PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM PRODUCER_ID = "ISAS/JAXA" LABEL_REVISION_NOTE = " 2016-10-18, K. McGouldrick, S. Murakami: Initial version; 2017-03-01, K. McGouldrick: cleaned to conform to PDS format requirements; 2017-05-05, S. Murakami: Revised; 2018-08-15, S. Murakami: Revised; " OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = "VCO" INSTRUMENT_ID = "LIR" OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "LONGWAVE INFRARED CAMERA" INSTRUMENT_TYPE = {"BOLOMETER", "CAMERA"} INSTRUMENT_DESC = " This summary of the LIR camera/bolometer is compiled primarily from [TAGUCHIETAL2007, FUKUHARAETAL2011]. Instrument Overview =================== The LIR imager detects thermal emission from the cloud top over a rather wide wavelength region of 8 -- 12 um in order to map the cloud-top temperature. Unlike the other imagers onboard Akatsuki spacecraft, the LIR imager is able to take both dayside and nightside images with equal quality and accuracy. The cloud-top temperature map will reflect the cloud height distribution, the detailed structure of which remains unknown, except in the high latitude observed by the Pioneer Venus (TAYLORETAL1980B), as well as the atmospheric temperature distribution. Specification of LIR -------------------- The specification of LIR are summarized as follows: - Wavelength region 8 -- 12 micrometers - Field of view 16.4 degrees x 12.4 degrees - Spatial resolution 0.05 degrees - Optics - F-number 1.4 - Focal length 42.2 mm - Detector - Bolometer uncooled micro-bolometer array (UMBA) - Number of pixels 328 x 248 - Pixel size 37 micrometers - Detector control - Data depth 12 bit - Target temperature 210 -- 240 K - Noise Equivalent Temperature Difference (NETD) 0.3 K (at 230 K) - Absolute accuracy 3 K - Weight 3.5 kg - Size 200 mm x 130 mm x 110 mm - Power consumption 18 W (nominal) Scientific Objectives ===================== The temperature at the cloud-top altitude varies with time and location and can be identified as the inhomogeneity of thermal infrared radiation from meso-scale to planetary scale. Images taken by the LIR imager will visualize the cloud height anomalies that originate from convection cells and various waves within the cloud layer. Furthermore, the tracking of blocky features in successive images will yield wind vectors including those on the nightside, which have been inaccessible in previous missions. The targets range from large-scale temperature variation due to diurnal change, planetary waves, gravity waves, tides, and climate change for global to synoptic scale phenomena, cloud clusters and cloud cells. The cloud-top temperature is typically as low as 230 K. The LIR imager has the capability to resolve temperature differences as small as 0.3 K for such a cold target, corresponding to a difference of a few hundred meters in cloud height. The absolute accuracy of temperature measurement will be 3 K. Measured Parameters =================== The measured parameter is thermal emission from the cloud top over a rather wide wavelength region of 8 -- 12 um in order to map the cloud-top temperature that corresponds to the spectral region with most sensitive to temperature variation and least sensitive to variation in sulfur dioxide, which has an absorption band at 8 -- 9 um. Subsystems ========== LIR was manufactured by NT Space Co. Inc., under the guidance of Mr. Higashino, Mr. Kashikawa, and other engineers. LIR consists of a sensor unit LIR-S which manages the function of image acquisition, an external power supply unit LIR-AE which converts the primary electric power to several voltages to distribute them to a regulator and a mechanical shutter, and a baffle which keeps direct sunlight away from the optical aperture. Detectors ========= An uncooled micro-bolometer array (UMBA) (TANAKAETAL2000) for a commercial infrared camera is used for LIR as an image sensor. The commercial camera is designed to give the best performance when it views room-temperature objects (WADAETAL1998). For LIR, the electronics and the driving parameters must be optimized for the low-temperature targets of this mission. The UMBA has 328 x 248 pixels with a pixel size of 37 micrometers, and its temperature is stabilized at 313 K by a Peltier temperature control system. An UMBA does not need cryogenic apparatus, which is commonly used for photodiode-type infrared detectors onboard spacecraft, LIR is relatively small and light in weight. Electronics =========== The LIR-AE is a power supply that converts the bus voltage supplied by the power control unit (PCU) to voltages necessary to drive the LIR-S. The LIR-AE is an independent unit because the variation in the heat generated by its DC/DC converter might result in instability of the sensor. The LIR-S has functions of driving the bolometer array sensor, stabilizing the sensor temperature using the Peltier cooler/heater and sensor, driving the shutter, acquiring image data, and onboard-correction of the data. Power for the survival heaters, which is activated independent of the power status