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Accurate vehicle position determination over 50 by 50-km service area accomplished by low-frequency-radio navigational system comprised of four frequency/phase comparison transmitters and receivers. Use of grid-calibration table permits accurate position determination in vehicle receiver system.
A method of continuous wave navigation using four transmitters operating at sufficiently low frequencies to assure essentially pure groundwave operation is described. The transmitters are keyed to transmit constant bursts (1/4 sec) in a time-multiplexed pattern with phase modulation of at least one transmitter for identification of the transmitters and with the ability to identify the absolute phase of the modulated transmitter and the ability to modulate low rate data for transmission. The transmitters are optimally positioned to provide groundwave coverage over a service region of about 50 by 50 km for the frequencies selected in the range of 200 to 500 kHz, but their locations are not critical because of the beneficial effect of overdetermination of position of a receiver made possible by the fourth transmitter. Four frequencies are used, at least two of which are selected to provide optimal resolution. All transmitters are synchronized to an average phase as received by a monitor receiver.
This paper describes the failure detection and isolation performance of a sensor fault tolerant system for the NASA Terminal Configured Vehicle (TCV) research aircraft in a Microwave Landing System (MLS) environment. The objective of the fault tolerant system is to detect failures in navigation-aid instruments and on-board sensors and to provide reliable estimates for the aircraft states in the possible presence of these sensor malfunctions. Analytic redundancy, which exists between the various sensor outputs due to the aircraft point mass equations of motion, is used to identify sensor failures. State estimates are used by an automatic guidance and control system to land the aircraft along a prescribed path.
Unlike their synchronized counterparts, redundant channels in an asynchronous flight system can, under no-fault conditions, exhibit cross-channel data disparities. Sources of these errors are examined in terms of the general, individual functions of the flight control and navigation application in the asynchronous digital environment. The effects of asynchronism on trajectory programmers, dynamic control algorithms and data reconstruction processes are examined in terms of data skews, data latencies, and clock rate uncertainties. An example is presented in which time corrections are applied to reduce the data disparities. Practical limitations of the approach of the example are discussed.
The Space Station System is defined as a network of space and ground assets which work together to support a variety of missions including commercial missions, science and applications missions, and technology development missions. The elements of the Space Station System include a Space Station Base, Space Platforms, Free Flyers, a Teleoperator Manuevering System (TMS), Orbital Transfer Vehicles (OTV), Orbiter Berthing Equipment, and Ground Support Equipment and Facilities. Guidance, navigation, and control (GNC) subsystem requirements are considered along with configuration trades.
Efforts to assure communications continuity and navigational accuracy for the ISEE-3 (now called the International Cometary Explorer - ICE) spacecraft as it heads toward a passage through the Giacobini-Zimmer (G-Z) comet tail are reviewed. Sufficient energy was gained with a series of lunar swingbys to take the rendezvous attempt possible. Communications will be by an S-band link though the upgraded Deep Space Network. The G-Z encounter is scheduled for September 1983. A 512 bps data rate will be supported by 64-3 m dishes at various global sites. The tracking will continue for 6 mos as the spacecraft moves into an orbit that parallels that of Halley's Comet. Ground-based recovery of the G-Z location in the spring of 1984 provided data for calculating the ephemerides required for configuring a major course correction for ICE 100 days before rendezvous.
A diverse collection of unmanned missions to explore the inner planets, outer planets, and small bodies of the solar system is being considered by the National Aeronautics and Space Administration for the remainder of this century and beyond. This paper describes the key navigational problems which are anticipated for some of these missions and the techniques which are likely to be used to solve these problems. Emphasis in this paper is placed on those missions which take place entirely within about 2 Astronomical Units of the sun. The missions studied include Orbiters of Venus, Mars, the moon, and an earth-approaching asteroid, probes of the atmosphere of Venus and the surface of Mars, fast flybys of comets, and sample return missions from Mars and a short-period comet.
A covariance analysis has been performed for the proposed QUASAT, an earth orbiting radio telescope. Conventional (Doppler and range) and advanced (diifferenced and doubly differenced one-way range) radio metric data types are used to evaluate the navigation accuracy that can be achieved. It is found that the mission requirements are met by conventional data types, while advanced data types can provide an order of magnitude improvement with less measurements, provided the three NASA DSN stations are augmented by more closely spaced stations.
Acceptable accuracy obtained with short alignment time. Report describes successful helicopter tests of laser inertial navigational equipment. Tests conducted over 3-year period, both in laboratory and flight. Inertial system used as position/velocity/attitude indicator and later also served as part of automatic flight-control system.
This paper describes the Orbital Maneuvering Vehicle (OMV) concept and its intended role. It recaps the past activities leading up to the current concept and summarizes the present status and plans. The various types of missions and operating modes required by the OMV are described as the basis of the guidance, navigation and control (GN&C) requirements. The general GN&C problem is outlined with potential hardware solutions.
The navigational capabilities of the TDRSS for the operational support of user spacecraft is evaluated. A batch-weighted least-squares algorithm is used to fit tracking measurements and to generate orbit solutions for the TDRS-E spacecraft and the following user spacecraft: (1) the SMM, (2) the Landsat-5, (3) the Earth Radiation Budget Satellite (ERBS), and (4) the Solar Mesosphere Explorer (SME). TDRS-E orbit accuracy was evaluated by using the consistency of consecutive orbit solutions as a measure; user orbit accuracy was estimated by comparing orbits determined from TDRSS tracking with orbits obtained from ground tracking for the same time periods. It was found that the predicted accuracy of the TDRS-E orbit propagation was improved by as much as seven times by refining the spherical harmonic expansion model of the earth's potential field. The results obtained for SMM were fairly consistent, while those obtained for Landsat-5 were consistent to better than 80 meters. The orbits obtained using ERBS tracking were consistent to better than 60 meters and the SME orbits for two arcs with 14 and 20 passes of TDRSS tracking had a maximum position difference of 140 meters.
