Satellite Observation System for Space Station Freedom - Abstract
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Satellite Observation System for Space Station Freedom


Greg Cabe, et. all


December 10, 1992

Introduction

Degobah Satellite Systems (DSS), in cooperation with the University Space Research Association (USRA), NASA - Johnson Space Center (JSC), and the University of Texas, has completed the preliminary design of a satellite system to provide inexpensive on-demand video images of all or any portion of Space Station Freedom (SSF). DSS has narrowed the scope of the project to complement the work done by Mr. Dennis Wells at Johnson Space Center. This three month project has resulted in completion of the preliminary design of AERCAM, the Autonomous Extravehicular Robotic Camera, detailed in this design report.

This report begins by providing information on the project background, describing the mission objectives, constraints, and assumptions. Preliminary designs for the primary concept and satellite subsystems are then discussed in detail. Included in the technical portion of the report are detailed descriptions of an advanced imaging system and docking and safing systems that ensure compatibility with the SSF. The report concludes by describing management procedures and project costs.

Design Objectives

Video access to any structural component and ORU (Orbital Replacement Unit) of the Station is an essential aid for crew operation and maintenance of Space Station Freedom. Versatile viewing capabilities also offer unique opportunities for enhancing the public image of the SSF program and its vital role in space exploration.

The video system currently baselined for use on SSF is composed of up to 14 cameras positioned in fixed locations throughout the Station truss. The cameras offer limited viewing angles and require extensive data and support cables. Though some of the cameras can be moved, this operation requires support from the Space Station Remote Manipulator System (SSRMS) and extravehicular activity (EVA) crew members. The system is therefore cumbersome, limited in its mobility, and complicated to service or modify.

The DSS design team has, consequently, undertaken the design of a system more appropriate to space applications and based on advances in a number of technical areas. A few major goals and constraints were established at the beginning of the project to guide the design. The primary task of the AERCAM system, as determined by DSS, is to provide on-demand detailed video access to all or any portion of SSF. In order to accomplish this task, a number of important design objectives were identified. These objectives were ultimately met by the final AERCAM design. The following is a brief list of those objectives:

The DSS design team made three major assumptions, each of which implies that AERCAM will be a dedicated element of the SSF baseline. The first assumption is that there will be dedicated human support for control of the satellite. The second assumption is that limited scarring of the Station will be allowed and some SSF resources (propellant and power) will be available to AERCAM. Finally, the AERCAM will require some communication and data handling support from SSF.

Concept Selection

The DSS design team studied four design concepts before selecting a final design. The concepts were evaluated heavily on their safety, reliability, simplicity, and the viewing detail they could provide.

A self-reboosting satellite was considered a possibility for a long-life satellite that would not require modifying the Station truss for a satellite support structure. Although the system would require very little maintenance and retrieval, the mass would have to be greater than 200 lb., and the satellite would require a stand-off orbit greater than a kilometer, reducing the resolution and versatility of the imaging system.

A disposable satellite was also considered. For this concept, an inexpensive satellite would orbit the Station in a manner similar to the self-reboosting satellite, and then re-enter the atmosphere by means of a drag balloon after one or two SSF reboosts (~200 days). Hardware that is inexpensive enough not to prohibit disposal would not be of a high enough quality to provide adequate imaging or maneuvering systems, however; and storage and deployment of replacement units was also a problem.

To avoid scarring of the Station truss, while allowing for a re-usable satellite, DSS studied using an independent docking platform designed to orbit behind the Station on the SSF velocity vector. The added costs of an external docking platform, which would need periodic maintenance, and the added complexity of multiple autonomous vehicles interacting with the docking platform resulted in the rejection of this concept.

DSS ultimately decided to base the preliminary design on a satellite which would nominally dock to the Space Station truss structure, either autonomously or with human assistance, for periodic servicing. This satellite could be smaller due to reduced propellant requirements, and more weight and power could be dedicated to improving imaging and maneuvering systems. This concept was also evaluated to be the most reliable.

The Concept

The AERCAM has been designed as a refuelable, free-flying spacecraft, based from a docking/servicing platform on the SSF truss and housing an advanced imaging system. The spacecraft bus weighs about 130 lbs. and is 0.7 cubic meters in volume. It is powered by NiH2 batteries, which provide 40 Watts of power for up to 4 hours of operation at peak usage. Portions of the exterior panels of the satellite are removable to provide access to a number of modular components, including the imaging system. The spacecraft features a docking mechanism compatible with the Space Station arm end effectors and a cold-gas propulsion system using compressed waste gas from the Station. The spacecraft is shown in Figure 1, including a front view, a view of the docking mechanism on the right and a view of the interface to the SSF arm end effector on the left.

