Serendipitous: Unexpected Uses of the Global Positioning System for Earth and Space Science

Speaker:

Seebany Datta–Barua, associate professor of mechanical and aerospace engineering, Illinois Tech 

 

Description:

Global navigation satellite systems such as the Global Positioning System (GPS) are ubiquitously integrated into contemporary life to be almost as invisible as the radio signals they transmit. Designed for providing precise position, navigation, and timing, GPS ranging signals have also emerged as a valuable sensor for Earth and space science. Fundamentally, the radio waves transmitted from the satellites and arriving at a receiver on the ground may undergo refraction, diffraction, or reflection, each of which can be used to sense the environment with which they interact. In this talk, I introduce some of the ways in which GPS signals are used for remote sensing.

 

The signals transmitted at 1575 MHz and 1227 MHz dispersively refract as they pass through the ionized layer of the upper atmosphere, known as the ionosphere. Worldwide networks of GPS receivers are routinely used to image the ionospheric plasma density.

In turn, these densities can be used to estimate the forces that drive the plasma motion: electric fields and neutral winds. GPS signals may be diffracted through small-scale plasma density variations in the ionosphere in some parts of the world. The amplitude and phase of a diffracted signal rapidly fluctuate when received on the ground, rapid fluctuations, known as scintillation. Closely-space arrays of receivers can be used to deduce the speed, motion, and shape distribution of the irregularities and the height at which they occur. More recently, reflections of GPS signals scattered from Earth’s surface are no longer sources of unwanted navigation errors known as multipath. The GPS signals scattered off surfaces and received now are usable as a form of bistatic radar, in which the transmitter is freely available. Recent fieldwork collecting signals scattered off surface lake ice holds promise in being able to distinguish water phase changes. Reflected GPS signals may become more widely used in the future, fielded in remote places where climate monitoring is of interest.