Applications for imaging and display in defense and aerospace seem almost limitless. Many are now performed at the individual level by the ubiquitous PC, either in the deskbound environment, or in the field using ruggedized laptops and PDAs. But there are still a large number of applications that are performed by dedicated sensor, imaging, and graphical display systems across airborne, land-based, and shipboard environments.
A typical example of these might be a maritime patrol helicopter which bristles with an array of sensors like radar, sonar, magnetic anomaly detection, diesel fume sniffers, electronic countermeasures, data links, TV cameras, and infrared thermal imaging. Operators at multifunction display consoles can select one or many of these sensors for display to help build a picture of the situation developing around and below the helicopter. Next generation army or amphibious scout vehicles will have very similar capabilities to the helicopter making use of the latest advances in imaging and graphical systems.
Multifunction display example
The multifunction situational display is a good example of the various technologies that must be deployed to offer the operator the information he needs in a digestible form. The objective of this kind of display may be to build and maintain a perceived air-picture, in which case the operator may choose to display tracks, plots, and raw or filtered radar images concurrently with information from identification friend or foe transponders, and radio emission lines from electronic warfare receivers. These may be displayed in windows that the operator uses to update the overall air picture, which is synthetically generated as graphics by the situational display processor. The air picture may be viewed from a number of selectable origins, and may even be overlaid onto a moving map.
Achieving just these fairly limited objectives introduces a considerable degree of complexity into both the imaging and graphical parts of the system. Today, most military LCD panels are derived from commercial displays that have been been re-engineered to operate within the harsh military environment. Resolution is typically 1024 x 768 pixels at the low end, and rises to 2k x 2k at the top end.
Radars, dependent on their vintage, do not produce convenient raster video output. Newer, digital solid-state radars can be energized in many different patterns depending on the mode in which they are operating. However, older radars with a rotating antenna must be scan converted from polar to raster format to enable them to be displayed on an LCD panel. For both radar types, there is an intermediate step which converts the radar returns into a displayable raster image using a convenient format such as NTSC or RGB. The video can then be sent to the operator for display. However, if the radar image is to be displayed in a variable-size and moveable window, then a number of further steps must be performed:
- Scaling: Converts the radar image, which is probably a fixed format of 1024 x 768 pixels, to the correct number of pixels (or cells) to fit into the selected window size on the LCD panel. This is a very complex task to perform in real-time, and the solution in the past has been to use an Application Specific Integrated Circuit (ASIC). Now there are a number of commercially available devices that can be used.
- Synchronization: The video output of the scaler must be synchronized to the required input bit and refresh rates of the LCD panel.
- Mixing: The background graphical display must be mixed with the radar window to provide the complete digital video stream to the LCD panel. Again, this is a hardware function that may be performed by the graphics processor itself, or by dedicated hardware downstream of the graphics chip.
Often the operator can select from multiple video sources to be displayed in a number of scalable windows. This increases the complexity still further as it requires video switching at the front end, and replication of the scaling device(s) for each displayable window instance. An example is the Cobra Ax Video Windowing board from Curtiss-Wright Controls Embedded Computing (Figure 1).
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VITA 46 is crucial
This application is also a good example of why the new backplane connectors proposed for VITA 46 are so important. Although current digital video streams from the radar and visual sensors can use the VMEbus P2 connectors without significant signal degradation, future Gbps video rates will require the new backplane connectors.
Refresh rate requirements
Amid the introduction of high-performance graphics display devices such as the 3D Labs Permedia3 and ATI M9000 Radeon, digital full color moving map displays are becoming more common in combat aircraft. While storage and retrieval of map data is no longer an issue, it is still necessary to dedicate a PowerPC processor, or equivalent, to drive the graphics display device to achieve a high refresh rate of the map image. This high refresh rate is important in order to display a smoothly scrolling map image, particularly for combat aircraft that can travel at Mach 2 and have excessive rates of turn.
Moving map requirements
As with the situational display, the operator can select different origins and scales for the moving maps, which greatly increase processor load as the map is ranged in. A map that is presented on a 4:3 aspect ratio screen will also distort as it is rotated, unless it is redrawn at a high refresh rate to compensate. The simplest solution is to just display a square map with the edges of the screen unused, or this area may be used for soft buttons and controls.
