We report a new determination of the orbits of the irregular Saturnian satellites. We fit their numerically integrated orbits to a data set containing Earth-based observations and imaging data from the Cassini spacecraft. We include the statistics of the observation residuals, the satellites’ orbital elements, and projected accuracies of the satellites’ positions. We also provide astrometric positions derived from the Cassini imaging. Two of the satellites are considered lost because they have not been observed for more than one epoch and have indeterminate uncertainties in their positions. Three of the satellites appear to be in a Kozai resonance, with one being the first irregular satellite of any planet found to be in a 270° rather than 90° resonance.

The Advanced Pointing Imaging Camera (APIC) is designed to obtain high-resolution imaging data to measure a target's geophysical and geodetic properties. The development of APIC originates from NASA's Homesteader program of technology development for candidate New Frontiers missions. The unique science enabled by APIC derives from its ability to simultaneously take images of the target and star field, allowing high-precision camera pointing knowledge with each high-resolution target image. APIC is small (28 cm x 18 cm x 24 cm encompassing volume), light-weight (6 kg total), and moderate in power (13 W maximum) while being high performance and robust to long missions in deep space. APIC incorporates two imagers, one narrow-angle camera (NAC) and one wide-angle camera (WAC) that can operate simultaneously. Both cameras utilize the CMOS-based Mars 2020 Engineering Camera technology with an option of either clear or Red-Green-Blue colors and have wide apertures to enable short exposures and thus perform a a wide range of targets. The NAC has a pixel resolution of 18 mu rad and 4 degrees field of view and the WAC has a pixel resolution of 82 mu rad and 18 degrees field of view. APIC also has two gimbals, allowing rapid camera pointing updates without the need to change the spacecraft attitude; thus, not interfering with other onboard sensors or spacecraft operations. Both gimbals are capable of compensating for relative spacecraft-target motion (i.e., image motion compensation) with an angular speed of up to 3075 (i.e., 0.5 rad/s). Many of APIC components are commercial-off-the-shelf (COTS), or adapted from other NASA flight programs, which makes APIC very competitive in cost and gives it a high technical maturity. APIC's high-resolution images enable the determination of high-accuracy topography for geologic studies. This paper presents details of APIC's characteristics and functionalities as well as specific science objectives that APIC data can address, such as measuring a geometric tidal flexing through estimating the tidal Love number, h(2) and l(2), and small rotational effects, such as libration and precession, of natural satellites and small bodies (i.e., asteroids and comets) that are key to exploring a planetary body's interior. Improved knowledge of spacecraft orbit via landmark tracking using the APIC data would also improve the recovery of low-degree gravitational parameters such as k(2). In this paper, the performance of APIC is presented by showing how well the tidal deformation and libration measurements can be recovered with realistic mission scenarios and configurations.

Long-range passive optical detection of an orbiting inert sphere by a robotic Mars orbiter is investigated and trades are described in terms of detectability via reflected visible light in the presence of orbit uncertainty, gravitational perturbations, and camera electronics noise. A new approximate equation for signal-to-noise ratio (SNR) is developed to include most relevant camera imperfections, diffraction, and stray light from the Mars limb as relevant to this scenario. Using this method, a notional camera suite is designed to meet detection, navigation, and redundancy requirements for an example mission scenario. Results from a simulation tool demonstrate the long-range initial detection strategy in the presence of perturbations from various sources. Navigation analysis shows that the information gathered using passive optical detection is sufficient to begin orbit matching. Finally, applicability of this sensor suite is examined relative to later phases.

The DPS-Navigator concept is a self-contained autonomous navigation hardware and software system that provides spacecraft on-board navigation throughout the solar system. It answers the question "where am I?" like the Global Positioning System (GPS), but without the need for the satellite infrastructure. For the lunar Gateway, DPS-Navigator would observe lunar landmarks to determine position information and compute orbital maneuvers to maintain the Gateway orbit when the crew is not present or to reduce the crew's dependence on ground-based mission control. The optical-only design is small (25 x 12 x 12 cm) and lightweight, less than 5 kg. Power requirements are less than 12 W with self-contained processing. Data link requirements (infrequent for set-up, monitoring, and maintenance) are less than 50 MB per day. DPS-Navigator leverages prior flight demonstrations of autonomous navigation (DS-1, Deep Impact, Stardust) to provide a more general and robust on-board solution. DPS-Navigator provides precise lunar landmark measurements using narrow angle field of view (FOV) optics and precise pointing knowledge using wide angle FOV optics. A more robust configuration of the DPS-Navigator uses optical and radiometric sensing. For the lunar Gateway, the optical-only version would be sufficient given the abundance of optical targets in the form of lunar surface landmarks. On-board navigation performance results using lunar landmarks are presented in this paper and shown to provide an alternative to traditional deep space network Earth-based radiometric techniques; thus, freeing Earth tracking stations and ground personnel for other support.

