In March 2014 China launched its third Dongdiao class AGI (Auxiliary General Intelligence, or electronic reconnaissance) ship. These 4,800 ton vessels are 119 meters (390 feet) long and with a crew of about a hundred sailors and technicians. The ships sports several domes protecting antennae and the ship is crammed with computers and signals processing gear. The Dongdiao class are replacing at some of older AGI type ships that entered service in the 1970s, even those these older ships have had their electronics and other information gathering gear upgraded.
China has about a dozen AGIs of varying sizes and ages. The Dongdiaos are the largest and most modern. AGI ships are mainly about electronic reconnaissance and collection. Just keeping track of the enemy's electronic devices has become a major operation, especially since no one knows exactly how everyone’s electronic equipment will interact until there is a sustained period of use. Such use does not occur in peacetime, when the EW equipment is used infrequently for training and testing. All electronic equipment has a unique electronic signature. Even equipment that is not broadcasting will appear a certain way to various sensors like radar or sonar. Thus a critical peacetime function is to determine what these signatures are. For this reason navies and air forces devote a significant amount of their time tracking other nation’s capabilities.
As a counter to ESM (Electronic Support Measures), equipment is disguised where possible. Signals can be varied in some circumstances. For equipment that is detected by shape and composition, like aircraft and ships, their shape and substance can be designed to minimize detection. This is the essence of the stealth technology that the United States is applying to a number of vehicles, especially aircraft. Small ships, aircraft, helicopters and vehicles loaded with sensors do most of the collecting. Low flying satellites are useful for catching signals deep inside a nation’s territory. Drones and RPV aircraft are used also, plus robotic sensors that are left on the ground or sea bottom. Collection involves more than sensors. Recording devices, foreign language interpreters and signal processing equipment also come into play.
Computers are increasingly crucial in sifting through the ocean of data swept up. Huge libraries of signals are collected, analyzed and boiled down to manageable amounts of data friendly troops and weapons can use. ESM has been so successful that one entire class of sensors, active sensors, has become endangered. Active sensors detect things by broadcasting a signal. When this signal bounces off something, the sensor detects the bounce back and knows something is out there. This is the basis of radar, which broadcasts microwaves, and sonar, which broadcasts sound. Because of the signal being broadcast, a passive sensor can detect it.
Passive sensors just listen. Because active sensor signals must reach an object in sufficient force to bounce something back, a vehicle carrying a passive sensor will detect a vehicle carrying an active sensor first. This is what happens you use a radar detector in your car to detect police speed trap radars. You usually have time to slow down before you illegal speed is detected by the police radar. As users of these devices well know, there is constant competition to come up with better radars and countermeasures. Passive sensors are the hot item in research and development these days, and for obvious reasons. Passive sensors are nearly impossible to detect. Passive sensors can also pick a wide variety of signals. Infrared sensors can detect heat, including something as faint as body heat or the hot skin of an approaching jet aircraft.