Since its observation in 2019, the first image of a super-massive black hole using Very Long Baseline Interferometry (VLBI) with an Earth-scale baseline has generated much scientific and public interest. Work is now underway to extend the baseline into space to obtain higher image resolution. Operating one or more VLBI nodes in space will require the use of frequency standards that are space-qualified, greatly reducing the number of options available. The coherence function is the metric usually used to determine the viability of a frequency standard. Here we show that is a useful but not sufficient metric for gauging frequency standard performance in VLBI and instead derive an expression for the clock-limited VLBI visibility S/N. We evaluate this expression for real frequency standards and find only the Ultra-Stable Oscillator (USO) and hydrogen maser to be viable for upcoming high-frequency VLBI with the USO only useful for very limited integration times (30 seconds at 90 GHz, 10 seconds at 230 GHz, 5 seconds at 345 GHz, and not viable at 630 GHz). The maser extends these, but may have prohibitive size for a space mission. We also evaluate emerging frequency standard technologies and find the optical local oscillator portion of optical clocks to be very promising (conservatively >100 seconds at 90 GHz, 60 seconds at 230 GHz, 40 seconds at 345 GHz, and 22 seconds at 630 GHz) when accounting for both performance and potential operation in space.

For perturbed two-body motion—such as small bodies in heliocentric orbits—the Cartesian state uncertainty alternates between near-Gaussian and strongly non-Gaussian regimes over the orbit. This behavior motivates hybrid estimators that adapt to the changing character of the uncertainty. Implementation of a hybrid filter hinges on the mechanisms that control the transitions between the two frameworks. In this paper, we introduce an unscented Kalman filter/particle filter (UKF/PF) hybrid prediction strategy that uses the normalized Euclidean distance to switch from the UKF (moment-based) to the PF (sample-based), and the Henze-Zirkler statistic to switch back, with dynamics sourced from ASSIST, an ephemeris-quality integrator. This autonomous switching technique requires only the abstractions propagated during prediction, enabling complete independence from measurement updates. We demonstrate the capability of the approach by propagating the state uncertainty of the asteroid 2024 YR_4 from its orbit solution in November 2025 to its close approach of the Moon in December 2032. The hybrid UKF/PF accurately and efficiently predicts the lunar impact probability, velocity, angle, and potential cratering of 2024 YR_4 , providing all the information necessary to assess the potential fallout of the ejecta created from such an impact event. The proposed hybrid prediction method effectively balances and integrates the complementary strengths of the moment- and sample-based approaches in a measurement-free environment.

A next-generation Mars Network is investigated to determine a configuration optimized for both communications and positioning performance for surface users. A previously proposed 6000 km altitude, 3-satellite equatorial constellation that was found to be optimal for communications to surface users located in the latitude range from 60°S to 60°N is shown to be deficient for surface positioning. Inclining the 3-satellite configuration between 30° and 50° improves positioning performance to users in this latitude range; however, due to a lack of coverage this improvement is primarily seen for positioning when using tracking data collected over long timespans. Moving to an inclined 6-satellite case and using a Walker 6/2/0 delta configuration, at selected inclinations and altitudes, greatly improves the positioning solution performance over shorter timescales, with the best performance obtained with orbits inclined at 50°. Also examined were continuous coverage global constellations that were compared to the Walker 50°: 6/2/0 configurations. The single fold continuous coverage Walker 55.7°:7/7/5 constellation slightly improves the positioning performance and provides more uniform and continuous coverage to the poles, which the Walker 50°:6/2/0 case cannot. Finally, a Walker 57.1°: 8/8/2 constellation that provides continuous twofold coverage was examined; however, the high altitude required for this case reduces its communication performance and yields poorer positioning performance relative to the Walker 55.7°:7/7/5. It is concluded that a next generation Mars Network with focused support to users between 60°S and 60°N that the Walker 50°: 6/2/0 is the best positioning and communications performance while, for continuous coverage global coverage, the Walker 55.7°: 7/7/5 is superior.

