Thermal Scanner

Many multispectral (MSS) systems sense radiation in the thermal infrared as well as the visible and reflected infrared portions of the spectrum. However, remote sensing of energy emitted from the Earth's surface in the thermal infrared (3 ^m to 15 ^m) is different from the sensing of reflected energy. Thermal sensors use photo detectors sensitive to the direct contact of photons on their surface, to detect emitted thermal radiation. The detectors are cooled to temperatures close to absolute zero in order to limit their own thermal emissions. Thermal sensors essentially measure the surface temperature and thermal properties of targets.

Thermal Imagers are typically across-track scanners that detect emitted radiation in only the thermal portion of the spectrum. Thermal sensors employ one or more internal temperature references for comparison with the detected radiation, so they can be related to absolute radiant temperature. The data are generally recorded on film and/or magnetic tape and the temperature resolution of current sensors can reach 0.1 °C. For analysis, an image of relative radiant temperatures is depicted in grey levels, with warmer temperatures shown in light tones, and cooler temperatures in dark tones.

Table 1. Thermal Sensors

HCMM

TM

Operational period

1978-1980

1982 to present

Orbital altitude

620 mm

705 km

Image coverage

700 by 700 km

185 by 170 km

Acquisition time, day

1:30 p.m.

10:30 a.m.

Acquisition time, night

2:30 a.m.

9:30 p.m.

Visible and reflected IR detectors

Number of bands

1

6

Spectral range

0.5 0 - 1.1 pm

0.4 - 2.35 pm

Ground resolution cell

500 by 500 m

30 by 30 m

Thermal IR detector

Spectral range

10.5 - 12.5 pm

10.5 - 12.5 pm

Ground resolution cell

60 m by 60 m in Landsat 7

0 0

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