Prediction of uncertainties in atmospheric properties measured by radio occultation experiments

Abstract

Refraction due to gradients in ionospheric electron density, N_e, and neutral number density, n_n, can shift the frequency of radio signals propagating through a planetary atmosphere. Radio occultation experiments measure time series of these frequency shifts, from which N_e and n_n can be determined. Major contributors to uncertainties in frequency shift are phase noise, which is controlled by the Allan Deviation of the experiment, and thermal noise, which is controlled by the signal-to-noise ratio of the experiment. We derive expressions relating uncertainties in atmospheric properties to uncertainties in frequency shift. Uncertainty in N_e is approximately (4πσfcm_eɛ₀/Ve²)√{2πH_p/R} where σ is uncertainty in frequency shift, f is the carrier frequency, c is the speed of light, m_e is the electron mass, ɛ₀ is the permittivity of free space, V is speed, e is the elementary charge, H_p is a plasma scale height and R is planetary radius. Uncertainty in n_n is approximately (σc/Vfκ)√{H_n/2πR} where κ and H_n are the refractive volume and scale height of the neutral atmosphere. Predictions from these expressions are consistent with the uncertainties of the radio occultation experiment on Mars Global Surveyor. These expressions can be used to interpret results from past radio occultation experiments and to perform preliminary design studies of future radio occultation experiments.