This works by synchronized mechanical and electronic scanning, so that the effects of dim imaging targets on the sensor can be integrated over longer periods of time.
The most used way to perform TDI is called dTDI from digital Time Delay Integration, which is software-based and independent of the type of underlying imaging sensor.
Its principle of operation is to rely on mechanical scanning, so that a single linear CCD element gets exposed to different parts of the object to be imaged, sequentially.
CCDs can also be manufactured in configurations that are tolerant to the wide fluctuations in radiation and temperature, characteristic of space environments, and scanning ones can be made extra robust by the inclusion of multiple lines.
At the same time, the continuous operation and slow, line-discrete readout also leads to a problem: if anything moves within the scene to be imaged, there will be blurring and tearing between lines.
This leads to what is in cinematography called motion blur, and since the readout of the multiple lines of the typical CCD array occurs at different successive times, it also causes screen tearing.
This is effectively the same as spinning the spacecraft or other platform to match the viewing angle towards an object; it yields time integration in the digital domain, instead of the physical one.
A designated TDI CCD improves upon the single-line-scan system by adding up multiple measured photocharges over its more complicated sensor, and by more comprehensive analysis of the interaction between continuous lines and discrete column structure.