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Deep Space Navigation
Deep space navigation enables missions to precisely target distant solar system bodies, as well as particular sites on those bodies. This navigation not only takes place in real time for control and operation of the spacecraft, but also in many cases includes later higher-fidelity reconstruction of the trajectory for subsequent trajectory corrections, as well as scientific and operational purposes.
Spacecraft navigation comprises two aspects: (1) knowledge and prediction of spacecraft position and velocity which is orbit determination, and (2) firing a rocket motor to alter the spacecraft’s velocity, which is flight path control. Since the Earth’s own orbital parameters and inherent motions are well known, the measurements made of the spacecraft’s motion as seen from Earth can be converted into the sun-centered (heliocentric) orbital parameters needed to describe the spacecraft’s trajectory. The meaningful measurements we can make from Earth of the spacecraft’s motion are:
• Its distance or range from Earth,
• The component of its velocity that is directly toward or away from Earth
• To the extent discussed below, its position in Earth’s sky. (Delta-Differential One Way Ranging (Delta-DOR)
Interplanetary trajectory design leverages electric propulsion to reduce project risk.
Delta-Differential One-way Ranging is a radio-tracking technique that is very useful in the orbit determination of some spacecraft. Using an interferometric technique, Delta-DOR requires two Deep Space Stations located at different complexes for a single measurement to determine positions in the plane-of-the-sky.
Existing technologies — Doppler, range, Delta-DOR, onboard optical — have been used in varying degrees since the late 1950s to navigate spacecraft with ever-increasing precision and accuracy. With our increasing understanding of solar system dynamics and spacecraft, methods of designing ever-more complex trajectories have been developed that, in turn, can drive requirements on spacecraft design. JPL’s expertise in deep space mission design and navigation has enabled many successful planetary missions, such as multiple missions to Mars using orbiters and landers, complex missions at both Jupiter and Saturn with probes and long-term orbiters, and missions to comets and asteroids along with sample-return segments. Missions that use the complicated gravitational interaction of the Sun and Earth to accomplish specific mission objectives and constraints (Genesis, Spitzer Space Telescope) have also been accomplished.
Future missions will need to build on these successful developments to meet tightening performance requirements and growing demands for autonomous response of spacecraft to new environments (atmospheric winds, comet outgassing jets, high radiation, etc.). Missions consisting of multiple spacecraft would require coordinated navigation. Missions in the New Frontiers and Discovery classes would require development of low-thrust and low-energy mission design and navigation capabilities, and more extensive search capabilities for multiple flyby trajectories, enabling efficient and economical exploration. This is particularly important for proposed sample-return and Outer Planet Flagshipmissions. Methods must be developed to efficiently explore complex satellite tour designs, innovative science orbits, and efficient capture of these orbits. This would also apply to missions which would use any type of low-thrust propulsion — including solar electric, nuclear electric, solar sail, and plasma sail — for any mission segment. Future small-body sample-return missions and interior-characterization missions would require further reductions of uncertainties in navigation delivery to small bodies by an order of magnitude. Finally, missions that would need very high accuracy relative to the target (planet, satellite, asteroid, or comet) to achieve science goals, reduce mission costs for ground resources, and release ground resources for other applications would require the continued development and extension of the multimission, autonomous, onboard navigation system (AutoNav) to be
a complete AutoGNC (autonomous guidance, navigation, and control) system.
Selected Research Efforts
Deep space navigation technologies have enabled every deep space mission ever flown. As these technologies have advanced, ever-more complex missions have been successfully accomplished. The advancement of these technologies would allow missions that were barely conceivable only a few years ago to be accomplished efficiently and effectively, resulting in scientific insights and understanding far beyond what is currently in hand.
Mission Design and Navigation Methods
Deep space mission design encompasses the methods and techniques used to find the existence of, develop the specific details of, and outline the operational considerations and constraints for a specific concept necessary to accomplish a set of scientific objectives. This is usually done initially within the context of an “envelope” of potential designs generally meeting the overall desires. Navigation methods include both the analysis of real-time data received during actual mission operation and a simulation in the design phases as part of the overall mission design. For both mission design and navigation, a large set of software tools and analysis techniques is necessary at a variety of precision and fidelity levels for different stages of design from early pre–Phase A concept studies through flight operations. This set includes tools and techniques for propagating and optimizing trajectories; reducing observational quantities using mathematical filtering algorithms; and simulating spacecraft guidance, attitude control, and maneuvering capabilities.
Extension of current methods for finding and navigating complex trajectories involving multiple flybys, low-thrust trajectories, and trajectories involving lengthy three-body arcs is necessary to meet the requirements of many future mission scenarios. In some cases, all three of those aspects may be involved in a single mission. Algorithms are required that would provide rapid and highly accurate orbital thrust profiles for maintaining orbit about a small body. In addition, advances are needed to decrease the time required to compute small-body landing trajectories in a highly complex gravity and topography field from several months to a few hours or less. Most, if not all, missions to small bodies would arrive at their destination with no detailed knowledge of the gravitational and topographical characteristics of that body. The algorithms, both onboard and on the ground, to analyze and appropriately control the spacecraft in this unknown environment must be adaptable and flexible enough to ensure spacecraft safety and accomplish the mission objectives.
