Introduction| Introduction |
Table of Contents
1.
Introduction
2. History
3. Working Group
4. Objectives and Mission Profiles
4.1 Alouette 1
4.2 Explorer 20
4.3 ISIS X
4.4 ISIS 1
4.5 ISIS 2
5. Summary of Technological Acomplishments
6. Unique Aspects of the Alouette-ISIS Program
7. References
1. Introduction
The Alouette-ISIS program is the one of earliest1 and probably one of the best examples of international cooperation in space research by the National Aeronautics and Space Administration (NASA). Specific directives for such cooperation were included in the Congressional Act2 which created the National Aeronautics and Space Administration (NASA) on July 29, 1958. When this legislation was enacted, the United States was one of 66 nations engaged in an unprecedented joint effort to understand the earth and its environment under the programs of the International Geophysical Year (July 1, 1957, to December 31, 1958). It is not surprising therefore that the Space Act of 1958 reflects the IGY spirit of international cooperation. One highlight of the IGY cooperation was the very successful U.S./Canadian rocket program conducted at Fort Churchill, Canada, which, in a sense, was a precursor to the Alouette-ISIS program.
A joint U.S./Canadian effort to investigate the global structure of the upper ionosphere was initiated at the end of 1958. The basic concept of the experimental approach was to explore the upper (or topside) ionosphere from a satellite by the same ionosonde (or sounder) technique which had been used for several decades from the ground. The satellite version of the ionosonde became known as the topside sounder (Franklin and Maclean, 1969), and, until 1963, the related U.S./Canadian effort was named the Topside Sounder Program. This program led to the first topside sounder satellite, Alouette3 launched on September 29, 1962. This satellite was designed and built in Canada. The launch vehicle was provided by the United States. On December 23, 1963, Canada (the Defense Research Board) and the United States (NASA) agreed to a continued and expanded program of satellite studies of the ionosphere (International Satellites for Ionospheric Studies, ISIS). The expanded program led to three additional Canadian-built, U.S.-launched, satellites: Alouette 2, ISIS 1, and ISIS 2. The United Kingdom was involved in the program from the beginning. International participation was increased later to include France, Japan, and Norway after 1966; India, New Zealand, and Australia after 1971; and Finland after 1977. To date, over 50 research groups and agencies have made use of topside sounder data4 to aid or supplement their own research. The topside sounder, however, was only one of many experiments included in the Alouette-ISIS program. The satellites of the series (Alouette 1, Explorer 20, Alouette 2, Explorer 31, ISIS 1, and ISIS 2), particularly the last two, contained a diversity of mutually supporting experiments, selected to provide a more complete understanding of the ionosphere. Collectively, these satellites have provided continuous observations of the topside ionosphere from 1962 until now (1986), representing over 50 satellite-years of ionospheric data.
The overall coordination of the program was provided by an international Working Group, which was named the Topside Sounder Working Group from January 1960 until December 1963 and later was known as the ISIS Working Group. The Working Group has provided (for over 20 years) the framework for the extensive international cooperation which was unquestionably the most important factor for the remarkable success and duration of the program.
2. History
The history of topside sounding appears to have begun in July 1958, when the Space Science Board of the National Academy of Sciences of the United States, under the chairmanship of L. V. Berkner, sent out a request for suggestions for satellite experiments. At a meeting in October 1958, called by H. G. Booker of Cornell University to discuss ionospheric experiments in satellites, a number of groups in the United States and Canada indicated an interest in topside sounding. In particular, this meeting stimulated a proposal from the Defense Research Telecommunications Establishment (DRTE) at Ottawa, Canada, which came to NASA at the end of 1958. NASA accepted the proposal as a joint undertaking between Canada and the United States, each country paying its own costs in the project. Canada agreed to supply the satellite instrumentation and the United States the launch vehicle. The United States also agreed to provide tracking and telemetry support from a number of NASA stations which eventually included (at the time of the Alouette 1 launch) Antofagasta, Chile; College, Alaska; East Grand Forks, Minnesota; Fort Myers, Florida; Quito, Ecuador; St. John's, Canada; South Point, Hawaii; Winkfield, England; and Woomera, Australia. Canada was to establish telemetry stations needed in Canada (at Ottawa, Ontario; Prince Albert, Saskatchewan; and Resolute Bay, Northwest Territories). A joint announcement of this arrangement was made by both countries on April 20, 1959. Canada subsequently assumed full responsibility for the topside sounder spacecraft with the exception of the environmental tests which were conducted at the Goddard Space Flight Center (GSFC).