The critical aspect of optical navigation (opnav) is the ability to accurately derive the centers of stars and target bodies. Sets of opnav images can be stacked and co-added to reduce noise and improve accuracy. However, this does not capitalize on the additional sampling information that exists due to the sub-integer pixel offsets between images created by spacecraft attitude drift. This paper details how, by utilizing simple subsampling techniques for the New Horizons flyby of (486958) 2014 MU69, this additional spatial information was recovered, thus enabling more accurate centers and earlier resolution of multiple bodies. Introduction. The New Horizons flyby of Kuiper Belt Object (486958) 2014 MU69,1 nicknamed “Ultima Thule” (UT), presented significant challenges when trying to derive accurate opnav observations. At 43 AU away from the sun, there was roughly a 6-hour one way light travel time, with light levels that were less than 0.06% that of at Earth. In order to reduce the amount of opnav data that would have to be transmitted to the ground, and to boost the signal in each image, images were 4×4 binned. And while this tactic successfully boosted the signal in each image and significantly reduced the communications load, it also resulted in undersampled point spread functions (PSFs). On top of that, the relative position of New Horizons with respect to UT during approach lined up with the Milky Way, resulting in a dense background of field stars. Another concern was the possibility of UT being a binary system, for which detecting the secondary as early as possible would be critical for accurate orbit determination. New image process techniques were investigated in order to tackle these challenges, with success found in the application of the linear reconstruction techniques referred to as subsampling here. These techniques were simple in concept and application and yet significantly improved both the accuracy of the opnav centers and the time of initial detection. Classical Image Processing. First, to quickly give some context, when dealing with a low signal target, an opnav campaign will involve sets of multiple images being taken in succession. The typical processing of these images is as follows2: each image is first processed individually and the centers of cataloged stars are located, from which a pointing solution can be determined. With their associated pointing solutions, each image is shifted and aligned into a stack. A median filter is then applied to the stack to improve the signal-tonoise ratio and eliminate cosmic ray strikes. A new pointing solution is calculated from the filtered result. Then the background star field can be subtracted using a filtered background image generated in the same manner. And finally, the center of the target is located. Subsampling. When subsampling, also referred to as linear reconstruction, the steps are identical up until the alignment and generation of the filtered image. At this point, two subsampling methods can be applied; ‘shiftand-add’ or ‘drizzle’3 both of which operate to overlay the individual images onto a finer grid of subpixels. By doing this, additional spatial information is extracted from how the pixel values vary as they are shifted subpixel amounts by spacecraft attitude drift. Shift-and-Add. When shift-and-adding, each pixel is replicated into a finer grid of pixels with the same value, shifted into place, and added to the output. This method has the advantage of being simple, with the only parameter to vary being the amount to subsample. However, as this method retains the original undersampled structure of the images, the output will still be convolved by the original pixel sampling and will increase correlated noise. This shows up visually in the subsampled images appearing slightly blurry and having a square-ish shape like the original images. Drizzle. The drizzle method attempts to reduce these issues of correlated noise and original pixel convolution by shrinking the pixel size before mapping and adding. This is illustrated in Fig. 1, where the red outline is the original pixel and the blue outline is the shrunken ‘drop’ pixel that is then mapped to a fine output grid. Figure 1. Drizzle method schematic When drizzling, the two parameters that are controlled are the amount to shrink the pixel, referred to as ‘picfrac’, and the amount the output is subsampled. What these

