Recognition of Aerial Photography

Photogrammetry, the science of making measurements of photographs, has enhanced man’s ability to produce extremely accurate and detailed maps.

Throughout World War II the demand for intelligence information and aerial survey was extremely great and this led to further improvements in camera design, exposure control, processing techniques, film characteristics and film analysis. In more recent times, thermographic and radiographic sensors, the data from which is also recorded on photographic film, have been added to the information­ gathering equipment inventory. Such sensors, along with their optical predecessors, now have applications in aircraft, remotely piloted vehicles (RPVs) and satellites employed in the reconnaissance role.

Australia has had a continuing involvement in photographic reconnaissance since the beginning of World War I. However, the RAAF has not employed an operational squadron full-time in the reconnaissance role since the Mosquito photographic squadron was disbanded in the early 1950s. Since then, full recognition has been given to the importance of reconnaissance, but the RAAF, like many sister air forces, has given higher priority to other aircraft requirements. Nevertheless, No. 2 Squadron, flying Canberra Bombers, has retained a limited reconnaissance capability by deploying a single framing camera. Some of the squadron’s aircraft are also fitted with special survey cameras used for map making. Modified Mirage fighters carry a low-altitude panoramic camera in a special nose cone, but they are limited today to only low-level reconnaissance. Some maritime photography is conducted with handheld cameras. Strike cameras are also fitted to F-111C and P3C aircraft.

Thus, while the RAAF has always recognised the need for aerial reconnaissance, it has only been able to retain a limited capability since the early 1950s. The modification of F-111C ike aircraft to accept reconnaissance sensors will redress this situation and for the first time give the RAAF a day and a tactical and strategic, long-range reconnaissance equipment with the ability to survive in a hostile environment.

Tactical Sensors

To put the F-111C’s future reconnaissance capability into perspective an understanding of sensor function is required.

For reasons of space, developmental costs, state-of-the-art technology and aircraft mission, not all types of reconnaissance sensors have been included in the F-111C package. Optical, thermographic (infrared) and TV equipment will comprise the sensor fitment; radiographic sensors (eg, sideways-looking radars) and devices for gathering electronic emission intelligence have been lauded. Because survey operations demand an accurate quality of reproduction and camera lenses require special calibration for scaling and accurate maps, survey cameras, are not fitted but provision has been made for them if required at a later date.

Like survey cameras, aerial reconnaissance cameras are designed to meet the specific requirements of the mission profiles that may be flown. Operational heights and flight feeds must be considered and certain features must be incorporated into the camera design. For example, if the mission is to involve a high-speed dash at minimum altitude over an extended area, then a low-level, high area coverage camera will be required. Thus, to meet a complex set of requirements, aerial cameras employed in the reconnaissance sphere are now configured to provide vertical, oblique, fan, continuous strip or panoramic imagery.

The main advantage of vertical photography is its relatively small scale and azimuth errors, and ground features appear in perspective and with minimum distortion. A disadvantage is that it represents an image from an unfamiliar point of view. The main applications are for taking a single picture or stereo-pairs of a particular area (spot photography); a strip of overlapping photographs taken at a constant altitude between two points (reconnaissance strip); and, two or more strips that also overlap laterally (mosaics). Cameras are installed vertically in the aircraft.

For oblique photography, the camera axis is set between the vertical and horizontal in such a way that the aircraft does not have to fly directly over the area to be photographed. A high oblique shows the horizon and much more area than a vertical photo taken with the same focal lengths lens and from the same altitude. However, the dimensional scale diminishes from foreground to horizon.

Fan photography is obtained from a combination of vertical and oblique camera installations. It provides wide lateral coverage along a single flight line. However, like oblique photography, the scale of individual strips diminishes from the centre of the film to the edges.

A continuous strip, as the name implies, is a continuous aerial picture of the terrain passing beneath the aircraft’s flight path. The film is moved across the focal plane behind a variable-width slit and is synchronised with the speed of the aircraft over the ground. The strip produced may comprise either single or stereoscopic imagery.

There are several methods of mechanising cameras for panoramic photography, including the fitment of a prism rotating in front of the camera lens. The result is the same as if the camera itself were swung to photograph the terrain from horizon to horizon. Sector scan is an additional refinement that permits the aircraft crew to photograph only one sector of the horizon-to-horizon field of view, that is, have the camera operate as a vertical or oblique on demand. Again, imagery suffers from a change of scale from foreground to horizon, but compensation for this distortion can be made during ground interpretation.

The operator who specifies his optical sensor requirement thus has a range of options available to him. For the F-111C these options were examined in a feasibility study conducted by General Dynamics, Fort Worth Division. Two RAAF officers assisted in the study. Twin vertical framing cameras capable of accepting 3, 6 and 12-inch lenses, for low, medium and high altitude photography respectively, were selected. Installation design also allowed the same cameras to take oblique photographs. Two panoramic cameras – one for low-level reconnaissance, the other for higher altitude work – were also selected. The sector scan for a feature on the high altitude camera gave added operational flexibility.