Concurrently, NASA had requested that the Central Radio Propagation Laboratory (CRPL) of the National Bureau of Standards, Boulder, Colorado, examine the topside sounder proposals received by NASA for scientific merit and engineering feasibility and recommend immediate and long-range approaches to this area of research. In June 1959, a CRPL study report recommended the fixed-frequency system as a first-generation experiment and suggested that DRTE be encouraged to develop its swept-frequency system as a second-generation experiment. This second recommendation was, in fact, a concurrence by CRPL with the decision already reached between NASA and DRTE. NASA accepted the CRPL recommendation to develop the fixed-frequency topside sounder at the same time as the swept-frequency sounder. This project was started in 1960 and placed under the scientific supervision of CRPL. GSFC was made responsible for the NASA management of the two topside sounder projects. In view of the similarity of objectives and techniques in the two projects, a joint working group was set up early in 1960.
The United Kingdom then expressed an interest in participating in the topside sounder program. Under an arrangement of March 1961, the United Kingdom agreed to support the program by operating telemetry stations in the South Atlantic and Singapore. In return for this assistance, the Radio Research Station (RRS) at slough, England, was given immediate access to the topside sounder data.5 The early cooperative agreements between Canada and the United States were extended in 1964 to include the ISIS program. The RRS participation also was extended subsequent to this new agreement. The international participation was increased during 1965 and 1966 to include agencies in France, Norway, and Japan; in 1971 and 1972 to include agencies in India, New Zealand, and Australia; and in 1977 to include Finland. Specific arrangements differed in detail, but basically all these nations supported the program by providing telemetry services and by participating in the reduction and analysis of topside ionograms. The French telemetry stations used in the Alouette-ISIS program include Brazzaville, Congo; Bretigny, France; Colomb Bechar, Algeria; Kerguelen island; Kurou, French Guyana; Las Palmas, Canary Is.; Ouagadougou, Upper Volta; Pretoria, S. Africa; and Terre Adelie, Antarctica. Other nations have provided telemetry services at Tromso (Norway), at Yashima (Japan), at Ahmedabad and Thumba (India), at Lauder (New Zealand), at Darwin (Australia), and at Sodankyla (Finland).
The phenomenal growth of the international participation has resulted in the creation of an ISIS telemetry network (outside of the NASA telemetry network) which became capable of acquiring most of the data desired from the Alouette-ISIS satellites. Consequently, in the early seventies, NASA reduced considerably its telemetry support of the ISIS program. March 9, 1984, was the date when ISIS operations were terminated in Canada. The Radio Research Laboratories (Tokyo, Japan) then requested and received permission to reactivate ISIS 1 and ISIS 2. Regular ISIS operations were started from Kashima, Japan, in early August 1984. The ISIS data processing facilities in Ottawa were kept operational until March 1985.
3. Working Group
The Topside Sounder Working Group, later to become the ISIS Working Group, was organized to coordinate and guide the rather diverse activities involved in planning and implementing an international program of ionospheric sounding from satellites. In Working Group meetings both the scientific goals of the program and the proposed solutions to the associated engineering problems were debated. A valuable consequence has been that the scientific, engineering, and administrative personnel involved developed an understanding of all the important issues.
The Working Group was concerned with the typical scientific, engineering, and operational problems of a satellite project. In addition, the Working Group had to devise, recommend, and carry out preliminary space investigations to establish the feasibility of topside soundings and to obtain a number of design parameters which previously were unknown. For example, it was evident when the project was initiated that the topside sounder would require antennas about one order of magnitude longer than previously had been used on space vehicles. Major advances in the existing technology were needed to build suitable antennas, and the reliability of the proposed system had to be established. Since the space environment could not be suitably simulated on the ground, the Working Group recommended that a special rocket test be conducted to test these unusually long antennas. Also, the power required from the sounder transmitter was unknown because the reflection coefficient of the topside ionosphere and the level of ambient noise at the sounding frequencies were both unknown. A radiometer for measuring the intensity of radio noise in the topside ionosphere at 3.8 MHz was constructed and installed in Transit 2A (Chapman and Molozzi, 1961), which was launched on June 22, 1960. The measured value of the cosmic noise provided the required design information.
