EnMAP HYPERSPECTRAL IMAGER (HSI) EnMAP is a hyperspectral imager of push broom type with two separate spectral channels: one for VNIR range from 420 to 1000 nm and one for the SWIR range from 900 to 2450 nm. Both channels share a common telescope (TMA) with a field splitter placed in the telescope focal plane. The field splitter features two entrance slits – one for each spectral channel. By placing a micro mirror directly behind the entrance slit of the SWIR channel both channels can be separated and fed into distinct spectrometer branches. Furthermore both spectrometers are designed as prism spectrometers thus providing highest possible optical transmission with low polarization sensitivity.
3-D cut view illustrating the internal accommodation of the optical system inside the IOU and showing the light path.
The sensor covers a swath width (= across-track) of 30 km, with a Ground Sampling Distance (GSD) of 30 x 30 m. The 2nd dimension is given by the along-track movement and corresponds to about 4.4 ms exposure time. This leads to a detector frame rate of ca. 230 Hz, which is a performance driving parameter for the detectors as well as the instrument control unit and the mass memory. The chosen sun-synchronous orbit and a +/-30° off-nadir pointing feature, each point on earth can be investigated and revisited within 4 days. The sun-synchronous orbit rather more enables the satellite to pass over any given point of the Earth's surface at the same local solar time, which results in a consistent illumination.
Split FOV Pushbroom Imager
The sensors are able to collect up to 5000 km data length per day, which means up to 1000 km within one orbit. To provide a fast transfer of this data down to earth, the downlink works via X -band with 320 Mbps. The estimated average duration for the transfer of the data of one day is 31 minutes.
In order to achieve the required SNR, detectors and front-end electronics are being developed with low noise and high quantum efficiency characteristics. The SWIR detector has to be actively cooled down to 150 K (-123°C) using a pulse tube cooler with flexure bearing compressor. The VNIR detector is thermally controlled to 0.1 K by means of thermoelectric cooling (Peltier). 14 bit quantification will be applied to be compliant with the high dynamic range of the input signal.