General description of Polar Orbiting Satellites
Illustrations and material provided by: David B. Johnson, NCAR/MMM.
Polar orbiting satellites are an important class of meteorological
and geophysical satellite. Typically, these satellites are placed
in circular sun-synchronous orbits. Their altitudes usually range
from 700 to 800 km, with orbital periods of 98 to 102 minutes. Satellites
in this category include the NOAA Polar-orbiting Operational Environmental
Satellites (POES), satellites of the Defense Meteorological Satellite
Program (DMSP), Landsat, and SPOT. The DMSP and NOAA/POES satellites
are operational meteorological satellites. Imagery from successive
orbits overlay with each other, giving global daily coverage from each
satellite. Landsat and SPOT, on the other hand, are intended for
geophysical remote sensing, with an emphasis on high-resolution and
multispectral imagery, at the cost of daily global coverage.
POES Orbits
This figure illustrates the orbital track for a sun-synchronous satellite
in near-polar orbit. The orbital track relative to the Earth's surface
is due to a combination of the orbital plane of the satellite coupled with
the rotation of the Earth beneath the satellite. To achieve a
syn-synchronous orbit, the orbital plane is inclined slightly away
from a true north-south track to introduce a slow precession in
the orbital plane, roughly one degree per day. This
precession ensures that the equatorial crossing times of the satellites,
in terms of the local solar time, remain nearly constant throughout the
year. This means that a satellite can make repeated global observations
from a single set of sensors with similar illumination from pass to pass.
Note that the orbit is slightly tilted towards the northwest and does not
actually go over the poles. While the red path follows the earth track
of the satellite, the transparent overlay indicates the coverage area
for the AVHRR imaging instrument carried by NOAA/POES satellites. This
instrument scans a roughly 3000 km wide swath. The map projection used
in this illustration, a cylindrical equidistant projection, becomes
increasingly distorted near the poles, as can be seen by the seeming
explosion of the viewing area as the satellite nears its northern and
southern most orbital limits. For a more realistic view of the satellite
orbit in the polar regions, it is better to use a differend map projection,
such as the polar stereographic
(see examples, 73 k).
POES/NOAA
Reprinted from Johnson et al., 1994 (Bulletin of the
American Meteorological Society, Volume 75, pp. 5-33).
Sun-synchonous orbits are typically described by their equatorial
crossing times. Having a sun-synchronous orbit, however, does not
mean that the solar illumination angles are constant throughout the
orbit. Most obviously, the sun will generally be lower in the sky
as you move northward or southward towards the poles. In addition,
the local solar time will also vary during the orbit. The upper panel
in the above figure
illustrates the local solar time at the subsatellite point throughout
one entire orbit of a sun-synchronous satellite. The "ascending" portion
of the orbit corresponds to that portion of the orbit when the satellite
is moving from south to north, while the "descending" part of the orbit
corresponds to north to south movement. This specific example is based
on the NOAA-11 satellite, with orbital parameters from July 1993.
The furthest poleward excursion of the satellite is at 81 degrees
latitude. The equatorial crossing times are at 0400 and 1600 LST. While
the equatorial crossing times are precisely 12 hours apart, passes at
other latitudes are not evenly spacen in time. For this example, the
satellite will cross 40 degree north latitude at 0431 and 1529 LST.
The bottom panel of this figure shows the time offfset from the
equatorial crossing time as a function of latitude. Unlike the top
panel, which strictly speaking, is only applicable to a single satellite
and date, the bottom panel is more general and can be used to make a good
first approximation of the equatorial crossing times at any latitude
up to 60 degrees for any of the NOAA/POES, DMSP, Landsat, or SPOT
satellites.
With an orbital period of about 100 minutes, these satellites will
complete slightly more than 14 orbits in a single day.
POES Orbits
This figure duplicated the orbital track shown in an earlier figure,
but with 14 additional orbits drawn in yellow. This figure gives a good
indication of the daily coverage of a single satellite in sun-synchronous
orbit. Note that the satellite does not make an integer number of
orbits in a single day, so that there is a slight offset in the orbital
tracks after 14 orbits. This means that although the equatorial
crossing time in terms of the
local solar time
is constant, the
clock time
for the satellite overpass at a fixed
location will vary from day to day, as will the distance and azimuth to
the satellite.
Coverage in Polar Regions:
The above maps give a good view of the equatorial regions, but distort
the polar regions rather grossly. Since polar orbiting satellites
are naturally of particular importance in polar regions (due to a
combination of poor coverage by geostationary satellites and the
frequent overflights of the satellites) it is useful to look at the
orbits in a polar stereographic point of view.
North Pole
South Pole
