Deep space navigation enables missions to precisely target distant solar system bodies and particular sites of interest on them. Navigation takes place in real time for spacecraft operation and control. It can also be used for creating higher-fidelity reconstructions of a craft’s trajectory for future course corrections, and for scientific and operational purposes.
Spacecraft navigation includes two primary activities:
- Orbit determination, or estimation and prediction of spacecraft position and velocity
- Flight path control, or firing a rocket engine or thruster to alter a spacecraft’s velocity
Since the Earth’s own orbital parameters and inherent motions are well known, measurements of a spacecraft’s motion as seen from Earth can be converted into sun-centered orbital parameters, which are needed to describe the spacecraft’s trajectory. The meaningful measurements we can make from Earth of the spacecraft’s motion include distance from Earth, the component of its velocity that is directly toward or away from Earth, and its position in Earth’s sky.
Existing navigation techniques — Doppler, range, Delta-DOR, onboard optical — have been used in varying degrees since the early 1960s to navigate spacecraft with ever-increasing precision and accuracy. JPL’s expertise in deep space mission design and navigation has enabled many successful planetary missions to targets across the solar system using flybys, orbiters, landers, rovers and sample return spacecraft.
Future missions will build on these successful developments to meet tightening performance requirements and growing demands for spacecraft that can respond autonomously to new environments including atmospheric winds, comet outgassing jets and high radiation.
Some examples of future capabilities being explored:
- Missions consisting of multiple spacecraft could require coordinated navigation.
- Missions in the New Frontiers and Discovery classes could require development of low-thrust and low-energy mission design and navigation capabilities, as well as multiple-flyby trajectories.
- Future small-body sample-return and interior-characterization missions could require further reductions of uncertainties in navigation delivery to small bodies by an order of magnitude.
- Missions that could need very high accuracy relative to their targets will require the continued development and extension of the multimission, autonomous, onboard navigation system (AutoNav) to be a complete autonomous guidance, navigation, and control system, or AutoGNC.
Selected Research Efforts
Deep space navigation technologies have enabled every deep space mission ever flown, and JPL has been at the forefront of developing this critical capability for NASA since the agency’s beginning.
Mission Design and Navigation Methods
Deep space mission design encompasses the methods and techniques used to develop concepts for accomplishing specific scientific objectives. Navigation activities include simulations during the mission design phase and the analysis of real-time data received during mission operations. Both mission design and navigation require a large set of software tools and analysis techniques, at a variety of precision and fidelity levels, for different stages in the lifetime of a mission. These include tools and techniques for propagating and optimizing trajectories; reducing observational quantities using mathematical filtering algorithms; and simulating spacecraft guidance, attitude control and maneuvering capabilities.
JPL is investing in a comprehensive software tool suite called the Mission-Analysis Operations Navigation Toolkit Environment (MONTE) that is capable of supporting initial proposal studies through operations support and post mission trajectory reconstructions.
Precision Tracking and Guidance
Many missions, especially those that make use of gravity assists, 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.
Ground-based atomic clocks are the cornerstone of spacecraft navigation for most deep space missions because of their use in forming precise two-way coherent Doppler and range measurements. Until recently, it has not been possible to produce onboard time and frequency references in interplanetary applications that are comparable in accuracy and stability to those available at Deep Space Network (DSN) tracking facilities.
JPL currently is leading the development of the Deep Space Atomic Clock, or DSAC, a small, low-mass atomic clock based on mercury-ion trap technology that can provide the unprecedented time and frequency accuracy needed for next-generation deep space navigation and radio science. DSAC will provide a capability onboard a spacecraft for forming precise one-way radio metric tracking data (i.e., range, Doppler, and phase), comparable in accuracy to ground-generated two-way data. DSAC will have long-term accuracy and stability equivalent to the existing DSN time and frequency references. By greatly reducing spacecraft clock errors from radio metric tracking data, DSAC enables a shift to a more efficient, flexible, and extensible one-way navigation architecture.
In comparison to two-way navigation, one-way navigation delivers more data (doubling or tripling the amount to a user) and is more accurate (by up to 10 times).
DSAC also enables a shift toward autonomous radio navigation where the tracking data are collected (from the DSN uplink) and processed on board. In the current ground-processing paradigm, the timeliest trajectory solutions available on board are stale by several hours as a result of light-time delays and ground navigation processing time. DSAC’s onboard one-way radio tracking enables more timely trajectory solutions and an autonomous GN&C capability. This capability can significantly enhance real-time GN&C events, such as entry, descent, and landing, orbit insertion, flyby, or aerobraking, by providing the improved trajectory knowledge needed to execute these events robustly, efficiently, and more accurately.
Onboard autonomous guidance, navigation and control requirements have been met in the past by the JPL-led 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 comet nucleus, 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.
Future challenges include the need to provide systems capable of orbital rendezvous, sample capture, and, eventually, sample return. This could 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.
JPL is investigating the development of an autonomous navigation system called the Deep Space Positioning System (DPS, like Earth bound GPS but for interplanetary space). The DPS is a collection of integrated hardware components including: a software defined radio (X-band with a computer for hosting autonomous navigation software), an optical camera and a chip scale (or DSAC) atomic clock. This unit will provide a bolt on interplanetary onboard navigation capability beyond low Earth orbit.
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.
* The above description incorporates material from “Guidance, Navigation, and Control Technology Assessment for Future Planetary Science Missions - Part I. Onboard and Ground Navigation and Mission Design” (NASA, 2012)