Data Relay Satellite Research Articles

The Columbus Programme

THE Columbus space station programme is an essential element of the European long‐term scenario adopted by the ESA Council Meeting at Ministerial Level in 1987, comprising also the new launcher Ariane 5, the space plane Hermes, a Data Relay Satellite, ground operations centres as well as corresponding utilisation programmes.

Multiple‐access phased array antenna simulator for A digital beam‐forming system investigation

AbstractFuture versions of data relay satellite systems are currently being planned by NASA. Being given consideration for implementation are on‐board digital beam‐forming techniques which will allow multiple users to simultaneously access a single S‐band phased array antenna system. To investigate the potential performance of such a system, a laboratory simulator has been developed at NASA's Lewis Research Center. This paper describes the system simulator, and in particular, the requirements, design and performance of a key subsystem, the phased array antenna simulator, which provides realistic inputs to the digital processor including multiple signals, noise and non‐linearities.

ESA Dual-Standard S-band Transponder: A versatile TT&C equipment for communications via a data relay satellite or directly with the ground network

This paper describes the new ESA Dual-Standard S-band Transponder designed for users of the European Data Relay System (DRS). Two contracts were placed with European companies for the design and development of an elegant breadboard of this equipment on the basis of Hermes spaceplane communication needs. They were completed by the end of 1991. The equipment can be operated in either of two modulation modes: the standard mode allows for direct interface with the existing ground networks whereas the spread spectrum mode is dedicated to operations with Data Relay Satellite Systems. The Dual-Standard Transponder offers new facilities to the user, such as frequency channel programmability, which allows to tune the equipment anywhere within the 2 GHz space research band, or telecommand data rate flexibility. The challenging Hermes requirements justified by the fact that all communication links with the ground are ensured by this equipment lead to a rather complex design. However, thanks to its modular concept, descoped versions can be easily derived to meet the needs of less demanding unmanned spacecraft.

Organizing large space activities

Since 1965 there has been a progressive shift in the organization of major space activities away from the original space agency model towards more commercial management principles. This article identifies nine basic organizational models, ranging from government-run programmes such as NASA's Tracking and Data Relay Satellite System right through to privately owned systems such as Asiasat and Astra, and notes that they by and large represent a historical evolution. A close look at the success of Intelsat as it has developed in an increasingly competitive world suggests that a functional focus on technical and commercial objectives, commercial management techniques, competitive procurement and recruitment on merit have been among the most important factors in the organization's success.

Emphasizing conflict resolution versus conflict avoidance during schedule generation

Abstract Classical planning systems attempt to solve a planning problem by avoiding possible conflicts before the actions are put on a timeline. This is computationally very expensive and the search for all possible future conflicts may be prohibitive. A conflict resolution approach can check for immediate conflicts and try conflict resolution strategies as each activity is put on a timeline without regard for possible future conflicts. A more practical approach is to use a combination of conflict avoidance and conflict resolution based upon heuristics which limit the amount of search required when either is used. Because humans are not good at solving problems which require complex lookahead, this combined approach, with emphasis on conflict resolution, is what human schedulers actually use when they develop schedules. A system which simulates this human approach to scheduling has been developed at NASA's Goddard Space Flight Center for scheduling satellite activities. This system, which includes the Planning And Resource Reasoning (PARR) shell, allows expert schedulers to specify conflict resolution strategies as well as conflict avoidance strategies to be used during the scheduling process. PARR has been used since May 1987 to schedule the Tracking and Data Relay Satellite System services for the Earth Radiation Budget Satellite. PARR will also be used to schedule platform resources on the Explorer Platform, scheduled for launch in early 1992 . This paper describes the advantages of using a combined conflict avoidance and resolution approach in a satellite scheduling system.