GlobeTrotter is a concept for a universal airbag-based robotic hopper for rapid, low-cost, and robust exploration of the surface and subsurface (pits, caves) of the Moon, Mars, Phobos, Deimos, and other small bodies. This paper presents an application of GlobeTrotter to lunar exploration, in particular as a versatile scouting capability in support of humans returning to the Moon. Introduction: To date, less than 0.002% of the 38 million km of the Moon’s area has been explored from the surface. Vast tracks of lunar highlands and maria, including discrete regions and features such as impact basins and craters, volcanic centers, lunar rilles, lunar pits and caves, and the permanently shadowed regions (PSR) of the lunar poles, remain unexplored from the surface. The maria are topographically benign at large scales (hm to km) but can present significant roughness at small scales (m to dm), e.g. large boulder fields [1]. The highlands present slopes at hm scales that commonly exceed 15-20 and often reach angle of repose (32-35) [2]. Their surface roughness at small scales replicates that at larger scales [1]. The youngest large impact craters on the Moon present even steeper slopes and rougher terrain [3]. The lunar poles, where the H2O ice-rich PSRs are located and where NASA aims to land astronauts by 2024 (South Pole) are in rough and steep-sloped highland terrain. Robust in-situ robotic precursors capable of rapidly scouting out large areas of the lunar surface, including the cold, dark, deep interiors of the PSRs, and requiring little development time or cost, are needed. Meanwhile, lunar pits and caves, which will also attract human explorers for science and might offer shelter and potential resources too, present novel exploration challenges [4]. Conventional concepts such as robotic rovers, walkers, danglers and landers all face the major risk of interacting with poorly known, likely rough terrain, as well as limited dwell times in cold, dark, and comms-denied caves. Drones have been proposed [4], but alternative options should also be considered. New Approach: GlobeTrotter is an airbag-based robotic vehicle that could robustly and quickly explore vast areas of the Moon via “leaps and bounds”, tolerant to terrain roughness and using slopes to its advantage (Fig.1). The approach emphasizes aerial coverage (range) and access to extreme terrain (trafficability) while offering diverse science focus options (mission versatility). GlobeTrotter could, at higher latitudes, keep pace with the shifting lunar terminator and investigate terminator processes such as charged dust transport. GlobeTrotter could also drop into pits/caves, hop around inside, and fly out of them again (Fig. 1). Figure 1. GlobeTrotter on the Moon. Top: Surface exploration. Bottom: Lava tube exploration.

The coming decades of planetary science and deep space exploration will likely have a combination of more ambitious missions and ever more constrained budgets. The outer solar system, in particular, poses a challenge for lower mission cost as the principal need for a robotic craft — a source of energy — is difficult to supply through conventional means (solar energy). Even as far from the Sun as Saturn, the solar energy density is only 1% of that at Earth. Not viewed typically as a power source for low-cost missions, radioisotope power systems (RPS) may well fill that role by providing power to small spacecraft in the outer solar system. And the outer solar system beckons with extremely compelling science. The rich dynamics of the atmospheres of the gas giants are continuously operating laboratories of extreme weather processes, examples of which in miniaturized scale may become more familiar here on Earth. Enceladus, a small moon of Saturn, has been seen by the Cassini mission to be a site of continuous high geologic activity, with plumes of water vapor and particles pumped hundreds of kilometers above the surface, indeed into Saturn orbit. The internal heating mechanisms of this activity beg for explanation, and more importantly, initial measurements by the Cassini spacecraft give tantalizing clues that the geo-thermal source of the heating is, in fact, maintaining a global sub-surface ocean, which in combination could provide a habitat for life. This paper will explore how existing and currently available RPS elements may make mission concepts to explore the intriguing science of Enceladus economically tractable, and at the same time provide a generic platform for other small but highly capable spacecraft to explore the outer solar system.

There is no equivalent of GPS in “deep space,” that is beyond the Earth Moon system (and use of GPS beyond LEO and MEO is already problematic.) NASA has been awarded a patent on the concept of the “Deep-space Positioning System” (DPS) which is a self-contained instrument concept that would use in-situ observations to provide an automated and autonomous means of determining its position in the Solar System, and at least as important, its position relative to a target, whether that target is a planet, asteroid, moon or comet. In addition, the DPS system computes trajectory change parameters, as necessary to keep a spacecraft on a planned trajectory. Implementation options include the use of a radio receiver to receive a frequency beacon from a non-in-situ source for additional navigation information over and above that which can be provided by the cameras that are the principal data source of DPS. This paper discusses the DPS instrument concept, and its potential use in deep space.

