The Airborne Hyperspectral Imager (AHI) was built to serve as the demonstration sensor for DARPA’s Hyperspectral Mine Detection (HMD) Program. The HMD program was charged with investigating, developing and demonstrating a hyperspectral infrared capability for remote buried mine detection. Beginning in late 1994, the program collected extensive non-imaging and imaging data of buried mines and mine surrogates in the 0.4 to 14 micron wavelength region. From the point spectrometer studies, it was determined that the long wave infrared region from 8 to 12 micrometers provided the most robust phenomena, with variations in the strength the silicate reststrahlen feature at 9.2 micrometers being the central spectral observable.

The phenomenology effort was used to define set of sensor requirements for the airborne demonstration system (Table 1). The nature of the silicate feature dictated that a sensor was needed that could spectrally resolve that feature and extend far enough to the longer wavelengths to obtain data entirely outside the feature. The requirement developed was to cover the spectral region from 8.3 - 11.0 micrometers with a spectral resolution greater than 32 wavenumbers. The point spectrometer data, and in particular the imaging spectrometer data, provided the basis for the detection algorithm development. These studies suggested that the allowable sensor noise level (expressed as noise equivalent spectral radiance, NESR) should be less than 0.01 watts/m2-sr-mm at 300K. These studies also showed that the mine detection observable was not always distinct from false alarm sources in the scene, such as certain mixtures of vegetation and undisturbed soil, but it was found that the statistical behavior of disturbed sites over emplaced mines was distinct from backgrounds with similar colors. This statistical phenomenon imposed a requirement for multiple pixels to be obtained for each mined site. A minimum number of 30 pixels on target was set to arrive at stable measurements of variance over mine-like areas.
 
Table 1. Phenomenology Driven Requirements
   
Spectral Range 8.3-11.0 microns
Spectral Resolution <32 wavenumbers
NESR <0.01 watt/m2-micron-sr
Pixels per target >30

In addition to the phenomenology-driven requirements, the HMD program imposed other requirements on the HMD Demonstration sensor (Table 2). These programmatic requirements included that the sensor be airborne and provide real-time detection of mines, that the demonstration be conducted in a realistic scenario, that the sensor design demonstrated a clear path to an operational system, and that the sensor be configured in such a way as to be able to be used as a general LWIR hyperspectral phenomenology collection platform.
 
 
Table 2. Programmatic Requirements  
   
Airborne  
Real-time detection  
Realistic demonstration scenario  
Path to an operational system  
Parallel use as phenomenology 
data collection platform		  

The AHI (Airborne Hyperspectral Imager) is the demonstration sensor built in response to the the requirements listed in Tables 1 and 2. AHI is an LWIR pushbroom hyperspectral sensor with gyroscopic image stabilization and radiance calibration. When configured for mine detection it is contained in an aerodynamic pod for external mount to a helicopter. The system also has a configuration for installation on fixed wing aircraft. The system includes a boresighted digital 3-color CCD linescan camera. The on-board data collection/ processing system includes a data processor producing data calibrated to radiance in real-time, and a near-real time algorithm processor producing mine detections. The on-board storage system is a RAID disk with 12 Gbyte capacity. The system also includes a ground data handling, archive and analysis system. Some of the system characteristics are shown in Table 3.
 
Table 3. AHI System Characteristics  
   
Spectral Range 7-11.5 mm
Spectral Resolution
and sampling		 125nm (32 bands)
100nm (256 bands)
Angular Resolution .9 by 2 mrad
Swath width 13 degrees (256 pixels)
Sensitivity (NeDT) <0.1 K