PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = "K. Bender, 2002-10-01; K. Murray, 2002-10-01; P. Christensen 2002-10-01" RECORD_TYPE = STREAM OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = ODY INSTRUMENT_ID = THEMIS OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "THERMAL EMISSION IMAGING SYSTEM" INSTRUMENT_TYPE = "CAMERA" INSTRUMENT_DESC = " Instrument Overview =================== The THEMIS flight instrument is a combined infrared and visible multi-spectral pushbroom imager [CHRISTENSENETAL2002]. It has a 12-cm effective aperature telescope and co-aligned infrared and visible area arrays. The imaging system is comprised of a three-mirror anastigmat telescope in a rugged enclosure, a visible/infrared beamsplitter, a silicon focal plane for visible detection, and a microbolometer for infrared detection. A major feature of this instrument is the use of an uncooled IR microbolometer array operated at ambient temperature, eliminating the need for complex passive or active cryogenic coolers. A small thermal electric cooler is used to stabilize the detector temperature to 0.001K. A calibration flag, the only moving part in the instrument, provides thermal calibration and a DC restore capability, and will also be used to protect the detectors from unintentional direct illumination from the Sun when the instrument is not in use. The electronics provide digital data collection and processing as well as the instrument control and data interface to the spacecraft. Infrared data will be collected in 9 wavelengths centered from 6.6 to 15.0 microns at 100 meter per pixel resolution; the 6.6 micron band is collected twice to result in a 10 band image. Visible data will be collected in 5 spectral bands at a resolution of 18 meters per pixel. The instrument weighs 11.2 kg, is 29 cm by 37 cm by 55 cm in size, and consumes an orbital average power of 14W. Optical Design -------------- In order to integrate the visible and IR bands into a single telescope, a fast, wide field-of-view reflective telescope has been used. The 3.5 degree (down-track) by 4.6 degree (cross-track) field of view is achieved with a 3 mirror f/1.6 anastigmat telescope with an effective aperature of 12 cm and a 20-cm effective focal length. The design allows for excellent baffling to minimize scattered light. It is based on a diamond-turned bolt-together approach to telescope design, fabrication, alightment and testing. The manufacture utilized high-precision machining capabilities that allowed the entire optical stage to be machined and assembled without manual optical component adjustments, and achieved diffraction-limited performance in both the visible and infrared. The optical surfaces were machined with extremely tight tolerances (0.0002in). The optical surfaces were machined directly from high order aspheric equations. The telescope was manufactured with aluminum to reduce cost and to be significantly light-weight. Nickel plating and automated post polishing were used to keep the surface scatter to levels unobtainable with conventional diamond turning techniques. The system was optimized to match the high signal performance required for the IR imager and the spatial resolution needed for the visible camera. The 9 micron pitch of the visible array maps to a ground sample distance (GSD) of 18 meters with an MTF of approximately 0.1 at Nyquist. Similarly, the 50 micron pitch of the IR focal plane array maps to a GSD of 100 meters. Focal Plane Assemblies ---------------------- The THEMIS infrared design is based on a Raytheon hand-held imager developed for rugged military use. The microbolometer array contains 320 pixels cross track by 240 pixels along track, with a square 50 micron pixel pitch. The microbolometer arrays were grown directly on the surface of Readout Integrated Circuits (ROIC) which are designed by Raytheon Santa Barbara Research Center (SBRC) and utilize custom Digital Signal Processing electronics. Spectral discrimination in the infrared is achieved with ten narrowband stripe filters. Each filter covers 16 lines in the along track direction with an 8-line "dead-space" between filters. The stripe filters were fabricated as separate stripe filters butted together on the focal plane. The along-track detectors under a common spectral filter are combined by the use of time-delay and integration (TDI) to improve the instrument's signal-to-noise performatnce. The calculated dwell time for a single pixel, at a martian orbit of 400km and a 100-meter footprint is 29.9 msec, which closely matches the 30Hz frame rate for the standard microbolometer. The ten stripe filter produces nine ~1 micron wide wavelength bands from 6.6 to 15 microns. Two filters (bands 1 and 2) cover the same spectral region centered at 6.6 microns. The nine IR wavelengths include eight surface-sensing wavelengths (bands 1 - 9) and one atmospheric wavelength (band 10). The visible camera was supplied by Malin Space Science Systems (MSSS) and is a derivative of the MS'98 MARDI camera. It consists of a small (5.5 x 8.5 x 6.5 cm, <500 gm) unit incorporating a focal plane assembly with five color filters superimposed on the CCD detector, a timing board, a data acquisition subsystem and a power supply. The visible sensor utilizes a Kodak KAI-1001 CCD. This detector has 1024 by 1024 9-micrometer pixels (1018 x 1008 photoactive). The visible imager used a filter plate mounted directly over the area-array detector on the focal plane. On the plate are multiple narrowband filter strips, each covering the entire cross-track width of the detector, but only a fraction of the along-track portion of the detector. The five filter bands are centered near 425, 550, 650, 750, and 860 nanometers. Band selection is accomplished by selectively reading out only part of the resulting frame for transmission to the spacecraft computer. The imager uses 5 stripes each 192 pixels in along-track extent. The entire detector is read out every 1.3 seconds. Electronics Design ------------------ Both the visible and infrared cameras utilized commercial, off-the-shelf electonics with modifications to accommodate space environmental requirements. Dedicated, miniaturized electronics provide ultra-stable, low-noise clock and bias signals to the focal planes, stabilize IR focal plane temperature with 0.001 degree C, and perform analog and digital processing of the output signals. The microbolometer readout electronics includes an initial 8-bit analog DC offset correction which occurs on the focal plane, an Analog-to-Digital Converter (ADC) coverts the signals to 12-bit words, which are then corrected for gain and offsets. The correction is provided by the electronics of the IR camera and consists of a 12-bit fine offset and 8-bit gain and responsivity adjustment, performed in real time on a pixel-by-pixel basis. This process eliminates all the significant noise elements with the exception of the fundamental random noise term. This noise is reduced by applying TDI to the corrected digital data. Internal THEMIS data processing in firnmware includes a 16:1 TDI processing and lossless data compression for the IR bands using a hardware Rice data compression algorithm chip. The visible sensor requires 7 clock signals: a two-phase vertical clock (V1/V2), a two-phase horizontal clock (H1/H2), a sub-state clear clock (S), a reset clock (R), and a fast-dump clock (F). In addition, the ADC requires a convert clock. The amplified CCD signal is digitized by an Analog Devices AD1672 12-bit ADC running at its maximum rate of 3 MSPS. For each pixel, both reset and video levels are digitized and then subtracted in the digital domain to perform correlated double sampling (CDS). The digital electronics are responsible for clock generation, sampling of the CCD signal, conversion of the 12-bit samples to 8-bit encoded pixels, storage of the pixels, and finally readout of the pixels to the spacecraft. The zero reference ("reset") level for each pixel is digitized and stored in a register. The sum of the video plus zero reference ("video") level is then digitized, and an arithmetic subtraction is performed to produce the final result. The CCD output only requires scaling to the ADC range; no analog sampling, delay or differencing is required. The digital signal processor within the visible sensor generates the CCD clocks, reads the reset and video levels from the ADC, performs the correlated double sampling subtraction, reduces the pixel from 12 to 8 bits, applies lossless (2:1) first-difference Huffman compression, and transmits it digitally with handshaking over the serial communications interface to the spacecraft CPU. The spacecraft interface electronics supply final processing of the focal plane data, a data and command interface to the spacecraft, and overall instrument power conditioning. The bulk of the interface electronics is performed using Actel Field Programmable Gate Arrays (FPGAs), that are packaged using a mixture of conventional, and Sealed Chip-On-Board, High-Density Multiple Interconnect technology and chip stack memory. The visible and IR subsystems have independent power supplies, the IR power supply uses off-the-shelf modules and requires only a few discrete components for input filtering to assure electromagnetic compatibility with the rest of the spacecraft. The spacecraft processor performas final data stream formatting for both the IR and the visible data. Mechanical Design ----------------- The THEMIS main frame is composed of aluminum and provides the mounting interface to the spacecraft as well as the telescope assembly, thermal blankets, and thermal control surface. The focal plane assemblies are mounted in the main frame using brackets that provide for the necessary degrees of freedom for alignment to the telescope. The calibration shutter flag is stored against a side wall that will maintain a known temperature of the flag for calibration