If scale is not included, the user can often calculate it from objects of known size. Football fields, baseball fields and section lines make good standards for calculating scale. Electronic systems use electromechanical means to recieve data. These systems are used primarily on orbiting platforms. Electronic systems can be either active or passive.
Active systems emit their own radiation.
SLAR and SIR systems emit radar waves from an antenna and record the reflectance electronically. Passive systems rel on the sun's radiation for illumination. Landsat has a photosensitive grid with an oscillating mirror. SPOT uses a linear array to recieve data. In general, commercial satellites with higher spatial resolution have lower spectral resolution.