The AHI optical system was designed and constructed by the University of Hawaii’s Hawaii Institute of Geophysics and Planetology. The AHI system consists of an LWIR pushbroom hyperspectral imager and a boresighted 3 color visible high resolution CCD linescan camera. The IR sensor consists of four subsystems: Telescope, spectrograph, background suppressor and FPA and associated electronics. The telescope is a 2 element diffraction limited transmission lens with 111 mm focal length and an 35 mm clear aperture. The spectrograph is an uncooled commercial reflective f/4 imaging spectrograph with gold coated optics. We are able to use an uncooled spectrograph and still achieve high performance because of the AHI background suppressor described below. The AHI uses a 256x256 element Rockwell TCM2250 HgCdTe focal plane array (FPA) mechanically cooled to 56K. In addition to having high quantum efficiency and pixel yields, it has excellent residual nonuniformity after correction (0.08%), crucial to our application.

AHI Calibration

The AHI IR system is radiometrically calibrated using three flat panel blackbodies deployed in a rotating pan which brings them in front of the entire optical train. Two of these panels are used to produce pixel-dependent gain and offset corrections from DN to radiance. One of these two panels is set to the air temperature near the ground to include most of the temperature of vegetation and the second panel is adjusted to the approximate mean temperature of hot objects in the scene, usually soil or man-made objects illuminated by the sun. The third panel is set to a temperature intermediate between the two and is used to calculate signal to noise ratio. Data for the intermediate temperature blackbody is collected and reduced to radiance as if it were target data, and the signal to noise is calculated over all the calibrated pixels for each wavelength so that spatial nonuniformity noise is included in the calculations. For many sensors spatial nonuniformity artifacts are the major source of noise so it is important to include this term in SNR measurements. Calibrations are performed frequently as we have found that calibration degrades relatively rapidly with time, with signal to noise dropping by a factor of two over the period of an hour and 10% in the first 5 minutes owing to increased residual nonuniformity noise.

Wavelength calibration is accomplished by viewing our highest temperature blackbody through two types of filters, polystyrene plastic film and a liquid indene cell. The polystyrene film is convenient to use in the field and has sufficient spectral absorptions to discern any problems with wavelength calibrations. The indene cell is less convenient, but possesses 11 very sharp absorptions in the AHI wavelength range and is used for construction of the wavelength table.

Processing Hardware

The AHI on-board processor has several tasks: sensor control, generation of calibration coefficients, geometric preprocessing, real time radiometric calibration, near real time detection algorithm processing, user interface and data recording. Figure 3 shows a block diagram of the system. The flight data processing system inside the helicopter is linked to the pod with a 200Mbit/s fiber optic data link. The processor receives the raw 12-bit digital data from the FPA and passes the raw data to a Xilinx field programmable gate array (FPGA) which spectrally bins the data into a 256x32 data array or temporally bins 8 frames into a 256x256 average array. In the spectrally binned mode, the process excludes pixels which have been previously defined to be "bad" due to poor correctibility. The Xilinx passes the binned data to a Sharc digital signal processor (DSP) which applies a set of previously defined set of calibration coefficients to output data calibrated to radiance (calibration coefficients are derived from observations of the on-board calibration sources). These data are passed to a second Sharc DSP which computes the principal components of the data. The principal components are passed to a third Sharc DSP which applies the detection algorithm to the principal components. The raw (binned) and calibrated data, and the principal components are passed via an output FIFO on a fourth Sharc to the Pentium PC and RAID array for storage.

Along with the hyperspectral data, output from a boresighted CCD linescan camera is also placed into the data stream. The 3-band color CCD camera has twice the swath width of the LWIR hyperspectral data, twice the resolution, and is sampled at twice the rate (300Hz) to provide a very high quality color context video. Both the optical and IR hyperspectral systems view the scene through a 3-axis (roll, pitch and yaw) gyroscopically stabilized mirror system to remove the effects of aircraft rotations. The AHI on-board computer has custom software to control the sensor, control the digital signal processors, save data to disk and to provide a real time display. The sensor control functions include control of the pod environmental shutter, the blackbodies, as well as all measurement modes. A series of sensor functions is also continually monitored in real time. The on-board software controls the functions of the DSP boards, including downloading programs and switching modes. The real time calibrated and uncalibrated data can be displayed in a series of waterfall plots. These plots show the infrared or color CCD data as it is being collected. Several plot functions are also available to determine the performance of the sensor as the data is collected including noise histograms, and radiance plots and histograms.