AIAA/USU Conference on Small Satellites

Astronaut Deployable Satellite

William Clapp — Mon, 27 Aug 1990 01:00:00 PDT

The Astronaut Deployable Satellite (ADSAT) is an educational satellite being designed and built by faculty, local engineers, and students at Weber State University. The ADSAT is our third satellite project after the success of two others, NUSAT I (Challenger April 85) and WEBERSAT-OSCAR 18 (Ariane-Jan 90. The ADSAT is designed to be tossed into space by an astronaut. The 16" X 16" X 4" AnSAT is self-contained and is designed to ride into space in a mid-deck stowage locker on the shuttle. When launch is desired, the astronaut removes the AnSAT from the locker and carries it outside. The astronaut then deploys the antennas and throws the ADSAT into space. The ADSAT is designed to send to Earth voice messages concerning onboard experiments. The voice messages will be generated by an onboard speech synthesizer that verbally relays the data to low-cost scanners on the ground. ADSAT is being designed to be thrown in a couple of years if NASA will agree to the concept.

The Insurance Climate for Small Satellites and ELVs: 1990

Peter Stark — Mon, 27 Aug 1990 01:02:00 PDT

The space insurance industry provides coverage for physical damage and liability risks to which space ventures are exposed as part of their business. The Insured party obtains insurance coverage through a Broker, who represents the Insured in soliciting coverage from the various Insurers in the world market. Physical damage insurance is designed to cover the value of an asset or the revenue it may provide, while liability insurance covers damage to the person or property of parties unrelated to those involved in launch activities. To date, five commercial launches of small ELVs have been conducted, and the necessary insurance has proven to be available at affordable rates. The forecast growth in the market for small spacecraft should prove attractive for insurers, and coverages should continue to be readily available for some time.

SURFSAT: Supporting Deep-Space-Network Research and Development with a Student-Designed Small Satellite

Jonathan Chow — Thu, 29 Aug 1991 11:30:00 PDT

SURFSAT is a small satellite designed primarily by students at the California Institute of Technology under Caltech's Summer Undergraduate Research Fellowship (SURF) program and is sponsored by the Telecommunications and Data Acquisition Office at JPL, the research and development arm for the Deep Space Network (DSN). The project was initiated in the summer of 1987 and has involved several dozen undergraduate students. SURFSAT is designed to radiate at either the milliwatt or microwatt level in four bands: Sband (2.29 GHz), X-band (8.45 GHz), Ku-band (15.33 GHz) and Ka-band (32 GHz). The signals will be received by a new 34-meter DSN research antenna at Goldstone, California. Performance of the new Ku-band link will be analyzed and compared to the performance of the more standard X-band link. The S-band and Ku-band signals will be used to support DSN spacecraft acquisition tests and training. Other experiment objectives have also been identified, including spacecraft position and orbit determination demonstrations utilizing ground-based connected-element interferometers. It will also carry an optical beacon to demonstrate the possibility of communicating at infra-red wavelengths. The basic SURFSAT satellite is a solar powered cube, 12 inches on each side, that will tumble through space. Attitude stabilization is not required, and the satellites will not have a propulsion subsystem or batteries. Signals are to be radiated in all directions. Redundant command receivers will enable transmit modes of one milliwatt, one microwatt, and OFF. At least two SURFSATs are scheduled for launch as secondary payloads on a Delta rocket in May 1994; the primary payload for this mission is LAGEOS III.

A Comparison of Digital Modulation Methods for Small Satellite Data Links

Daniel Mulally et al. — Thu, 29 Aug 1991 11:15:00 PDT

The selection of a good modulation scheme for a satellite data link should involve careful consideration of several factors. Bit-error-rate (BER), initial cost, power consumption, circuit complexity, channel linearity, reliability, and bandwidth must be considered and weighed in the selection process. This paper examines and compares various modulation methods applicable to small satellite data links. The performance of frequency-shift keying (FSK), bi-phase-shift keying (BPSK), quadrature-phase-shift keying (QPSK), offset QPSK (OQPSK), minimum-shift keying (MSK), and on-off keying (OOK) are compared. The use of a non-linear transmitter amplifier is normally desirable because of its power efficiency. Because of this, a near constant envelope modulation scheme is desired. Power efficiency and bandwidth efficiency may also be important. In regards to these and other criteria, OQPSK has good characteristics and is recommended.

