Remote sensing is a technique for obtaining information about something from a distance, without physical interaction. Digital remote sensing is a more refined version of this method that relies on advanced electronics to garner information and interpret it. This is distinct from conventional remote sensing that encompasses analog devices as well as methods as basic as visual perception, or simply the act of looking at something.
The applications that involve digital remote sensing can vary wildly in scope. Weather satellites and ultrasound machines are both types of remote sensing devices, for example, though the subjects of their scrutiny are extremely different. The important thing to remember is that scale is irrelevant when it comes to remote sensing. Its primary benefit is the ability to obtain data about objects that are either inaccessible or effectively intangible, no matter their size.
A Magnetic Resonance Imaging (MRI) machine, for example, can scan the interior of the human body through the skin, eliminating the need for exploratory surgery in a wide number of cases. In much the same way, devices called photometers measure the amount of radiation emitted or reflected by far away objects, like stars. Electron microscopes measure some of the tiniest of objects known to man, that can otherwise only be assessed in huge quantities.
Digital remote sensing, like remote sensing in general, can be broken down into two main types — passive and active. A passive digital remote sensing device merely receives visual information that is projected by whatever subject is being observed. A digital camera is an example of a passive sensor, in that it records the light radiated by an object and stores it.
Active digital remote sensing, by contrast, works by sending out a signal that scans a subject and sends back results. Active sonar, which involves emitting sound waves and interpreting the way they bounce off objects, is a type of active digital remote sensing. Radar, which involves measuring the distance and direction radio waves take when bouncing of targets in their path, is another example.
In almost all cases of digital remote sensing, the information of interest cannot be measured directly. Instead, a principle known as the inverse problem is employed, whereby a known relative quantity is measured and used to derive the desired results. A basic example of this is how a police detective would assess tire tracks left at a crime scene to help determine what kind of car may have been used in a robbery. Since no physical signs exist of the car itself, the inverse problem method uses a known quantity to help draw a conclusion.