On June 14, 1961, a rocket test was made of the technique of extending from a spinning vehicle the 75-ft antennas required for efficient radiation of the sounder transmissions (Molozzi and Richardson, 1967). Two antenna units were flown on the rocket; one of the antennas extended its full length, the other extended three-quarters of its length. The engineering information obtained was adequate for specifying the modifications required in the final mechanical design of the antennas used in the Alouette and Explorer 20 satellites. Feasibility of the topside sounding technique was established by rocket tests instrumented by Airborne Instruments Laboratory (AIL). Each of the rockets carried one or more fixed-frequency sounders. The first was launched on June 24, 1961, during the day into a quiet ionosphere that had smooth reflection surfaces; the second was launched on October 31, 1961, at night into moderately disturbed ionospheric conditions during spread F (Knecht et al., 1961; Knecht and Russell, 1962). The required engineering information was obtained and, in addition, important new phenomena were observed. The first of these rockets obtained evidence of plasma resonance phenomena; the second provided the first strong evidence of ducted propagation along the magnetic field in the ionosphere. A third rocket, launched to observe the properties of the topside winter ionosphere, failed; the heat shield did not detach.
The Working Group has also contributed significantly to the optimum use of the Alouette-ISIS satellites, by coordinating the acquisition, processing, publication and exchange of the data. Periodic reporting by the member organizations of technical and scientific developments has provided the information necessary for effective collaboration between experimenters, especially between those associated with different experimental equipment. Fifty Alouette-ISIS Working Group meetings were held during the period January 1960 to September 1972. A comparable number of separate engineering or experimenters' meetings have also been held, representing a more specialized extension of the Working Group activities. During the period January 1973 to December 1980, Working Group meetings were held approximately once a year, and Experimenters' meetings were held about three times per year.
4. Objectives and Mission Profiles
The broad objectives of the Alouette-ISIS program were to conduct a comprehensive synoptic study of the topside ionosphere over a complete range of solar activity, and to provide the basis for a theoretical understanding of the observations. The discussion of objectives and mission profiles does not include detailed descriptions of spacecraft and experiments. The present discussion is concerned primarily with the scientific purposes of the various missions, with the various technological problems encountered, and with the methods used to solve these problems.
4.1 Alouette 1 (launched September 29, 1962)
The primary purpose of the Alouette 1 mission was to investigate the geographic and diurnal properties of the topside ionosphere at altitudes up to 1000 km. These considerations led to the choice of a circular orbit at 1000 km with an inclination of 80 degrees prograde which provided a complete coverage of all geomagnetic latitudes, while making it still possible to achieve a diurnal variation in a period of 3 months. Design parameters for the sounder were based partly upon existing knowledge and partly upon additional space experiments conducted primarily to supply the needed design data. The maximum sounder frequency was based upon the known maximum densities to be encountered at hmaxF2, while the minimum sweep frequency was essentially the minimum gyrofrequency at 1000 km. This made it theoretically possible to obtain echoes through the total altitude range on at least the extraordinary mode. Secondary objectives included cosmic noise measurements, VLF studies and energetic particle investigations (electrons in the 40 keV to 3.9 MeV energy range; protons in the 0.5 to 700 MeV energy range).
Satellite technology was still in its infancy when the design of Alouette 1 was initiated by the Canadian Topside Sounder Group at DRTE, Ottawa. To optimize the chances of success, undue complexity was avoided in the payload design, and redundancy of vital components was stressed. Thus, data storage was not provided in the spacecraft, but spare batteries were included. The conservative approach used in the design led to the remarkable 10-year life of Alouette 1. One consequence of the decision not to have satellite-borne data storage was that provisions had to be made for a large network of telemetry stations. Operation of the satellite (for 10-minute periods) was initiated by a command signal from the ground, when the satellite came within the telemetry range of a ground station and when the turn-on had been scheduled by the satellite controller. The master ground station was at DRTE (now CRC6), Ottawa, and it was there that the satellite controller monitored the engineering status of the spacecraft and prepared operating schedules. The Ottawa station also transmitted the more complex commands such as those required to switch spare batteries or to select certain operating modes.
One complexity which could not be avoided was the sounder antenna system which had to be capable of radiating efficiently signals in the frequency range from 0.5 to 12 MHz. To satisfy this requirement, extremely long antennas had to be provided. A dipole 45.7 meters long was used for the band 0.5 to 5 MHz, while one 22.8 meters long was used from 5 MHz upward. The two dipoles were perpendicular to the spin axis and to each other. Such long antennas had never been used previously on a satellite, and the successful mechanical design of these antennas represents a unique and major contribution to the field of spacecraft engineering (Mar and Garrett, 1969).
