# Probing Dust and Water in Martian Atmosphere with Far-Infrared Frequency Spacecraft Occultation
## 3 Results  
### Instrument Sensitivity and Phase Noise Uncertainty  
A direct assessment of instrument sensitivity to absorption losses in the far-infrared band is not possible. Thereby, we utilize the observed S- and X-band absorptivities in Venus Express radio occultation experiments (VeRas) [40] as a range to test the potential detection of dust absorptivity by the receiver during occultation mode. Sulfuric acid absorptivities provide the baseline for comparing the magnitude of atmospheric losses that the signal can undergo during transmission through the Martian atmosphere. It provides an optimum range for which the signal undergoes partial attenuation in the atmosphere, enough to be resolved by the receiver.  
We do not have any uncertainty quantification for far-infrared occultation instruments. Therefore, we used the sensitivity of the S- and X-band instruments to model the uncertainties in phase noise at the 1 and 10 THz frequencies. In this study, we consider the SmallSat Iris v2.1 transponder proposed for S- and X-band cross-link radio occultation to model uncertainties at the THz frequency. As the free space loss is proportional to transmission frequency, higher losses are expected for THz transmission between satellite and Earth. Therefore, we consider the case of O2O cross-link occultation consisting of small satellites as a standard case for THz occultation. Past radio occultation missions relied on a highly accurate USO (Allan deviation of \(10^{-13}\) or better), but for a satellite constellation mission, a less accurate clock must be used to realistically fit within the satellite bus. We employ a dual one-way approach (DOW), previously used in gravity range experiments, to sum the phase of two slightly offset frequencies while simultaneously sending and receiving [52]. The effect of noise reduction is shown in Figure 2A, where the chosen radio frequency is shown as a solid line, and the corresponding DOW transmission is shown as a dashed line. For a single clock, the total noise profile for each transmission frequency is drastically lowered with a DOW filter.  
Using the Monte Carlo simulation code developed by [47], we ran 25 iterations comparing X-band to THz frequency radio occultation observations. The expected noise values combine the thermal uncertainty in the table above along with clock noise generated by an ultra-stable oscillator (USO) power spectral density function as calculated in [52]. The chip size clock we plan to use has an Allan deviation of \(10^{-12}\). The results of the simulation code (Figure 2B) show that the combined bending angle noise is about two orders of magnitude  
Figure 2: **(A)** Phase noise estimations for 1 and 10 THz for one-way and dual one-way occultation. The estimations are compared with Iris V2.1 transponder at UHF (5 MHz), X (8.4 GHz) and Ka (32 GHz) band noise; **(B)** Monte Carlo simulations of expected bending angle noise at 8.4 GHz, 1 THz, and 10 THz at Allan deviation of \(10^{-12}\). Expected bending angles for 1 THz and 10 THz do not differ by a significant magnitude. Thereby, the lines overlap with each other.  
lower for 1 THz than X band. The lower total noise value allows for higher accuracy when retrieving pressure, temperature and number density values.