purposes. Aluminum covers are installed over the electronics circuit cards to provide EMI, RFI, and radiation shielding as required. There is no reliance on the spacecraft for thermal control of THEMIS, other than the application of replacement heater power when the instrument is off. The themal control plan includes the use of multi-layer insulation blankets and appropriate thermal control surfaces to provide a stable thermal environment and a heatsink for the electronics and the TE termperature controller on the focal plane arrays. Performance Characteristics --------------------------- The predicted performance for the infrared bands produced noise equivalent delta emissivity values ranging from 0.007 to 0.038 when viewing Mars at surface temperatures of 245K to 270K. The measured SNR values for each band at a reference surface temperature of 245K are as follows: band 1,2 = 45 band 3 = 107 band 4 = 169 band 5 = 193 band 6 = 187 band 7 = 194 band 8 = 167 band 9 = 128 band 10 = 120 SNR ratios for the visible imager were computed for a low albedo (0.25), flat-lying surface viewed at an incidence angle of 67.5 degrees under aphelion conditions. The SNR values for this case vary from 200 to 400. Software -------- The flight software for the IR imager resides on the spacecraft computer and performs the formatting and data packetization. Instrument commanding will be done using discrete spacecraft commands to the THEMIS instrument over an RS-232 command line. These commands will consist of: 1) IR camera on/off/standby; 2) visible camera on/off/stand-by; 3) calibration flag shutter control and electronics synchronization; 4) instrument parameter settings (gain, offset, integration time, etc.). The visible imager software runs on two processors: the main spacecraft CPU and the internal DSP. The CPU will be responsible for instrument operational commands and image post-processing and compression. The DSP is responsible for generating the CCD clocks, emulating the required analog processing and transmitting the data output to the CPU. Lossless predictive compression is implemented as part of the DSP firmware. The algorithm employed compresses each image line independently by encoding first differences with a single, fixed Huffman table. Selective readout and pixel summing can also be performed by the DSP software. The result of an imaging command is a stream of raw or compressed 8-bit pixels. Experience has shown that the volume of data likely to be returned from a spacecraft often evolves during a mission. Implementing data compression in software on the spacecraft computer provides the maximum flexibility for the science and spacecraft team to trade-off data return and buffer space usage. The compression nodes developed are: 1) lossless predictive (capable of applications by the SDP in real-time); 2) a relatively fast discrete cosine transform compression (applied by the spacecraft CPU in "near realtime" a few tens of seconds); 3) high-quality lossy wavelet compression (applied by the CPU on a longer timescale). Each compression node has optional pixel summing. Scientific Objectives ===================== The objectives of the THEMIS experiment are: 1) to determine the mineralogy and petrology of localized deposits associated with hydrothermal or sub-aqueous environments, and to identify future landing sites likely to represent these environments. 2) to search for pre-dawn thermal anomalies associated with active sub-surface hydrothermal systems. 3) to study small-scale geologic processes and landing site characteristics using morphologic and thermophysical properties. 4) to investigate polar cap processes at all seasons using infrared observations at high spatial resolution. 5) to provide a direct link to the global hyperspectral mineral mapping from the Mars Global Surveyor TES investigation by utilizing the same infrared spectral region at high (100m) spatial resolution. Calibration =========== The THEMIS instrument was radiometrically, spectrally, and spatially calibrated prior to delivery. Three categories of calibration were performed: 1) spatial calibration; 2) spectral calibration; and 3) radiometric calibration. The radiometric calibration included the absolute rediometry, the relative precision (SNR), and the calibration stability during an image aquisition. The data returned from the instrument in-flight will be converted to scene radiance by: 1) adjusting for the gain and offset that were applied in the instrument to optimize the dynamic range and signal resolution for each scene; 2) correcting for drift or offest that occur between observations of the calibrations flag; 3) converting signal to radiance using the instrument response function determined prior to launch. The response functions necessary to perform this calibration were acquired prior to instrument delivery using a thermal vacuum chamber at the SBRS facility. See calibration report for details on IR and visible image