PRISM comprises a pushbroom imaging spectrometer covering the near UV to near IR range (350-1050 nm) and a separate spot radiometer covering two short wave infrared (SWIR) bands at 1240 and 1610 nm respectively.

The complete sensor comprises the optical head assembly (OHA) and control electronics. At about 40 kg, the OHA comprises the vacuum vessel with the spectrometer and SWIR radiometer inside, an aircraft interface/mounting plate, an INS/GPS unit attached to the optical head for enabling precise orthorectification and geolocation, as well as the preamplifier and A-D converter for the detector electronics. The vacuum vessel shell is temperature-stabilized to around 26°C with external strip heaters. Required vacuum level for stable operation is < 3 mTorr. The instrument retains this vacuum level through a typical flight without on-board pumping. Three commercial vibration isolators prevent high frequency vibration from transmitting to the optics and detectors.

The control electronics can be configured in one or two racks. The control computer passes the high speed data stream to a CORE® DVR data recording unit where the data is stored on solid state drives with a total capacity of 1.5TB (expandable). The drives are removed after the flight and the data downloaded to another computer for same-day processing. The remaining electronics are dedicated to thermal control and data logging. The temperature of the instruments inside the vacuum vessel is kept at 25°C through bi-directional thermoelectric coolers. The spectrometer detector is running at a temperature of about 7°C. Shutters for the SWIR radiometer and spectrometer provide dark frame acquisition.

The operator’s task is to click on a software “record” or “stop” button at the beginning and end of a flight line. Dark frames are acquired automatically at those times. If desired, the computer displays temperature data from the various sensor locations in the OHA, in addition to a “waterfall” display of the data being acquired. Data are digitized to 14 bits and output to an IO Industries CORE® recorder utilizing removable solid state drives. Software control of the PRISM instrument is provided by a specially developed Windows application, which communicates with IO Industries software through an IOI-provided library. The software is used to start and stop recording, and to export image data post-flight. Geolocation data are acquired from a Systron-Donner C-MIGITS III unit and correlated to the FPA readout within ±100 μs to ensure pointing knowledge within one IFOV.

In deployment, PRISM is accompanied by a calibration monitoring assembly comprising a 12” diameter integrating sphere with customized port, stabilized tungsten halogen source, calibrated broadband detector, and low-pressure Hg lamp source. The integrating sphere is hermetically mated to the spectrometer and SWIR radiometer window using a simple clip-on mechanism. To confirm stability, a reading of the sphere output may be taken by the instrument before and after the flight without removing it from the aircraft mount.

Optical Sensor Schematic

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PRISM Optical Head Assembly

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PRISM control electronics, separated here into an operator rack and a control rack. An Uninterruptible Power Supply (not shown) provides power between hangar and aircraft power during take-off and landing.

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Integrating sphere for radiometric and spectral calibration.