On the reflectivity of complex mesh surfaces (spacecraft reflector antennas)

Poorer than expected surface reflectivity was observed in an early Tracking and Data Relay Satellite System antenna utilizing a tricot mesh weave. This poor reflectivity was determined to be caused by inadequate electrical contact at wire crossover points. A proper mathematical and numerical approach to assess the impact of wire junctions on reflectivity performance is developed. A mathematical method is presented for computing the surface reflectivity of complex mesh configurations like those on unfurlable-type spacecraft antennas. The method is based on the Floquet mode expansion to establish an integral equation for mesh wire currents. The equation is solved using the method of moments with triangular basis functions. It is observed that it is necessary to give special attention to the junction treatment among different branches of the mesh configurations. A vector junction current approach that resulted in satisfactory solutions for the current is described. The results of numerical simulations are compared against measured data and excellent agreement is observed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Using C to build a satellite scheduling expert system: Examples from the explorer platform planning system

Recently, many expert systems have been developed in a LISP environment and then ported to the real world C environment before the final system is delivered. This situation may require that the entire system be completely rewritten in C and may actually result in a system that is put together as quickly as possible with little regard for maintainability and further evolution. With the introduction of high-performance UNIX and X-windows-based workstations, a great deal of the advantages of developing a first system in the LISP environment have become questionable. This paper describes a C-based artificial intelligence (AI) development effort based on a software tools approach with emphasis on reusability and maintainability of code. The discussion starts with simple examples of how list processing can easily be implemented in C and then proceeds to the implementations of frames and objects that use dynamic memory allocation. The implementation of procedures that use depth first search, constraint propagation, context switching, and a blackboard-like simulation environment are described. Techniques for managing the complexity of C-based AI software are noted, especially the object-oriented techniques of data encapsulation and incremental development. Finally, all these concepts are put together by describing the components of planning software called the Planning And Resource Reasoning (PARR) shell. This shell has been successfully utilized for scheduling services of the Tracking and Data Relay Satellite System for the Earth Radiation Budget Satellite since May of 1987 and will be used for operations scheduling of the Explorer Platform in November of 1991.

Efficient Loads Analyses of Shuttle-Payloads Using Dynamic Models With Linear Or Nonlinear Interfaces

A technique is presented for conducting efficient loads analyses of Shuttle-payloads systems with linear or nonlinear attachment interfaces. The technique relies on representing the Space Shuttle and the payloads with physical and modal coordinates. Further, by invoking a standard algorithm of numerical integration of equations of motion, the kinematics of the interface degrees of freedom at a given time are determined without calculating the modes of the combined system involving the Space Shuttle and the payload. If the Shuttle-payloads interface coupling induces a linear dynamic model for the loads analysis, the equations of motion of the Shuttle and the payload are integrated separately step-by-step in time. If the dynamic model is nonlinear, the equations of motion of the Shuttle and the payload are again integrated separately. However, in the latter case an iterative procedure is used within a time step to converge to reliable values of the nonlinear terms of the equations of motion. The usefulness of the proposed technique is demonstrated by conducting a loads analysis for the Shuttle abort landing event with the Inertia Upper Stage (IUS) booster carrying a Tracking and Data Relay Satellite (TDRS) in the payload bay. This combined system has at its interface dry friction and hydraulic nonlinear dampers. For the analysis of this system, the discontinuous signum function used traditionally in modeling dry friction is replaced by an expeditious continuous approximation. Because of its efficiency and versatility, the new technique deserves serious consideration for becoming a standard tool for linear or nonlinear analysis of combined systems, in general, and of Shuttle-payloads systems, in particular.

The European data relay system: present concept and future evolution

After briefly reviewing the users' requirements and the results of traffic scenario simulations needed to size the system, the design of the European data relay system (DRS) (ground and space segment) is presented in detail. The interrelationship with the other data relay systems (NASA's tracking and data relay satellite system (TDRSS) And the Japanese data relay and tracking satellite system (DRTSS)) is discussed together with the current status of systems interoperability. The evolution of DRS into a second generation, foreseen for operation at the beginning of next century, is outlined in terms of user scenario, improved spacecraft and system configurations, and technology developments needed. >

