The radio-occultation technique can provide vertical profiles of planetary ionospheric and atmospheric parameters, which merit the planetary-climate and space-weather scientific research so far. The Tianwen-1 one-way single-frequency radio-occultation technique was developed to retrieve Mars ionospheric and atmospheric parameters. The first radio-occultation event observation experiment was conducted on 5 August 2021. The retrieved excess Doppler frequency, bending angle, refractivity, electron density, neutral mass density, pressure and temperature profiles are presented. The Mars ionosphere M1 (M2) layer peak height is at 140 km (105 km) with a peak density of about 3.7 × 10¹⁰ el/m³ (5.3 × 10¹⁰ el/m³) in the retrieved electron-density profile. A planetary boundary layer (−2.35 km~5 km), a troposphere (temperature decreases with height) and a stratosphere (24 km–40 km) clearly appear in the retrieved temperature profile below 50 km. Results show that Tianwen-1 radio occultation data are scientifically reliable and useful for further Mars climate and space-weather studies.

Stellar occultation instruments detect the transmission of stellar spectra through the planetary atmosphere to retrieve densities of various atmospheric components. This paper introduces an idea of using instruments with bandpass filters for stellar occultation detection. According to the characteristics of the occultation technique for oxygen density measurement, a full-link forward model is established, and the average transmission under a typical nocturnal atmosphere is calculated with the help of the HITRAN database, occultation simulation and a 3D ray-tracing program. The central wavelength and bandwidth suitable for 760 nm oxygen A-band absorption measurement are discussed. This paper also compares the results of the forward model with GOMOS spectrometer data under this band, calculates the observation signal-to-noise ratio corresponding to different instrument parameters, and target star magnitudes. The results of this paper provide a theoretical basis for the development of a stellar occultation technique with a bandpass filter and guidance on the instrument design.

The stellar occultation technique is capable of atmospheric trace gas detection using the molecule absorption characteristics of the stellar spectra. In this paper, the non-iterative and iterative retrieval methods for oxygen and air density detection by stellar occultation are investigated. For the single-band average transmission data in the oxygen 761 nm A-band, an onion-peeling algorithm is used to calculate the effective optical depth of each atmospheric layer, and then the optical depth is used to retrieve the oxygen number density. The iteration method introduces atmospheric hydrostatic equilibrium and the ideal gas equation of state, and it achieves a more accurate retrieval of the air density under the condition of a priori temperature deviation. Finally, this paper analyzes the double solution problem in the iteration process and the ideas to improve the problem. This paper provides a theoretical basis for the development of a new type of atmospheric density detection method.

GNSS ionospheric radio occultation (IRO) is an important technique for remote sensing of Earth’s ionosphere. Previous validation of GNSS IRO has often been based on comparisons with ground-based observations, such as ionosondes or in-situ observations, which are assumed to be true representations of the real ionospheric state. But these reference observations commonly used for validation also have errors of their own, which are often neglected in such comparisons. In this study, we employ the three-cornered hat (3CH) method to analyze error statistics and separate errors present in Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) IRO foF ₂ observations. This provides a better characterization of the errors since it avoids the need to know the true value of the quantity of interest. To form the necessary triplet datasets, ionosondes, International Reference Ionosphere (IRI), NeQuick, and the Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM-X) data are used in addition to COSMIC. To verify the feasibility of this method under the error conditions of the data used in this paper, we initially verified the method using simulated data. The error characteristics are analyzed according to local time and geomagnetic latitude. IRO electron density error characteristics are found to be primarily related to inversion assumptions. The COSMIC IRO errors are smallest in summer and largest in winter, and the errors in high solar activity years are larger than the errors in low solar activity years. When understanding error results derived from the 3CH method, special attention should be given to the impact of error correlation and representativeness errors. The results show that the 3CH method provides a better characterization of the errors in COSMIC IRO observations, is helpful for the specification of errors in data assimilation systems, and is suitable for estimating errors in geospace observations.