Ground-based atomic clocks have been the foundation of the Deep Space Network's (DSN's) ability to provide high-precision tracking to deep space users for navigation and radio science since its inception in the mid-1960s. This paper describes the development of space clocks that could aid the DSN and the solar system exploration enterprise (such as by being the basis of a Lunar or Mars navigation system—similar to ground clock's role for the DSN—and by forming an in situ timescale). The paper reviews the most promising technologies potentially available in the next few years that could be used to realize these benefits for the DSN and exploration/science in the coming decade. Specifically, advances with the Deep Space Atomic Clock (DSAC) make it the most viable technology at this time for realizing a new space clock with orders of magnitude better long-term stability than existing space clocks. Other technologies, such as a space-capable fully optical clock with further improvements in performance, are likely more than a decade away from a space demonstration. Thus, with investment now, a follow-on to DSAC (that we label generically as DSAC-FO) could be ready for demonstration and commercialization in a few short years and made available for wide-scale use by NASA and the DoD this decade.

The methods of trapping and cooling of atoms and ions have been transformative for atomic clocks due to the reduction, and in some cases elimination, of major systematic frequency shifts. Continuously operating atomic clocks based on trapped mercury ions have existed for decades but until recently have been restricted to terrestrial applications. The recently completed Deep Space Atomic Clock (DSAC) mission demonstrated the first trapped ion clock operation in space. Here we review DSAC as well as follow-on improvements towards the realization of high stability, long life Hg ion atomic clocks for foreseen ground and flight application.

The atmospheric radio occultation (RO) technique was developed six decades ago for planetary missions and has since resulted in numerous scientific discoveries throughout the Solar System. The traditional experimental configuration utililizes spacecraft communication links with phase-stable radio signals transmitted from a spacecraft orbiting or flying past a planet and received at a ground station after propagating through the atmosphere. Alterations in the phase and amplitude are used to infer properties of that atmosphere. A reverse configuration with a ground-to-spacecraft one-way link has been used with the appropriately instrumented mission to Pluto to overcome severe signal-to-noise ratio limitations. A more recently investigated spacecraft-to-spacecraft occultation technique utilizes UHF communication links between Mars landers and orbiters to measure ionospheric properties.In 1995, the Earth science community demonstrated a variation of this technique that enabled remote sensing of Earth’s atmosphere and ionosphere. By tracking spacecraft-to-spacecraft crosslinks utilizing the global navigation satellite constellations as the transmitting terminals and specially instrumented satellites as the receiving terminals, high resolution vertical profiles are routinely retrieved with valuable utility in numerical weather prediction and atmospheric dynamics. Applying crosslinks more generally to planetary missions would allow large increases in spatial and temporal coverage of atmospheric structures and composition; however, this has been prohibitively costly until recent technological advances and breakthroughs.The advances of the required technologies for crosslink RO experiments currently enable realistic planning of their use in future planetary missions. These crosslinks could use orbiting small spacecraft for one or multiple links, and possibly probes, aerobots (e.g., balloons), and landers equipped for relay communications. These technologies and techniques include:1.Small software-defined radios2.Multi-frequency communication links3.Smaller ultra-stable oscillators4.Advancedsignal processing and holographic methods5.Interplanetary flight, orbit insertion, and formation-flyingThis paper presents the state of the art of these technologies and introduces lower cost mission concepts to Venus, the giant planets, and other planetary targets in the context of recent Decadal Surveys. These multi-spacecraft missions provide natural opportunities for international collaboration. Design requirements, recent simulations, radio-holographic analysis methods, and lessons from an opportunistic UHF crosslink demonstration at Mars will be discussed.

In June 2022, the Electra Relay Operations team began conducting radiometric phase and power data collection during relay overflights of the InSight Lander by the ExoMars Trace Gas Orbiter (TGO). This mode of the Electra radio has potential applications for both navigation and radio science, but is relatively untested in flight. Here, we present both one-way (non-coherent) and two-way (coherent) Doppler observations conducted during 32 TGO relay overflights. Then we explore the ability to find the surface position of the lander with each data set and compare this against other methods.Looking forward to Mars Sample Return, the capability to do one-way Doppler tracking would be essential for monitoring the launch performance of the Mars Ascent Vehicle and determining the final trajectory of the Orbiting Sample container. Testing these positioning capabilities now will help us to prepare for that anticipated need.

