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Deep Space Navigation
JPL’s expertise in deep space navigation has led to many successful planetary missions. Missions planned in the future will not only rely on current deep space navigation capabilities, but will require substantial extensions of existing technologies. Researchers are developing new techniques and technologies in the areas of low-thrust navigation, precision tracking and guidance, and autonav to make these future missions possible.
Spacecraft navigation comprises two aspects: (1) knowledge and prediction of spacecraft position and velocity which is orbit determination, and (2) firing the rocket motor to alter the spacecrafts velocity, which is flight path control.
Since the Earths own orbital parameters and inherent motions are well known, the measurements made of the spacecrafts motion as seen from Earth can be converted into the sun-centered (heliocentric) orbital parameters needed to describe the spacecrafts trajectory. The meaningful measurements we can make from Earth of the spacecrafts motion are:
Its distance or range from Earth,
The component of its velocity that is directly toward or away from Earth, and
To the extent discussed below, its position in Earths sky. (Delta-Differential One Way Ranging (Delta-DOR)
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.
Some spacecraft can generate a fourth type of navigation data:
Optical navigation, wherein the spacecraft uses its imaging instrument to view a target planet or body against the background stars or to view known landmarks on the target body
By repeatedly acquiring these three or four types of data, a mathematical model may be constructed and maintained describing the history of a spacecrafts location in three-dimensional space over time. The navigation history of a spacecraft is incorporated not only in planning its future maneuvers, but also in reconstructing its observations of a planet or body it encounters. This is essential to constructing SAR (synthetic aperture radar) images, tracking the spacecrafts passage through planetary magnetospheres or rings, and interpreting imaging results. It is also essential for accurate entry into a planetary atmosphere and/or landing on the surface of a target body.
Another use of navigation data is the creation of predicts, which are data sets predicting locations in the sky and radio frequencies for the Deep Space Network (DSN), to use in acquiring and tracking the spacecraft.
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. For more information about the JPL Ephemeris, click here: http://www.cv.nrao.edu/~rfisher/Ephemerides/ephem_use.html or here for contact information: http://ssd.jpl.nasa.gov
Selected Current Projects
Cassini Prime Mission Orbits - Deep Space Navigation
During the period from January 2006 through the end of the Prime Mission, which ended on July 1, 2008, Cassini performed 36 encounters. Thirty-four of these were with Titan; one was a 1650 km flyby of the interesting moon Iapetus on September 10 of 2007; and one daring 50 km flyby of Enceladus on March 12, 2008. This diagram shows the entire Saturn satellite tour of Cassini. Cassini flew by Phoebe on the way in to Saturn, entered into Saturn orbit on June 30, 2004, flew by Titan in October 2004, released the Huygens probe on Christmas 2004 which successfully landed on Titan’s surface January 14, 2005. The orbits highlighted in green depict the orbits completed during the first two years of the mission; those in red are the last two years.
During the period from Jan. 2006 through the end of the Prime Mission which ended on July 1, 2008 (shown in red), Cassini performed 36 encounters. Also shown are encounters with Enceladus, Titan, and Phoebe.
Autonomous Navigation and Guidance System Technology
Image showing recent successes in autonomous navigation and guidance system technologies at JPL.
The challenges of performing onboard autonomous Guidance Navigation and Control (GN&C) have been met in the past by the Deep Space 1, Stardust and Deep Impact missions, which, between them, have captured all of NASAs 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, and computing the spacecraft position and correcting the camera body pointing to keep the comet nucleus in view. Future missions, such as sample return from a comet or asteroid, accurate and safe landing on the Moon or Mars, and rendezvous on Mars orbit with a sample canister wouldl require much more extensive knowledge and modeling of target body surfaces and integration of navigation with vehicle attitude control and guidance systems. To meet the first challenge, AutoNav is being equipped with an extensive array of surface feature recognition capability, which provides extensive and accurate terrain-relative navigation. To meet the second challenge, AutoNav is advancing to an AutoGNC concept, where navigation, guidance and attitude control functions are integrated. Having such a critical element of a missions flight software pre-developed and ready for a wide range of mission scenarios presents many opportunities for new and economically developed missions, but presents new challenges for integration of this large capability into existing flight software and operations systems. To meet these additional challenges, and to provide the high degree of intelligence, adaptability, "self-awareness" and fault recovery that these missions will require, the higher-level "brain" functions of AutoGNC are being reformulated in an advanced version of an autonomy-enabling command and sequence system, called "Virtual Machine Language," an earlier version of which is currently operating many of JPLs active missions.
Planetary Landing Contours
This plot was created by JPLs landing site analysis program MarsLS. The program convolves landing probability, based on navigation, atmospheric and other uncertainties, with terrain-based information (such as hazards or areas of scientific interest) to compute the probability of landing in desirable/undesirable areas. In the figure, a magenta contour is plotted on the Phoenix Mars lander Certified Safe Zone. This contour defines the region in which the Navigation Teams landing estimate will or will not violate the criteria set forth by the project. The program has been used in a similar fashion on MER, Genesis, and Stardust (for Earth sample returns) and is currently being used by the MSL project.
Phoenix landing ellipses and contours that helped the Mars lander find a suitable place to set down on the Martian surface.
Global Ionospheric Maps
A representative global map of vertical total electron content (TEC) is shown here plotted as a function of local time and geographic latitude.
Data from over 100+ continuously operating GPS receivers in a global network are being used to produce global maps of the ionospheres total electron content (TEC). These Global Ionosphere Maps (GIM) provide instantaneous "snapshots" of the global TEC distribution, by interpolating, in both space and time, the 6-8 simultaneous TEC measurements obtained from each GPS receiver every 30 seconds. The maps can be produced unattended in a real-time mode, with an update rate of 5-15 minutes.
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.
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