Precision Tracking and Guidance
Currently, precision tracking and guidance are primarily required for delivery of landers to the surface of a body or to minimize the propellant necessary to insert an orbiter into the desired orbit. Maintaining an orbit both in a knowledge and control sense also requires high precision. Missions utilizing flybys of gravitating bodies during the mission to accomplish their objectives also require high-precision tracking and guidance, since even very small delivery errors at the intermediate body or bodies are greatly magnified and must be corrected right after the flyby with potentially costly midcourse maneuvers. Future missions would require the characterization of small body internal/subsurface physical characteristics required to model the complex gravity field of a non-spherical body as well as the characterization of surface composition. This also requires navigational tracking measurements, currently performed using the vehicle’s X-band communication systems, and involves measurements of two-way Doppler shifts, two-way ranging, and interferometric measurements of angular offsets from stellar radio sources (Delta-DOR).
Future migration to Ka-band, spacecraft-to-spacecraft tracking, and optical communications would offer new challenges as well as opportunities for tracking measurement accuracy improvements. The goal is to achieve navigation accuracy to 1m in the vicinity of a small body. This would allow very close orbiting, hovering, “touch-and-go” sampling of the surface, and safe landing on the surface. Future spacecraft development with advanced capabilities during atmospheric flight would allow landing on the surface of a planetary body with an atmosphere to within tens of meters rather than tens of kilometers. Hazard avoidance would also be possible. This would be enabled with active control and guidance during the atmospheric portion of the flight and would require the development of analysis tools to design such trajectories.
Onboard Autonomous Navigation
Onboard autonomous guidance navigation and control requirements have been met in the past by the Deep Space 1, Stardust, and Deep Impact missions, which, collectively, have captured all of NASA’s close-up images of comets. For those missions, a system called AutoNav performed an autonomous navigation function, utilizing images of the target body (a comet), computing the spacecraft position, and correcting the camera-body pointing to keep the comet nucleus in view. In the case of Deep Space 1 and Deep Impact, AutoNav corrected the spacecraft trajectory as well; and for Deep Impact, this was used to guide the impactor spacecraft to a collision with the nucleus. The challenges for future missions would be to provide systems capable of orbital rendezvous, sample capture, and, eventually, sample return. This would require autonomous systems that interact with observation systems, onboard planning, and highly accurate onboard reference maps, and would include an extensive array of surface feature-recognition capabilities to provide accurate terrain-relative navigation. Autonomous system error-detection and self-maintenance are integrated with autonomous navigation, guidance, and attitude control functions into pre-developed mission flight software, providing a high degree of robustness, intelligence, adaptability, “self-awareness,” and fault recovery (AutoGNC).
Altair lunar landing and "touch and go" on comet Wirtanen, AutoGNC simulations.
Positions in Space for Celestial Navigation
The JPL Solar System Ephemeris specifies the past and future positions of the Sun, Moon, and eight planets in three-dimensional space. Many versions of this ephemeris have been produced to include improved measurements of the positions of the Moon and planets and to conform to new and improved coordinate system definitions.
Planetary positions are generated by a computer integration fit to the best available observations of the positions of the Sun, Moon, planets, and five largest asteroids. The computer integration involves step-wise computation of the position of each planet as determined by the gravitation of all of the other objects in the solar system. The planets position is stepped both forward and backward in time from some chosen epoch. Minor adjustments are made to the masses and shapes of the Moon and planets to get best agreement with their observed position of the last 80 years or so. The observations are mainly derived from transit circles since 1911, planetary radar ranging since 1964, lunar laser ranging since 1969, and distances to the Viking Lander on Mars since 1976. The computer calculations have been extended as far as 3000 BC to 3000 AD, but positions for the 1850-2050 ranges are the most accurate.
Subtle differences exist between the best ephemeris model coordinates and the standard definitions of B1950 and J2000, between the coordinate systems defined by star positions and the B1950 and J2000 standards, and between the coordinate systems defined by stars and radio sources. These differences, which start at the level of a couple of milliseconds and a few tenths of an arc second, are very important to pulsar timing and radio interferometry. Hence, one needs to be very careful about comparing observations reduced on the basis of different ephemerides and coordinate systems. With care and consistency, all-sky accuracies of a few hundred nanoseconds and a few milliarcseconds are currently being achieved.
Selected Research Projects
NASA Propagation Program
The objectives of the NASA Propagation Program at JPL, carried out in past years by many individuals at many institutions, were to enable the development of new commercial satellite communication systems and services by providing timely data and models about propagation of satellite radio signals through the intervening environment and to support NASA missions for high rate knowledge delivery communications technology. In partnership with industry and academia, the program leverages unique NASA assets to obtain propagation data to serve the NASA propagation community. The findings of the study are disseminated through refereed journals, NASA reference publications, workshops, electronic media, and direct interface with industry.
The NASA Global Differential GPS (GDGPS) System
The NASA GDGPS is a complete, robust real-time GPS monitoring and augmentation system. Employing a large ground network of real-time reference receivers, innovative network architecture, and award-winning real-time data processing software, the GDGPS System provides decimeter (10 cm) positioning accuracy and sub-nanosecond time transfer accuracy anywhere in the world, on the ground, in the air, and in space, independent of local infrastructure.