The sensors can be divided into two independent suites according to the area scanned as the aircraft moves along its flight trajectory on a data collection run: the first is a forward-looking suite, with the sensor field-of-regard pointing generally ahead of and somewhat below the direction of flight; the second is a down-looking suite, with sensor field-of-regard pointing generally at the ground directly below the aircraft.
For a more complete description of the IRAR sensors, see the IRAR sensor description document.
The forward-looking suite comprises an optical system housed in a unit suspended below the aircraft fuselage and a millimeter-wave radar unit housed in the nosecone of the aircraft. The forward-looking suite can be operated in several modes which combine left-to-right scanning (azimuth variation) with vertical scanning (elevation variation) to generate images. The millimeter-wave radar unit is aimed so that its field of view coincides with that of the optical system, and it is slaved to the optical system so that they scan together. The optical system comprises active (laser radar) and passive imaging subsystems; the active long-wave infrared laser radar system, operating at 10.6 microns, employs coherent detection, and is capable of collecting range, intensity, and Doppler (relative velocity) information. The passive system operates in the 8-to-12-micron wavelength range. The active and passive subsystems share optics and are thus pixel-registered; that is, for each pixel of active data there is a corresponding pixel of passive data simultaneously recorded. There is also a video channel showing the output of a boresighted video camera to provide some idea of ground truth. The forward-looking suite is thus capable of producing up to four channels of scene information: boresight video, passive IR, active intensity, and active range, where active intensity represents the strength of laser radar returns, and active range represents the estimated range delay of the laser radar signal.
To Forward-Looking Sample Images
The Forward-Looking system supports two scan modes:
For a more complete description of the Forward-Looking Sensor Suite of the IRAR, see the appropriate sections of the IRAR sensor description document.
The down-looking suite comprises active and passive imaging systems. The active system
has two channels in the later data sets, one corresponding to near-visible infrared (0.85
microns), and one corresponding to long-wave infrared (10.6 microns). Both active systems
employ direct-detection receivers. The passive system
comprises a single channel in the long-wave infrared (LWIR, 8-to-12-micron) band. The
down-looking sensor is scanned in azimuth for cross-track variation, and the flight motion
of the aircraft provides the along-track variation required for imaging. The system scans
from -45 degrees to +45 degrees in azimuth, but for most of the data sets only the portion
of each scan from -22.5 to 22.5 degrees was recorded due to bandwidth limitations of the
recording equipment. The scan rate is controlled to provide contiguous ground coverage with
minimal pixel overlap. All four optical sensors in the down-looking suite are
pixel-registered.
In its most complete form, the down-looking suite had both GaAs and carbon-dioxide laser radars operational and provided long-wave infrared passive imaging as well. In this form the system was designated the Multispectral Active/Passive Sensor system (MAPS) and was used extensively to study the problem of distinguishing targets in clutter.
For a more complete description of the Down-Looking Sensor Suite of the IRAR, see the appropriate sections of the IRAR sensor description document.