Satellite Characteristics: Orbit (Near-Polar and Sun-Synchronous Orbits) and Crossing Time (Especially for GATE-Geospatial 2022)

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Remote sensing instruments can be placed on a variety of platforms to view and image targets. Although ground-based and aircraft platforms may be used, satellites provide a great deal of the remote sensing imagery commonly used today. Satellites have several unique characteristics which make them particularly useful for remote sensing of the Earth՚s surface.

Orbit of Satellites

The path followed by a satellite is referred to as its orbit. Satellite orbits are matched to the capability and objective of the sensor (s) they carry. Orbit selection can vary in terms of altitude (their height above the Earth՚s surface) and their orientation and rotation relative to the Earth. Satellites at very high altitudes, which view the same portion of the Earth՚s surface at all times have geostationary orbits. These geostationary satellites, at altitudes of approximately 36,000 kilometres, revolve at speeds which match the rotation of the Earth so they seem stationary, relative to the Earth՚s surface. This allows the satellites to observe and collect information continuously over specific areas. Weather and communications satellites commonly have these types of orbits. Due to their high altitude, some geostationary weather satellites can monitor weather and cloud patterns covering an entire hemisphere of the Earth.

Satellite Characteristics Orbits and Swaths

Geostationary satellites are placed at high altitude viewing the same portion of the Earth՚s surface at all times (Left) , sun-synchronous satellites have near-polar orbits cover each area of the world at a constant local time of day called local sun time (Right)

Near-Polar and Sun-Synchronous Orbits

Many remote sensing platforms are designed to follow an orbit (basically north-south) which, in conjunction with the Earth՚s rotation (west-east) , allows them to cover most of the Earth՚s surface over a certain period. These are near-polar orbits, so named for the inclination of the orbit relative to a line running between the North and South poles.

Many of these satellite orbits are also sun-synchronous such that they cover each area of the world at a constant local time of day called local sun time. At any given latitude, the position of the sun in the sky as the satellite passes overhead will be the same within the same season. This ensures consistent illumination conditions when acquiring images in a specific season over successive years, or over a particular area over a series of days. This is an important factor for monitoring changes between images or for mosaicking adjacent images together, as they do not have to be corrected for different illumination conditions.

Most of the remote sensing satellite platforms today are in near-polar orbits, which means that the satellite travels northwards on one side of the Earth and then toward the southern pole on the second half of its orbit. These are called ascending and descending passes, respectively. If the orbit is also sun-synchronous, the ascending pass is most likely on the shadowed side of the Earth while the descending pass is on the sunlit side. Sensors recording reflected solar energy only image the surface on a descending pass, when solar illumination is available. Active sensors which provide their own illumination or passive sensors that record emitted (e. g. thermal) radiation can also image the surface on ascending passes.

Resource Satellite Data Acquisition Based on Crossing Time

As a satellite in a near-polar sun-synchronous orbit revolves around the Earth, the satellite crosses the equator at approximately the same local sun time every day. Because of the orbital velocity, all other points on the globe are passed either slightly before or after this time.

  • For a sensor in the visible portion of the spectrum, an early morning crossing time would have the sun at a very low angle in the sky and would be good for emphasizing topographic effects but would result in a lot of shadow in areas of high relief.
  • A crossing time around noon would have the sun at its highest point in the sky and would provide the maximum and most uniform illumination conditions. This would be useful for surfaces of low reflectance but might cause saturation of the sensor over high reflectance surfaces, such as ice. Also, under such illumination, ‘specular reflection’ from smooth surfaces may be a problem for interpreters. In the mid-afternoon, the illumination conditions would be more moderate. However, a phenomenon called solar heating (due to the sun heating the surface) , which causes difficulties for recording reflected energy, will be near maximum at this time of day. In order to minimize between these effects, most satellites which image in the visible, reflected, and emitted infrared regions use crossing times around midmorning as a compromise.

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