Due to its eccentric orbit about Jupiter, Europa experiences periodic tidal deformation, which causes changes in its gravitational field and induces both radial and transverse displacements of the surface. The amplitude and phase of these tidal changes are diagnostic of internal structure, and can be measured with sufficient radiometric and optical tracking of a spacecraft during a series of flyby encounters with Europa. This paper presents results of the simulated accuracy for recovery of the tides of Europa through measuring the second-degree tidal Love numbers k2, h2, and l2. A reference trajectory, which consists of a total of 45 close flybys, was considered and a detailed covariance analysis was performed. The study was based on Earth-based Doppler tracking during ±2h of each periapsis passage and surface imaging data taken below 500km altitude. The result shows that the formal uncertainty of the second-degree tidal Love numbers can be estimated to be σk2=0.01, σh2=0.02, and σl2=0.01, which is sufficient to constrain the global ice thickness to about 10km under reasonable assumptions. Moreover, the forced librations of Europa can be measured to 0.3″ accuracy, which can further constrain Europa's interior structure.

After 40 years of solar system exploration by spacecraft, the origin of Mars's satellites, remains vexingly unknown. There are three prevailing hypotheses concerning their origin: H1: They are captured small bodies from the outer main belt or beyond; H2: They are reaccreted Mars impact ejecta; H3: They are remnants of Mars' formation. There are many variants of these hypotheses, but as stated, these three capture the key ideas and constraints on their nature. So far, data and modeling have not allowed any one of these hypotheses to be verified or excluded. Each one of these hypotheses has important implications for the evolution of the solar system, the formation and evolution of planets and satellites, and the delivery of water and organics to Early Mars and Early Earth. Determining the origin of Phobos and Deimos is identified by the NASA and the NRC Decadal Survey as the most important science goal at these bodies.

This paper presents an evaluation of the onboard GN&C capabilities and technologies needed for future missions pursuing NASA's planetary science goals. In particular, this assessment covers attitude estimation and control in general, as well as the estimation and control of vehicle flight paths when flight path and attitude dynamics are strongly coupled or performed primarily onboard (as is the case during certain critical phases, such as entry, descent, and landing, in some planetary missions). This work first surveys the technologies, appraises their applicability to future NASA planetary missions, and then quantitatively assesses priorities for NASA based on likely need, relevance and optionally cost.

Future planetary explorations envisioned by the National Research Council's Vision and Voyages for Planetary Science in the Decade 2013-2022 seek to reach targets of broad scientific interest across the solar system. Advancements in guidance, navigation, and control and mission design ranging from software and algorithm development to new sensors will be necessary to enable these future missions. This paper describes the general categories of mission design capabilities that need further development in support of future planetary science missions: multiple-encounter tour design, close-proximity trajectory design for small-body missions, low-energy trajectory design and optimization, multiple-spacecraft trajectory optimization, and low-thrust trajectory design and optimization. The paper also describes a number of ways in which deep space navigation may be enhanced in the future, including precise one-way radio metric tracking, based on use of the proposed Deep Space Atomic Clock; autonomous navigation (in particular, its application to autonomous aerobraking, outer planet tours, and primitive body/lunar proximity operations and pinpoint landing); evolutionary improvements in Deep Space Network radio metric data accuracy; and derivation of metric tracking data from optical communication links.

The Altair lunar lander navigation system is driven by a set of requirements that specify a need to land within 100 m of a designated spot on the moon and to be capable of a safe return to an orbiting Orion capsule in the event of loss of Earth ground support. These requirements lead to a design for a capable onboard navigation system that works in conjunction with an Earth ground navigation system. The resulting system relies on combining a multiplicity of data types including navigation updates from the ground, passive optical imaging from a gimbaled camera, a stable inertial measurement unit, and a capable radar altimeter and velocimeter. It is shown that such a system is capable of a safe delivery to the moon and then a subsequent landing that meets the 100 in requirement. Furthermore, parametric trades on system components (i.e., onboard instruments not working, loss of the Earth ground system, etc.) demonstrate that the system is robust in the presence of failures and able to maintain sufficient knowledge of the trajectory for safe operations.