Alouette 1 was spin-stabilized with the spin axis (at the time of launch) normal to the plane of the ecliptic. The initial spin rate after antenna deployment was 1.4 rpm. The spin rate, however, decreased at a much faster rate than expected and it was down to 0.9 rpm at the end of 1 year (Mar and Garrett, 1969). After a few years, this rapid decay in spin rate caused Alouette 1 to become gravity-stabilized with the long antennas aligned with the local vertical. The spin rate decay did not, however, cause a significant loss of data.
4.2 Explorer 20 (launched August 25, 1964)
The fixed-frequency topside sounder satellite, Explorer 20, was developed in the United States as a part of the International Topside Sounder Program. The spacecraft was built at the Airborne Instruments Laboratory (AIL), Deer Park, N.Y., and the data analysis was done at CRPL. Explorer 20 was conceived initially as a first-generation topside sounder satellite because of its simplicity relative to Alouette 1. The Explorer 20 sounder was designed for operation at only six fixed frequencies, namely 1.5, 2.0, 2.85, 3.72, 5.47, and 7.22 MHz. These frequencies were spaced logarithmically to optimize the resulting low-resolution sampling of the exponentially shaped topside profile. The CRPL/AIL sounder was designed to complete its 6-frequency sounding in 1/10 s, during which time the satellite would travel less than 1 km along its orbit. The time required for a complete sounding on Alouette 1 was about 5 to 10 s, corresponding to a horizontal displacement of 35 to 70 km. Thus, the Explorer 20 sounder was designed to provide a horizontal resolution considerably greater than that of Alouette 1. The frequency resolution (or, equivalently, the vertical resolution), however, was two orders of magnitude greater on Alouette 1, since approximately 700 discrete and closely spaced frequencies were used on Alouette 1 during the complete 0.5 to 12.0 MHz sweep. Clearly, the two sounder techniques were complementary.
The Explorer 20 spacecraft was built on a schedule paralleling that of the Canadian Alouette 1 spacecraft, and the Explorer 20 launch was planned for the summer of 1962, i.e., slightly ahead of the Alouette 1 launch. The Explorer 20 program, however, was delayed by problems with the Scout launch vehicle, and the Canadian satellite was launched first. The fixed-frequency topside sounder was continued in spite of the successful launch of Alouette 1 and the excellent in-orbit performance of the Canadian sounder, because of the complementary nature of the two sounder techniques.
Explorer 20 was launched into an orbit similar to that of Alouette 1 and it has provided useful data for the period August 1964 to January 1966. The fixed-frequency sounder has yielded data on the fine structure of ionospheric irregularities and plasma resonances which are impossible to obtain with a swept-frequency sounder. The fixed-frequency sounder data also could be used to calculate the electron-density (N) as a function of height (h) (Lawrence and Hallenbeck, 1965). The Alouette 1 ionograms, however, were plentiful7 and much better suited for N(h) calculations. Consequently, the data from the Explorer 20 sounder were used almost exclusively to study small-scale ionospheric irregularities and the fine structure of plasma resonances.
The Explorer 20 spacecraft also included a spherical ion probe designed to measure ion concentrations and ion temperatures in the immediate vicinity of the satellite. Experiments of this type that provide various parameters of the ambient medium (such as density, temperature, and composition) by using sensors at the satellite surface are known as direct measurement experiments. These experiments require that local disturbances be minimized in the vicinity of the sensors. These disturbances are due primarily to electrical potentials on the surface of the spacecraft. Theoretical considerations indicated that these effects might be sufficiently minimized at the sensor locations by the use of blocking capacitors at the antenna feed points, which would isolate the VxB potentials induced on the large antennas.
The spherical probe on Explorer 20 was the first attempt to combine direct measurements with topside soundings on a satellite. Although ac-coupling was used to connect the sounder antennas to the spacecraft, this proved to be inadequate and most of the spherical probe data were unusable. The topside sounder project, however, benefited from this experience, and sheath guards were added to the sounder antennas on Alouette 2.
