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petroleum exploration. Measuring 9.4 m long by 2.2 m wide and weighing 180 kg, SIR was a side-looking 'synthetic-aperture' radar that looked for all the world like an enormous rectangular dinner table filling almost half of Columbia's payload bay.

Assembled from spares left over from NASA's 1978 Seasat mission, it was mounted on its own truss structure which, in turn, was affixed to the Spacelab pallet, providing a viewing angle 47 degrees from nadir, so the Shuttle oriented itself to aim SIR at the ground. It covered the radio frequency of 2.175 GHz (L-band) and a wavelength of 23.5 cm and could construct two-dimensional radar images of the surface. It worked by transmitting microwave signals and receiving reflected 'echoes', recording data onto computer tapes for post-mission analysis. During STS-2, it would prove hugely successful and acquire some intriguing results.

Somewhat less visible on the Spacelab pallet, yet still capable of generating a tremendous amount of valuable scientific data, were the Shuttle Multispectral Infrared Radiometer (SMIRR), the Feature Identification and Location Experiment (FILE), the Ocean Colour Experiment (OCE) and the Measurement of Air Pollution by Satellite (MAPS).

Integration of these five experiments onto the Spacelab pallet was completed in the Operations and Checkout Building at KSC in the early part of 1981 and on 1 July the full OSTA-1 payload was loaded on board Columbia. A series of tests verified its compatibility with the Shuttle, and the ability of the crew to switch it on and off, and command the instruments, from the flight deck. On 10 August, she was rolled over to the VAB for attachment to her ET and SRBs and from thence to Pad 39A for final pre-launch preparations.

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