Satellites can orbit the earth in a number of ways. Lower orbits are faster, meaning the satellite will travel more quickly around the planet. Higher orbits are slower. Geosynchronous (GEO) orbits—where the satellite does not change position relative to the earth’s surface—are quite high (35,786 km/22,236 mi above sea level). NASA’s GOES (Geostationary Operational Environmental Satellite) orbits at this altitude.
GPS satellites, which we use for navigation, fly at a medium earth orbit (MEO) of roughly 20,200 km/12,552 mi altitude. Satellites which complete one full trip across the globe per day (sun-synchronous) fly at a low earth orbit (LEO) of 705 km/438 mi above the earth’s surface. A popular orbit for satellites observing the poles is the Russian-invented Molniya, or high elliptical (HEO) orbit, both highly eccentric and at a high level of inclination from the Earth’s equator.
Current NASA satellite altitude missions orbit between low (LEO and medium (MEO) altitudes, depending on the specific mission. They rely on GPS satellites and land stations to constrain their altitude and so improve the quality of their measurements, which is typically within 2 cm (meaning the satellites can monitor not only topography and ice features but even dynamic features like ocean waves).
The principle of altimetry is simple: the satellite emits a
signal, usually a radio wave at 13.6 GHz (billions of cycles per second). The
speed of light through the atmosphere is precisely known, and the two-way
travel time of the signal is converted into an altitude. The characteristics of
a reflected radio wave are also well known, so return signals can be correlated
precisely against the model and so be made that much more accurate. A number of
other factors must be known ahead of time, or backed out of the data, to make
this precision possible. These include:
- Atmospheric moisture content by altitude (measured by microwave)
- Pitch (in both axes) of the satellite
- Ocean tides
- Solid earth tide (response of the earth’s crust to the moon and sun’s gravity, usually about 30 cm)
- Tidal effects of the moon and sun on the satellite itself
Some of these can be modeled ahead of time, such as tidal
effects on the satellite’s orbit, but some must be removed in post-processing
after analysis. The full course of a satellite’s passage over the world’s
oceans (including the Arctic) can provide a global mean sea level accurate to
within a few millimeters.
Tomorrow: changing plant hardiness zones in the US.
Be brave, and be well.
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