Elsevier

Acta Astronautica

Volume 177, December 2020, Pages 103-110
Acta Astronautica

Mars’ atmospheric calibration of radio tracking data for precise orbit determination

https://doi.org/10.1016/j.actaastro.2020.07.019Get rights and content

Highlights

  • Mars atmosphere strongly perturbs spacecraft radio tracking data.

  • We propose a method to fully calibrate atmospheric effects on Doppler data.

  • This calibration method is well-suited to recover ~10% of the entire MRO dataset.

  • Highly accurate navigation in orbit around Mars will require this data calibration.

Abstract

The accurate navigation of Mars' orbiters requires the precise modeling the radio tracking measurements. The Martian atmosphere perturbs the optical path of the radio links leading to significant Doppler shifts (i.e., up to 5–10 Hz) that affect the spacecraft orbit determination solutions. To process the data occulted by the Martian atmosphere, we present a method that fully calibrates the path delays induced by the neutral atmosphere and ionosphere. Mars’ atmospheric models are used to predict the refractive index of these media, and the estimation of scale factors enables a complete compensation of these perturbative effects. This technique allowed us to reanalyze MRO radio tracking data that were previously discarded to avoid aliasing in the results of our gravity investigation. This precise calibration of the Martian atmosphere will also impact the navigation of future missions during aerobraking phases and science operations at low altitudes.

Introduction

The planet Mars has been visited by several orbiters that have enabled outstanding scientific investigations (e.g., Ref. [1]) and data relay communications to ground for landers and rovers [2]. Spacecraft dedicated to science have been equipped with instruments designed to study Martian interior, climate, and surface. Precise orbit determination (POD) of these orbiters has been fundamental to fulfill the mission requirements and to accomplish the science goals. Accurate georeferencing of high-resolution imaging and altimetric data, for example, strongly relies on the spacecraft reconstructed trajectory. Sophisticated radio science subsystems have then been included onboard those spacecraft to collect precise radio tracking data (i.e., range and range rate) from Earth stations (e.g., NASA's Deep Space Network, DSN). The analysis of these measurements through POD software yields accurate estimates of spacecraft orbit evolution. The quality of the reconstructed orbit severely depends on the modeling of conservative (i.e., planetary gravity field) and non-conservative (e.g, atmospheric drag) forces, and of the measurements (e.g., Earth's troposphere and ionosphere, solar plasma, and Mars' atmosphere and ionosphere).

Dynamical perturbations are compensated by using accurate theoretical and empirical models, which can also be adjusted in POD solutions. The radio tracking data are processed after calibration of Earth's troposphere and ionosphere effects. Solar plasma, which can be compensated only by using multi-frequency radio systems [3], provides a major contribution to the data noise. The Mars Reconnaissance Orbiter (MRO) has been the only spacecraft to Mars with a technology demonstration to enable X-band up- and downlink, and Ka-band downlink to mitigate solar plasma effects [4]. However, this experiment has never worked properly, and MRO radio tracking data have been affected by high levels of noise during solar conjunctions (i.e., a few weeks per year) as have the other Mars' missions (e.g., Mars Global Surveyor – MGS, 2001 Mars Odyssey, Mars Atmosphere and Volatile Evolution – MAVEN).

Mars’ troposphere and ionosphere are also responsible for significant errors in the range rate measurements during atmospheric radio occultations. These events may occur frequently (i.e., once per orbit) in the case of sun-synchronous eccentric orbits (e.g., MGS, Mars Odyssey, and MRO). A modeling of the Martian atmosphere is then important to calibrate the radio tracking data. Atmospheric predictions, however, may not be accurate enough because of uncertainties in seasonal variabilities, global circulation patterns, latitudinal and longitudinal asymmetries, and the presence of local and global dust storms [5]. Scientific investigations of the Martian atmosphere provided crucial information on the lower (e.g., Ref. [6]) and upper atmosphere [7]. Nevertheless, a significant lack of information exists at mid-altitudes (i.e., 10–200 km), and model predictions are not always sufficient to fully compensate data perturbations due to the neutral atmosphere and ionosphere.

In this paper, we present a method to calibrate the radio tracking data during Mars' atmospheric occultations. This technique is applied to MRO radio tracking data that were occulted by the atmosphere. By correcting these measurement errors, we are able to enhance the quality of the reconstructed trajectory. To mitigate the errors due to atmospheric occultations, Genova et al. ([8]), ruled out these data from the POD solution that was focused on the estimation of Mars’ static and seasonal gravity field. The method proposed here will enable including those data in future POD solutions dedicated to spacecraft orbit and gravity determination.