calibration methodologies [CHRISTENSEN2002] THEMIS images will be calibrated using periodic views of the internal calibration flag. This flag will be closed, imaged, and reopened at selectable intervals throughout each orbit. Calibration data are expected to be acquired every 3-5 minutes. However, the optimum spacing of these observations that meets the calibration accuracy requirements, while minimizing the loss of surface observations, will be determined in Mars orbit. Operation of THEMIS =================== The THEMIS instrument is operated from ASU, building on the facility and staff developed and in place for the MGS TES investigation. No special spacecraft operation or orientation is required to obtain THEMIS data. The instrument alternates between data collection (<3.5 hours per day) and idle modes based on available DSN downlink rates. These modes will fall within allocated resources (e.g. power), and will not require power cycling of the instrument. All instrument commands are generated at ASU, delivered electronically to the Odyssey Project, and transmitted to the spacecraft. Image Collection ---------------- IR images can be acquired at selectable image lengths, in multiples of 256 lines (25.6 km). The image width is 320 pixels (32 km from the nominal mapping orbit) and the length is variable, as given by ((#frames)*256 lines) - 240. The allowable number of frames varies from 2 to 256, giving minimum and maximum image lengths of 272 and 65,296 lines respectively (27.2 km and 6,530 km from the nominal mapping orbit). The bands returned to the ground are selectable. THEMIS visible images can be acquired simultaneously with IR images, but the spacecraft can only transfer data from one of the two THEMIS imagers at a time. The IR image transfers data as it is being collected, while the visible images are stored within an internal THEMIS buffer for later transfer to the spacecraft CPU. Visible images can be acquired in framelets that are 1024 samples crosstrack by 192 lines downtrack in size. The images can be composed of any selectable combination of bands and image length that can be stored within the 3.734 Mbytes THEMIS internal buffer. The size of an image is given by: (1024 * 192) * #framelets * #bands < 3.734 Mbytes. Thus, for example, either a single-band, 19 framelet (65.6 km) image or a 5-band 3-framelet (10.3 km) image can be collected. This buffer must be emptied to the spacecraft before a subsequent image can be acquired. Data Allocation --------------- THEMIS data collection will be distributed between the mineralogic, temperature, and morphologic science objectives in both targeted and global mapping modes. A baseline observing plan has been developed to prioritize the total data volume returned by THEMIS between the different objectives. This plan currently devotes 62% of the total data to the IR imager and 38% to the visible, averaged over the course of the Primary Mission. In the baselne plane the IR data will be further sub-divided into 9-wavelength daytime mineralogic observations (47% of total data return) and 2-wavelength nighttime and polar temperature mapping (15% of total data). The visible data will be sub-divided into monochromatic images (36% of total data) and 5-band multi-spectral images (2%). We have assumed a lossless data compression factor of 1.7 for the IR imager and a combination of lossless (40%), and lossy with compression factors of 4 (30%) and 6(30%) for the visible imager. With these allocations, the Science and Relay phases of the mission will fully map Mars in daytime IR and will map the planet 1.5 times in nighttime IR. The visible imaging will cover 60% of the planet at 18 meter resolution in one band (80,000 18x50Km frames) and <1% in 5-band color. Tradeoffs between monochromatic and multi-spectral imaging, as well as variations in the degree of lossy compression , will be made to maximize the science return from the visible imager. THEMIS data volume return varies significantly will mission phase due to variations n the Earth-Mars distance. In addition, the equator crossing local time will vary between ~15.5 H (24 H equals one martian day) and 18 H over the course of the mission. The first ~180-day phase of the mission provides some of the best viewing conditions for the IR imager and will allow ~40% of the planet to be observed. IR images acquried during this period will be carefully selected using the TES global mineral maps to focus on the sites of highest mineralogical or morphological interest. During a second IR mapping phase, when the data rates are again high and the local time close to 16.5 H, the remaining ~60% of the planet will be mapped. Principal Investigator ====================== Philip R. Christensen Arizona State University" END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "CHRISTENSEN2002" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "CHRISTENSENETAL2002" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END