What is TDI?
TDI (Time Delay Integration) is a method for increasing signal when there is fast relative motion between the camera and the object being imaged. We have four different TDI choices: 128, 64, 32, and 8 lines, so that we can image a spot up to 128 times and add up the signal. This is also the method we use to vary our exposure level, since our line time is fixed with altitude. With TDI an image is read one row at a time, from the bottom of the CCD to the top. As each row is read, signals are shifted. If we do not properly match our TDI readout rate (same as line time) to the velocity of the spacecraft, our images will be blurred.
