Fig. 6-21. Typical Photodiode Schematic

Fig. 6-21. Typical Photodiode Schematic

Detectors which respond to the longer wavelengths of infrared blackbody radiation are based on the operating principles of the thermistor, thermocouple, or pyroelectric crystal [Barnes Engineering Co., 1976]. A bolometer is a very sensitive resistance thermometer, or thermistor, used to detect infrared radiation. Thermistors consist of fused conglomerates, or sinters, of manganese, cobalt, and nickel oxide formed into flakes, typically 0.5 mm by 0.5 mm by 10 pm thick, bonded to a heat-dissipating substrate or heat sink. Impinging radiation heats the flake and alters the resistance, typically by 3.5%/°K, which is sensed by conversion to a voltage and amplification. When radiation is removed from the flake, its temperature returns to that of the heat sink with a time constant depending on the thermal conductance of the flake-substrate bond. A typical time constant is 3 ms. Bolometers are able to sense temperature changes of 0.001 °K due to radiation despite ambient temperature changes four orders of magnitude greater [Astheimer, 1976], The minute temperature change is observed by modulating the incoming radiation by, for example, scanning across the target, and thereby removing the effect of ambient temperature changes on the output voltage by capacitance coupling to the amplifier.

A bolometer may have either one or two flakes in the focal plane of the optical system. The two flakes of a dual-flake system detect radiance originating from different regions of the celestial sphere. Consequently, the two output signals may be combined in an electronic AND circuit to provide Sun rejection if the separation between the flakes is such that the Sun cannot be seen by both flakes simultaneously. Thermistors are often immersed in or surrounded by a germanium lens (transparent to infrared radiation) to increase the intensity of radiation at the thermistor.

A thermopile consists of a string of thermocouple junctions connected in series. Each thermocouple consists of a hot junction and a cold junction. The hot junctions are insulated from a heat sink and coated with a blackening agent to reduce reflection. The cold junctions are connected direcdy to a heat sink. When exposed to impinging infrared radiation, the hot junction is heated and yields a measurable output voltage. Thermocouple junctions commonly use bismuth and antimony. Thermopile detectors are simple, requiring minimal electronics and no moving parts; however, they suffer from a slow response time and are used only in nonscanning systems.

Pyroelectric detectors consist of a thin crystal slab, such as triglycine sulfate, sandwiched between two electrodes. Impinging radiation raises the temperature of the crystal, causes spontaneous charge polarization of the crystal material, and yields a measurable potential difference across the electrodes. Pyroelectric detectors may be used in scanning systems because they are fast and have a high signal-to-noise ratio with no low-frequency noise.

The output from a scanning horizon sensor is a measure of the time between the sensing of a reference direction and the electronic pulse generated when the radiance detector output reaches or falls below a selected threshold. The reference direction for a body-mounted sensor is generally a Sun pulse from a separate sensor, whereas wheel-mounted sensors typically use a magnetic pickoff fixed in the. body. If the detector output is increasing across the threshold, the pulse corresponds to a dark-to-light transition or acquisition of signal (AOS). If the detector output is decreasing across the threshold, the pulse corresponds to a light-to-dark transition or loss of signal (LOS), the AOS and LOS pulses are also referred to as in-crossings and out-crossings, or in-triggering and out-triggering, respectively.

Various electronic systems provide the reference to AOS time (t, = tA0s_ 'ref)> the reference to LOS time (/0 = /L0S- fREF), the Earthwidth (V= 'los~ 'aos)' and the reference to midsLan time* (tM = (('los+'aos)/2-'ref)- The percentage of the scan period that the radiance is above threshold is the duty cycle. Figure 6-22 illustrates the various possible outputs. Knowledge of the scan rate or duty cycle allows the conversion from time to angle either onboard or on the ground. As described in Sections 4.1, 7.2, and 7.3, the horizon crossing times depend on the sensor field of view, the radiance profile of the scanned body, the transfer function, and the locator. The transfer function relates the radiation pulse incident on the detector to the electronic output of the horizon sensor. The transfer function includes the thermal response time of the detector and time constants associated with pulse amplification and shaping. Typically, sensors are designed and calibrated such that the system output may be used directly for attitude control and determination within a specified accuracy under normal conditions. The electronic technique used to define the threshold for horizon detection, called the locator, can significantly affect the overall attitude accuracy of the system. Many locators have been studied [Thomas, 1967] and two are widely used: the fixed threshold locator specifies the observed detector output which defines the horizon; the fixed percentage of maximum output or normalized locator redefines the threshold for each scan period as a fixed percentage of the maximum output encountered by an earlier scan. Better results are obtained with the normalized threshold locator because it is less sensitive to seasonal or geographical variations in radiance (see Section 4.2 for specific radiance profiles of the Earth). A slightly modified locator has been proposed for SEASAT which continuously resets the threshold to 40% of the mean detector output observed on the Earth between 5 and 11 deg from the located horizon. The thresholds for AOS and LOS are determined

•The negative of the reference to midscan time is the split-to-index lime.