15 GHz space VLBI observations using an antenna on a TDRSS satellite

A 4.9 m diameter antenna in earth orbit, part of the Tracking and Data Relay Satellite System, was previously used for space VLBI observations at 2.3 GHz. It has now been used successfully with two ground antennas for VLBI measurements at 15 GHz. Although the sensitivity (gain/system temperature) of the orbiting antenna was a factor of 6 poorer at 15 GHz than at 2.3 GHz, 11 out of 22 extragalactic sources were detected on space-ground baselines, up to a maximum baseline of 1.63 earth diameters. The results suggest that the distribution of source visibilities at 15 and 2.3 GHz are similar for these baseline lengths. Model-dependent brightness temperatures of 1-2.5 x 10 to the 12th K were derived for six sources. This suggests that sources with brightness temperatures in this range are as common at 15 GHz as at 2.3 GHz. The coherence on space-ground baselines (this includes effects due to the phase transfer from the ground to the satellite and the reconstruction of the spacecraft orbit) was 0.76 for 340 s integrations.

Space Applications of GPS

The country's space systems ground command and control network is rapidly becoming antiquated, and will not be able to handle the expansion of space traffic projected for the twenty-first century. There is a heavy reliance on vulnerable overseas ground locations that are not guaranteed. A cooperative satellite system architecture based on GPS would significantly alleviate the complexity of a future command and control structure. Specifically, if each satellite were to carry a GPS receiver as part of its telemetry, tracking, and command subsystem, all space vehicles could maintain accurate and reliable time, ephemeris, and pointing estimates. This satellite navigation and status data could then be transmitted on a regularly scheduled basis to a ground monitoring station or a Tracking and Data-Relay Satellite System (TDRSS) Satellite. In this way, normal operations would be reduced to merely reviewing the telemetered data. In case of anomalies in the data or a space emergency, a dwell mode would be initiated in which the ground segment would begin active tracking of the space vehicle. This paper outlines the structure of such a GPS-based cooperative satellite monitoring system.

VLBI using a telescope in Earth orbit. I - The observations

A VLBI experiment has been conducted at a frequency of 2.3 GHz, using an antenna in Earth orbit. The observational procedure for the orbiting antenna, part of the Tracking and Data Relay Satellite System, is described. Nonstandard VLBI procedures were necessary to obtain adequate phase stability and to correlate the data. Several technical concepts necessary for a dedicated space VLBI mission were successfully tested.

SPACE SHUTTLE: U.S. makes successful return to space

Thirty-two months after the Challenger space shuttle disaster of January 1986, the U.S. is back into space. Last week, the shuttle Discovery and its five-man crew landed after four nearly flawless days in orbit around Earth. The hundreds of revamped shuttle components passed their first flight test with only a few minor glitches, and the crew successfully deployed its primary payload, a Tracking & Data Relay Satellite (TDRS-C). Moreover, the astronauts carried out smoothly all the scientific experiments contained in the 11 secondary payloads. Meanwhile, the same day that Discovery was launched, Sept. 29, two other events emphasized that the U.S. now has strong competition in space. By coincidence, the U.S. and 11 other nations signed agreements to build a $23 billion, permanently manned, international space station to orbit Earth by the late 1990s. Cosigners were Belgium, Canada, Denmark, France, West Germany, Italy, Japan, the Netherlands, Norway, Spain, and the U.K. And by coincidence or ...

Antenna system alternatives for data relay satellites with multiple steerable beams

In this paper, antenna system alternatives for data relay satellites are presented. To that end, the European Data Relay Satellite (DRS) as planned by the European Space Agency (ESA) is described in terms of its mission. Following that, the requirements on the antenna system are outlined. In the DRS two frequency bands are employed at S- and Ka-Band to track a total of four low orbit satellites simultaneously with four independently steerable beams, two at S-Band and two at Ka-Band. The goal of the work presented in this paper is to satisfy the mission requirements with only two large aperture antennas. Constraints on the antenna system are discussed such as imposed by the launcher; the mission as planned foresees an ARIANE Double Launch/Bottom Position. A major design driver has been that the scanning range at Ka-Band amounts to 80 half-power-beamwidths. No undue gain degradations are to be incurred over this range. Another design driver has been the minimizing of the complexity of the mechanisms needed for deployment of the large aperture antennas and their accommodation under the volume-limited launcher shroud. The potential solutions are quantitatively assessed. Various possibilities of combining the S-Band and Ka-Band are outlined, including combined feeds, dichroic reflectors and subreflectors, auxiliary reflectors, beam-waveguide systems, and phased arrays. For the determination of the preferred system, a catalog of trade-off criteria and associated weighting factors has been established. A considerable number of candidate antenna concepts satisfying the mission requirements are presented (in deployed and stowed configuration on a SPACEBUS-200 type of spacecraft) and their trade-off scores listed. Finally, the winning candidate will be depicted in more detail.

