Near-Polar Orbits

The other popular orbit type for remote sensing is near-polar. The name is fairly self-explanatory; these are orbits with high inclination angles with an sub-orbital track that almost goes across the poles. Their altitudes are in the range 400-1000 km, which is considerably lower than the 35,800 km of a geostationary satellite. This lower altitude means that the period of these satellites is typically 90-120 minutes. The figure below shows the path of a typical near-polar orbit.

Figure 1: A near-polar satellite orbit

The advantages offered by these orbits include:

• almost complete global coverage, as the Earth rotates on its axis under the orbit. Unlike geostationary spacecraft, near-polar orbiters can provide images of the poles and high latitude regions;
• good surface resolution, since the spacecraft are at low altitudes they can provide considerable surface detail (for example, SPOT can image to 10 metre resolution).
• orbits can be carefully chosen to ensure that variations of solar illumination angle are compensated for (sun-synchronous orbits).

There are, however, some disadvantages:

• the repeat cycle of the orbit can be long, meaning, for example that the interval between direct fly-over of a region by the spacecraft can be quite long and so monitoring of very short time-scale events is not possible;
• data can only be received when the satellite is above the horizon of the receiving station and this is only for a short time, so images from other regions must be obtained via a network of receiving stations or stored on-board.

There are many examples of near-polar spacecraft, such as Landsat, SPOT, and the NOAA series. The table below presents a brief summary of the orbital parameters of each of these.