The Magellan mission used SAR and gravity measurements to study Venus. Visit The Magellan Venus Explorer's Guide for results. For those of you who just can't get enough, this tutorial from NASA will keep you busy for quite a while, and when you're finished, you will know all about remote sensing. We do this by means of studying the interaction between our object of interest and the electromagnetic spectrum, or other phenomena. Remote sensing in which the object of study is subjected to electomagnetic radiation emitted from the data collection device is active remote sensing.
Radar is an example of active remote sensing.
Passive remote sensing is done by recording the naturally-occurring interaction between the object of study and the source of electromagnetic radiation, such as photography. Electromagnetic energy has distinctive properties. The most familiar form of electromagnetic energy is visible light. For practical purposes, we can consider the speed of light a constant. The electromagnetic spectrum is the name we give to the continuum of energy that travels through the universe at the speed of light.