CHRIS INSTRUMENT

Concept
The CHRIS instrument (Figure 2) is a conventional imaging spectrometer with a telescope forming an image of the Earth's surface onto the entrance slit of a spectrometer, and an area-array detector at the spectrometer focal plane. It operates in a push-broom mode. The detector is a thinned, back illuminated, frame-transfer CCD, with the CCD rows being assigned to different wavelengths and the CCD columns to separate resolved areas on the Earth's surface.

The platform will be required to provide pointing in both across-track and along-track directions, for target acquisition and for BRDF measurements. The platform will also be required to provide slow pitch during imaging in order to increase the integration time of the instrument. This increase in integration time is needed to achieve the target radiometric resolution, at the baseline spatial and spectral sampling interval.

The spectral waveband covered by the instrument is limited to the band 400nm to 1050nm, which can be achieved using a single CCD area-array detector. In a later development, there are plans to add the short-wave IR range up to 1700nm (for additional vegetation and moisture features) or to 2500nm (to include features of minerals). For this reason, the design form selected for the spectrometer is capable of extension to cover the whole spectral range from 400nm to 2500nm by addition of a SWIR detector array.

The instrument will be radiometrically calibrated in flight by the use of (a) a dark scene, and (b) sunlight deflected into the instrument field, through optics of stable transmission and geometrical spread. This will provide data in calibration mode that will be used for flat-fielding and for absolute and relative spectral response measurement. Wavelength calibration may be corrected using data generated in flight from the atmosphere oxygen absorption band at 762nm.

Design
The instrument optical design is shown in Figure 3. The system comprises a catedioptric telescope and an imaging spectrometer. There are no moving parts.

A catedioptric design is used for the telescope, to provide the required spectral range without aspherics or off-axis elements. All refracting elements in the present design, including the large refracting element near the focal plane, are made of fused quartz. The secondary mirror, which is cemented to the first large refracting element, is also fused quartz. The telescope primary mirror is made of common optical glass. This choice is made, in order to athermalise the telescope (i.e. to limit change of focus with temperature change). Athermalisation depends mainly on the materials of the primary mirror and the telescope structure: a titanium structure, with a glass mirror, will allow athermalisation over a large temperature range.

The spectrometer is a design recently patented by Sira Electro-optics Ltd. It uses "prisms" with curved surfaces integrated into a modified Offner relay. Curved prisms have been suggested on many occasions, but they have seldom been used in practice because of problems with performance and/or construction. The design used by Sira has only spherical surfaces, and uses only one material - fused quartz - for the prisms. The spectrometer mirrors are made in a common optical glass. The baseline design shown in Figure 2 has 3 mirrors and 2 curved prisms. As for the telescope, all surfaces are spherical. The dispersion of the spectrometer varies from 1.25 to 11nm across the spectrum, with the highest dispersion at 400nm and the lowest in the near infrared at 1050nm. Measurements of the spectrometer registration, during ground tests, were shown to be better than 5% of the pixel both spectrally and spatially, with resolution essentially limited by the detector pixel size.

   
The CCD detector is an MAT device with the following features:
  • CCD area array
  • Frame transfer
  • Thinned and back-illuminated (providing good blue response)
  • 770 nominal columns
  • 576 lines rows of which approx. 200 are assigned for spectral sammpling in the CHRIS instrument - others are used for smear correction)
  • dump gate (providing fast parallel dumping)

The instrument electronics will include:

  • programmed line integration and dumping on chip for spectral band selection
  • pixel integration on chip for spatial resolution control
  • correlated double sampling (noise reduction circuit)
  • dynamic gain switch for optimum usage of 12 bit ADC resolution.

The platform will receive demands from ground control for:

  • target location - requiring roll manoeuvres to point across-track
  • viewing directions for each target in one orbit - requiring pitch manoeuvres to point along-track
  • spectral bands and spectral sampling interval in each band
  • spatial sampling interval

The platform will perform the required pitch and roll manoeuvres, and transmit control signals to CHRIS to initiate and terminate imaging with the required spectral and spatial characteristics.

The platform will receive digitised data from CHRIS, store the data in a mass memory unit and transmit to ground on command.

The platform will operate at an altitude of approximately 830km in a near circular, polar and sun-synchronous orbit.

Data Specifications
CHRIS will provide hyperspectral images with ground coverage of 17.5km x 17.5km at an altitude of 580km. The standard mode of operation will be with 18 spectral bands at the highest spatial resolution. Specifications of the CHRIS data to be acquired within the PROBA/CHRIS experiment are as follows:

Spatial sampling interval17.5m on ground at nadir (580km altitude)
Image area13km x 13km (744 x748 pixels) @ 580KM ALTITUDE
Number of images5 per download (nominal)
Data per image (13x13km2)131 Mbits
Spectral range400nm to 1050nm
Number of spectral bands19 @ 25m, 62 @ 50m
Spectral resolution1.25 to 11nm (varies across spectrum)
Selectable spatial resolution25m or 50m across track
25 to 50m along track (via integration time)
Digitisation12 bits
Signal-to-noise ratio200 (@target albedo 0.2)
Power consumptionapprox. 9 W primary power
Massapprox. 14 kg
Volume790 x 260 x 200 mm3

Platform and Operational Characterisics
Details of the satellite orbit, pointing and data transmission capabilities are as follows:
Platform ESA PROBA:
Orbit558-676km, circular, sun -synch.
Inclination and period97.8, 97 min
Repeat periodapprox. 16 days
Pointing:
along-trackup to 38 (@satellite)
across-trackup to 25 (@satellite)
yaw steeringup to 24 (@satellite) n.b. only to be used for calibration
Image ProcessingTemic (DSP) TCS21020
Uplink S-band2 to 4 kbit/s packet TC link
Downlinkup to 1 Mbit/s packet TM link

 

 

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