There are two types of satellites that collect imagery - optoelectronic and radar. Optoelectronic satellites are sensitive to infrared wavelengths and visible objects. Imagery from an optoelectronic satellite can be made in panchromatic or multispectral modes. This is helpful for land-use monitoring, mapping, city planning, etc. Radar satellites use ultra-high frequencies to remotely sense the Earth's surface. Ultra-high frequency allows these satellites to collect data through clouds, smoke, haze and sometimes through vegetation. It also can be operated at day or at night.