Polar orbiting and geostationary satellites are the two different types of weather satellites. Both satellite systems have distinctive traits and generate extremely dissimilar results. The two polar orbiting satellites survey the same area of the Earth twice a day, once during the day and once at night, from their north-south orbits.
Polar Orbiting Environmental Satellites (POES)
Due to the Earth’s rotation, it is possible to use near-polar orbiting satellites, which have an orbital plane that crosses the poles, to combine the benefits of low-altitude orbits with worldwide coverage. These satellites, known as Polar Orbiting Environmental Satelliites (POES), are launched into orbits at high inclinations* to the Earth’s rotation (at low angles with longitude lines), so that they travel through high latitudes close to the poles.
Most POES orbits are circular or slightly elliptical and range in distance from the geoid between 700 and 1700 km (435 and 1056 miles). They move at various speeds while at various heights. A subsatellite point with a “high inclination” moves north or south along the earth’s axis’ surface projection.
Due to the gravitational pull of the Earth and its rotation, the ground track of the POES is shifted to the west after each orbital period. The orbital period determines this shift in longitude (often less than 2 hours for low altitude orbits). In essence, the area of the planet at the subsatellite point will experience either complete darkness or complete illumination while the satellite rounds both poles. In contrast to the h you determined in Homework 1 for the GOES, such spacecraft need a low earth (h) orbit to balance the effects of gravitational acceleration and centrifugal acceleration.
Geostationary satellites circle the earth in the same direction as the planet and are located at an altitude of roughly 35,800 kilometers (22,300 miles) over the equator (west to east). One orbit takes place at this altitude in 24 hours, which is how long it takes the earth to complete one rotation of its axis. The term “geostationary” refers to how a satellite of this type looks to an observer on the ground to be almost stationary in the sky. Geostationary satellites are used by the new worldwide mobile communications network, BGAN.
A single geostationary satellite can see around 40% of the planet’s surface in a straight line.
used, interference from other satellites and ground-based sources is reduced.
Located at an elevated altitude in earth’s atmosphere, a geostationary satellite Except for small circular areas centered at the north and south geographic poles, three of these satellites, each separated by 120 degrees of longitude, may cover the whole planet.
A directional antenna, often a tiny dish, can be used to access a geostationary satellite if it is pointed at the area of the sky where it appears to be hovering.
This type of satellite’s main benefit is that an earthbound directional antenna can be pointed and then left in place without additional adjustment. Another benefit is that interference from ground-based sources and other satellites is reduced because to the use of highly directed antennas.
There are two primary drawbacks for geostationary satellites.
First, the number of satellites that can be kept in geostationary orbits without interfering with one another is constrained since the orbital zone is a very tight ring in the plane of the equator.
Second, a geostationary satellite’s electromagnetic (EM) signal must travel a minimum of 71,600 kilometers (44,600 miles) to and from it.
A round-trip EM transmission from the ground to the satellite at a speed of 300,000 kilometers per second (186,000 miles per second) introduces a latency of at least 240 milliseconds.
Two other, less critical issues with geostationary satellites exist.
First, due to gravitational interactions with the earth, sun, moon, and extraterrestrial planets, a geostationary satellite’s precise position in relation to the surface varies slightly throughout the course of each 24-hour period. The satellite moves around the box, a rectangular area of the sky that can be seen from the ground. Although the box is small, it restricts the sharpness of the directed pattern and, consequently, the power gain that can be incorporated into earth-based antennas.
Second, due to the sun’s strength as a source of energy, there is a noticeable rise in background EM noise as the satellite approaches the sun, as seen from a receiving station on the ground.