Encyclopedia Astronautica
1998.07.10 - TMSAT


Customer: Thailand (Thai Microsatellite Company and MUT). Thailand's first microsatellite built through a technology transfer programme with Surrey Satellite Technology Ltd. Carried store and forward and Earth observation payloads. Still operational as of 2000.

TMSAT Cameras

Wide Angle Cameras (WAC): The TMSat WAC used a CCD sensor made by EEV Ltd. with 568 x 560 active imaging pixels. The camera was fitted with an ultra wide angle lens (focal length 4.8 mm) to give a coverage area of 1500 x 1050 km at a mean resolution of 2 km per pixel (from TMSat's 816 km orbit). The FASat-Bravo mission carried an Identical WAC.

The camera was fitted with a near-IR optical filter (810-890 nm) to provide strong contrast between land, sea and clouds (NB. Although invisible to the human eye, these wavelengths were very close to the visible range, and behave in the same way. Thus the camera was recording light from the Sun, reflected off the Earth's surface and atmosphere. This should not be confused with thermal infra-red imaging, where the Sun's energy was absorbed by the Earth and then reradiated)

Narrow Angle Cameras (NAC) This camera design was a new development for the TMSat mission and used a 1020 x 1020 pixel CCD sensor manufactured by Eastman-Kodak. The three NACs were electrically identical, as were the lenses (75 mm focal length) used to provide 100 metre resolution from TMSat's 800 km orbit.

However, the cameras were fitted with different optical filters, each sensing in a different part of the optical spectrum. These were NAC0 in the near-IR (810 - 890 nm), NAC1 in the red (610 - 690 nm) and NAC2 in the green (510 - 590 nm). When the cameras were operated simultaneously, the three images can be combined to produce multispectral (false-colour) data set.

The misalignments (caused by x-y translations, rotations, scaling and pincushioning) must be analysed and compensated for. This process involved manually identifying several dozen 'ground reference points' common to all three images, and computing the appropriate inverse transformations. Fortunately, this labour-intensive process would only need to be performed a few times at the start of the mission, allowing a 'standard' transformation matrix to be defined.

Whereas the WAC images were 330 kBytes, each of the NAC images was 1 MByte. This would place a significant burden on the downlink capacity of the microsatellite, re-emphasising the need for both on-board image compression and faster downlink data rates.

CMOS VIDEO CAMERA

Image Size: 382 x 287 Pixels, Resolution: 400 m Field of View 10.5o x 7.8o Coverage Area: 150 km x 110 km Optical Band: 350 nm - 750 nm (Visible) Lens: 25 mm, f/8 Pixel Size: 12 mm x 12 mm Array Size: 4.6 mm x 3.4 mm Frame Rate: 50 frames per second, CCIR TV Format. Exposure: Automatic (On-Chip H/W Controlled - 25,000:1 Dynamic Range) Payload Power: 200 mA at 5V = 1W Continuous Operation. Payload Storage: 4 Mbytes (= 30 Images)

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