The spacecraft was originally built for launch by the Space Shuttle, but was modified for launch on the Atlas I vehicle after the Challenger accident. These modifications included the removal of a large orbit transfer stage and removal of one-half of the original chemical canister payload. The orbiter cradle was replaced with a payload adapter to mate with the Centaur upper stage of the Atlas I. The solar panels were relocated to fit into the 14-ft diam Atlas I fairing. The initial CRRES orbit was 350 x 33,584 km with an inclination of 18.1°. The initial apogee altitude was approximately 2000 km lower than the targeted geosynchronous altitude of 35,786 km. Following orbit insertion and just prior to separation from the Centaur, CRRES was oriented with its spin axis lying in the ecliptic plane and pointed 12° ahead of the Sun's apparent motion and spun up by the Centaur to its nominal initial spin rate of 2.2 +- 0.2 rpm.
CRRES was acquired by the Air Force Satellite Control network (AFSCN) Indian Ocean Tracking Station approximately 40 min after launch. Initialization and checkout of the vehicle subsystems and instrument payloads, including boom deployments, was accomplished on schedule, within 30 days after launch. Prior to deployment of the long wire booms and the Astromast boom, the vehicle was spun up to 20 rpm; these booms required the centrifugal force of the higher spin rate for the deployment. After the booms were fully deployed the vehicle was spun down to its nominal spin rate of 2.0 rpm, using a sequence of phased spin-down maneuvers. Separating the spin-down pulses by one-half of a boom swing period cancelled the side-to-side motion of the booms, significantly reducing the minimum time required between successive maneuvers.
Normal on-orbit operations began immediately after initialization and checkout was completed. Nominally the majority of the science instruments remain powered and active except during long occulation periods during which duty cycling of selected instruments is required. Normal spacecraft operations include maintaining the spin axis between 5 and 15 ° from the Sun; autonomous battery charge control by an onboard power control unit and both passive and active thermal control for maintaining the spacecraft temperatures within their specified limits.
The specified mission duration was 1 year with a goal of 3 years. During this time the CRRES will travel through the severe radiation environment of the Earth's inner and outer radiation belts. There are three primary mission objectives:
The primary focus of these studies is on the natural radiation environment and the effects of this environment on microelectronic components. CRRES is traveling through the inner and outer radiation belts of the Earth, exposing state-of-the-art microelectronic components to this radiation environment to establish their capabilities for use in future space missions. Also, the radiation belts are being accurately mapped so that a direct correlation can be made between the exposure and microelectronics performance. More than 40 instruments are operating to support these studies. These include an experimental new generation of high-efficiency solar panels and instruments which are investigating the effects of solar flares and cosmic rays on the Earth's magnetosphere and radiation belts.
The CRRES payload complement included 24 chemical canisters, 16 large and 8 small, which were released during the first 13 months of the CRRES mission at altitudes varying from near apogee and near perigee over ground observation sites and diagnostic facilities. These releases formed large clouds of metal vapor, about 100 km in diameter, which interacted with the ionospheric and magnetospheric plasma and the Earth's magnetic field. These releases were studies with optical, radar, and plasma wave and particle instruments from the ground, aircraft, and CRRES. The three chemical release campaigns were:
LASSII is studying naturally occurring and artificially produced ionospheric perturbations and the effects of ionospheric perturbations on communication paths. The LASSII measurements are being made near perigee of selected orbits. In addition, LASSII made observations of the low-altitude chemical releases. The onboard set of LASSII instruments consists of two pulsed plasma probes, a very low frequency wave analyzer including two electric field antennas and magnetic hop antenna, and a quadruple ion mass spectrometer.
Perturbations to the CRRES orbit have played an important role in the design and planning of the CRRES mission. Specifically, perturbations due to the Earth's oblateness (J2 perturbations) cause cumulative secular variations (i.e., increasing with time) in the argument of perigee and the right ascension of the ascending node. These variations, coupled with the apparent 1°/day motion of the Sun, result in a new rotation of orbit perigee and apogee toward earlier local time, as the mission proceeds. Apsidal rotation also produces a periodic variation (36° peak to peak) in the latitude of perigee with a period of ~525 days. These two motions, given the initial local time of apogee, determined when and where, in local time and latitude, significant mission events such as the CRRES chemical releases occurred.
Third body influences of the Sun and Moon, along with atmospheric drag, cause periodic and secular variations in the semimajor axis, eccentricity, and inclination. Third body effects and atmospheric drag are highly coupled and can have a dramatic effect on the stability of high eccentricity orbits, especially those slightly more eccentric or inclined than CRRES. Thousands of orbits in the neighborhood of the CRRES orbit were investigated in a study of high-eccentricity orbit stability and evolution. No eccentric re-entries were found to be possible for the range of CRRES orbits of interest.