Moving map displays will also incorporate additional symbology such as mission parameters, vectors, weapons data, enemy dispositions, and position of other aircraft. It is important to be able to display this symbology clearly as an overlay to the map. Drawing lines and symbology on a raster display presents further difficulty to ensure smooth straight lines without visible jagged transitions through the pixels of the display screen. Hardware anti-aliasing is provided by most high-performance graphics display devices so that each line drawn on the display is perceived by the operator to have no transitions across pixel boundaries.
Safety requirements
Moving map displays can be used in many platform types such as tankers, helicopters, air cushion vehicles, and even tanks, though their performance requirements will generally be less than that required by combat aircraft. Many of these applications will be safety critical where, for example, an aircraft might be navigated by reference to the moving map. This safety criticality is shared with another application for high performance display systems such as cockpit displays. Although cockpit flight displays will require a higher level of safety certification than moving maps, there are some parallel requirements that graphics display products for these applications must address during system design.
OpenGL has become the industry-standard graphics language for writing display applications. Portable across many development and runtime environments, it provides the user with hardware obsolescence protection when using the latest graphics technology designed for other, shorter life-cycle markets. OpenGL, certifiable to DO-178B, is now available from a number of vendors (notably ALT Software and Seaweed), and can be incorporated into certifiable runtime operating systems.
Another safety requirement, also applicable to weapons systems, is the ability to verify that the image displayed by a monitor is equivalent to the image that is generated by the display computer. A graphical display is in an open loop unless there is a feedback mechanism that feeds test patterns to the display computer.
Helmet Mounted Display
These previous examples have concentrated on typical flat panel LCD applications. A newer application is the high-performance graphical display controller used with a Helmet Mounted Display (HMD) as shown in Figure 2.
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The displays are primarily used in helicopters and combat aircraft, and less sophisticated displays will be used in the battlefield by soldiers. The purpose of a HMD is to convey flight, tactical, and weapons data to the pilot with a minimum of distraction caused by looking down at cockpit flight displays and tactical displays. This increases the pilot’s efficiency and safety under highly stressed combat conditions.
A fully integrated HMD allows the pilot to fly blind without direct reference to any visual external cues at all; the view from the cockpit is displayed as an image on the screen as well as the other essential data needed to fly the aircraft. A HMD uses a small LCD panel that is projected to a display directly in the operator’s line of sight. The HMD detects movement of the pilot’s head and eyes, and moves the projected images and graphics accordingly.
Very high refresh rates are required to ensure smooth scrolling of the displayed images and graphics, which requires the use of a powerful processor and graphics display device. Very fast response is also required when the pilot moves his head. The display must follow the movement within the next frame time, or the pilot will be confused due to lag time.
In order to more accurately simulate the pilot’s field of view from the cockpit and his natural visual ability, the image is progressively compressed towards the edges of the display. This imitates the natural field of view, which is much broader than the field of view from a single TV camera.
Graphic PMCs
From the limited number of examples, it is clear that there are many factors that will affect the implementation of a graphics and imaging system. Often graphics and imaging is incorporated as a function into, for example, the mission computers of a combat aircraft. This makes it an ideal candidate for the use of VME or PCI Mezzanine Card (PMC) products for efficient and cost-effective implementation. The PMC-704 graphics PMC from Curtiss-Wright Controls is shown in Figure 3.
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In addition to the scan conversion and scaling products identified earlier, major COTS VME companies like Curtiss-Wright Controls offer end-to-end solutions from the sensors to the displays. These solutions include DSPs, SBCs, and high-performance graphics PMCs that can drive two displays.
Duncan Young was educated in the UK and has worked in the defense industry for almost 40 years. Duncan was part of the management buyout team that formed Radstone Technology, and he initiated product development of the world’s first conduction-cooled VMEbus modules. He has also served on a number of standardization committees. Duncan is now an independent consultant, but writes this column on behalf of Curtiss-Wright Controls Embedded Computing (CWCEC).