Tracking Small Satellites using Translated GPS

Don Lefevre et al. — Thu, 29 Aug 1991 10:59:00 PDT

This paper discusses using translated GPS for tracking small satellites, the technical trade-offs involved, and the position and timing accuracies which are achievable using translated GPS. The Global Positioning System (GPS) uses the relative times-of-arrival of multiple spread-spectrum signals at an antenna to determine the position of the antenna. The system can also determine the time the antenna was at that position. The direct sequence spread spectrum signals are transmitted from GPS satellites whose orbital position and timing (the ephemeris data) are accurately known. Once the GPS signals are received at the antenna, their relative timings are fixed, and are not changed in subsequent amplifications and signal processing. This fact can be used to track small satellites to a high degree of accuracy without placing a GPS receiver on the satellite. A GPS antenna is placed on the satellite, and the received signals are amplified, converted to a new frequency, and retransmitted to a ground-based GPS receiver. In the most cost-effective arrangement, the ground GPS receiver consists of a low-noise amplifier, a frequency converter to the original GPS frequencies, and an off-the shelf GPS receiver. The off-the-shelf receiver may be coupled to a computer for extensive post-processing. Such post-processing is currently available for IBM PC's. The advantages of this translated-GPS tracking are that high order GPS receivers with large amounts of post-processing can be used for tracking spacecraft, while leaving the spacecraft electronics very simple. This allows highly precise, inexpensive, and power-efficient GPS systems to be used on small satellites without: the risk of non space-qualified GPS receivers: the expense of space-qualified receivers: or the power-consumption of high-order receivers.

Earth-Space LEOSAT Communication using Optical Frequencies

Kenneth Baker et al. — Thu, 29 Aug 1991 10:45:00 PDT

This paper introduces the concept of a high bit rate burst type communications system as a means of increasing the communications throughput on LEO spacecraft. The discussion concerns the use of optical frequencies due to the decrease in size, weight and prime power of LEO satellite communication packages at these wavelengths. The penalty is an increase in pointing and tracking complexity. An example link design is presented.

SOESAT Low Cost Telemetry Ground Station

David Horton — Thu, 29 Aug 1991 10:30:00 PDT

The School Of Engineering SATellite (SOESAT) will be fabricated and donated by American-Microsat Inc. of Sunnyvale, California. SOESAT will carry a radiation sensor that will measure the secondary emission internal to the satellite caused by the impinging of external primary radiation. At the time of this abstract SOESAT has not been manifested on a launch vehicle. A fourth quarter '92 launch into a geotransfer orbit on a European Space Agency Ariane is being investigated. Low earth orbit launch alternatives on various boosters are also being investigated. Command and control of SOESAT will be accomplished using the telemetry ground station built and operated on site at San Jose State University. The total cost of this system is less than $8000.

Spaceborne Application Multiprocessor Operating System

Gary Grisbeck et al. — Thu, 29 Aug 1991 09:45:00 PDT

Control Data Corporation, as part of a joint development program with the Boeing Electronics Company, built a nine node, three active module multiprocessor called SPA-l or Spaceborne Processor Array-I. (SPA was the processing system concept proposed as the heart of a multi-mission autonomous surveillance satellite in a paper presented at the 4th Annual AIAA/Utah State Small Satellite Conference in 1990). Since then, an operating system called the Operational Kernel, or OK, has been completed, tested, and functionally validated in a demonstration using the SPA-l. This demo featured fully autonomous on-board control of data movement, fault detection, fault isolation, hardware reconfiguration, application re-start, and load balancing/redistribution. The demo application consisted of ephemeris calculations being performed for two satellites, in each of three independent processors at different nodes on the SPA-I; this data (and SPA-l health status) was sent via a serial I/O (input/output) channel to a host machine for display. With the demo software running in the three active processing nodes, observers were invited to cause random nodal interconnect or processing hardware elements to fail by selection of switches on a fault injection panel. SPA-I, under the aegis of the OK, detected that a failure had occurred, isolated it, reconfigured around it, redistributed the processing load (to the two or one remaining active processors) and continued with the application processing, all without operator intervention of any sort. The OK is written in Ada. Support of the execution of Ada programs is provided for by the Ada Run Time System (RTS), provided by the Ada compilation system. The RTS provides very basic services such as tasking and memory management, and exception handling. The OK consists of Ada packages that are run on top of the RTS. This collection of packages include lower level services that involve message buffering, interrupt handling, and individual configuration commands. Operational high level services include: a block I/O facility that uses protocols to ensure the integrity of data transfers between modules at different nodes on the SPA network; a configuration facility that provides a high level set of operations to configure the network; and a health check facility to support application controlled detection and isolation of failed SPA elements. The SPA hardware, with processing elements operating essentially asynchronously at each node on the network, supports many concurrent activities. The OK handles this with Ada tasks. The number of tasks is application dependent. Approximately sixty tasks were employed in the SPA-l demo.