The CRRES is composed of two basic components: the satellite and the payload adapter. The adapter interfaced with the launch vehicle both mechanically and electrically. The total weight of the CRRES system at launch, including the adapter, was 1,753 kg. The satellite weight at launch was 1,716 kg, whereas the total payload weight was 678 kg including the chemical canister payload. The total weight of the 24 chemical canisters including chemicals and release control units was 425 kg.
The satellite consists of the structure; deployable mechanisms (booms and chemical canisters); the telemetry, tracking and command (TT&C) subsystem; the electrical power and distribution (EPDS) subsystem; the attitude determination and control (ADCS) subsystem; the thermal control subsystem; the chemical module/canister assembly subsystem; and the scientific payloads. Because of the sensitivity of the scientific payloads, very stringent electromagnetic compatibility and magnetic cleanliness controls were maintained on the spacecraft.
Contact with the CRRES spacecraft was lost on 12 Oct 1991 and was presumed to be due to onboard battery failure.
The magnetic electron spectrometer on CRRES, also known as the MEA (Medium Electrons A), has a long history. It was originally built in 1968 as a backup for the magnetic electron spectrometer flown on the U. S. Air Force scientific satellite OV1-19. At that time, it was completely checked out and calibrated, ready for flight. When the OV1-19 was successfully launched in 1969, the backup instrument was put into storage. The MEA originally covered the energy range 300 keV to 5.2 MeV in sixteen differential channels, each using a discrete lithium-drifted silicon detector. A seventeenth detector measured penetrating particle and bremsstrahlung background in two separate channels. In the early seventies, the instrument was modified to make measurements on a rocket to be launched in the auroral zone. The modification consisted of a reduction of the magnetic field of the instrument from 2.1 kilogauss to 850 gauss (to lower the energy range of the instrument), a corresponding reduction in the electronic thresholds set on each detector channel, and milling down of the chamber wall to lighten the instrument. With the reduction in magnetic field, the thickness of the yoke could be reduced. The rocket launch location was changed to a low-latitude site where there was no possibility of observing electrons. The instrument was removed from the rocket and placed back into storage. In 1983, it was removed from storage, the electronic calibrations were rechecked, and the instrument was left powered-up for a two-year period during which noise levels in the detectors were checked periodically. Power consumption was only 650 mw, so the long 'burn-in' was innocuous. In 1986, the instrument was calibrated with electrons at the NASA Goddard tandem Van de Graaff accelerator and delivered to Ball Aerospace for integration into the CRRES spacecraft.
At the time the instrument was recalibrated at Goddard, testing of suspected count rate limitations in lithium-drifted silicon detectors was also performed. These tests disclosed that some inherent mechanism in the detector itself restricts count rates to the order of 25000 c/s-cm2. For the MEA, this was a limitation of about 40000 c/s per channel, since each of the detectors were 1 cm x 1.5 cm. With the Challenger disaster and the 3 year delay in launch of CRRES, a decision was made to replace the lithium-drifted detectors with ion- implanted silicon detectors which have no such inherent limitation in count rate. Since the new detectors required a bias voltage of -100v as compared to the +75v that the lithium-drifted detectors used, a new power supply was required. Also, the new detectors produced a pulse of opposite polarity from that of the previous detectors, so all of the electronics had to be replaced. Thus, the instrument finally flown on CRRES, while originally built in 1968, retained only the original magnetic chamber, magnet, and collimator. The instrument, as flown, will be described below.
In a 180° magnetic electron spectrometer, particles entering an aperture encounter a uniform solenoidal magnetic field and travel a circular path in the plane transverse to the field. After being bent through 180°, the particle is detected by a planar array. First order focussing occurs in the plane. There is no focussing in the vertical plane. The focussing in the transverse plane occurs because the length of a chord subtending angles near 180° in a circle does not change rapidly with a change in the angle subtended (the chord is similar to the diameter in length). Thus, the MEA instrument incorporates a sensor design which had had extensive use in low energy nuclear research laboratories through the early years of the atomic age and is very well under- stood. The measurement principle used is momentum analysis in a solenoidal magnetic field. In a uniform magnetic field, the radius of curvature of a charged particle, r, is defined by the charge on the particle, the mass of the particle, the velocity, and the component of the magnetic field perpendicular to the motion of the particle:
where B is the transverse magnetic field, m is the mass of the particle, q is the charge on the particle, and v is the particle velocity. This equation just equivalences the electric force on the charge q due to motion across a magnetic field B with the centrifugal force on a particle of mass m and velocity v gyrating in a circle with radius r. Thus, in a magnetic electron spectrometer, the energy analysis is done by geometric means and the information derived from the energy deposit in the detector can be used for other purposes. In the case of the MEA, this information is used to increase the efficiency of detection to approximately 100% and reduce penetrating particle backgrounds (cosmic rays, energetic protons in the inner Van Allen zone).