4.3 ISIS X (launched November 29, 1965)
The primary purpose of the ISIS X mission was to extend the scope of the Alouette 1 mission both in altitude coverage and in the number of ionospheric parameters to be investigated. Secondary objectives included cosmic noise measurements, VLF studies, and energetic particle investigations. The new ionospheric parameters to be measured included electron temperature, ion temperature, and ion composition. These parameters are most readily obtained using direct measurement techniques, and a sufficient number of these techniques were incorporated in the ISIS X mission to provide at least two independent measurements of each parameter. As explained earlier, a difficult spacecraft potential problem had to be overcome in order to conduct successful direct measurements on a satellite containing a topside sounder. Although the blocking capacitors used on Explorer 20 between the spacecraft and the antennas did not solve this problem, the use of these capacitors was a step in the proper direction. Additional precautions taken in the ISIS X design included the use of sheath guards on the sounder antennas and the use of insulation on the spacecraft skin of all exposed metallic surfaces with nonfloating potentials (such as the solar cell interconnections).
Modifications also were made to the antenna system in an attempt to correct the excessive spin rate decay experienced on Alouette 1. A theoretical study by Etkin and Hughes (1967) indicated that the observed spin decay on Alouette 1 could be explained by taking into consideration the flexibility of the long antennas. When this was done, additional de-spin torques were obtained from the action on the antennas of (1) the combination of the thermal and pressure fields of the sun, and (2) the combination of the thermal field of the sun and atmospheric drag. The temperature difference between the sunlit side of the antenna and the shadow side causes differential expansion and bending of the antenna. As a result, the center of mass and the center of pressure separate, allowing the Alouette satellites to experience torques due to solar radiation pressure and also due to atmospheric drag when below 1000 km. To counteract the solar radiation de-spin torque on ISIS X, highly reflective end plates were installed on the ends of the long antennas. The high reflectivity ensured that most of the incident radiation was reflected specularly, resulting in a net spin-up torque on the satellite.
The ISIS X objectives required that the sounder experiment be extensively modified. Since the sounder would operate during a period of increasing solar activity, the maximum density in the ionosphere would be greater than for Alouette 1, and the highest frequency of the sounder had to be raised to 13.5 MHz. On the other hand, the much higher apogee planned for ISIS X would bring the sounder into regions of much lower electron density and magnetic field, requiring that the lowest frequency of the sounder be reduced from 0.5 to 0.2 MHz. The decision to reduce the lower frequency limit required in turn that the length of the longer sounder antenna be increased from 45.7 to 73.0 meters.
The ISIS X mission was designed to test the spacecraft modifications outlined above, while at the same time ensuring that the basic scientific objectives would be met. This was accomplished by launching two satellites simultaneously into the same orbit (3000 km apogee, 500 km perigee, 80 degree inclination): Alouette 28 (a modified version of Alouette 1) and Explorer 319 (Direct-Measurements Explorer A or DMEA), a spacecraft of shape and size known to be suitable for local sensing of ionospheric parameters. For the safe placement in orbit and deployment of spacecraft appendages, the two ISIS X spacecraft had to be provided with a small but sufficient separation velocity. A separation velocity of 8.75 km per day was achieved, resulting in a period of about 4 months during which the two spacecraft could perform essentially simultaneous measurements. This "close proximity" period turned out to be five times longer than specified. One of the Explorer 31 temperature probes was duplicated on Alouette 2, and comparison between the two identical experiments showed that the Alouette 2 structure would be satisfactory for direct sensing experiments.
In keeping with the conservative approach used to ensure the success of the ISIS X mission, undue complexity again was avoided and redundancy of critical components was emphasized. For example, there was no provision for data storage on either Alouette 2 or Explorer 31, but spare batteries were included in both spacecraft. The lack of data storage facilities in the ISIS X spacecraft required that a large network of telemetry stations be used with Alouette 2 and Explorer 31. The ISIS X mission utilized essentially the same telemetry network as that used for Alouette 1. In some cases, the telemetry station capability had to be increased to permit simultaneous command and telemetry of Alouette 2 and Explorer 31. The Alouette 1 master station at DRTE, Ottawa, was used also as the Alouette 2 master station. It was at DRTE that Alouette 2 housekeeping data were obtained and more complex commands were executed. The master station for Explorer 31 was the APL station at Laurel, Maryland. The satellite controller at the APL station monitored the operational status of Explorer 31 and issued the special commands required for housekeeping and attitude control.
