Cameras for Space and Aeronautical Applications
Aerospace imaging applications require specific functionality and performance from airborne cameras. Dr. Petko Dinev, CEO of Imperx, a designer and manufacturer of high performance cameras, describes these requirements and explains how camera vendors are working to successfully meet these evolving demands and challenges.
Variety of implementations
Airborne systems come in a variety of implementations based upon the application. For example, a persistent surveillance system includes a cluster of six to eight cameras and a large server with a lot of storage. The cameras are synchronized to produce a 100-plus megapixel image covering a 4 x 4 mile area with ~ 1 foot resolution, which is compressed and stored locally on the server. This system records continuously for the duration of the flight, and can identify and track multiple objects. Custom software then extracts the required information from the recording. In another implementation, a system includes a single camera or multiple independent cameras – connected to a common computer/server – that each generates an independent video stream. Common to all applications is the fact that the cameras are not passive; there is constant communication between the camera and the computer and between the camera and the lens. In the majority of implementations, cameras use GEV output and may or may not support in-camera image compression.
Three categories of airborne cameras
Historically, cameras for aerospace imaging have been divided into three categories: low-resolution imagers below 16 megapixels, medium imagers between 16 and 50 megapixels and large 50 megapixel plus imagers.
Recently, this division has started to blur as space imaging applications undergo rapid expansion due to the decreasing costs of launching small satellites. Typically, large 50-plus megapixel imagers were used in high altitude, high profile space applications (i.e., NASA and Google), primarily because of price and complexity. The remaining applications used low resolution sub-16 megapixel sensors. Imperx is now finding that 29 or 47 megapixel imagers are the default choice in almost all our new projects, regardless of whether it’s space or airborne.
Benefits of using high resolution cameras
High image quality and high resolution cameras are always preferred in airborne applications.
A higher resolution sensor offers many operational and cost benefits to military applications since the aircraft can make fewer passes and/or fly at higher altitudes. For example, when flying a mission in a hostile environment, it is imperative that the airborne system acquire as much information as possible in the shortest amount of time. If this system has low resolution cameras, it would have to fly at a lower altitude and make more passes to collect the same information, which increases its likelihood of being identified and taken down. High-resolution sensors offer these same benefits to civilian applications. The cost of the camera is a small fraction of the overall system cost so it is important to collect the necessary information as quickly as possible.
Battle between resolution and speed
For airborne applications, resolution and speed are in a constant battle that must take into account what is being imaged. In many aerial missions, resolution prevails because of on-board hardware limitations. Also, if the speed of the craft is relatively low, having a higher frame rate does not offer a significant benefit. For example, most Imperx applications use 29 megapixel cameras running at 4 fps or, in some cases, 1 or 2 fps because the on-board computer cannot process the data faster. If the platform is a rocket launch vehicle, the frame rate is very important because of what is being imaged. In most cases, we work with our clients to understand the objectives of the mission so we can modify the camera’s operation to satisfy their resolution and speed requirements.
Importance of image quality
Image quality is very important in airborne systems since images are used to extract valuable information from target locations, identify objects, measure distances, etc. Noise is a critical factor that adversely affects the performance, reliability and repeatability of these systems. Identifying objects and patterns or extracting analytical data is much more difficult with an increased noise floor. Therefore, camera manufacturers use a variety of methods to reduce noise. The camera must also be designed to have very low noise so that it maintains its performance across a wide range of temperatures. Implementing automatic exposure, gain and iris control, followed by nonlinear transformations, image equalization and enhancements are essential tools that help ensure better image quality.
Rapidly changing light conditions
Low light and variable lighting conditions present significant challenges to aerospace imaging. It is vital that airborne cameras have the ability to adapt to these rapidly changing conditions. As explained previously, camera vendors rely upon a variety of techniques to optimize image quality. One such method used by Imperx is dynamic iris control although not all lenses allow such control. As well, an adaptive automatic exposure and gain algorithm is an essential part of every flying system we have developed. When the camera is flying, the effective exposure range is very limited – on the upper side it is limited by motion smear (~ 2.5 ms) and on the lower side it is limited by vertical smear (~ 700 μs).
We have a similar situation with the automatic gain control – because of the above mentioned exposure limitations we have to use dynamic gain control but higher gain can introduce noise and reduce the dynamic range performance. Also, the algorithm has to be fast enough to react to rapid scene changes. A camera’s sensitivity and dynamic range are also very important for image quality, especially in low-light situations. In fact, Imperx is developing a low light camera with dynamic range > 80dB. In all airborne applications, we work very closely with customers to fine-tune the algorithms and strike the right balance between exposure and gain. What works on the ground in a static environment usually doesn’t work “up in the air”.
Environmental and operating challenges
Airborne cameras are affected by many environmental and operating challenges that are application and altitude-dependent. The camera’s robustness (i.e., its ability to withstand shock, vibration, extreme temperatures and weather) is a very important factor in military and civilian applications. In an airborne application, the camera has to endure take-off, landing and turbulence, and operate in high temperature gradients during the fast ascent and descent. For instance, the temperature on the ground might be +40°C but within a few minutes when the aircraft reaches an altitude of 10 km the temperature drops to -50°C. During descent, the camera is subject to very high condensation because of the rapid reversed temperature change. For a space application, the camera and lens have to survive lift off where vibration can reach up to 70g – in addition to tolerating extremely low temperatures. If the application is a space telescope, the camera has to operate in extreme hot and cold while in a vacuum. Also, for most space applications, camera power is important because of the limited capacity of the satellite’s solar panels.
The future of airborne cameras
Looking to the future, we predict that the demand for higher and higher resolution airborne cameras will continue to grow. Since these cameras operate in a low to medium vacuum environment, power management will, however, remain a challenge. There will also be requirements for cameras to be small and light weight, operate in extended temperature ranges and offer a rigid construction capable of withstanding powerful launches.