Single Event Upset Error Protection for Solid State Data Memory on Microsatellites

M.S. Hodgart et al. — Thu, 29 Aug 1991 09:30:00 PDT

Many microsatellite missions rely upon large arrays of CMOS static RAM for data storage. Even a satellite with strict power and volume limitations can carry several Mbytes of SRAM. Experience has shown that these memory arrays are useful for scientific data collection image storage, store and- forward message switching and spacecraft telemetry monitoring. This paper describes the authors' design and implementation of error protection codes which protect large CMOS RAM arrays from radiation-induced single event upsets.

Protocols for Store-and-Forward Message Switching via Microsatellites

J.W. Ward et al. — Thu, 29 Aug 1991 09:15:00 PDT

The authors have developed a suite of protocols specifically optimized for use on store-and-forward microsatellite communications missions. The protocols support networks in which user terminals directly access the store-and-forward satellite to transfer electronic mail. The authors' PACSAT Protocol Suite includes a message format standard, a virtual-circuit directory and file transfer service, and a datagram based point-to-multipoint "broadcast" protocol. An implementation of this suite has been operating on the UoSAT-3 PACSAT Communications Experiment (PCE) for twelve months and on the AMSAT Microsats for seven months. During this period, hundreds of small and medium-sized ground terminals in all parts of the world have accessed the satellites. On-board software monitors network activity closely, and usage statistics for UoSAT-3 are gathered regularly at the UoSAT Command Station. Based on this data and the reported experiences of regular system users, we compare the effectiveness of the various protocols. From these comparisons we make some recommendations which are generally applicable to store-and-forward microsatellite missions. This paper describes the design of the PACSAT Protocol Suite and the UoSAT-3 and Microsat implementations. It summarizes the in-orbit performance results, and concludes with recommendations for future store-and-forward microsatellite missions.

DATAware; A Graphical Approach for Decoding and Displaying Small Satellite Data and Telemetry

George Starcher — Thu, 29 Aug 1991 09:00:00 PDT

Today's small satellites are capable of producing vast quantities of data from experiments and engineering telemetry. The requirements of the small satellite operator define how science and engineering data is treated. The small satellite operator typically does not have the financial resources for a worldwide network of downlink ground stations. These constraints require that the satellite's status must be ascertained within a relatively short window during a pass over a single or a few ground stations. Any anomalous operation needs to be quickly identified so that corrective action may be taken. In addition, the satellite operator is often the end user of the data gathered from the onboard experiments. Currently, ground stations display data in a list format. The display requires decoding and analyzing the data two distinctly different software algorithms. There is a need for a single software package to decode and display the experiment data and engineering telemetry in an easy to interpret format. A single software package will ease the burden of understanding satellite data decoding, calibration, and display. DATAware is being developed to fulfill this need.

Optical Processing of Microwave Signals for Small Satellite Payloads

Dan Sullivan — Thu, 29 Aug 1991 08:45:00 PDT

For low altitude small satellite applications performing electronic surveillance or communication transponding missions highly capable payloads are needed. Generally these spacecraft have low dwell times over the areas of interest and must receive, or search for, signals over a wide frequency band. This paper presents an approach to the implementation of optical processing in complex electronic systems intended to receive and operate on multiple radio frequency (or microwave) signals. The goal is to exploit the rapidly expanding field of linear and nonlinear optics to synthesize transponders and receiving systems for satellites and other platforms. The inputs are assumed to be microwave. The outputs are assumed to be microwave or electronic (digital). In between, the signal operations are performed optically. The focus of the effort is in the architecture for the electronic functions, that allow optical component realization. These elements perform the signal processing operations of: pulse signal detection and pulse parameter estimation; modulation and demodulation of AM, PM, and PM carriers; phase locked loop signal tracking; carrier element mixing (frequency shifting); signal filtering; and signal matched filter detection. The spatial optical processing of ordinary time waveform signals offers significant potential benefits. It inherently provides wide bandwidth, high carrier frequency, and fast response processing capability. A signal Fourier transforms can be performed with a simple lens. The second spatial dimension for parallel processing enhances the capability for exhaustive search of a signal space for parameters of interest. The two-dimensional optical implementation of switching and routing matches the channelized nature of many current communication systems. Increased optical implementations of electronic systems can take advantage of the rapid technological growth in applications and devices in this parallel discipline of optics to effect greater capabilities for the 1990's.

