316
Chapter 30
These indications demonstrate the small degree of freedom in the
instrument’s design approach in terms of selectable or scalable parameters. A
high radiometric resolution (either expressed in terms of
with low
values being
or in terms of encoding bits) at a given orbit altitude and
Sun zenith angle more or less automatically results in large aperture
requirements.
The main design-driving parameters and requirements are briefly
discussed below. We should underline that we have, based on frequent
contacts with various science and application users, a rather fair
understanding of these requirements:
Orbit altitude: Impact on radiometric performance, see above, but also
relative velocity (decreasing towards higher altitudes); impact on analog
and digital readout electronics frequency
Ground pixel size (or IFoV): Together with radiometric resolution most
decisive parameter for radiometric aperture, effective focal length
definition. Hence this affects instrument size and mass, as well as analog
and digital processing electronics. Example: a 30 m SSP compared to 50
m brings either the radiometric performance down by a factor of 3.6, or
the aperture needs to be increased by factor of 1.9 to maintain the
performance, with a nearly doubled
FoV or Swath width: At least for the pushbroom type of instrument, a
two-fold critical parameter is relevant. In terms of choice of optics,
mostly introducing aspheric elements to yield FPA planarity and slit
curvature correction, plus limited number of spatial pixels for 2-d
detector arrays, for wavelengths
typically to
spatial
pixels unless optical butting is applied. For IS, the large number of
spectral channels combined with a medium to high geometric resolution,
also a design driver for the onboard storage or telemetry data rate
Swath variation or pointing (across-track for more rapid target access,
and/or along-track, e.g., for BRDF measurements): Particularly 2-axis
mechanisms are technically cumbersome in space. A single axis pointing
leaves the choice of rotation in the optical axis with the negative effect
of image distortion at increasing angles, and rotation of the mirror
surface causes incidence angle depending transmission and polarization
changes, which need to be calibrated out
Radiometric resolution (in
or
or bit,
):
Impact on radiometric aperture, see above, optical throughput, i.e.,
choice of optics, particularly the disperser, ADC, signal processing
electronics, and, most of all, calibration accuracy
Radiometric accuracy (mostly expressed in % or
Impact on
calibration unit and choice, thermal stability particularly of the FPA,
widely dependent on detector noise characteristics
Chapter 30
These indications demonstrate the small degree of freedom in the
instrument’s design approach in terms of selectable or scalable parameters. A
high radiometric resolution (either expressed in terms of
with low
values being
or in terms of encoding bits) at a given orbit altitude and
Sun zenith angle more or less automatically results in large aperture
requirements.
The main design-driving parameters and requirements are briefly
discussed below. We should underline that we have, based on frequent
contacts with various science and application users, a rather fair
understanding of these requirements:
Orbit altitude: Impact on radiometric performance, see above, but also
relative velocity (decreasing towards higher altitudes); impact on analog
and digital readout electronics frequency
Ground pixel size (or IFoV): Together with radiometric resolution most
decisive parameter for radiometric aperture, effective focal length
definition. Hence this affects instrument size and mass, as well as analog
and digital processing electronics. Example: a 30 m SSP compared to 50
m brings either the radiometric performance down by a factor of 3.6, or
the aperture needs to be increased by factor of 1.9 to maintain the
performance, with a nearly doubled
FoV or Swath width: At least for the pushbroom type of instrument, a
two-fold critical parameter is relevant. In terms of choice of optics,
mostly introducing aspheric elements to yield FPA planarity and slit
curvature correction, plus limited number of spatial pixels for 2-d
detector arrays, for wavelengths
typically to
spatial
pixels unless optical butting is applied. For IS, the large number of
spectral channels combined with a medium to high geometric resolution,
also a design driver for the onboard storage or telemetry data rate
Swath variation or pointing (across-track for more rapid target access,
and/or along-track, e.g., for BRDF measurements): Particularly 2-axis
mechanisms are technically cumbersome in space. A single axis pointing
leaves the choice of rotation in the optical axis with the negative effect
of image distortion at increasing angles, and rotation of the mirror
surface causes incidence angle depending transmission and polarization
changes, which need to be calibrated out
Radiometric resolution (in
or
or bit,
):
Impact on radiometric aperture, see above, optical throughput, i.e.,
choice of optics, particularly the disperser, ADC, signal processing
electronics, and, most of all, calibration accuracy
Radiometric accuracy (mostly expressed in % or
Impact on
calibration unit and choice, thermal stability particularly of the FPA,
widely dependent on detector noise characteristics