Particularly during the World War II era, most of the enemy detection was by line of sight, affected primarily by interfering terrain, and lighting conditions, and weather
But this did not necessarily mean that ground units had to be involved.
A large amount of recon missions was carried out by air force branches of the involved countries. This procedure goes even back to the first year of World War 1, where planes, balloons and zeppelins were used primarily for reconnaissance, initially.
While it was almost impossible to handle bulky cameras on planes, photography equipment improved quicly and got smaller, so that planes then took pictures of frontlines, force build-ups and factories in the hinterland, in addition to the already performed roles as artillery observers and spotters (dropping notes over friendly territory, due to the absence of radios). Balloons were used to recon the frontline and the immediate rear area and as artillery observer vehicles, and Zeppelins covered 3 roles: Observer, Recon (especially over the Channel and the North Sea) and long range bomber (Paris, London, etc.).
In 1940, the Germans used recon planes (equipped with cameras) to assess the level of damage of British airfields, aircraft factories, Air Force HQs, radar installations and assets in cities (eg. London docks) after they had been struck - during the Battle of Britain and later on. In the Russian campaign, aerial recon and local scout planes (ie. Fieseler 156 Storch) were used to identify targets (strongpoints, hubs, large enemy units) for future advance operations.
Germany's longe range planes (FW 200, a civilian design for airlines), which were used in all theaters for long range recon missions, and which were actually acquired by the Air Force for a heavy long range bomber role (which did not materialize at large scale, due to defects in the design, low output numbers, the complicated manufacturing process, the lack of armor protection for crews, due to the absence of a tail gunner/turret, etc. etc.) were also used as maritime long range bomber and recon plane, tasked to search for Allied convoys or fast single ships in the North Atlantic and they even formed Kampfgruppen with German submarines. For these missions the FW 200 used its "Hohentwiel" radar to detect Allied ships and then dropped its bombs. So, in these missions, planes did not even depend on LOS detection. In this role, Churchill called the FW "Scourge of the Atlantic" as it inflicted heavy losses on Allied shipping. In 1940 a FW 200 bombarded the Empress of Britain (used as troop transport), which caused massive fires which then forced the captain to evacuate the ship eventually, and where then an attempt to tow the ship was prevented by a German submarine, which sent it to the bottom. Just between August 1940 and February 1941, one of the FW 200s managed to sink 85 Allied ships (amounting to 363,000 BRT). The pilot could operate the MGs in the nose, using a reticle for aiming, so the plane could also strafe enemy ships. In 1943, FW 200s from the 2nd Group of KG 40 dropped glide bombs (Henschel HS 293, which were guided) on 2 troop transports, namely the California and the Duchess of York, causing massive fires that forced the Allies to abandon these 2 large ships.
The main workhorses for recon were German medium bombers, smaller tactical bombers, and the Fi 156, where the leightweight Fieseler 156 served in a triple role, a) as artillery observer and recon plane, b) as standard liaison and courier plane for the Luftwaffe and c) as medical evac plane (with the air ambulance version that could hold 1 stretcher, when one or 2 seats were removed). The plane offered space for 1 pilot and 2 observers.
While often employed in a local recon role, the Fi 156 could also conduct medium and long range missions, as it could be equipped with disposable/auxiliary fuel tanks, increasing the max. range to 1,010 kilometers. Locally, when winds were strong enough, and if enemy AA and fighter threat was low, the Fi 156 could "hover" over a sector for a while, turning it into an almost stationary observer asset, pretty much like a balloon, it was famous for its very low stall speed. The Fieseler could land on pretty much most terrain types, and it just needed a strip of 50 meters for takeoff and 20 meters for landing. Since the wings were foldable, it could be transported by a truck or even slowly towed by Luftwaffe or Army vehicles.
Due to its low stall speed, it could circle over target areas for hours (if using the disposable tanks and if flying at low speed). The Luftwaffe used this ability extensively when searching and tracking partisan movements in Yugoslavia and Russian regions with partisan activity.
Charles D. Melson describes in "Red Sun: A German airborne Raid, May 1944" (2000), The Journal of Slavic Military Studies. Routledge, how the Germans tried to pin-point Titos HQ and how they employed Fieseler planes in preparation for an airborne operation near Drvar, the actual location of Tito's HQ:
It seems that a single German Fieseler Fi 156 flew a number of parallel runs up and down the Una valley over Drvar at around 600 metres; activity consistent with conducting low altitude aerial photography.
Lt. Col. Wayne Eyre, (Canadian Army) adds in "Operation Rösselsprung and the Elimination of Tito, May 25, 1944: A Failure in Planning and Intelligence Support" (2006), that the aircraft paid particular attention to the villages of Prinavor and Trninić Brijeg where the British military mission and American military personnel were located. This was observed by Street, the acting commander of the British military mission, who assumed it was spotting for a bombing raid and advised both Tito and the Americans. Both Allied missions moved their locations as a result.
Balloons were also used as recon and artillery obs assets, until around 1942.
Only after German air force contingents in Russia had been split and moved to Italy and North Africa (ie. to help out in the North African theater) and after aviation fuel had to be rationed (partially starting in 1942, and probably severely kicking in around 1943 before Citadel , but not later than after the massive amount of sorties during Operation Citadel at Kursk), the German recon regime in Russia started to falter.
The lack of intel about the Russian build-up at Stalingrad, which enabled the Russians to carry out a two-prongued attack that created the Stalingrad pocket, is still subject to discussion among historians, as - in theory - the Germans possessed sufficient numbers of recon or even fighter planes (which could have been used for recon, as well) in that sector. Still, the Germans appeared to be rather blind, as they either could not or just didn't gather sufficient intel.
Several possible reasons:
- The Russians had moved AA units near the eastern bank of the Volga as cover for the Russian artillery, (which performed surprisingly accurate shelling of German troops on the western bank, and even successfully performed some counterbattery missions, while it was difficult for the German artillery - in the main - to reach the last line on the west bank, right at the river, as the angle of the steep dunes and elevations [downhill slope pointing towards the beaches] to score hits on trenchlines and tunnels), turning recon over the eastern bank into a dangerous business.
- Russian air units increased the number of sorties (they were conspicious by their absence during vital phases of the German onslaught in Stalingrad, though)
- German recon (initially FW 200 at high altitude, later on Stukas, Fi 156 and medium bombers) focused on documenting the progress of house-to-house figting, instead of screening the hinterland of the eastern bank.
- Re-deployment of units to the Italian and North African theater, decreasing the number of planes in the Russian theater.
- early Rationing of aviation fuel that effected certain Luftwaffe units (not sure if the particular Luftflotte(n) was (were) affected). .
So, the level/quality of aerial recon could highly influence the outcome of battles.
In contrast, the number of Russian aerial recon missions north and south of Stalingrad increased, so that - coupled with other means of intel collection - the Russians possessed quite some knowledge about the number, sizes and dispositions of German units deployed in and behind Stalingrad. Interestingly, during the Kursk operation, even though the Russians sometimes (locally) deployed an impressive amount of ground support aircraft and fighter planes, aerial and land-based recon was piss-poor, if not non-existent, which led to the severe decimation of the 5th Guards Tank Army, as its elements ran into German tanks waiting at Prokhorovka, losing at least 235 tanks in the process, whereas the Germans lost only 3 (according to recent research by retired Colonel and military historian Karl-Heinz Frieser, who reviewed Russian and German documents and used daily actual strength reports providing the number of operational tanks - in contrast to Glantz' bloated numbers, who kept using ration strength reports and who didn't take reinforcements into account when detailing the total number of German troops and tanks available before the start of Citadel and the number of German tank losses during citadel).