Autonomous Data Acquisition using the ACTEL 1020 FPGA

Ron Novick — Thu, 29 Aug 1991 08:30:00 PDT

The Actel 1020 Field Programmable Gate Array (FPGA) is a low power CMOS device equivalent to 2000 logic gates. It is a one-time programmable device utilizing non-volatile PLICETM anti-fuse programming. The 2-micron, two level metal CMOS process has been tested Rad Hard to 400K Rad total dose Si making it suitable for space applications. In the Barnes Dual Cone Scanner Earth Horizon Sensor, it has been used (with an A/D converter) to autonomously sample radiance data and store it in pseudo-dual port memory. The Actel chip is also used to control bus access to the memory. The Barnes implementation replaces many discrete logic components and frees up processor time by putting the data in memory autonomously. This architecture is applicable to any system requiring low power data acquisition in a minimum amount of board space.

Space Science and Microsatellites - A Case Study Observations of the Near-Earth Radiation Environment using the Cosmic-Ray Effects and Dosimetry (CREDO) Payload On-Board UoSAT-3

C.J. Underwood. et al. — Wed, 28 Aug 1991 16:30:00 PDT

The University of Surrey's technology demonstration microsatellite: UoSAT-3 launched in January 1990. carries on-board a Cosmic-Ray Effects and Dosimetry (CREDO) payload for characterizing the low-Earth orbit (LEO) radiation environment. Measurements made with this payload are correlated with radiation effects observed in the spacecraft's microelectronics, in particular, the occurrence of single event upsets (SEUs) in solid-state memory devices. The CREDO payload consists of two Sub-systems, the Cosmic Particle Experiment (CPE) and the Total-Dose Experiment (TDE). The CPE houses an array of large-area PIN diode detectors, connected to a pulse-height analysis network. Particles incident on the detector are counted and logged according to their linear-energy transfer (LET). Results are integrated over five minute intervals and the data are stored in the PACSAT Communications Experiment (PCE) memory. The TDE consists of specially manufactured p-channel MOSFETs which are monitored for changes in threshold voltage due to accumulated radiation dose. During the first year's operation, CREDO has provided measurements of the cosmic-ray background, the trapped particle population of the South Atlantic Anomaly (SAA), and has observed a number of large solar proton events - most recently, the major events of June 1991. This paper reviews the results obtained so far and comments on the suitability of microsatellites for this kind of small-scale space science mission.

A Spacecraft Computer for High-Performance Applications

John Stone et al. — Wed, 28 Aug 1991 16:15:00 PDT

A high-performance processor circuit called the SC-3 has been developed to meet the requirements of advanced experiment and attitude control applications. It is based on the 16 MHz Intel 80386/80387 chip set and implements a dual bus system configuration which allows high-speed, 32-bit wide memory and low-speed. 16-bit wide Input Output(I/O) circuits to be separated. This separation maintains compatibility with a wide range of current I/O circuit designs while exploiting the high-bandwidth memory access capabilities of the 80386. Performance is further enhanced by means of a cache on the 32-bit bus. Gibson, Whetstone, and Dhrystone instruction mixes have been used to evaluate performance under various operating modes. When the SC-3 is constrained to execute from 16-bit memory. the Gibson mix indicates a 32% performance improvement compared to previous 16-bit processors. An average of 1.1 million Whetstones per second are performed over the typical range of memory wait states. The average Dhrystone performance improvement between 32-bit non-cached and 32-bit cached operation over a typical range of memory wait states is 115%. The initial application of this processor circuit is on Stanford University's Gravity Probe-B experiment.