The Iris software radio has been updated to collect one-way Doppler and range data for potential use with deep space autonomous navigation. One-way radiometric data have found limited use because a typical radio oscillator is not sufficiently stable for use in navigation. However, Iris has been paired with a chip-scale atomic clock (CSAC) via an input signal of one pulse per second. With superior stability relative to a typical oscillator, the CSAC has the potential to provide onboard tracking data with sufficient accuracy to support a small satellite mission with modest navigation requirements. In this paper, we develop models of the Iris radio one-way Doppler and range data and analyze their performance in lab testing prior to a future inflight test on NASA’s CAPSTONE mission to the Moon. The test results confirm theoretical predictions for range precision measured between 0.38 m and 2.21 m with a range rate of 11 mm/s at 60 s.

The NASA Discovery-class mission VERITAS, selected in June 2021, will be launched towards Venus after 2027. In addition to the science instrumentation that will build global foundational geophysical datasets, VERITAS proposed to conduct a technology demonstration for the Deep Space Atomic Clock (DSAC-2). A first DSAC successfully operated in low-Earth orbit for more than two years, demonstrated the trapped ion atomic clock technology, and established a new level of performance for clocks in space. DSAC-2 would have further improvements in size, power, and performance. It would host a $1\times{10}^{-13}$ grade USO to produce a frequency output with short-term stability of less than $2\times{10}^{-13}/\sqrt\tau$ (where $\tau$ is the averaging time). However, due to funding shortfalls, DSAC-2, had to be canceled. The initially foreseen presence of an atomic clock on board the probe, however, raised the question whether this kind of instrumentation could be useful not only for navigation and time transfer but also for fundamental physics tests. In this work, we consider the DSAC-2 atomic clock and VERITAS mission as a specific example to measure possible discrepancies in the redshift predicted by General Relativity by using an atomic clock onboard an interplanetary spacecraft. In particular we investigate the possibility of measuring possible violations of the Local Lorentz Invariance and Local Position Invariance principles. We perform accurate simulations of the experiment during the VERITAS cruise phase. We consider different parametrizations of the possible violations of the General Relativity, different operational conditions, and several different assumptions on the expected measurement performance. Our analysis shows the scientific value of atomic clocks like DSAC-2 hosted onboard interplanetary spacecraft.

High–sensitivity shaken lattice interferometry (SLI) based sensors have the potential to provide deep space missions with the ability to precisely measure non–gravitational perturbing forces. This work considers the simulation of the OSIRIS-REx mission navigation in the vicinity of Bennu with the addition of measurements from onboard SLI–based accelerometers. The simulation is performed in the Jet Propulsion Laboratory’s (JPL) Mission Analysis, Operations and Navigation Toolkit (MONTE) and incorporates OSIRIS-REx reconstructed trajectory and attitude data from the Navigation and Ancillary Information Facility (NAIF) database. The use of the reconstructed data from NAIF provides realistic true dynamical errors and JPL’s MONTE software allows for a high–fidelity simulation of an integrated trajectory for the filter. The navigation performance and reduction of tracking and complex modelling enabled by the onboard SLI–based sensor are presented for two orbital phases of the OSIRIS–REx mission. Overall, the results show that the addition of SLI–based accelerometer measurements improves navigation performance, when compared to a radiometric tracking only configuration. In addition, results demonstrate that highly–precise accelerometer measurements can effectively replace at least one day of DSN passes over a three–day period, thereby reducing tracking requirements. Furthermore, it is shown that lower–fidelity surface force modeling and parameter estimation is required when using onboard SLI–based accelerometers.

