SWS - Short Wave Spectrometer

Using simple telescopes and a 45° mirror in a rotating scan drum, the instrument measures visible and near-infrared radiation from 303.4nm to 1706.5nm (resolution 3.2nm up to 948.7nm, 6.3nm thereafter). The scan drum allows the instrument to be pointed upwards and forwards 64°, and downward and rearwards 60°. The variable downward scan positions are so that different path lengths through the atmosphere can be measured. The variable upward positions are so that the mirror can be pointed at the sun (by flying directly into the sun), to use the instrument as a sun photometer. In each scan position the instrument will have a total field of view of 3° to 4°.

Instrument

The SWS is a visible/near infrared radiance spectrometer. In its concept it is a combination of Monolithic Miniature Spectrometer (MMS) modules from Carl Zeiss Ltd with a scanning optic head and controlling software designed by the Met Office.

The MMS modules consist of a spectrometer body made of UBK 7 glass with an aberration-corrected grating, a fibre cross section converter as optical entrance and a diode array. The cross-section converter and the diode array are fixed to the glass body.

The optic head is mounted on a window blank of the BAe 146-301 (see Figure 1 below) on the starboard side of the aircraft aft of the wings. The optic head can scan through 360 degrees, though some viewing angles are restricted by the wings and tailplane.

The optic head reflects incoming light through 90 degrees onto two telescopes (See Figure 2 below). Optical fibres take the light from the telescopes to the spectrometer modules. At present the instrument has 2 MMS modules:

  • a visible module covering the wavelengths 300-1134 nm.
    This module has a silicon diode detector array with 256 pixels;
  • a near-infrared module covering the wavelengths 941-1706 nm.
    This module has an indium gallium arsenide detector array with 128 pixels.

The modular design means that any other MMS module can easily be connected to the optics. The spectrometer modules are housed in a fridge (see Figure 3) to ensure temperature control of the electronic components which increases the signal to noise ratio.

SWS Optical Head
Figure 1: Location of the optic head for the SWS on the BAe 146-301. The optics window is pointing downwards. Figure 2: Section through the SWS optics. Light enters through the vertically facing window, is reflected through 90 degrees to the telescopes. Figure 3: The SWS rack and operator's seat on the BAe 146-301. The spectrometer modules are in the fridge on the bottom of the rack. The optics are outside the green window blank. Fibre optic cables link the optics to the spectrometer modules.

Further Details

For mor details contact Rob King, Met Office

Additional information