To optimize the direct measurements on Explorer 31, an elaborate magnetic attitude stabilization and control system was included on this satellite. The magnetic spin-up system was designed to maintain a 3-rpm spin rate with less than a 10 percent duty cycle of the attitude control system. The spin-axis orientation system was designed to maintain the spin axis orthogonal to the orbital plane. The detectors were mounted perpendicular to the spin axis.
The ISIS X mission achieved all of its scientific and technological objectives. The modifications made to the Alouette 2 antenna system reduced the spin rate decay by one order of magnitude, showing that the highly reflective end plates installed on the long Alouette 2 antennas had effectively counteracted any rapid de-spin of this satellite. The success of the ISIS X compatibility test showed that the Alouette 2 structure was satisfactory for direct measurements. The next step was to combine all the measurements on a single spacecraft. This was done on ISIS 1.
4.4 ISIS 1 (launched January 30, 1969)
The objectives of ISIS 1 were to make measurements similar to those of ISIS X during a period of maximum and declining solar activity. The selected ISIS 1 orbit (3500 km apogee and 565 km perigee), therefore, was similar to that of the ISIS X satellites. The ISIS 1 objectives had to be accomplished with a single spacecraft instead of the two satellites required for the ISIS X mission. One advantage of the single spacecraft approach was that simultaneous measurements would no longer be limited to a few months of "close proximity" as was the case for ISIS X. In fact, based upon the performances of Alouettes 1 and 2, it seemed likely that the ISIS 1 mission could provide simultaneous measurements for several years. Therefore, comprehensive ionospheric data were desired from ISIS 1 over essentially the same altitude range and geographic areas as those selected for the ISIS X mission. In addition, data were also desired from several very large areas of the world which could not be explored by the previous satellites of the series, since only real-time telemetry was available for the earlier missions.
The ISIS 1 spacecraft included basically the same experiments as those of ISIS X. In addition, it contained a fixed-frequency sounder similar to that of Explorer 20, a VLF transmitter used to excite various VLF phenomena in the vicinity of the spacecraft, a Beacon experiment, and instrumentation to measure electrons and positive ions in the 10 eV to 10 keV energy range. This last addition represented both an increased awareness of the importance (to ionospheric phenomena) of particles in this energy range and an increased emphasis on the development of suitable technology to make the required measurements. The ISIS 1 objectives required a -spacecraft far more complex than the earlier spacecraft of the Alouette-ISIS program. The ISIS 1 satellite was the first of the Alouette-ISIS series to incorporate the following features: (1) swept- and fixed-frequency sounder techniques combined with a complete set of direct measurements; (2) active spin maintenance and spin-axis attitude control; and (3) onboard data storage. To meet these new requirements the spacecraft design used on Alouette 2 had to be extensively modified. New facilities had to be added to the ISIS 1 spacecraft, and the capabilities of earlier facilities had to be greatly expanded.
When the design of the ISIS 1 spacecraft was initiated (1964), the results of the ISIS X mission were not yet known. However, the spacecraft potential problem was sufficiently understood to expect a successful compatibility test on ISIS X and to proceed with plans for a complete selection of direct measurement experiments on ISIS 1. The theory of the antenna spin decay on Alouette 1 was still in a tentative stage, and even if the passive spin decay compensation planned for Alouette 2 were successful, changes in spin-axis orientation were a certainty and these changes would be excessive for direct measurement experiments. Active spin rate and attitude controls, therefore, were incorporated in the ISIS 1 spacecraft. Magnetic torquing techniques were used to control the spin rate within the range 1 to 3 rpm and to correct the spin-axis attitude (when necessary) at a rate of 3 degrees per orbit.
The spacecraft tape recorder was a 4-track unit capable of storing data from all ISIS 1 experiments simultaneously for several periods, for a total of 64 min. The playback data were telemetered to the master ground station at DRTE. The playback speed was four times the recording speed. In order to acquire data over locations inaccessible to the ground-based telemetry network, it was necessary to provide an onboard programmer that could switch on the desired experiments and the tape recorder at pre-selected times. A total of five commands could be stored together with their times of execution. These commands could be selected from a group of 10. The actual times at which the data were obtained by the tape recorder were provided by a clock that could be reset. The greater number of experiments to be controlled on ISIS 1, the addition of a programmer and clock, and provisions for active spin and attitude controls required that the command capability be expanded from the 24 commands used on Alouettes 1 and 2 to the 216 commands used on ISIS 1.
