What are remote physical sensors?
When Satellites (and UASs) collect data, people usually talk about sensors, although the resulting images look somewhat like photographs. A different sensor is needed for each frequency. Strictly speaking, even a camera in a phone senses three different frequencies: red (longest wavelength), green, and blue (shortest wavelength). Phones have these three sensors for each pixel, and there are typically some tens of millions of pixels, hence megapixels (kilo stands for thousand, mega for million). In satellites, the mechanism normally works differently, but the idea of having different sensors for different frequencies is the same.
A lot of work is done to collect more information by adding sensors to satellites and UASs. The first satellites had blue, green, red, and near infrared, and that was mostly it. Now some satellites are hyperspectral, meaning they have a large number of frequencies. Even the modern versions of conventional multispectral satellites, such as used in the current Landsat mission, collect data for 11 different frequencies.