The CHRIS Instrument

CHRIS (Compact High Resolution Imaging Spectrometer) is a physically compact payload as its name implies (weighing under 15 kg) and operates in the 'push-broom' mode shown schematically in figure 1. The telescope in the CHRIS instrument is nadir pointing as is shown in figure 1. CHRIS has the additional advantage of being relatively cheap and easy to manufacture since it has no moving parts. Its main applications will be in environmental monitoring, forestry inventory and precision farming. A schematic representation of the instrument optics is shown in figure 2 and a computer aided 'cut-away' drawing is displayed in figure 3.

The first version of the instrument will fly on the PROBA (Project for On Board Autonomy) platform. It will carry the instrument in a sun-synchronous elliptical polar orbit, at a mean altitude of about 600 km . All parts of the Earth's surface will be accessible when PROBA's across track pointing ability is used.

From a 600 km orbit, CHRIS can image the Earth in a 14 km swath with a spatial resolution of 18 m (this is somewhat variable as the altitude varies artound the orbit). Using PROBA's agile steering capabilities in along and across track directions enables observation of selectable targets well outside the nominal field of view of 1.3^o . Images will generally be acquired in sets of 5, these being taken at along track angles of +/- 55 degrees. +/- 36 degrees, and as close to nadir as possible.

CHRIS operates over the visible/near infrared band from 400 nm to 1050 nm and can operate in 63 spectral bands at a spatial resolution of 36m, or with 18 bands at full spatial resolution. Spectral sampling varies from 2-3 nm at the blue end of the spectrum, to about 12 nm at 1050nm. Sampling is about 7nm near the red edge (~690-740nm). The instrument is very flexible and different sets of bands can beused for different applications. The bandsets principally used during the mission can be found here.

Key Characteristics:
 
 

Spatial sampling interval	18m on ground at nadir
Image area	14 km X 14 km  (748 X 748 pixels)
Number of images	Nominal is 5 downloads at different view angles
Data per image  (for a 14 X 14 km2)	131 Mbits
Spectral range	410nm to 1050 nm
Number of spectral bands	19 bands at a spatial resolution of 18m, 63 at 36m
Spectral resolution	1.3 nm @ 410nm  to 12 nm @ 1050nm (i.e it varies across the spectrum)
Programmable operation:
Across track pixel size	18m or 36m
Along track pixel siz	finest resolution is 18m but can be made coarser by changing the integration time
Spectral	variable bandwidth and location
Digitisation	12 bits
Signal-to-noise ratio	200 @ a target albedo of 0.2

There is a trade-off between the number of bands that can be output and the image spatial resolution because there is a finite data transfer rate between the groundstation and the platform. The spatial and spectral resolutions can be altered by 'binning 'charges on the CCD detector array. Data rates can be reduced by limiting the number of across track pixels output or by operating at a coarse along track resolution. The along track resolution is determined by the detector integration time. By controlling the pitch of the platform the integration time can be varied so that signal to noise ratios can be increased for low albedo targets.

The instrument will be calibrated for absolute response in flight by

i) Using sunlight collected by the solar calibration device.
ii) Vicarious calibration using sites with stable reflectance properties and using measurements of surface reflectance and atmospheric properties at the time of overflight.

The instrument will be calibrated spectrally in flight by

i) Using known atmospheric absorption features when the instrument is staring at a 'bland' earth scene.
ii) Vicarious calibration using sites with stable spectral reflectance properties.

Page last modified 9 March 2002