With a growing number of radar manufacturers offering their products to airports for security, it is increasingly important to understand the critical tradeoffs to selecting the right ground radar for airport perimeter security. Radars are designed for many applications such as weather, military, research, navigation, traffic, and surveillance, and they are not all a good fit for airfield surveillance. Features vary widely in areas like antenna design, frequency, and processing. Fortunately, only a handful of these tradeoffs are important in selecting a ground radar for airports. If you get these right, then you will likely have good results and be happy with your system. This article identifies these tradeoffs and discusses the pros and cons of each.
The tradeoffs in selecting ground radar for airport perimeter security
- Spread Beam Versus Narrow Beam
- Rotating Versus Fixed
- High Frequency Versus Low Frequency
- FMCW Versus Doppler
- Processing Software
Spread Beam Versus Narrow Beam. Most ground surveillance radars have a narrow vertical beam width to focus the energy and maximize the range of the radar. Think of a long range, narrow beam flashlight. While these radars will see objects that are in the beam, they will miss objects below the beam. To make matters worse, obstructions like runway signs and equipment shelters can block the view of objects behind them. Raising the beam to see over the obstructions only increases the blind spot below the beam. Some ground surveillance radars use what’s called a Spread Beam, where the bottom half of the beam is spread down, and the top half continues to long range. Spread beam radars are more like a streetlight than a flashlight, looking down into your area of interest. A spread beam radar can be mounted higher, up to 30 feet, to see over obstructions and into elevation changes in the airfield. In a perfectly flat environment with no obstructions, you could make the case for narrow beam, but on a typical airfield with low spots and runway signs, the spread beam design provides much better coverage.
Rotating Versus Fixed. About half the ground radars available are flat panel fixed radars, and about half are rotating. The fixed radars have a field of view of 90 or 120 degrees, and the rotating radars have a field of view of 360 degrees. Intuitively you would think that for surveillance, 360 degrees would be better because you get 3 or 4 times the coverage, and you would be right. While it’s true that you can arrange 3 or 4 flat panels to cover 360 degrees, a single rotating radar is a much simpler solution. Panel manufacturers sometimes point out that they require less maintenance because of fewer moving parts, but with the newest rotating radars having a 5-year maintenance interval, the savings is negligible compared to the benefits of 360-degree coverage. While there may be special cases where a fixed radar is a better fit, in general a rotating radar is much more suited to airfield surveillance than a fixed radar.
High Frequency Versus Low Frequency. Ground radars advertised for airfield use vary in frequency from a low of 3GHz (S band) to a high of 77GHz (W band or millimeter wave). The benefits to low frequency radars are longer range, and they are typically used in weather and navigation applications to see hundreds of miles. The downside to low frequency radars is low resolution, which is not important when you’re looking at storms a hundred miles away, but is very important when you’re tracking a fence climber at three quarters of a mile. High frequency radars were designed specifically for tracking people, and present a much higher resolution radar image to the software, reducing the chance that an important object like an intruder will disappear into the clutter of a fence or a building. Since the useful line of sight ranges on an airfield are typically a mile or less, high frequency – high resolution radars are best suited for airports.
FMCW Versus Doppler. This tradeoff refers to how objects are detected and tracked by a radar system. Doppler radars use the frequency change in the reflection from a moving object to detect it and calculate the speed. The limitation with Doppler is that only objects moving toward or away from the radar change the frequency and are detected. Think of a police radar alongside a highway. The most accurate readings are those with a small angle between the radar and the speeders, and aiming the radar at cross traffic would result in no readings at all. This lack of cross beam tracking could be overcome on an airfield by adding additional radars to cover the same area, for instance one for North/South tracking and one for East/West tracking, but that would greatly increase the cost and complexity of the system. FMCW stands for Frequency Modulated Continuous Wave, which has some inherent advantages such as lower power and higher resolution, but in this tradeoff, the important point is how objects are detected and tracked. FMCW radars send a high resolution radar image to the processing software every sweep. The software knows from previous images what objects are stationary and what are moving, and calculates the location, speed, and direction of the moving objects from the images. All moving objects are tracked, regardless of the direction. For airfield surveillance, FMCW is the clear winner over Doppler.
Processing Software. Most radar surveillance systems come with basic Command and Control (C2) software that has features like map display of tracked objects, camera following, protection zones, and alarms on entering a protection zone. The sensitivity of the radars is set at the radar level, and do not take into account differences of the surface conditions under surveillance. The sensitivity settings present a tradeoff between more nuisance alarms from say vegetation and wildlife, and lower probability of detecting a real threat. This type of C2 could be somewhat effective with a dedicated operator to assess and dismiss the nuisance alarms, and be on the lookout for a real threat.
Other radar systems have advanced C2 software that can automate the surveillance function without a dedicated operator. They allow sensitivity settings to vary based on the surface conditions under surveillance (grass, asphalt, water, sand, etc.), which greatly reduces nuisance alarms. They also provide sophisticated user defined rules that allow the system to distinguish potential threats from normal operations.
Experience has shown that requiring a dedicated operator to monitor a surveillance system is not a viable long term solution. Operator fatigue sets in after a short time, and the system ends up being ignored. An automated system is required to let the C2 software do the work. Following is a list of features to look for in a C2 to reduce nuisance alarms and automate the assessment of potential threats:
- Unique tracking parameters for different areas of grass, rock, water, etc.
- Exclusion zones to prevent tracks from starting is noisy areas such as wildlife habitat
- Classification to distinguish between people and vehicles
- User defined rules that distinguish threats from normal operations, by assessing:
- Speed and direction
- Distance to an area or another object
- Other sensor inputs
- Duration of rule triggering
- Other rules
- Previous behavior
- Date and time
- Camera configuration to control what cameras are responsible for what areas
- Camera configuration to compensate for changes in elevation
In Summary. Of the many complex features and design attributes of different radars, making these five simple tradeoffs correctly will help ensure the success of your security radar project.
Spread Beam Versus Narrow Beam: Spread Beam – Better coverage across the entire site, can see over obstructions and into dips
Rotating Versus Fixed: Rotating – Complete coverage, fewer radars, simpler installation and better value for money
High Frequency Versus Low Frequency: High Frequency and High Resolution – Better detection near ground obstacles (i.e. trees, buildings etc.) and lower false alarm rate
FMCW Versus Doppler: FMCW – Better detection of targets moving in all directions, including when stationary
Processing Software: C2 with Fully Automated Surveillance – Intelligent assessment of target behavior to notify operator only when necessary