large choice of resolutions from 2.5 to 10 m.
Equation 1
r=hλ/d
where r is the resolution, h is the height of the satellite, λ is the wavelength of light and d is the diameter of the telescope lens or mirror.
Jay Jacobsmeyer is president of Pericle Communications Co., a consulting engineering firm located in Colorado Springs, Colo. He holds bachelor’s and master’s degrees in electrical engineering from Virginia Tech and Cornell University, respectively, and has more than 25 years experience as a radio-frequency engineer.