Very Long Baseline Interferometry Observations Using the Tracking and Data Relay Satellite as an Orbiting Radio Telescope

An antenna in geostationary orbit was used for VLBI observations at 2.3 GHz, in combination with ground antennas in Australia and Japan. 23 of the 25 observed sources were detected on orbiter-ground baselines, with baseline lengths as large as 2.15 earth diameters. Brightness temperatures between 1012 K and 4 × 1012 K were measured for 10 sources.

A rule-based systems approach to spacecraft communications configuration optimization

An experimental rule-based system for optimizing user spacecraft communications configurations has been developed at NASA to support mission planning for spacecraft that obtain telecommunications services through NASA's Tracking and Data Relay Satellite System. Designated ECCO (Expert for Communications Configuration Optimization), and implemented in the OPS5 production system language, the system has shown the validity of a rule-based systems approach to this optimization problem. This article discusses the development of ECCO and the incremental optimization method on which it is based. A test case using hypothetical mission data is included to demonstrate the optimization concept.

Antennas for satellites: Around the world with antennas made for three-axis stabilized spacecraft

Antennas for three-axis stabilized spacecraft that are in circular orbit and must communicate with one or more earth-based antennas are discussed. Antennas for many military satellites of a classified nature and for NASA's deep space probes are excluded. The fundamental characteristics of an antenna, namely radiation pattern, directivity, gain, and polarization, are defined. These properties are discussed in relation to the dipole and the conical spiral, both low-gain antennas. A consideration of the nature of the transmission of information over long, free space paths reveals that high-gain antennas are required when the information data rate is high, as in color TV transmissions. Some typical satellite antennas are briefly described, including those of the Global Positioning System's Navstar and the Telecommunications Data Relay satellites.

New shuttle schedule released

The Hubble Space Telescope has a tentative launch date of November 17, 1988, according to an announcement made October 3, 1986, by the National Aeronautics and Space Administration (NASA). The plan calls for the first shuttle launch to take place on February 18, 1988, when Discovery is slated to launch a second Tracking and Data Relay Satellite (TDRS). A TDRS satellite was aboard Challenger when it exploded on January 28, 1986.The $1.3 billion telescope would be launched aboard the fifth shuttle mission in 1988. After launching the TDRS satellite, NASA plans to send Atlantis and then Columbia spacebound with military payloads. In September 1988, Discovery would be launched again with a third TDRS satellite.

Centralized vs decentralized options for a european data relay satellite system

The European Data Relay Satellite (DRS) is now being planned to support future European missions in the nineties and in particular the various elements of the in-orbit infrastructure. Studies are being conducted to investigate the usefulness of the Relay System as well as to provide the basis for issuing technical specifications for a development and launch in 1993. This paper presents the results of a study issued by ESA on possible options for a DRS System, concentrating on the comparison between a centralized and a decentralized data distribution concept. After recalling the space programmes foreseen in Europe, the paper discusses the architecture and design of the various elements of the System: space segment, DRS ground segment and user ground segment for different options of data dissemination.

Satellite and launcher trajectory estimation using a data-relay satellite

Utilization of a data-relay satellite (DRS) to obtain range and range-rate measurements of a low Earth-orbiting satellite presents obvious visibility advantages compared to Ground stations. A feasibility study of the orbital estimation of a low-orbiting satellite by means of a Kalman filtering algorithm using DRS measurements has been performed, leading to positive conclusions. The estimation of a launcher ascent trajectory has been also investigated, but no definite conclusions could be stated to date.