Surveillance aircraft usually carry no armament, or only limited defensive armament. They do not always require high-performance capability or stealth characteristics, and may be modified civilian aircraft. Surveillance aircraft have also included moored balloons (e.g. TARS) and unmanned aerial vehicles (UAVs).
The Global Hawk family's US DoD designation - RQ-4 - may belie the Block 40's true calling. "R" is the Pentagon's designator for reconnaissance, .... But the true calling ... is surveillance, not reconnaissance. … Reconnaissance missions are typically more oriented for long-term intelligence-gathering purposes. The surveillance mission is much more integral to the kill chain, with more tactically oriented operations servicing the short-term decision-making process.
In order to be surveillance, it is critical for the collection system, the target, and the decision maker to be in contact with each other in such a way that the actions of the enemy are relayed in real-time to those who can make decisions to counter the enemy actions.
from USAF research report, 2001
The terms “surveillance” and “reconnaissance” have sometimes been used interchangeably, but, in the military context, a distinction can be drawn between surveillance, which monitors a changing situation in real time, and reconnaissance, which captures a static picture for analysis.
In 1794, during the Battle of Fleurus, the French Aerostatic Corps balloon L'Entreprenant remained afloat for nine hours. French officers used the balloon to observe the movements of the Austrian Army, dropping notes to the ground for collection by the French Army, and also signalled messages using semaphore.
World War I
One of the first aircraft used for surveillance was the Rumpler Taube during World War I, when aviators like Fred Zinn evolved entirely new methods of reconnaissance and photography. The translucent wings of the plane made it very difficult for ground-based observers to detect a Taube at an altitude above 400 m. The French also called this plane "the Invisible Aircraft", and it is sometimes also referred to as the "world's very first stealth plane". German Taube aircraft were able to detect the advancing Russian army during the Battle of Tannenberg (1914).
Pre war, the British identified a need for an aircraft that could follow and observe the enemy fleet at a distance. To this end the slow-flying Airspeed Fleet Shadower and General Aircraft Fleet Shadower designs were built and flown in 1940 but they were made obsolete by the introduction of airborne radar.
Spy flights were a source of major contention between the US and Soviet Union during most of the 1960s.
Unmanned (UAV) surveillance aircraft have been "deployed or are under development in many countries, including Israel, the UK, the United States, Canada, China, India, South Africa and Pakistan."Drones are increasingly used in conservation work to complete tasks such as mapping forest cover, tracking wildlife, and enforcing environmental laws by catching illegal loggers or poachers.
Unmanned surveillance UAVs include both airships—such as Sky Sentinel
and HiSentinel 80—and airplanes.
Most air forces around the world lack dedicated surveillance planes.
^Koh, Lian Pin and Serge A. Wich. 2012. “Dawn of Drone Ecology: Low-Cost Autonomous Aerial Vehicles for Conservation.” Tropical Conservation Science 5(2):121–32. Retrieved March 4, 2019 (https://journals.sagepub.com/doi/abs/10.1177/194008291200500202).