The primary data acquisition system for ISIS 1 continued to be the telemetry network used for Alouette 1 and for ISIS X. The tape recorder data were intended to be supplementary and obtainable without causing a reduction in the primary data acquisition. This was achieved by providing an additional telemetry link operating at 400 MHz and having a bandwidth of 500 kHz. This wide-band telemetry system also could be used to transmit real-time sounder or VLF data in the event of a failure of the wide-band 136 MHz telemetry link. Finally, to operate the additional experiments and spacecraft systems, the power system had to be greatly enlarged. The number of solar cells (n-on-p type) was increased from 6480 (Alouettes 1 and 2) to 11,000 (ISIS 1). It is seen from the above discussion of the spacecraft experiments and systems that the ISIS 1 satellite was much more complex than its predecessors. The greater complexity also resulted in a significant weight increase from 145 kg (Alouettes 1 and 2) to 241 kg (ISIS 1).
Another very important difference between ISIS I and the Alouette satellites was in the basic approach used for the design and construction of the ISIS 1 spacecraft. With the exception of the sounder antennas which were manufactured by de Havilland Aircraft of Canada, Ltd., and with the exception of some standard components available commercially, Alouettes 1 and 2 were both completely designed and built at DRTE, a laboratory operated and staffed by the Canadian government. The ISIS 1 spacecraft, on the other hand, was built almost entirely by Canadian industry under contract to DRTE. The prime contractor was RCA Victor (RCAV), Ltd., Montreal. SPAR Aerospace Ltd., Toronto, provided the mechanical structure and the sounder antennas. Thus, with the ISIS mission, private industry in Canada became a major participant in the Canadian space program. The overall management of the ISIS 1 mission, however, remained at DRTE. A close cooperation established between DRTE and RCAV ensured that RCAV derived maximum benefit from the extensive experience with space technology available at DRTE.
ISIS 1 has now (1986) been operating for over 17 years, and its longevity has exceeded the 10-year records of Alouettes 1 and 2. Because of its long life, ISIS 1 was also able to provide data during the 1975-1976 sunspot minimum and to participate in the IMS program (International Magnetospheric Study, January 1, 1976, to December 31, 1979). The ISIS 1 satellite was the oldest of the 27 IMS satellites.
4.5 ISIS 2 (launched April 1, 1971)
The official Canadian/U.S. statement of the ISIS 2 mission was as follows: "To inject the spacecraft into a near circular earth orbit which will permit the study of the topside of the ionosphere above the electron peak of the F region. To continue and extend the cooperative Canadian/U.S. program of ionospheric studies initiated by Alouette 1 by combining sounder data with correlative direct measurements for a time sufficient to cover latitudinal and diurnal variations during a period of declining solar activity."
An eccentric orbit such as the one selected for ISIS 1 was excellent for the exploratory purposes of the ISIS 1 mission, but it was not well suited for the ISIS 2 mission which stressed the study of latitudinal effects. When direct measurements were made on ISIS 1, it was often very difficult to separate latitude and altitude effects. The complete latitudinal (pole-to-pole) variation of the vertical electron-density distribution could not be satisfactorily derived from the ISIS 1 sounder data. At perigee, the sounder data were very limited in altitude range; and at apogee, they suffered from poor resolution. A circular orbit at 1400 km, which avoids these problems, was found best suited for the ISIS 2 mission.
To fulfill the primary objectives, measurements were planned to study: (1) the distribution of free electrons and of the various species of ions as a function of time and position; (2) ionospheric irregularities such as spread-F and field-aligned ionization; (3) the composition and fluxes of energetic particles that interact with the ionosphere; and (4) the velocity distribution of thermal electrons and ions. The ISIS 2 spacecraft included basically all the ISIS 1 experiments plus two new ones. Of the 10 experiments similar to those on ISIS 1, eight were almost identical and two provided essentially the same information as their ISIS 1 counterparts but with different instruments. Many of the additional objectives of ISIS 2, therefore, were similar to those of ISIS 1, including cosmic-noise measurements, VLF studies, and energetic particle investigations. The two new experiments were designed to study atmospheric optical emissions at 6300, 5577, and 3914 A. The optical experiments also required a circular orbit and attitude control.
