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

Several spaceborne altimeters have been built and flown, and others are bei ng developed, to provide measurements of ocean and ice sheet topography. Un til the launch of TOPEX (Ocean Topography Experiment), altimeters were sing le-frequency systems that were incapable of removing the effects of ionosph eric delay on the radar pulse. With the current state-of-the-art in satelli te altimetry, the ionosphere causes the largest single error when using sin gle-frequency altimeters. Ionospheric models provide the only recourse shor t of adding a second frequency to the altimeter. Unfortunately, measurement s of the ionosphere are lacking over the oceans or ice sheets where they ar e most needed. A possible solution to the lack of data density may result f rom an expanded use of the Global Positioning System (GPS). This paper disc usses how the reflection of the GPS signal from the ocean can be used to ex tend 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-t rack the reflected signal for an extended period of time over a variety of sea states. The theory of specularly and diffusely reflected radio frequenc y radiation from a rough surface is reviewed. Results of experiments to dem onstrate tracking of a reflected signal are presented for three aircraft fl ights over the Chesapeake Bay and the Eastern Shore of Virginia. The experi mental hardware consisted of two off-the-shelf receivers configured such th at one received the GPS signal in the conventional manner using a right-han d 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 r eflected from a random surface were observed in the carrier to noise ratio and predicted specular points were plotted which demonstrate reflection onl y from wet areas. These tests indicated tracking of reflected signals for e xtended periods of time at altitudes of up to 5500 m and sporadic signal ac quisition at higher altitudes.