of the LIR imager when the instrumental temperature decreases below the lowest limit, is supplied by the heater control unit (HCE). The design of electronics especially for the driver of the bolometer array is based on a commercial camera using the uncooled micro-bolometer array. For our instrument, however, all of the parts except for the detector are replaced by highly reliable parts, qualified for space missions. The house keeping (HK) status data, such as the temperatures of the sensor and shutter and image data, are transferred to the Digital Electronics (DE), which sends commands to the LIR-S. Filters and Optics ================== A bandpass filter is inserted at the pupil position of the optics. The pass band of bandpass filter should correspond to the spectral region that is most sensitive to temperature variation and least sensitive to variation in sulfur dioxide, which has an absorption band at 8 -- 9 um. In the early phase of design, the pass band was set to 9 -- 11 um in order to avoid the sulfur dioxide band. However, the pass band was found to be slightly too narrow to achieve the sensitivity requirement. Therefore, the pass band was widened to 8 -- 12 um, although a small influence remains due to sulfur dioxide especially when observed in the high latitudes. It should be also noted that there exists a band of carbon dioxide in the wavelength region of 9.5 -- 10.4 um. The brightness temperature is reconstructed from the data shown in [KNOLLENBERG&HUNT1980] and [RAGENTETAL1985]. The weighting functions were calculated for a typical cloud distribution case and no cloud case using a nominal temperature profile. The weighting function depends on actual temperature and cloud distributions. The effect of these gaseous components should be carefully taken into account by comparing image data obtained by the other imagers. Information on the atmospheric extinction profile may be reproduced by a technique using the limb-darkening effect (DINER1978). The filter transmittance profile is designed so that it changes within the permitted tolerance with variations in incident angle or filter temperature. The lens and filter are confirmed not to be thermally damaged by accidental exposure to direct sunlight, because most of the solar radiation power is blocked by the surface of the first lens. The LIR-S has a large baffle that keeps direct sunlight away from the optical aperture. Since the LIR imager operates in the thermal infrared region, visible light from the sunlit hemisphere of Venus or the Sun does not affect the imager. However, thermal emissions from the baffle itself may be a source of contamination. The temperature of the baffle could be very high when exposed to direct solar radiation and as a result may emit strong thermal infrared radiation. The heat from the baffle could also cause distortion in the mechanical structure and degrade the performance of the optics. In order to avoid these problems the baffle is thermally connected to the satellite structure and is insulated from the optics of the LIR imager. The optics in the sensor unit consists of three germanium lenses with the F-number of 1.4. The field-of-view of 16.4 degrees x 12.4 degrees is based on a common requirement with other cameras. The pixel resolution of 0.05 degrees corresponds to 26 -- 70 km on the Venus surface when the spacecraft views Venus from distances of 3 -- 8 x 10**4 km on an elliptical orbit. The temperature of the optics is kept within 293 -- 308 K by the heater-controlling electronics (HCE) of the spacecraft in order to prevent thermal distortions. The mechanical shutter which works not only as a sunlight shield but also as a blackbody for calibration is positioned just in front of the UMBA and is driven by a stepper motor. Operational Modes ================= LIR has six operation modes: Off, Stand-by, Protect, Idling, Parameter Setting, and Image modes, and can take an image only during the Idling mode. Before taking an image on-chip fixed pattern noise (OFPN) data are prepared in the LIR memory. The OFPN data are used to cancel the pixel- to-pixel non-uniformity in sensitivity and offset. The shutter is basically closed in all modes except for Idling mode in order to avoid accidental exposure of the UMBA to direct solar radiation. The shutter surface facing to the UMBA is blackened by anodic oxidation to be a quasi-blackbody with an emissivity ~0.89. Its temperature is kept at 27 to 30 degrees Celsius and monitored at the resolution of 0.01 degrees Celsius. Thus a shutter image can be used as a temperature reference for data analysis. Onboard Data processing ======================= The UMBA for LIR has large pixel-to-pixel inhomogeneities of offset and sensitivity. Although these inhomogeneities are partly reduced in the analog circuit before analog-to-digital conversion, by using the on-chip fixed pattern noise (OFPN) data which is preacquired before launch, they still remain in the raw data. In order to remove the pixel-dependent offset, LIR acquires a target data and a shutter data