Sensitivity analysis for airborne sounding of the troposphere by GNSS radio occultation

Abstract

The usual geometry for radio occultation sounding using global navigation satellite system (GNSS) signals has the receiver placed on a low earth orbit (LEO) satellite. We investigate a new geometric approach, assuming an airborne rather than a spaceborne receiver. Information on the refractivity structure and hence the pressure, temperature, and humidity can be retrieved from accurate airborne measurements of amplitude and phase delay of the signals occulted by the troposphere. We present some advantages and disadvantages for the concept of making measurements from commercial aircraft equipped with proper GNSS receivers and antennae compared to the spaceborne case. We simulated realistic airborne occultation observations and assessed the characteristics of their geometry and sampling. We also compared the dynamic range of the signal with the magnitude of error sources that affect the measurements. Findings include that an airborne system has the potential to provide more profiles per unit area below 10 km height than a constellation of up to 25 satellites over the North Atlantic (though with inferior global coverage), and that the signal to noise ratio (SNR) should be better below 5 km than in the LEO case. Though the receiver velocity error is larger than for the LEO system, it is still small enough relative to the signal level to retrieve useful information. The estimated sensitivity of the technique is better than 0.1% refractivity at 3 km altitude increasing to 0.5% refractivity at 11 km. Because of the large horizontal drift of the tangent point of up to 450 km, the assumption of spherical symmetry in the existence of significant 3D variations in structure is expected to be a major error source, in addition to the airplane velocity uncertainty.