Thermographic Sensors

One product of the evolution of electronic technology has been equipment that produces thermographic images (for video or photographic recording) by detecting infrared radiation. Infrared radiation depends upon the temperature and the emissivity of an object. Thus, any target the temperature of which is different from that of its surroundings can be detected. It also will be detected if it has a different emissivity from that of its background, even though they are both at the same temperature. Camouflage nets and trees make little difference and objects of different temperatures and emissivity can be clearly seen by infrared sensors. It also is possible to detect the condition of an object at a specific time. For example, aircraft or tank engines that are running will be hotter than the fuselage and will show up clearly. Similarly, the wake of a ship or submarine shows up because vessel movement warms the water.

The operating principle of an infrared sensor is simple optics scan a thin line of the scene beneath the aircraft at an extremely rapid rate and focus infrared radiation onto a detector. Variations in the intensity of radiation across the scene are detected and recorded. Such a sensor has three distinct advantages: it needs no external illumination for night operations, it is passive in that it does not rotate, and it often contains more “intelligence” information than its optical counterpart. In the reconnaissance role, it is essentially a low-level day and night device used primarily to obtain high-definition photographs of particular area targets. As a forward looking infrared (FLIR) sensor it produces real-time images of the scene as it appears in the infrared spectrum for display on a CRT screen.

An infrared line scan mounted vertically is to be fitted to the F-111C. Space has been provided for the possible future fitment of a FLIR set.

Television Sensors

low-cost television sensors can also be used for information gathering instead of the FLIR. However, they need some natural or artificial illumination to provide a suitable video image for recording or the aircrew uses two TV cables one mounted vertically, the other obliquely pointing forward, are to be fitted to the F-111C. This combination provides the aircrew with a video display that can be used 82 as a non-radiating ground tracking device or reconnaissance sensor aiming aid. Recorded video pictures can be played back on the ground for intelligence analysis or mission debriefing. The pictures do not have the same high definition of optical or infrared sensors.

Sensor Integration

While each sensor for the F-111C has been selected to cover a range of mission profiles and data-gathering requirements, none is of value unless it Is integrated with the aircraft systems to form an overall system. Each sensor requires external power, cooling vacuum supply, etc, to function correctly. Controls and indicators are needed for operation. Altitude above ground and ground speed are required for calculation of image motion compensation and outputs of image motion compensation must be correctly scaled for each sensor. latitude, longitude, altitude, heading pitch, roll and information plus additional data has to be fed to each sensor for recording to assist in the interpretation and exploitation of the reconnaissance information. Timing signals derived from aircraft velocity and altitude and specific sensor controls are also required for the actuation of the sensors finally, the environment in which each sensor operates must be controlled so that thermal shock, vibration, hum1drty and other environmental conditions do not degrade performance. Thus, it is the effort put into integration design that ultimately decides how well each sensor and the total system performs in the reconnaissance role.

The configuration chosen by the RAAF requires that all reconnaissance interfacing electronics, radar altimeters and sensors be installed on a new three-quarter weapons back door. The port vertical camera, although mounted on a common cradle with its “twin”, has its viewing window installed in the one-quarter weapons bay (maintenance) door. With this configuration, the aircraft can still be operated in its basic strike role if necessary.

General Dynamics, as part of their contract, is to modify the first aircraft and demonstrate, via a flight test program that each sensor meets specified performance criteria. One F111C aircraft has been positioned, along with ground crew at Fort Worth for this purpose. Further flight trials will becon­ conducted in Darwin to prove that the tropical atmosphere there has no detrimental effect on sensor and system performance Three other aircraft will be modified in Australia when prototype kit proofing and testing are completed.

Possession of an advanced technology dedicated to surveillance and reconnaissance fit for four F-111C aircraft will greatly enhance Australia’s ability to gather intelligence from areas not previously covered. The Mirage 1110 also has a limited tac­tical reconnaissance capability with a camera pack that fits aft of the Cyrano radar bay. (Defence PR)

Operations

Flight testing will prove system capabilities and limits within these, specially trained aircrews will plan and fly reconnaissance missions. Information recorded will be processed in newly constructed photographic processing and interpretation facilities (PPIF) being built at RAAF Base 1 Kimberley. A secondary airportable PPIF will be provided for deployed operations. These facilities will be manned by a dedicated group of photographers who must practise their art developing the latent imagery that conveys the intelligence information. There will also be dedicated photointerpreters whose jobs do not only entail taking measurements from photographs but involve recognising and identifying photographic images and judging their significance. This is the true military application of photographic reconnaissance and one in which the RAAF must gain expertise if it is to acquire a capability for independent operations.

The inherent operational capabilities of the F-111C aircraft will give the reconnaissance force its potency. But before it can reach its full potential, new skills have to be learnt by aircrew and ground staff. It is the fulfilment of the concept that machines, sensors, aircrew, technicians, photographers and photointerpreters must work together as an entity to provide another aid to the military planner, that will ensure the force’s success.

Article courtesy of “TRIAD”, Dept. of Defence