Since the instrument is well understood, the geometric factors, the energy responses, and the efficiencies of the individual channels can be determined with very good accuracy through computational means buttressed by judicious tests incorporating electron beams. In the usual case, the magnetic field in the chamber of a magnetic electron spectrometer is quite uniform. The two dimensional angular response can be checked quite accurately and the energy cutoffs of each channel can be determined with a precision that is limited only by the energy spread of the particle beam used in the determination. As a result, data from magnetic electron spectrometers can be used for absolute calibrations.
Figure 1 shows a schematic outline of the MEA analyzing chamber. The chamber consists of two halves which were milled out of Armco magnetic iron (a low coercive force oxygen-free material). The low coercive force material provides a relatively low weight yoke with a low fringing field. The permanent magnet utilized is made from Indox V, a ceramic material with high coercive force. A high coercive force is needed to ensure stability in the field intensity throughout launch vibration, temperature variations, rotation in the Earth's magnetic field, etc. The Indox V is stabilized by disassembling the yoke after the Indox V has been magnetized in it. The external collimator consists of a series of tungsten apertures held in an aluminum assembly. The internal collimator is entirely tungsten. A disk-loaded collimator is an absolute necessity for an electron spectrometer because of the ease with which electrons backscatter out of material. The disk-loaded collimator acts as a true collimator; a smooth wall in a collimator acts as a funnel. The MEA also incorporates anti-scatter structures within the chamber. The top and bottom of the chamber have aluminium face plates with milled ridges. The sides of the chamber have aluminium fins extending to the working area of the magnetic field. These ridges and fins insure that an electron must under go numerous scatter- ings in order to reach a detector unless its trajectory lies entirely within the collimator acceptance zone. The sole exception is a scattering from the top of one of the ridges or fins. The aluminium fins are coated with a black conductive paint to reduce light scattering to the detectors and to prevent charge buildup on the plates (which would cause unwanted, and uncontrolled, focussing in the vertical direction).
At the 180° focus, the electrons impinge upon a detector array consisting of six ion-implanted silicon plates mounted in three pairs on a thick circuit card, one of each pair in front and one in back with a window in the circuit card between them. Each of the silicon plates is nominally 1.55 cm wide, 6.05 cm long, and 0.50 mm thick. Each plate has six metallized areas nominally .95 cm by 1.45 cm with a 0.5 mm separation. Corresponding metallizations on the front and rear silicon plates are tied together electrically to form a single detector with a nominal thickness of 1 mm. Thus there are a total of 18 de- tection channels in the array. Each detector has a separate electronics channel which amplifies the pulses and passes the pulses through a window discriminator (one having a lower and upper threshold to define valid events).
Pulses with amplitudes below the lower threshold are considered noise or bremsstrahlung and are rejected. Pulses with amplitudes above the upper threshold are due to highly ionizing particles (or long path length traject- ories) and are rejected as unwanted background. In general, the lower thresh- old is set at approximately 50% of the minimum energy electron that can be focussed upon the detector and the upper threshold is set at 110% of the maxi- mum energy electron that can be focussed on the detector. The low threshold ensures efficient detection of electrons which backscatter out of a detector after depositing only part of their energy. The upper threshold ensures detection of valid events in the presence of noise or low energy bremsstrah- lung which add to the pulse heighth. For more energetic electrons, the lower threshold is set at the energy corresponding to a minimum ionizing particle traversing a minimal path through the detector. This assures efficient detect- ion of energetic electrons which pass through the detector with little scatter- ing. Table 1 provides a list of the channels with the various energy boundar- ies and electronic thresholds. The detector closest to the aperture is shield- ed from direct electron access. In the original instrument, small energy deposits in this channel were interpreted as being due to bremsstrahlung and large deposits as being produced by penetrating protons or cosmic rays. Since the new detector array had an eighteen-channel capacity, the bremsstrahlung channel was deleted and its telemetry used for a seventeenth electron channel.