Figure 1: Satellite Concept

Though configured for remote control by a user, AERCAM is fully capable of autonomous orbiting and safing. It features three fault tolerant components and an array of sensors and artificial intelligence for proximity sensing and contingency safing operations. The bus is also equipped with a passive energy attenuation system to prevent damage to the Station in the event of a collision.

The imaging system for AERCAM is a self-contained infrared and visual camera package, housed within a removable, spherical encasement. It provides 40° panning in all directions and zooming. Coupled with a number of flight modes ranging between 10m and 500m from the Station, the imaging system can provide a resolution as detailed as 1mm/pixel and can provide a field of view which includes the entire Station.

Operational Concept

AERCAM is nominally docked to the Space Station truss, on a dedicated docking platform, until it is needed by a crew member. The docking structure is a standard attach platform used for SSF ORU's, modified to include a power and propellant interface to refuel and recharge the satellite while it is docked. This platform is shown in Figure 2.

Figure 2: Standard Attach Platform

Once AERCAM is launched from its platform, it is capable of a number of flight modes. The satellite can be controlled by a hand controller and command console onboard the Station. The Station command console can control the operation of the imaging system, maneuvering system, and the internal functions of the satellite. The satellite is also capable autonomous flight modes. It can hold its position at any location with respect to the Station, including holds on the velocity vector and the radial vector. Nominally, the satellite is designed to translate and position hold ten meters from the Station, allowing for an adequate safety zone. In addition, it can maintain a sub-orbit about the SSF at one half a kilometer from the Station. Though capable of user-controlled operation, the satellite calculates its own trajectories and propulsion and attitude requirements, and it is actively aware of a dynamic environment, allowing for operation independent of constant human control.

Imaging System

The imaging system is housed in an independent, spherical encasement, capable of panning up to 40° in all directions. The system is shown in top and side views in Figure 3. The imaging system housing has two locking gimbals, coupled with two small actuators and gimbal stops, to provide the required pointing at a rate of 10° per second. The housing weighs approximately 10 lb., occupies 1.77 ft.3, and the system requires approximately 15 Watts during peak operation. The system is modular and easily accessed through a removable panel and detachable latches at the gimbals.

Figure 3: Imaging System

The imaging system features a small light, providing illumination for nearby viewing in dark conditions. It also features a flash shield, reactive in 2 milliseconds, designed to protect the camera components from intense radiation.

The cameras are based on digital charged coupled devices (CCD's) and produce digital images using a matrix of 2048 by 2048 sensors. A series of mirrors and filters separate the incoming electromagnetic radiation into two streams, one directed to a visual camera and the other to an infrared (IR) camera. The visual camera responds over a spectrum from 0.45 mm to 1.1 mm, and the IR camera responds over a spectrum from 1.1 mm to 15 mm.

The imaging system is designed to provide a resolution of 1 mm / pixel at a distance of ten meters from the Station, which corresponds to a field of view 2.048 meters. The imaging system can also provide a field of view as wide a 128 meters at 500 meters from the Station, which corresponds to a resolution of 62.5 mm / pixel.

Images are produced at a rate of 3 frames per second, generating at least 4.5 Megabits (Mbps) of data each second. This data is processed and compressed using a computing system derived from the Kodak DCS 200 and processors used for the Brilliant Pebbles system.

The performance of the AERCAM imaging system is summarized in Table 1, below.

Table 1: AERCAM Performance

Communication System

In order to provide adequate communications support, the satellite is capable of transmitting 100 Mbps of telemetered data and of transmitting and receiving 10 Kbps of commanding, health, and status data. In order to reduce the amount of data telemetered from the imaging system, AERCAM offers a number of data reduction and compression techniques, including Vector Quantization - a patented compression technique that can reduce data in a lossless manner by up to a factor of 12.

The AERCAM communication system is based on a space-to-space communication system originally designed for Space Station Freedom. It employs twelve antennas on the satellite and ten antenna arrays on the Space Station. Each antenna is a circular loop antenna, one wavelength in diameter. The satellite antennas draw 0.5 Watts of power for transmission, and the Station antennas draw less than 2 Watts. The carrier frequency ranges between 14.0 GHz and 14.9 GHz. Based on a transmission path length of 1 kilometer, the AERCAM link budget predicts a signal-to-noise ratio of 25 and a transmission margin of 18.5 dBWatts, both excellent characteristics for a communication system.