The paper first presents the atmospheric radio occultations that are calibrated with our proposed method then, after introducing the modeling of the atmospheric path delay, the proposed calibration method is developed and applied to MRO radio tracking data, with results showing the accuracy and usefulness of the method to accomplish a full calibration of the Martian atmosphere.

NASA missions MGS, Mars Odyssey, and MRO were designed to survey Mars from near-circular near-polar sun-synchronous orbits. Uniform illumination conditions are required by science instruments (e.g., high-resolution camera) to monitor the Martian surface, and by the electrical power distribution subsystem to enable spacecraft operations. The orbital eccentricity (e) and argument of pericenter (ωp) of the three spacecraft were opportunely defined to minimize the large secular perturbations induced by Mars' gravity zonal harmonics J2, J3, and J4 (i.e., frozen orbits). These orbit configurations led the spacecraft to be periodically occulted by Mars as seen by the Earth. The durations and the geometry of these events strongly depend on Mars-Earth relative position providing gaps in the radio tracking passages. The radio data acquired before (i.e., ingress) and after (i.e., egress) Mars’ planet occultations are significantly affected by the refractivity of the Martian neutral atmosphere and ionosphere.

The Doppler shift measured during atmospheric occultations provides information on tropospheric and ionospheric density profiles. A sophisticated Ultra-Stable Oscillator (USO) was onboard MGS to enable accurate one-way range rate data that were well-suited to retrieve atmospheric profiles [9]. A stable (i.e., 10−13 in terms of fractional frequency stability) radio signal was directly sent by the spacecraft to the DSN station during ingress and egress atmospheric occultations providing high-quality atmospheric sounding. Mars Odyssey and MRO host USOs that are one order of magnitude less stable than MGS USO [8]. Therefore, those one-way range rate data are not accurate enough to measure Mars’ atmospheric properties. Radio occultation operations were conducted with MRO in 2008 and 2012 by using two-way range rate data [10]. However, a coherent two-way link cannot be established quickly enough to enable egress occultations, and those campaigns yielded ingress occultation only. The limited scientific impact of those observations resulted in the planned atmospheric occultation activities for MRO not being pursued.

Our analysis of the full MRO radio tracking dataset shows that a significant number of DSN tracking passages are affected by atmospheric occultations. Fig. 1 shows latitude and areocentric solar longitude (Ls) coverage of the mid-point of ingress atmospheric occultations, which occur on both northern and southern hemispheres. The calibration of these data is fundamental to process all the available observations. Our previous work did not include those data in the POD solutions to avoid a detrimental effect on the quality of orbit reconstruction [8,11].

Section snippets

Methods

To incorporate the radio tracking data occulted by Mars' atmosphere in the POD processing, we propose a method that is based on the well-known technique applied to compensate for Earth's tropospheric and ionospheric path delays. The effects on the radio signals induced by atmospheric refractions are described in detail (Section 3.1) to introduce our calibration method of Mars' atmosphere that allows disentangling uplink and downlink contributions (Section 3.2). We also estimate the coefficients

Results and discussion

The atmospheric calibration technique presented in Section 3 allows us to extend the number of radio tracking data processed in the MRO POD solutions. Section 4.1 shows the benefits of the proposed method on two sets of atmospheric radio occultations of the MRO spacecraft. The observations that are occulted by the Martian atmosphere at altitudes below ~100 km are ~10% of the total number of MRO data. This percentage represents an average over an entire Martian year since the evolution of

Conclusions

Atmospheric occultations of radio tracking data represent a great opportunity to characterize the properties of planetary troposphere and ionosphere. These data allow to sample regions of Mars’ atmosphere that are still poorly known [10]. However, radio occultation campaigns require sophisticated instrumentation onboard the spacecraft (i.e, USO) to obtain accurate one-way data to measure atmospheric density, pressure, and temperature profiles. Two-way data have been barely used to study

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work has been partially funded by grants of the Italian Ministry of Education, University and Research (MIUR). G.C. is grateful to D. Durante (Sapienza University of Rome) for the many fruitful discussions and continuous support. A.G. thanks E. Mazarico (NASA, GSFC), S. Goossens (UMBC, CRESST), D. E. Smith (MIT), and M. T. Zuber (MIT) for their support in the analysis of MRO radio tracking data. The data used in this paper are available from the NASA Planetary Data System (PDS) at //pds-geosciences.wustl.edu/mro/mro-m-rss-1-magr-v1/mrors_0xxx/

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