The following table contains the geometric-energy factors for each of the channels. These geometric-energy factors are based on the laboratory calibration data obtained from the MEA just prior to final delivery in January, 1990. The data were obtained using a collimated electron beam consisting of a 90Sr source viewed through sets of collimators and a large bending magnet (90°) for energy selection. The energy response of each channel at 0° incident beam was matched to numerical calculations of expected energy response for a 180° magnetic spectrometer with the collimator geometry of the CRRES instrument. This was necessary because the field in the magnet chamber was non-uniform (the result of a small chip of Indox V magnet material being flaked off during a cold soak at -60 C). Prior to the cold-soak (and the fracture of the magnet from the yoke, due to differential thermal contraction), the chamber magnetic field was about 850 gauss and quite uniform. After rebonding the magnet pole piece to the yoke, remagnetizing (in a somewhat non- uniform field), disassembling the yoke to stabilize the magnet, and finally reassembling, the field varied from about 700 gauss to 850 gauss at various positions within the chamber. Thus a detailed recalibration was required. The calibration data were obtained after final "buttoning-up" of the spectro- meter with the flight detector array and used the flight data processor to obtain data simultaneously in all channels at all test energies.
The geometric-energy factors are calculated factors based on test data taken at intervals of 10 to 30 keV over the range 95 to 1739 keV and have an esti- mated accuracy of about 1%. The energy response is accurate to 1 or 2%, also, provided the energy of the input beam was accurate to within 1%. The input beam was calibrated with conversion electrons from a radioactive source at a number of energies. The ultimate limit on knowledge of the energy calibrations is due to the finite widths of the collimator apertures within the bending magnet, which translate into finite widths of energy spread in the electron beam (varying from about 10 keV at the lowest test energies to about 35 keV at the highest energies). The energy profile of the beam was roughly rectangular (sharp cutoffs in energy at both the low and high sides). Finally, the geometric-energy factor calculations assume ~100% efficiency in the detection process. This is known (by laboratory test) to be a reasonable assumption.
The ThL,U are the lower and upper electronic thresholds set on each channel pulse height discriminator. Emin,abs is the lowest energy electron that can reach the detector without being scattered into it. Emin is the nominal lower energy of the channel. The response at this point is 10% of the peak response. The Emax,... are similar maxima. Ecenter is the center of the response in that 50% of the GEF is above and 50% below this value. The peak response of the channel is very close to this value (within 1% or 2%). The nominal cutoff energies are determined in the following manner: The energy at which the peak response occurs is determined. The high and low energy cutoff values (10% of the peak value) are determined. A linear least-squares fit is made separately to the response between Emin,10% and Ecenter and between Ecenter and Emax,10%. The zero intercepts of these fits are then listed as the Emin,nom and Emax,nom. The GEF is the integral under the original curve.
For some purposes, a GF is used which is generated by constructing a rectangle between the lower and upper nominal bounds which has the same area as the integral under the original curve. This GF is simply GEF/DE, where the DE is (Emax,nom)-(Emin,nom). The energies are given in keV and the GEF is in cm2-ster-keV. Thus, the counts/second must be divided by this number to transform to flux. Note that the counts in the data stream are counts per 0.512 seconds.
The background count should be subtracted from the raw counts before conversion to flux. The flux in channel i is:
The actual collimator angles are larger than the nominal angles due to the finite length of the collimator. The actual instrument limiting angles in the spin plane (due to detector location in chamber) are:
All of these values are half-angles.
Remarks:
Frazier, W. E., R. Stone, and P. R. Thompson, Selection of Orbits for the CRRES Dual-Mission Satellite, AAS/AIAA Astrodynamics Specialist Conf., AAS Paper 85-403, Vail, CO, Aug., 1985.
Frazier, W. E., K. Saylors, F. Patton, and K. Stakkestad, Attitude Control Experience with the Combined Release and Radiation Effects Satellite, 14th Annual AAS Guidance and Control Conf., Keystone, CO, AAS Paper 91-070, pp.11-13, Feb. 1991.
Frazier, W. E.,Semi-Analytic Study of High Eccentricity Orbit Stability and Evolution, Ph.D. Dissertation, Colorado Center for Astrodynamics Research, CO, pp 125-127, Jun. 1989.
Gussenhoven, M. S., E. G. Mullen, and R. C. Sagalyn, CRRES/Spacerad Experiment Descriptions, Air Force Geophysics Lab., Rept. AFGL-TR85-0017, Hanscom AFB, MA, Jan 1985.
Gussenhoven, M. S., and E. G. Mullen, Space Radiation Effects Program, available from the authors.
Johnson M. H.,and J. K. Ball, Combined Release and Radiation Effects Satellite (CRRES): Spacecraft and Mission, J. Spacecraft and Rockets, Vol. 29, No. 4, pp. 556 - 563, Jul. 1992.
Reasoner, D. L., The Chemical Release Mission on CRRES, J. Spacecraft and Rockets, Vol. 28, No. 1, 1991.
Rodriguez, P., CRRES Low Altitude Studies of Ionospheric Irregularities, J. Spacecraft and Rockets, (to be published).