6.2.2 Horizon Sensor Systems

The simplest horizon sensor system is a body-mounted horizon sensor sensitive to visible light Such a system consists of an aperture and lens to define the field of view and a photodiode to indicate the presence of a lit body. Body-mounted sensors are cheap and reliable and have been used on IMP; slightly more complex versions, sensitive to the infrared spectrum, have been used on AE, SMS/GOES, CTS, and SI RIO. A body-mounted infrared sensor is shown in Fig. 6-23. Body-mounted sensors are suitable only for spinning spacecraft and their fixed mounting angle makes target acquisition a substantial problem for many missions. In this subsection we describe the operating principles of several more versatile sensor systems that have been used operationally.

Fig. 6-23. Body Mounted Horizon Sensor Used on the CTS Spacecraft (Photo courtesy of Barnes Engineering Co.)

Panoramic Attitude Sensor (PAS). The PAS, flown on IUE and ISEE-1 and planned for ISEE-C, is manufactured by the Ball Brothers Research Corporation and is a modification of the original design flown on RAE-2. The PAS is among the most versatile of all horizon scanners because of its ability to use either an internal scanning motion or the spacecraft rotation with a variable sensor mounting angle which may be controlled by ground command. Thus, the PAS may detect both the Earth and the Moon for virtually any attitude and central-body geometry. A slit Sun sensor and a visual wavelength telescope, both employing photodiode detectors, are included in the PAS assembly, as illustrated in Figs. 6-24 and 6-25. The telescope has an 0.71-deg FOV diameter, and its optical axis may assume any of 512 discrete positions with a specified positional accuracy of 0.1 deg. The PAS functions as a variable-angle body-mounted sensor when the spacecraft is spinning about an axis parallel to the X axis in Fig. 6-24. Outputs from the system in this

Fig. 6-24. Panoramic Attitude Sensor. (Photo courtesy of Ball Brothers Research Corporation,):'

Fig. 6-24. Panoramic Attitude Sensor. (Photo courtesy of Ball Brothers Research Corporation,):'

tr spherical mode are the times from the Sun pulse to AOS and LOS. The threshold for detection is specified as 0.1 times the maximum lunar radiance, which corresponds to a first- or third-quarter Moon as viewed from the vicinity of the Earth.

When the spacecraft is despun, the scanning motion in the planar mode is accomplished by rapidly stepping the turret. Various commands are available to control the operation of the PAS in both the spherical and the planar modes. The turret can be commanded to step continuously in either direction, to reverse directions at specified limit angles, or to inhibit stepping altogether. The detector records and stores, in a series of registers, the encoded steps at each dark-to-light or light-to-dark transition. The telescope is baffled to prevent detection of the Sun at separation angles between the telescope axis and the Sun of 12 deg or more.

Nonspinning Earth Sensor Assembly (NESA). The NFS A, built by TRW for the synchronous orbit CTS and the ATS-6 spacecraft, consists of two independent infrared sensors that scan across the Earth, measuring rotation angles to define the spacecraft attitude relative to the Earth from synchronous altitude. The detector senses radiation in the 13.5- to 25- /¿m spectral band and uses a fixed percentage of maximum output locator. A small Sun detector is located near the infrared telescope to identify intrusion of the Sun in the FOV. This sensor consists of a small mirror, two fixed mechanical apertures, and a silicon photodiode detector. The Sun detector FOV is concentric with, but larger than, the infrared detector

The sensor geometry near the mission attitude (pitch = roll=0) is shown in Fig. 6-26 with the spacecraft Z axis in the nadir direction. A scanning field of view, approximately in the spacecraft X-Z or Y-Z plane, is created by oscillating a beryllium mirror at 4.4 Hz. The scan plane is tilted 3.5 deg so that, for the mission attitude, the scan paths are slightly offset from the Earth center. The sensor geometry is chosen such that either the north-south (NS) or east-west (EW) scanner

Fig. 6-26. Nonspinning Earth Sensor Assembly. View from Earth toward spacecraft at synchronous

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