The ATS-6 spacecraft was designed to accomplish a wide spectrum of experiments dealing with communications, technology, meteorology, and science using a three-axes stabilized platform in geostationary orbit. Launch occurred from Cape Canaveral on May 30, 1974, and the science experiments were activated by June 15. The mission was divided into three major phases: (1) Operations during the first mission "year" from 94 degrees west longitude over the USA; (2) Operations during the second mission "year" from 35° east longitude with an emphasis on the subcontinent of India; and (3) Operations from 140° west longitude after return to the U.S. An end-of-mission activity occurred at the end of July, 1979 in which the satellite was moved 450 km out of the geostationary orbit and began an eastward drift of ˜6.l per day. At the same time the satellite was spun-up around one of its equatorial-plane axes and scientifically entered a fourth phase.
The objectives of the scientific experiments on ATS-6 were to study and to gain a better understanding of the environment in space at the synchronous altitude. This has been accomplished by monitoring the spatial and temporal variations of the various plasma regimes, the geomagnetic field, and energetic particles present in the vicinity of the spacecraft and the interactions of each of these components.
Low Energy Proton Experiment (LEPE)
The National Oceanic and Atmospheric Administration (NOAA) Low Energy Proton Experiment was included as part of the Applications Technology Satellite-F (ATS-F) Environmental Measurements Experiments (EME) payload. Professor Theodore A. Fritz of the Boston University Center for Space Physics was the principal investigator. The principal scientific objectives of this experiment are to study protons (>20 keV) which are thought to produce the storm time extraterrestrial ring current associated with main phase geomagnetic storms and to delineate the morphology of these storms and smaller substorms as a function of local time. Another principal objective of the experiment is to search for the existence of energetic heavy ions trapped and/or possibly energized within the magnetosphere. Determination of the presence of such ions and their relative abundances will provide answers in a definitive manner to questions of whether the magnetospheric particles are of solar or terrestrial (ionospheric) origin. The experiment employs four solid state detector telescopes consisting of two elements each in order to accomplish its scientific goals.
Low Energy Proton Section
This portion of the experiment used low-noise charge sensitive preamplifiers and commercially available low-noise solid state detectors. The detector-preamplifier-electronics had total system noise full width at half maximum (FWHM) at the planned operating temperature of approximately 5.5-keV equivalent particle energy. This design was incorporated into the three detector telescopes labeled A, B, and C in Fig. 1. Every effort was made to make the telescopes operationally identical. The front element, D1, of each telescope, (designated A1, B1, and C1 in Fig. 1) was positioned behind a separate broom magnet with a field strength of approximately 2.2 kG. These magnets swept away electrons with energies less than 300 keV. Any proton which entered the front telescope element produced a quantity of charge 'Q' proportional to the energy deposited in the silicon at a rate of 3.5 eV per charge pair produced. This charge produced a voltage step at the output of its preamplifier, the amplitude of which was proportional to the incident energy of the proton assuming it stopped in the detector. Each of the three D1 preamplifiers was multiplexed to a common set of follow on electronics. Particles entering the detector of the preamplifier being examined at any given time produced voltage steps which were amplified and shaped by the following linear amplifiers shown in Fig. 1. Single differentiation and single inductively peaked integration was used with 0.5 microsecond time constants producing a semi-Gaussian shaped pulse. These shaped pulses triggered a series of tunnel diode discriminators and monostable circuits arranged to perform a simple pulse height analysis. A strobe pulse was generated from the lowest level (El) discriminator and monostable. This strobe pulse was gated into a spacecraft accumulator through the appropriate digital line delta E(N) when the condition is satisfied that all discriminators below and including the E(N) discriminator have been triggered by the incoming pulse and all discriminators E(N+1) and above had not.
If the incoming proton had sufficient energy (22.8 MeV or greater) to penetrate detector D1 and leave sufficient energy in the back detector D2 to trigger the discriminator, D2L1, the strobe pulse was not produced. Thus high energy protons and electrons with energies greater than 300 keV which would have otherwise created an ambiguous response were eliminated from the low energy proton channels via this coincidence mode.
The heavy ion portion of the experiment used the properties of a very thin front detector (H1) in a fourth two-element telescope H to measure fluxes of ions with Z greater than or equal to 2. This telescope was mounted such that it "looked" in the same direction as telescope A. The front detector was a 3.8 micrometers thick surface-barrier totally depleted solid state detector with a 900 Angstrom self-supporting Al foil for light protection. The pulses from the preamplifier of this detector were delay-line clipped using a shorted 100 nanoseconds delay line to minimize proton pulse pileup effects in the alpha particle channels. The thin detector was essentially insensitive to electrons. Pulse height analysis was performed on the response of the H1 detector in a manner similar to that described above for the D1 detectors. The strobe pulse was generated from the response of the discriminator and monostable circuit labeled Al. In this manner the passbands delta alpha 1, delta alpha 2, L1, M1, and A5 were produced. The logic for these passbands and their primary particle response are presented in Table 1. The accumulation duty cycle for these passbands was 100 percent.
On a lower duty cycle (~19 percent) the response of the second detector in the H telescope, H2, is fed into the amplifier chain associated with the D1 detectors. The basis for this duty cycle is discussed in Section 3 below. During this period it was possible to use the responses of the H1 and H2 detectors in coincidence to perform a dE/dx and E analysis on the particles passing through the H telescope. In this manner, it is possible to uniquely identify the presence of helium in two energy ranges (delta alpha 3 and delta alpha 4) and to identify uniquely the presence of the groups of ions: lithium (Li), beryllium (Be), boron (B) and carbon (C), using the L2 channel and carbon (C), nitrogen (N), and oxygen (O) using the M2 channel. The logic for these passbands and their primary particle energy passbands are presented in detail in Table 1 and Fig. 2.
The resulting eight heavy ion data channels were fed from the digital logic section of the experiment into respective 7-bit accumulators. The digital state (0 to 127) of each accumulator was continually converted to an analog voltage (0 to 5.1 V) but never reset except by overflow of the 7-bit counter. These eight analog outputs were subcommutated within the experiment through four of the seven EME subcom positions originally designated to be used for housekeeping. Each individual heavy ion data channel had a duty cycle as high as 100 percent but was sampled only once every 128 seconds. The particular set of four data channels being sampled at any one time were uniquely identified using the most significant bit of a fifth subcom channel which monitored the experiment temperatures.
High Energy Proton Section
The remaining section of the experiment was the high energy proton portion and this used aspects of both of the other portions. As noted above, the response of the H2 detector was multiplexed into the amplifier chain associated with the D1 detectors. The response of this detector was pulse height analyzed in the same manner as described in the low energy proton section, except that the strobe pulse was generated in a different manner. A particle entering detector H2 must pass through detector H1. By setting a discriminator level on the H1 detector (H1P) that was sensitive to protons but insensitive to electrons and using this output in coincidence with the H2 response, an additional seven proton differential energy measurements were allowed. In effect, the energy response of H2 used the presence of the 3.8 micrometer H1 detector as a thick foil. Care must be exercised in the interpretation of these energy channel responses because effects due to the energy straggling process appear as a noise source and reduce the efficiency of these channels for responding to protons in the narrow passbands indicated in Table 1.