The Deep Space Atomic Clock (DSAC), a NASA Technology Demonstration Mission, was launched into low-Earth orbit on June 25, 2019 as a hosted payload aboard General Atomics' Orbital Test Bed (OTB) spacecraft. The DSAC mission has been conducting a two-year demonstration of a mercury ion atomic clock to characterize its space-based performance and to validate its utility for deep space navigation and radio science. Analysis of the collected data using JPL's GIPSY-OASIS software has shown DSAC's Allan Deviation (AD) at one-day to be near 3×10 -15 ; much better than required AD of 2×10 -14 . Such low spacecraft clock errors will enable one-way radiometric tracking data with precision equivalent to or better than current-day two way tracking data, allowing a shift to a more efficient and flexible one-way deep space navigation architecture. To verify this, an analog deep space navigation experiment was performed using JPL's operational navigation software (Monte). The experiment recovered orbit solutions with reduced data sets and geometric variations that are more representative of deep space missions, and showed that orbit determination using DSAC derived data is on par with more traditional two-way datatypes. As a technology demonstrator, DSAC's development focus has been on maturing the mercury ion trap clock technology rather than achieving the smallest size, weight, and power (SWaP). Over the course of DSAC's development the project has identified numerous improvements that could be made to significantly reduce SWaP for DSAC's next version. Indeed, DSAC-2 was recently selected by NASA for further demonstration on the VERITAS mission to Venus. This work will review the DSAC technology, mission, and results from its two-year mission.

With the advent of the Deep Space Atomic Clock, operationally accurate and reliable one-way radiometric data sent from a radio beacon (i.e., a DSN antenna or other spacecraft) and collected using a spacecraft’s radio receiver enables the development and use of autonomous radio navigation. This work examines the fusion of radiometric data with optical data (i.e. OpNav) to yield robust and accurate trajectory solutions that include selected model reductions and computationally efficient navigation algorithms that can be readily adopted for onboard, autonomous navigation. The methodology is characterized using a representative high-fidelity simulation of deep space cruise, approach, and delivery to Mars. The results show that the combination of the two data types yields solutions that are almost an order of magnitude more accurate than those obtained using each data type by itself. Furthermore, the combined data solutions readily meet representative entry navigation requirements (in this case at Mars).

The Cislunar Autonomous Positioning System Technology Operations and Navigation Experiment (CAPSTONE) mission is an upcoming lunar flight demonstration, with a targeted launch in early 2022. The primary objective of the mission is to test out navigation and operations in the same Near Rectilinear Halo Orbit (NRHO) that will be utilized by NASA’s Lunar Gateway. In addition to this objective, CAPSTONE contains a dedicated flight board for the Cislunar Autonomous Positioning System (CAPS), a framework for autonomous or near-autonomous navigation in cislunar space developed by Advanced Space. During the mission, CAPS will demonstrate onboard orbit determination via both one-way uplink measurements and crosslink radiometric measurements with the Lunar Reconnaissance Orbiter (LRO). This paper previews the expected performance of these two experimental data types with a set of high-fidelity simulations using realistic CAPSTONE concept of operations (CONOPS) and radio specifications. For CAPSTONE, the nominal tracking architecture will rely on the standard two-way radiometric measurements of the Deep Space Network (DSN) – CAPS is intended is a technology demonstration. However, solutions will be processed onboard, and the expected navigation performance of these experimental data types are compared against the expected ground-based navigation performance. The goal of CAPS is ultimately to provide a framework for autonomous navigation in cislunar space. The CAPS experiments onboard CAPSTONE will be the first on-orbit step towards this goal. While the early experiments will not replace the utilization of ground-based navigation, they should be able to supplement it and work towards a future where the reliance on two-way ground networks is greatly reduced. For the CAPSTONE orbit and radio specifications, the one-way uplink measurements are expected to provide navigation solutions with uncertainties on the order of single kilometers in position and single cm/s in velocity. These solutions could, in theory, be utilized to design stationkeeping maneuvers onboard the spacecraft and maintain its NRHO over time. Simulations of the expected crosslink measurements also show navigation performances similar to the expected performance of two-way tracking. In addition to examining the CAPSTONE operational CONOPS, the performance of a conceptual ground-based one-way beaconing system is examined, compared to standard two-way radiometric tracking, and used in conjunction with the expected crosslink performance to study what may be possible in terms of cislunar autonomous navigation provided by CAPS in the future.