BREM-SAT Attitude Control at Low Altitudes

Hans Koenigsmann et al. — Wed, 28 Aug 1991 16:00:00 PDT

The German University satellite BREM-SAT (scheduled for launch in early 1993) carries two experiments which require attitude control down to 150 km altitude and aerodynamic stabilization between 150 and 100 km altitude. These contradicting requirements and new ways to change the satellite's attitude and to keep it stable and controlled at very high disturbance torque levels. Complex numerical attitude simulation has shown the feasibility to maintain the required attitude with a momentum wheel and a single torque coil down to 150 km when optimized algorithms are used. To achieve the aerodynamic stabilization, the momentum wheel has to be ejected and flaps must be deployed just before loosing attitude control.

Flexible Booms, Momentum Wheels, and Subtle Gravity-Gradient Instabilities

J.W. Hunt et al. — Wed, 28 Aug 1991 15:45:00 PDT

A gravity-gradient boom and a momentum wheel provides a passive, three-axis attitude control system for a small satellite requiring 10° Earth-oriented pointing In a low Earth orbit. The Polar BEAR satellite Is a small satellite using just such a system that has experienced unexpected attitude instabilities during some of Its full-sun orbit periods. This paper examines the attitude dynamles and disturbances associated with gravlty-gradientlmomentum-wheel systems In an attempt to identify potential destabilizing mechanisms common to the configuration. Polar BEAR is not the only such configuration to experience problems In full sun. and several other examples are briefiy discussed. Although we place particular emphasis on trying to understand Polar BEAR's anomaly, Its performance may be symptomatic of problems with the Dexible-boom/momentum-wheel configuration.

The Commandable Rate Scanner: Precision Attitude Sensing for Spinning Spacecraft

David Skulsky — Wed, 28 Aug 1991 15:30:00 PDT

High performance Earth sensing for spin-stabilized vehicles can be achieved through a straightforward modification to a flight-proven conical Earth horizon sensor. The necessary hardware and therefore, cost and weight is essentially identical to hardware used by three-axis stabilized satellites making precision attitude sensing available to the entire spacecraft community, including small satellite builders. The Barnes Engineering Commandable Rate Scanner uses a rotating sensor on a spinning spacecraft to provide full-sky (4x steradian) Earth coverage with a single sensor. The sensor provides data for any spacecraft attitude and altitude from LEO to above GEO. In LEO, a data rate of several hundred horizon crossings per minute is typical, allowing for extremely accurate (better than 0.02 deg) attitude sensing. In addition, the principal biases which normally dominate attitude determination accuracy for spinning spacecraft are either eliminated or can be measured. Computational complexity is low, and example algorithms for sensor data processing are provided. Simulation results show that the sensor performs well even in the presence of substantial spacecraft nutation.

A Deployable Stiffened Biconvex Mast

Nancy Holt et al. — Wed, 28 Aug 1991 15:30:00 PDT

Deployable structures are a common spacecraft configuration feature. Deployables are used to configure spacecraft inertia properties, provide expanded antenna aperture and solar array area, or to provide separation between spacecraft components. Lightsat design would be especially enhanced by small size lightweight deployable structures. A review of the state of the art of deployable structures has been conducted and shows that these devices can be divided into two categories: one, rollable mast of the stem, bistem, or stacer types; and two, folding trusses. The rollable masts are generally of open section and susceptible to thermal excitation. Folding trusses are not available in small size. To address these design challenges, a rollable closed section biconvex mast was design, constructed, and experimentally evaluated. The mast was constructed of composites to obtain high stiffness and low weight and to exploit the relative ease with which a shaped cross section can be fabricated. The mast has a lenticular cross section that can be flattened to allow it to be stowed around a drum. Once on orbit, the mast is unrolled and opens up to the lenticular cross section. A novel stiffener has also been developed which increases the capability of the section. Design characteristics and test results are presented.

Electrical, Electronic, and Electro-mechanical Parts for Space Flight Use, A Review

Gerard Kiernan — Wed, 28 Aug 1991 15:15:00 PDT

This paper will review the various philosophies (e.g., established reliability, Class S) used in developing the military and NASA specifications for the various types of electrical, electronic, and electromechanical parts. The paper will show how the specification requirements can be combined with applications guidelines, such as derating tables given in various NASA and military documents, to choose appropriate parts to meet the reliability goal of the particular space flight project.