Atmospheric Sensing

Applications
Title	Atmospheric Sensing
Author(s)	GMV.
Level	Medium
Year of Publication	2011

Earth-orbiting satellites have been equipped with Global Positioning System (GPS) receivers for many years now, mostly as an aid for orbit determination. The receiver processes the signals from visible GPS satellites through one or more antennas mounted on the satellite. For GPS satellites that appear to the receiver to be close to the Earth’s horizon, the signals travel through the Earth’s atmosphere and are therefore less useful for orbit determination. However, such signals can be exploited for sounding the upper atmosphere by measuring how they are affected.

Application Architecture

During occultation of the transmitting satellite by the Earth’s horizon, a large part of the signal path traverses the atmosphere. This slightly reduces the speed of the radio waves compared to the speed of light in vacuum, apparently increasing the measured distance between the GPS satellite and the receiver aboard the low Earth orbit (LEO) satellite. The effect is greatest at the point where the signal is nearest to the Earth. As a result of the relative motion of the two satellites, the altitude of this point will decrease (in the case of a setting occultation) or increase (in the case of a rising occultation). While this atmospheric effect on the signal is a source of error when the data are used for precise positioning or orbit determination, it can yield useful information about the the upper atmosphere, such as temperature and pressure. Tracing the effect with time generates an atmospheric profile.

With precise orbits calculated for both the GNSS satellite and receiver satellite and the data (dual-frequency measurements) collected by the receiving satellite it is possible to calculate (usually in post-processing in the ground segment) data such as temperature, pressure and humidity.