VML (Virtual Machine Language) is an advanced computing environment that allows spacecraft to operate using mechanisms ranging from simple, time-oriented sequencing to advanced, multicomponent reactive systems. VML has developed in four evolutionary stages. VML 0 is a core execution capability providing multi-threaded command execution, integer data types, and rudimentary branching. VML 1 added named parameterized procedures, extensive polymorphism, data typing, branching, looping issuance of commands using run-time parameters, and named global variables. VML 2 added for loops, data verification, telemetry reaction, and an open flight adaptation architecture. VML 2.1 contains major advances in control flow capabilities for executable state machines. On the resource requirements front, VML 2.1 features a reduced memory footprint in order to fit more capability into modestly sized flight processors, and endian-neutral data access for compatibility with Intel little-endian processors. Sequence packaging has been improved with object-oriented programming constructs and the use of implicit (rather than explicit) time tags on statements. Sequence event detection has been significantly enhanced with multi-variable waiting, which allows a sequence to detect and react to conditions defined by complex expressions with multiple global variables. This multi-variable waiting serves as the basis for implementing parallel rule checking, which in turn, makes possible executable state machines. The new state machine feature in VML 2.1 allows the creation of sophisticated autonomous reactive systems without the need to develop expensive flight software. Users specify named states and transitions, along with the truth conditions required, before taking transitions. Transitions with the same signal name allow separate state machines to coordinate actions: the conditions distributed across all state machines necessary to arm a particular signal are evaluated, and once found true, that signal is raised. The selected signal then causes all identically named transitions in all present state machines to be taken simultaneously. VML 2.1 has relevance to all potential space missions, both manned and unmanned. It was under consideration for use on Orion.

This study sets forth conceptual mission design strategies for the ascent and rendezvous phase of the proposed NASA/ESA joint Mars Sample Return Campaign. The current notional mission architecture calls for the launch of an acquisition/caching rover in 2018, an Earth return orbiter in 2022, and a fetch rover with ascent vehicle in 2024. Strategies are presented to launch the sample into a nearly coplanar orbit with the Orbiter which would facilitate robust optical detection, orbit determination, and rendezvous. Repeating ground track orbits exist at 457 and 572 km which would provide multiple launch opportunities with similar geometries for detection and rendezvous.

In order to simulate physically plausible surfaces that represent geologically evolved surfaces, demonstrating demanding surface-relative guidance navigation and control (GN&C) actions, such surfaces must be made to mimic the geological processes themselves. A report describes how, using software and algorithms to model body surfaces as a series of digital terrain maps, a series of processes was put in place that evolve the surface from some assumed nominal starting condition. The physical processes modeled in this algorithmic technique include fractal regolith substrate texturing, fractally textured rocks (of empirically derived size and distribution power laws), cratering, and regolith migration under potential energy gradient. Starting with a global model that may be determined observationally or created ad hoc, the surface evolution is begun. First, material of some assumed strength is layered on the global model in a fractally random pattern. Then, rocks are distributed according to power laws measured on the Moon. Cratering then takes place in a temporal fashion, including modeling of ejecta blankets and taking into account the gravity of the object (which determines how much of the ejecta blanket falls back to the surface), and causing the observed phenomena of older craters being progressively buried by the ejecta of earlier impacts. Finally, regolith migration occurs which stratifies finer materials from coarser, as the fine material progressively migrates to regions of lower potential energy.

Human space exploration is expected to enter its next phase in the coming decades as the United States prepares to return to the Moon or perhaps venture even further with a crewed mission to a near‐Earth asteroid. Both mission classes are viewed by NASA as precursors of eventual crewed missions to Mars. In anticipation of extensive robotic and human presence in the space environment beyond the protection of the Earth's magnetosphere, it is important to better quantify and bound effects of earthward directed solar storms not just on the human body but also on engineering signals. In this paper, we study the effects of solar storms on S band (∼2.3 GHz) radio links in the near‐Earth environment, primarily for application to navigation. In particular, we are concerned with induced long‐period signatures on Doppler tracking data that could be confused with the Earth's gravity signature, resulting in perturbed trajectory solutions of returning spacecraft during Earth entry targeting. We have quantified “worst‐case” levels of such induced signatures on S band signal phase using model predictions based on measured in situ charged particle content from satellites and have compared these results with signatures seen in actual tracking data during periods of interplanetary coronal mass ejections (ICME) and related geomagnetic storms. We show that induced Doppler can mask Earth gravity field effects in navigation trajectory solutions at S band, a commonly used frequency for near‐Earth communications and navigation. Finally, we suggest a few ways that such effects can be identified, alleviated or eliminated in near real‐time.