sequentially, and the shutter data is subtracted from the target data. Output signals from the detector are digitized by a 12-bit analog-to- digital converter and stored as 16-bit signed integer. Raw target and shutter image data of up to 128 images thus obtained, respectively, may be accumulated at a frame rate of 60 frames/sec to improve signal-to- noise ratio (first accumulation). After acquiring both target and shutter images subtraction between them is performed to create a thermal image. Up to 32 thermal images may be accumulated again to improve signal-to- noise ratio (second accumulation). Then the data are converted to a 16-bit unsigned integer, compressed by HIREW, and divided into packets. These calculations are processed in the image processor in DE. The stability of the spacecraft's attitude, which is designed for the other cameras onboard Akatsuki, is within 0.015 degrees (NAKAMURAETAL2011B). Since they have a higher spatial resolution than LIR, there is no restriction for the observation of LIR. Calibration =========== There still remains a slight inhomogeneity of the sensitivity, even in a subtracted image. This will be corrected after being transmitted to the ground. The resultant image, which gives the difference of the brightness temperature between the object and the shutter, is further converted to a brightness temperature map by adding the temperature of the shutter to all pixel values. Noise Equivalent Temperature Difference (NETD) will be improved by averaging several tens of images taken within a few minutes [GEOFFRAYETAL2000]. The NETD variations with the numbers of the first and second accumulations are estimated as NETD = dT/(S/N), where dT is the actual temperature difference between the warmer and colder blackbodies, S the difference of the observed mean brightness between the warmer and colder blackbodies in the image, and N is the standard deviation of the observed brightness that is evaluated locally in the warmer or colder domains. The reduction of noise by the first accumulation is effective until m = 32 as long as the number of the second accumulation n is less than 5. When the number of second averaging is 5 or more, the noise compression becomes negligible. This suggests that the noise is not ideally random. When the first and second averaging were 32 and 32, respectively, substituting the observed values of dT = 4.22 K, S = 2922 and N = 247 into the equation above, we obtained the best NETD ~0.36 K, which meets the required specification. These numbers of averaging have been adopted for the flight model. After processing more than 8000 Venus images we found that mean temperature of the Venus disk correlates with temperature of the hood of LIR. This is regarded as a false temperature bias due to contamination by thermal infrared radiation emitted from the lens of LIR. The hood heated by solar irradiation emits thermal infrared radiation, of which a part is absorbed by the lens, resulting in increases of lens temperature and thermal infrared radiation emitted from it. This factor can be corrected using relation between the background temperature and the hood temperature. Images of the deep space were obtained by changing attitude of the spacecraft during the periods while observation of Venus was prohibited in September, 2016 and April, 2017, and pixel-to-pixel correction formulae are determined from the image data. Correction of the false background temperature bias will be added to the next version of data processing pipeline [FUKUHARAETAL2017]. The instrumental spectral response of LIR is well approximated by a rectangular shape function with sharp cut-offs at 8 and 12 um, which is used in the current pipeline. The actual overall spectral response function is a product of transmittances of the lens system, the bandpass filter and a Ge window of the detector package and spectral response of the detector. We have estimated the precise spectral response function. Susceptibility of retrieved brightness temperature to the shape of spectral response function will be evaluated, and, if necessary, the pipeline will be updated with the precise spectral response function. " END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "BAKERETAL1998" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "DINER1978" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "FUKUHARAETAL2011" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "GEOFFRAYETAL2000" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "IGNATIEVETAL2009" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "KNOLLENBERG&HUNT1980" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "NAKAMURAETAL2011B" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "TAGUCHIETAL2007" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "TAKADAETAL2007" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "TANAKAETAL2000" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "WADAETAL1998" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "ZASOVAETAL2007" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END