Due to budgetary constraints, the design changes on ISIS 2 were kept to a minimum. Consequently, as many systems and units as possible from ISIS 1 were incorporated in the design of the ISIS 2 spacecraft. Thus, attitude control, telemetry, command, data storage, and antenna and power systems on ISIS 2 were essentially the same as those of ISIS 1. A few changes were made in the sounder design to increase accuracy (onboard range marker and amplitude calibration), output power (two 400-W power amplifiers) and versatility (mixed mode, VLF/sounder mode, AIT mode). The AIT (Automatic Ionogram Transmission) mode allowed for the automatic operation of the sounder system once every 3 minutes. This gave an opportunity for small institutions to acquire topside ionograms with low-cost telemetry stations. The scope of the VLF experiment was increased to include antenna impedance measurements (sounder short dipole antenna measurements). The addition of two experiments and the various new features added to the previous experiments led to a slight increase in spacecraft weight from 241 kg on ISIS 1 to 256 kg on ISIS 2. The ISIS 2 spacecraft was built by Canadian industry under the same contractual arrangements as were used for the design and construction of ISIS 1.
The ISIS 2 mission was initiated in 1971 during a period of declining solar activity, and it has continued through the subsequent 1975-1976 sunspot minimum. In order to give the various ISIS 2 experiments the opportunity to acquire data under their "optimum operating conditions," the orientation of the spacecraft axis has been changed periodically from an orientation perpendicular to the orbital plane (cartwheel orbit) to an orientation parallel to the orbit plane (orbit aligned). It took about 10 days to accomplish these orientation changes. During this 10-day period, the orientation was unfavorable for most of the experiments, and the power available for experiments was significantly reduced. The orientation was changed typically once every 3 months. Thus about 10 percent of the total operational life of the satellite was spent for orientation maneuvers. The ISIS 2 satellite has now (1986) been operating for over 15 years, and it has participated in the IMS program with 10 of the 12 experiments still fully operational and with all spacecraft systems (except for the data storage capability) also fully operational. The ISIS 2 satellite has provided a unique and most comprehensive combination of experiments for ionospheric, auroral, and magnetospheric studies.
5. Summary of Technological Accomplishments
The scientific and technological objectives of the Alouette-ISIS program were met and exceeded in all five missions. The technological accomplishments include:
6. Unique Aspects of the Alouette-ISIS Program
For many reasons, the Alouette-ISIS program is probably the most outstanding of the international programs of NASA. The efforts of the unusually competent and dedicated members of the Canadian team, together with the wholehearted support of their U.S. counterparts, led to Canada's spectacular entry into the space age with Alouette 1 on September 29, 1962. The Canadian space program has since then maintained an unequaled record for overall excellence, in both the scientific and applications areas.
The summary of technological accomplishments given above did not take into consideration the international aspect of the Alouette-ISIS program. This aspect of the program also includes an impressive number of accomplishments.
7. References
1A cooperative effort between NASA and the United Kingdom led to the successful
launching of Ariel 1 on April 26, 1962, making Ariel 1 the first international satellite
of NASA.
2The National Aeronautics and Space Act of 1958, sections 102 and 205.
3The name "Alouette," the French word for a high-flying bird, the lark,
has connotations that extend deep into early Canadian colonial history. It is also the
title of one of the country's best known and most nonsensical folk songs, originally
brought to North America from France many centuries ago.
4Analysis of topside sounder data (topside ionograms) yields ionospheric
electron-density versus altitude from the satellite height down to the height of maximum
electron-density hmaxF2 (located typically at 300 km), as well as a wealth of information
concerning plasma and propagation effects.
5In the sixties, when a new satellite was launched, the scientific data were
considered proprietary to the principal investigators for a reasonable period of time,
usually 1 year. After the proprietary period the data were usually made available to the
scientific community. Participants in the Alouette-ISIS program were, in effect, given
principal investigator status.
6DRTE was transferred in April 1969 to the newly formed Department of
Communications, and DRTE became known as the Communications Research Centre.
7During the first 3 years in orbit, Alouette 1 was providing topside ionograms at
the rate of 1100 ionograms per day.
8Alouette 2, like Alouette 1, was designed and built at DRTE, Ottawa.
9Explorer 31 was designed and built at the Applied Physics Laboratory (APL),
Silver Spring, Maryland.
10J.E. Jackson and R. N. Horowitz, "Data Catalog Series for Space Science and
Applications Flight Missions, Volume 3B, Descriptions of Data Sets from Low- and
Medium-Altitude Scientific Spacecraft and Investigations," NSSDC/WDC-A-R&S 86-01,
April 1986.
Excerpted from document NSSDC/WDC-A-R&S 86-09
Reprinted courtesy of the National Space Science Data Center
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