We have neglected one final, rather important point. For high-spatial-resolution systems, satellite movement becomes an issue because of motion blur, just as occurs in regular photography of rapidly moving targets. A simple calculation can estimate exposure time. CCD arrays have sensitivity not unlike regular daylight film, with a standard speed defined as ISO 100. An old photographic rule of thumb is that the exposure time at f/11 tof/16 is 1/ISO, or in this case 1/100 s. Thef/14 IKONOS optics provide sufficient light for 1/100 s (10 ms) exposure. The satellite moves approximately 70 m in that time— much too far for a clear image. Modern sensors are somewhat more sensitive than given here, but the principle holds for systems like Quickbird and IKONOS.
Figure 3.45 NGDC made a stable-lights dataset of the USA using 236 orbits of nighttime DMSP data from the dark half of lunar cycles over six months. The image is resampled to 1 km resolution.
There are two approaches to the need for very short exposure times. One is to mechanically scan the optics to compensate for satellite motion or slew the spacecraft so as to reduce effective ground velocity.The second is to counter the motion electronically. This is done on IKONOS and Quickbird with a technique called time-delay integration (TDI) on the focal plane, by which electrons are moved along the focal plane in the direction of satellite motion, accumulating until sufficient exposure time has been reached (the same technology is used on some flatbed scanners). The Kodak-built focal-plane on IKONOS has up to 32 steps of TDI. Typically, twenty steps of TDI are needed to build up sufficient charge. The Quickbird satellite implements both TDI and mechanical slew of the whole spacecraft to reduce effective ground velocity.
