SMAP Orbital Motion
In order to achieve large coverage on the ground the antenna spins at 14.6 revolutions per minute (one revolution every 4 seconds). SMAP’s orbital motion combined with the spin of the antenna sweeps a small field of view in a series of overlapping loops that create a swath 1000 km (621 miles) wide. This large swath coverage allows SMAP to make complete soil moisture maps of the Earth repeated every 2 to 3 days. SMAP’s orbit is 685 km (426 miles) above Earth’s surface. Because Earth spins while SMAP orbits, swaths from each orbit are offset from each other and after 8 days, the same swath is repeated. Data taken by SMAP’s two instruments run from pole to pole. The orbit is oriented such that it crosses Earth’s terminator at the equator. (The terminator is the boundary between day and night, either at sunrise or at sunset).