Terrain-following flying

Terrain-following flying referred to a tactic in which starfighters or combat airspeeders would fly at altitudes that were perilously close to the surface of a planet, moon, or other celestial body. Often used to defy detection from varying types of sensors while en route to or making a run in on a target, a pilot had to possess superior situational awareness and reflexes in order to avoid crashing.

Characteristics
This type of piloting was particularly dangerous to both machine and being as the tactic was at its most effective when executed through unstable, unpredictable terrain, such as that of mountain ranges, cityscapes, asteroids, or the most famous instance of terrain-following flying, the Trench Run phase of the Battle of Yavin. Due to the nature of most types of sensors used throughout the galaxy, flying in such close proximity to the surface of a world essentially scrambled their readings; instead of detecting a starfighter executing the tactic, they would simply return a readout of the ground itself. This was a particular flaw of so-called "line-of-sight" forms of detection. Similar tactics included the Horizon approach.

History
The tactic of flying extremely close to the surface of a world to avoid detection was, perhaps, as old as the concept of flying itself, long before the age of space warfare and interplanetary conquest. The most famous instances of terrain-following flying were practiced by the starfighters of the Rebel Alliance and later the New Republic Defense Force, though they were by no means the only practitioners. Those who were the victims of this tactic soon learned to adopt it themselves; such flying was best-suited for small-scale raids against outlying targets or those with a minimum of air- or space-based defenses or those that were inadequate for defending against starfighters.

During the Battle of Yavin, Gold and Red Squadrons used terrain-following flying as they attempted to exploit a weakness in the design of the original Death Star. Flying through an equatorial trench that ringed the surface of the vast battlemoon, they attempted to elude the Galactic Empire's TIE Fighters as well as the guns on the surface; though the trench itself was also lined with turbolaser emplacements, they were too heavy and slow to effectively target the smaller X- and Y-wings of the Rebels. However, Darth Vader and his wingmates were able to do the same thing, downing two successive attempts by the Alliance forces to fire their proton torpedoes into the thermal exhaust port at the end of the trench. However, Luke Skywalker, utilizing his newfound powers in the Force, was able to succeed in destroying the battle station.

Following the victory at Yavin, the Rebels continued to utilize the tactic, adapting it for runs against Imperial capital ships in what came to be known as the Trench Run Defense. The heavy firepower of the Star Destroyers that made up the bulk of the heavy units within the Imperial Navy were largely ineffective against fast-moving Rebel craft, a weakness that was exploited at the Battle of Endor against a number of Imperial-class Star Destroyers as well as the Star Dreadnaught Executor. Flying through the canyons that lined the surface of the massive, nineteen-kilometer long vessel, a number of X-wings and A-wings were able to strike at the warship's command tower, whose bridge deflector shields had been brought down by an earlier attack pass. An A-wing flown by Arvel Crynyd dealt the final blow, smashing into the bridge and sending the ship plunging into the second Death Star. The tactic continued to be used throughout the subsequent years of the Galactic Civil War, including the Battle of Vladet and the Assault on Borleias two years later, as well as the Adumari Civil War in 13 ABY.

Appearances

 * Star Wars Episode IV: A New Hope
 * Star Wars Episode V: The Empire Strikes Back
 * Star Wars Episode VI: Return of the Jedi
 * X-wing: Rogue Squadron
 * ''X-wing: Starfighters of Adumar