There are a number of different orbit types. One, the geostationary orbit, allows a satellite to continually observe the same section of the Earths surface. A polar orbit allows observation of the whole of the Earths surface. Satellite orbits are tailored to maximise observation times of the areas of interest. Use the links below and a web search to find out more about orbit types.
Satellites operate above the atmosphere.
In order to observe the surface of the Earth we need to use sensors operating in 'atmosphere transparent' regions of the electromagnetic spectrum, known as atmospheric windows. Conversely, if we wish to make atmospheric measurements then our sensors should operate in absorbing regions of the atmospheric spectrum Using the links below see if you can find a diagram of the transmission spectrum of the atmosphere. This gives information about, for example, sea colour and vegetation . Emitted passive sensors observe the radiation emitted by Earth systems. See if you can find out about the terrestrial and solar emission spectra from the links below.