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 as they travel through the atmosphere.

Application Architecture

GNSS Satellites

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.

Application Characterization

Although the GPS signal was not designed for Atmospheric Sensing, some of its characteristics makes it very suitable for this applications. The GPS signal incorporates a time reference and by design the time taken for the signal to travel from the GNSS satellite and the receiving satellite is measurable. The GPS signal is available in 2 different frequencies and since the ionospheric interference varies with frequency following well known rules this can be used to remove the ionospheric interference. The use of an existing radio signal source allows for a active sensing infrastructure (that requires an emitter and a receiver) with the cost of a passive sensing infrastructure (that only requires a receiver). Also the GNSS signals are designed to not be affected by clouds and precipitation.

The deployment of new GNSS constellations (Galileo, COMPASS) will expand the number of radio signal sources usable for Atmospheric Sensing making available much more data that is currently available.

Application Examples

• GPS/MET - First Atmospheric Sensing experimental satellite managed by the University Corporation of Atmospheric Research (UCAR) and consisted of a 2 kg GPS receiver piggybacked on the MicroLab I satellite.
• CHAMP - CHAllenging Minisatellite Payload is a German satellite that was used for atmospheric and ionospheric research, as well as other geoscientific applications, such as Atmospheric Sensing.
• COSMIC - Constellation Observing System for Meteorology, Ionosphere, and Climate is a US-Taiwan program designed to provide advances in meteorology, ionospheric research, climatology, and space weather by using GPS satellites in conjunction with a constellation of 6 micro-satellites in low Earth orbit.