Recent advances with space navigation technologies developed by NASA in space-based atomic clocks and pulsar X-ray navigation, combined with past successes in autonomous navigation using optical imaging, brings to the forefront the need to compare space navigation using optical, radiometric, and pulsar-based measurements using a common set of assumptions and techniques. This review article examines these navigation data types in two different ways. First, a simplified deep space orbit determination problem is posed that captures key features of the dynamics and geometry, and then each data type is characterized for its ability to solve for the orbit. The data types are compared and contrasted using a semi-analytical approach with geometric dilution of precision techniques. The results provide useful parametric insights into the strengths of each data type. In the second part of the paper, a high-fidelity, Monte Carlo simulation of a Mars cruise, approach, and entry navigation problem is studied. The results found complement the semi-analytic results in the first part, and illustrate specific issues such as each data type's quantitative impact on solution accuracy and their ability to support autonomous delivery to a planet.

This study investigates methods for autonomous navigation of a deep-space spacecraft where one-way radiometric and on-board optical information are fused to create a fully informed state estimate. The specific focus is on using filter bank methods (i.e., Multiple Model Estimation [MME] and Mixture of Experts [MoE]) to detect when measurement and/or dynamical mis-modeling occurs. We develop a new χ2-based gating network for a filter bank that may be used to identify poorly performing filters (i.e., those with low weights), which may be used as a signal for mis-modeling in the system. In addition to defining and deriving this new weighting scheme, numerical simulations based on NASA’s InSight mission demonstrate this new algorithm’s performance with and without measurement and dynamical mis-modeling present.

The timing and frequency stability provided by the Deep Space Atomic Clock (DSAC) is nearly commensurate to the Deep Space Network's ground clocks and enables one-way radiometric measurements with accuracy equivalent to current two-way tracking data. A demonstration unit of the clock was launched into low Earth orbit on June 25, 2019, for the purpose of validating DSAC's performance in the space environment. Global Positioning System ( GPS) data collected throughout the two-year mission was utilized not only for precise clock estimation but also as a proxy for deep space tracking data to conduct the Deep Space Navigation Analog Experiment. Through careful selection and processing of GPS Doppler data and limited modeling fidelity representative of deep space navigation capabilities, the analog orbit solutions are compared to higher-fidelity solutions, demonstratingDSAC's viability as a navigation instrument in conditions typical for a low-altitude Mars orbiter. Onboard telemetry quantifying the ultrastable oscillator (USO) frequency correction is processed to demonstrate the orbit determination performance degradation when utilizing USO-based one-way radiometric tracking data.

Deep understanding of planetary habitability requires identifying key factors that govern the surface environment over time. Venus is the ultimate control case for understanding how Earth developed and maintained conditions suited to life. Venus very likely had elements essential to habitability such as past surface water and a dynamo. Tectonism and volcanism, which create chemical disequilibrium, very likely persist today. What caused Earth and Venus to diverge down different evolutionary paths? VERITAS would create foundational, co-registered data sets of high-resolution topography, imaging, spectroscopy, and gravity, on par with those available for Mercury, Mars, and the Moon. VERITAS would answer outstanding fundamental questions about the evolution of Earth's twin. The VERITAS payload consists of the Venus Interferometric Synthetic Aperture Radar (VISAR) and the Venus Emissivity Mapper (VEM), plus a gravity science investigation. VISAR is an X-band radar that provides: 1) a global digital elevation model (DEM) with 250-m postings and 6-m height accuracy, 2) Synthetic aperture radar (SAR) imaging at 30-m horizontal resolution globally, 3) SAR imaging at 15-m resolution for $> \boldsymbol{25\%}$ of the surface, and 4) surface deformation from repeat pass interferometry (RPI) with 2-cm vertical precision for $> \boldsymbol{12} \boldsymbol{(\sim 200\ \mathrm{x}\ 200\ \text{km})}$ targeted areas. VEM covers $\boldsymbol{ > 70\%}$ of the surface in six near-infrared (NIR) bands sensitive to iron composition located within five atmospheric windows, plus eight atmospheric bands for calibration and water vapor measurements. VEM would provide near-global maps of mafic to felsic rock type and will search for active and recent volcanism. VERITAS would use two-way Ka-band uplink and downlink from a low circular orbit $\boldsymbol{(< 250\ \text{km})}$ to create a global gravity field with 3-mGal accuracy of 155-km resolution (degree and order 123). An onboard technology demonstration, the Deep Space Atomic Clock (DSAC-2), may support radio science and navigation with one-way tracking. VERITAS data would enable estimation of elastic thickness (a proxy for thermal gradient) and density differences due to subsurface structures, as well as constraining interior structure, including core size and state. Lockheed Martin builds the spacecraft. VISAR is built by JPL, with the Italian Space Agency (ASI) providing the low power electronics. ASI also provides transponders and a high gain antenna for the telecom system. CNES provides the Ka-band traveling wave tube amplifiers (TWTA). The German Space Agency (DLR) provides VEM and contributes algorithms for VISAR ground and onboard data processing.

