As discussed in the previous post, a satellite must be very high in order to orbit at a rate equivalent to the earth’s rotation, making it geosynchronous. The diagram below shows the scale of this orbital distance relative to the diameter of the earth. “Nadir” is the point on the globe where a satellite is looking directly downward. For geostationary satellites, the longitude may vary, but the latitude will always be over the equator. Since the orbital distance is very far from earth, the viewing angle “off-nadir” initially changes slowly with increasing latitude, but then increases significantly near the edge of the visible earth disk. As a result, above 50 degrees north (and south) parallax displacement error grows significantly.
The parallax offset problem is not restricted to the poles and can occur near any edge of the full disk view of the earth. However, it is mitigated somewhat in lower latitudes by an array of geostationary weather satellites positioned strategically around the equator. While this provides most non-polar locations with narrow viewing angle options, it does not completely eliminate it. Here is an interesting blog from CIMSS that presents an example of parallax displacement differences in the mid-latitudes between three geostationary satellites.
In October 2016, the first in a series of next generation geostationary satellites, GOES-R, was launched. The advanced technology on this satellite has significantly improved detection and observation capabilities, directly affecting public safety and economic prosperity. Sensor capabilities on GOES-R are very similar to one of Japan’s geostationary satellites, Himawari-8, which is in orbit around 140° east. The example below is a “true-color” image from Himawari-8 in which the Australian continent is near the center of the southern hemisphere, while the Bering Sea and Aleutian Islands are near the far northern edge.
Although image resolution with the new GOES-R satellite will be four times greater, it is important to remember that parallax will continue to be a problem in polar regions. The graph at right is an easy way to estimate the effect of parallax cloud displacement in geostationary satellite data.
For northern latitudes, the high frequency of geostationary data is invaluable for tracking the movement and evolution of cloud features, however the displacement of these features due to parallax should always be taken into account. When following critical weather events, comparisons between geostationary and polar satellite data can be an important exercise in order to correctly determine the location of affected areas.
By Carl Dierking