In the proposed Cassini mission, a combined Saturn Orbiter/Titan Probe spacecraft will be launched from the Space Shuttle to arrive at Saturn around 2002, by means of a delta-VEGA trajectory. After Saturn-orbit insertion and a pericrone raise maneuver, the probe will be released to enter the Titan atmosphere and impact onto its surface. During its descent phase and impact onto Titan, the probe will maintain radio contact with the orbiter. Since the Titan-probe experimental phase lasts for only about four hours, probe-orbiter geometry and probe-delivery accuracy are critical to successful completion of this part of the mission. From a preliminary navigation analysis for probe delivery accuracy, it seems feasible to deliver the probe within 50 km (1-sigma value) of the desired aim-point in the Titan B-plane. The covariance study, however, clearly indicates the need for optical data, in addition to radio metric data. A Monte Carlo study indicates that a Delta-V capability of 98 m/sec for trajectory correction maneuvers will be sufficient to cover 99 percent of all contingencies during the segment from Saturn-orbit insertion to Titan-probe release.
Report discusses proposed low cost very-long-baseline interferometric (VLBI) system for use with Tracking and Data Relay Satellite System (TDRSS) in determining orbits of user satellites. Theoretical accuracies of orbits to be measured by new system compared with similar orbital determinations by other satellite-navigation system, based on anticipated state of VLBI art in year 1990.
The International Cometary Explorer (ICE) encounter with Comet Giacobini-Zinner took place 7 years after the spacecraft's original launch on 12 August 1978 as the International Sun Earth Explorer 3 (ISEE-3), part of a three-spacecraft project to study the interaction between the solar wind and the Earth's magnetosphere. Transfer to an interplanetary trajectory was performed via a 119-km-altitute, gravity-assist, lunar swingby on December 1983. Navigation support during interplanetary cruise and comet encounter was provided by orbit determination utilizing radio metric data from the DSN 64-meter antennas in Goldstone, California and Madrid, Spain. Orbit solutions yielding predictions of 50-km geocentric delivery accuracy in the target aim plane were achieved during interplanetary cruise and at comet encounter using 6-to-12-week data arcs between periodic attitude-change maneuvers. One-sigma two-way range and range rate residuals were consistently 40 meters and 0.2 mm/s or better, respectively. Non-gravitational forces affected the comet's motion during late August and early September 1985 and caused a 2300-km shift in the orbit of the comet relative to the spacecraft. This necessitated a final ICE orbit trim maneuver 3 days prior to encounter. Near-real-time assessment of two-way 2-GHz (S-band) Doppler pseudo-residuals during the June and July 1985 trajectory change maneuvers aided in calibration of the spacecraft's thrusters in preparation for this final critical maneuver. Post-flight analysis indicates tail centerline passage was achieved within 10 seconds of the predicted time and geocentric position uncertainty at encounter was less than 40 km.
Cooperative efforts between NASA and the European Space Agency (ESA) in supporting the flight of Giotto to Halley's Comet included prelaunch checks of ESA navigation software and delivery of validated DSN radio metric tracking data during the mission. Effects of drag from passing through the coma are seen in data received pre and post encounter. The post encounter Giotto trajectory provides a solar occultation in January 1988, prior to returning to the Earth in 1990 for possible retargeting to yet another comet.
Results of the VEGA Pathfinder concept which was used to successfully target the European Space Agnecy's Giotto spacecraft to a 600 km encounter with the comet Halley are presented. Pathfinder was an international cooperative navigation activity involving USSR, European and U.S. space agencies. The final Giotto targeting maneuver was based on a comet location determined from optical data acquired by the earlier arriving Soviet VEGA spacecraft. Inertial pointing angles extracted from optical images of the comet nucleus were combined with a precise estimate of the VEGA encounter orbits determined using VLBI data acquired by NASA's Deep Space Network to predict the location of Halley at Giotto encounter. This article describes the VLBI techniques used to determine the VEGA orbits and shows that the insensitivity of the VLBI data strategy to unmodeled dynamic error sources resulted in estimates of the VEGA orbits with an accuracy of 50 km.
The experiments carried out at MIT during 1984 focussed on two aspects of Loran-C relative navigation that would impact system performance at the sub-microsecond level of accuracy: tracking loop bandwidth and localized field deformation. Figures are given to illustrate the results from both effects.
Navigation strategies for determining highly elliptical orbits with VLBI are described. The predicted performance of wideband VLBI and Delta VLBI measurements obtained by orthogonal baseline pairs are compared for a 16-hr equatorial orbit. It is observed that the one-sigma apogee position accuracy improves two orders of magnitude to the meter level when Delta VLBI measurements are added to coherent Doppler and range, and the simpler VLBI strategy provides nearly the same orbit accuracy. The effects of differential measurement noise and acquisition geometry on orbit accuracy are investigated. The data reveal that quasar position uncertainty limits the accuracy of wideband Delta VLBI measurements, and that polar motion and baseline uncertainties and offsets between station clocks affect the wideband VLBI data. It is noted that differential one-way range (DOR) has performance nearly equal to that of the more complex Delta DOR and is recommended for use on spacecraft in high elliptical orbits.