Surface reflected signals from the Global Positioning System for ionospheric measurements: Experimental results at aircraft altitudes

Abstract

Several spaceborne altimeters have been built and flown, and others are being developed, to provide measurements of ocean and ice sheet topography. Until the launch of TOPEX (Ocean Topography Experiment), altimeters were single-frequency systems that were incapable of removing the effects of ionospheric delay on the radar pulse. With the current state-of-the-art in satellite altimetry, the ionosphere causes the largest single error when using single-frequency altimeters. Ionospheric models provide the only recourse short of adding a second frequency to the altimeter. Unfortunately, measurements of the ionosphere are lacking over the oceans or ice sheets where they are most needed. A possible solution to the lack of data density may result from an expanded use of the Global Positioning System (GPS). This paper discusses how the reflection of the GPS signal from the ocean can be used to extend ionospheric measurements by simply adding a GPS receiver and downward-pointing antenna to satellites carrying single-frequency altimeters. The viability of this concept hinges upon the ability to acquire and code-track the reflected signal for an extended period of time over a variety of sea states. The theory of specularly and diffusely reflected radio frequency radiation from a rough surface is reviewed. Results of experiments to demonstrate tracking of a reflected signal are presented for three aircraft flights over the Chesapeake Bay and the Eastern Shore of Virginia. The experimental hardware consisted of two off-the shelf receivers configured such that one received the GPS signal in the conventional manner using a right-hand circularly polarized (RHCP) antenna on top of the fuselage and the other could receive the reflected signal using a left-hand circularly polarized (LHCP) antenna on the bottom of the fuselage. Three tests were performed on the data to verify that the signals received in the bottom antenna were sea surface reflections: pseudorange double differences were compared against predicted geometric range double differences, characteristics of a signal reflected from a random surface were observed in the carrier to noise ratio and predicted specular points were plotted which demonstrate reflection only from wet areas. These tests indicated tracking of reflected signals for extended periods of time at altitudes of up to 5500m and sporadic signal acquisition at higher altitudes.