The Lincoln Calibration Sphere 1, or LCS-1, is a large, hollow, aluminum sphere with a precisely defined cross-section that has been on Earth orbit since 1965. It is considered to be the oldest functioning satellite even though it has no power and no electronics. This is because, this 3 feet 8 inch diameter metal sphere is still true to its original purpose—provide ground based radars with a target to calibrate their instruments.
To calibrate radar, it's necessary to have a reference point of known data against which the radar output can be compared. This enables fine-tuning of the radar to ensure that its output matches the expected results. Calibration spheres are designed to have a consistent Radar Cross Section (RCS) value, making them an effective tool for calibrating. However, if more accurate tracking tools, such as lasers, are available for satellite tracking, the same satellites could also be used to calibrate radar position data.
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Source: amusingplanet.com