Satellite Bus

The satellite bus is a 20 sided hexagonal structure, shown in Figure 1, composed of an internal support structure and 20 external panels, 8 of which are removable. The structure is designed for modularity and ease of access to the internal components. The propulsion system includes a 0.5 cubic foot tank, containing compressed carbon dioxide. It provides up to 150 ft/sec of ∆V. Twenty-four NiH2 cells supply 40 Watts of power, and can operate up to 4 hours at peak usage. Twenty Watts are allocated to the imaging system for nominal operation, and the computer system and propulsion/attitude control systems are always guaranteed 20 Watts, respectively, for emergency operations.

One end of the satellite is equipped with a docking mechanism compatible with the standard attach platform, and the opposite end of the satellite is equipped with a standard SSF grapple fixture, both shown in Figure 2, to allow for grappling and maneuvering using the Special Purpose Dexterous Manipulator. The docking mechanism provides interfaces with a power fixture and propellant valve to allow recharging and refueling of the satellite while it is docked.

Safety is one of the most important elements in the AERCAM mission. In order to maintain autonomous proximity detection and collision avoidance, the spacecraft is equipped with an intelligent logic system, capable of adapting to a dynamic world model. To support this system and to generate the dynamic world model, the satellite is equipped with sensor clusters on each face of the satellite. These clusters provide both ranging and relative velocity information for the satellite.

The following critical components of the satellite have been designed for three fault tolerance: computer system, thruster/propulsion system, and power system. Key elements in the communication system have also been designed for fault tolerance.

Should the satellite collide with the Station, the bus has been equipped with an energy attenuation device. The method of energy attenuation which DSS chose to implement requires that the vertices of the satellite be fitted with compartmentalized "bumpers," filled with an inert gas. These bumpers represent a passive system, and therefore will always be operational, despite other failures within the spacecraft.

The insurance of safety is perhaps the most significant hurdle to overcome for acceptance of a satellite of this nature. Therefore DSS has been committed to identifying the most reliable methods of operation, control, and safing. We believe that the satellite's design will offer reliability and safety measures which will be acceptable to the SSF program.

Recommendations

Due to the limited duration of the project and the scope of work which the DSS design team undertook, some areas of design have been left for future work. Some crucial areas of future design work are listed below:



Conclusion - The AERCAM Advantages

Advances in automation, robotics, and microtechnology, as well as the modern proficiency for designing inexpensive, reliable systems with multiple fault tolerance offer a unique opportunity to take a large step forward in space imaging and servicing technology. The current baseline video system for SSF uses the same design philosophy used for video surveillance on Earth - multiple, fixed cameras linked through a large system. While this design philosophy is a proven one, Degobah Satellite Systems believes that a new technology, more appropriate to space application, is not only feasible, it is also more versatile and powerful and represents a stepping stone to automated robotic maintenance in space.

The AERCAM is a more mobile system than the fixed video system. It can be moved more rapidly and does not require crew or SSRMS support to change locations. Its viewing is not limited by physical obstacles, and access to all or any portion of the Station is available on very short notice. It offers much greater versatility in its viewing options, and can even be attached to the SSRMS as additional mode of viewing.

AERCAM requires only a single platform for docking and servicing, and two (as opposed to 14) units must ever be modified or serviced. It also offers superior imaging through higher resolution, wider spectral response, and a larger field of view.

AERCAM's application can be extended far beyond the SSF project. The camera, even based from the Space Station, can be used to provide imaging for nearby spacecraft and to assist in any docking or proximity operations of multiple spacecraft. Further, an easily modified or replaced docking panel and the advanced proximity detection and maneuvering capabilities of the satellite make it adaptable to use on future orbiting facilities.

Finally, if AERCAM were implemented, it would represent the first major step toward using fully automated robotics for space maintenance. The imaging, proximity detection and maneuvering, and intelligent logic systems are the major design hurdles in developing advanced space robotic devices. AERCAM's "hands-off" viewing operations offer an excellent opportunity to safely test these three crucial systems before progressing to actively interacting with Station hardware. It is then a small step to equip one or more of AERCAM's unused external panels with robotic arms and end effectors for use as an autonomous maintenance system. Such robotic systems will be invaluable in terms of reducing crew work time and increasing crew safety. full text


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