CASI

• Background information:

  The CASI 2, produced by Itres Research of Canada, is a two-dimensional CCD array-based pushbroom imaging spectrograph.

  The CASI "hyperspectral imager" - spectral range: 400-915nm - is fully programmable prior to, and during flight operations, and can be operated in a series of different modes depending on the requirements of the project application. Its operating modes consist of:

  • Spatial Mode: 512 pixels across swath, up to 18 spectral bands (fully programmable).
  • Spectral Mode: Full spectrum (288 channels) for up to 39 look directions spread across swath (4-, 8-, 12-, or 16-pixel spacing between look directions). This also includes a monochromatic image at full spatial resolution - the Scene Recovery Channel.
  • Enhanced Spectral Mode: Full spectrum (288 channels) in a block of 101 adjacent spatial pixels.
  • Full Frame: 512 pixels across swath x 288 spectral pixels (~1-2 sec. integration time ). This limits this mode to laboratory calibration or ground-based field use.

  Specification history:

  • Original specification

    Property	Details
    Field of View	38.4 ° across-track (custom fore-optics lens)
    Aperture	f/2.8 - f/11 (Automated iris control)
    Spectral range	401 - 915 nm
    Spatial sampling	512 spatial pixels
    Spectral sampling	288 at 1.8 nm intervals (3 nm FWHM @ 650 nm)
    Dynamic range	12-bit (4096 levels)

    Table 1. Specification of the original CASI sensor flown by the NERC Airborne Research and Survey Facility (ARSF) from March 1994

  • Subsequent Modifications
  • From February 1995, Field of view: 42 °, Motorised iris.
  • From February 1996, Field of view: 44.6 °, Spectral range: 405 - 948 nm.
  • From January 2000 - CASI 2 - Field of View: 37.8 °, Spectral Range: 428 - 976 nm.
  • From January 2001, Field of View: 54.4 °, Spectral Range: 405 - 950 nm.
  • Data acquisition modes:

    Although it is possible to image all 512 spatial pixels and all 288 spectral channels simultaneously (Full Frame Mode), in practice a compromise is required to avoid excessive smearing of along track pixels caused by the length of time taken to perform the read out for all elements of the array. The compromise takes the form of using either of two main operating modes - spatial and spectral modes - which reduce the data recording requirements.

  • Spatial Mode & Default Bandsets:

    Spatial mode records data from all 512 spatial dimension pixels, but from a limited number of programmable spectral bands (max. 18) by selection and summation of the 1.8 nm resolution detector elements. This provides an equivalent, spatially contiguous, multi-band data set to that recorded by the other airborne sensor - the Airborne Thematic Mapper (ATM). A spatial bandset can be formed from single detector elements or unique summations of two or more adjacent detector applications. elements located throughout the entire spectral range of the casi (400-915 nm). A single detector element cannot be summed into more than one spatial band, even if the spatial bands are adjacent. The number, locations and bandwidths of the different bands can be set up, according to the user application, to cover either broad spectrally different regions eg. blue, green, red, and near-infrared, or at precisely placed narrow wavelengths to measure specific phenomena eg. chlorophyll fluorescence, red-edge position, or atmospheric absorptions.

  • Spectral Mode & Scene Recovery Channel:

    In spectral mode the full spectral profile of 288 channels can be recorded, but from a limited number of look directions, or pixel positions, spread as a rake across the ground (max. 39). The 39 'tines' of this 'push-rake spectrometer' mode can be located with either 4, 8, 12 or 16 non-imaged pixels (look spacing) between each recorded spatial pixel, according to the user requirement; concentrating pixels together for high spatial resolution or spreading them out across the entire swath for good spatial coverage. Due to the missing spatial pixels it may sometimes be difficult to precisely locate the rake of spectral pixels within a scene. To offset this problem the Spectral Mode simultaneously produces a single full spatial band, formed from a single detector element, which is inherently co-registered with the separate look-directions. Using this Scene Recovery Channel (SRC) it is possible to locate the pixels positions of the spectral data within the scene. When selecting Spectral Mode for an airborne remote sensing mission the operator needs to program the casi with the spread of the push-rake, either 4, 8, 12 or 16 non- imaged pixels between the 'tines', and the channel wavelength (nm) for the SRC.

  • Enhanced Spectral Mode

    In the Enhanced Spectral Mode the full 3D data cube can be alternatively sliced into a single block of 101 adjacent spatial pixels imaged for all 288 spectral channels. This mode can be further extended into a series with increasing spatial coverage (more spatial pixels) at the expense of decreasing spectral resolution (fewer, wider channels). Each output band is formed by either a single detector element (101 x 288) or summations of adjacent detectors ie. summations of CCD row l+2, etc. or l+2+3, etc. thus maintaining a consistent integration time. The next table shows some typical examples of the look directions versus number of spectral channels, indicating the effect on frame rate and pixel resolution.

  • Geometrically Corrected Data Products

    Casi data acquired by the NERC Airborne Research and?Survey Facility from July 1994 can be geometrically registered to a map coordinate system using real-time measurements of aircraft attitude during project overflights.

    Look directions	No. of spectral channels	Spectral resolution (nm)	Approximate frame rate (frames/sec)	Spatial Resolution WxL (m)	Altitude (m above ground level)
    101	101	203	303	511	3049
    288	48	144	96	48	1000
    1.8	10.8	3.6	5.6	10.8	3049
    7	38	7	7	9	3049
    3.7 x 7.3	1.2 x 1.2	3.7 x 7.3	3.7 x 7.3	3.7 x 7.3	3049

    Table 2. Typical possible configuration in Enhanced Spectral Mode. Note: Aircraft speed over ground is assumed to be 100knots (51 m/s) for the above calculations.

    (However, both the Spectral and Enhanced Spectral Modes have a distinct drawback, since integration times are much longer than for Spatial Mode. This precludes the general use of these high data rate modes at low altitudes where fast integration times are required to maintain the squareness of pixels (pixel widths being smaller because of the low altitude). If this is not attainable at the altitude required, an aspect ratio (pixel length to width) of 2:1 or more may have to be selected by the operator).

  • Full Frame Mode:

    Full Frame Mode is generally only used during laboratory calibrations since the integration times (1-2 seconds) are too long to be used during airborne flights without tremendous smearing of the image. However, it may be possible to use this mode when the casi is mounted on a ground-based motion tripod to view static objects for use in, for example, colour perception studies.

  • Additional CASI Sensor

    The NERC ARSF CASI System also incorporates an Incident Light Sensor (ILS) measuring downwelling radiation at the aircraft; this is Lumogen coated for enhancement of the blue response below 450 nm.