Atomic clocks, which lock the frequency of an oscillator to the extremely stable quantized energy levels of atoms, are essential for navigation applications such as deep space exploration1 and global navigation satellite systems2, and are useful tools with which to address questions in fundamental physics3–6. Such satellite systems use precise measurement of signal propagation times determined by atomic clocks, together with propagation speed, to calculate position. Although space atomic clocks with low instability are an enabling technology for global navigation, they have not yet been applied to deep space navigation and have seen only limited application to space-based fundamental physics, owing to performance constraints imposed by the rigours of space operation7. Methods of electromagnetically trapping and cooling ions have revolutionized atomic clock performance8–13. Terrestrial trapped-ion clocks operating in the optical domain have achieved orders-of-magnitude improvements in performance over their predecessors and have become a key component in national metrology laboratory research programmes13, but transporting this new technology into space has remained challenging. Here we show the results from a trapped-ion atomic clock operating in space. On the ground, NASA’s Deep Space Atomic Clock demonstrated a short-term fractional frequency stability of 1.5 × 10−13/τ1/2 (where τ is the averaging time)14. Launched in 2019, the clock has operated for more than 12 months in space and demonstrated there a long-term stability of 3 × 10−15 at 23 days (no drift removal), and an estimated drift of 3.0(0.7) × 10−16 per day. Each of these exceeds current space clock performance by up to an order of magnitude15–17. The Deep Space Atomic Clock is particularly amenable to the space environment because of its low sensitivity to variations in radiation, temperature and magnetic fields. This level of space clock performance will enable one-way navigation in which signal delay times are measured in situ, making near-real-time navigation of deep space probes possible18. Operating in space, NASA’s Deep Space Atomic Clock, a trapped-ion clock, is shown to have long-term stability and drift that are an order of magnitude better than current space clocks.

Illustration of a two-member constellation of small spacecraft at Venus with crosslinks for radio occultations that can lead to global coverage with high spatial and temporal resolutions. SummaryFrom Mercury to the outer reaches of the solar system, the past six decades have witnessed a vast set of discoveries utilizing radio science (RS) methods.For example, based on key gravitational evidence, sub-surface oceans have been inferred at Titan, Enceladus, and Europa, where potential future missions may search for life.

The problem of constellation design presents many difficulties in the form of multiple, competing objectives and a complex design space. Past approaches to solving this problem have been to limit the number of objectives and/or simplify the geometry (such as restricting the design procedure to using only circular orbits) so that a tractable solution is obtainable. The genetic algorithm (GA) has been used in many design problems, but has seen limited application to the problem of constellation design. The GA has the ability to handle objectives of mixed (continuous and discrete) type and complex design spaces that do not have convenient analytical representations. With these capabilities, it becomes possible to consider the use of eccentric, inclined orbits for designing constellations efficiently. A method to evaluate coverage associated with ellipitical orbits via a streets-of-coverage approach was derived for this effort. This paper describes a method for applying a GA in a gaming approach for multiobjective design of satellite constellations for zonal coverage. The algorithm is applied to a northern hemisphere coverage problem, a CONUS-like coverage problem, and an ELLIPSO™-like coverage problem. Results are presented that suggest elliptic orbits may be beneficial to reduce the number of satellites needed for certain coverage problems.