About Radar Technology
There are 4 technologies used frequently for security:
- Pulsed Doppler
- Pulse Compression
FMCW (Frequency Modulated Continuous Wave) is a very popular security radar technology. As the radar transmits, the frequency of the radar changes in a repeated cycle. If the frequency is plotted, it appears like a sawtooth pattern over time. The wider the span of frequency change (bandwidth), the higher the resolution of the radar.
Resolution can be thought of like pixels in a camera image. The greater the density of pixels, the more clear and crisp the image will be. Similarly, the greater the resolution of a radar, the easier it is to make out an outline of a building or object. And the greater the resolution, the easier it is to detect closely spaced objects (like two people standing close).
FMICW (Frequency Modulated Interrupted Continuous Waveform) is a special case of FMCW radars. One of the problems with FMCW is susceptibility to multipath or multibounce. By continually radiating, energy can bounce many times on its way to and back from an intruder. This causes the signal from the intruder to appear at more than one place (usually directly behind the object), which appears as false targets to the radar. This problem is magnified for FCMW over all other technologies discussed. To help reduce the multipath effect, FMICW has a gap between the frequency ramps.
FMCW technology has three strengths. (1) It can accomplish longer range with less radiated peak power. (2) It achieves higher resolution than many other technologies. (3) The low processing demands allows it to spin faster than a Pulsed Doppler radar, but slower than a Pulsed radar. (The faster the spin rate, the more often the intruder position can be updated.)
Besides multipath, the FMCW does not reject clutter as well as a Pulsed Doppler. It usually needs to be mounted higher and away from sources of reflection nearby. It works best is open areas and over water (given the frequency is X-Band or less).
FMCW is usually implemented with two antennas — one is transmit and one is receive. This prevents the radar from blinding itself. The best frequency for security purposes is 9-10 GHz, which allows for a reasonable radar size and very good all-weather performance.
The Dorado and Dorado Lite are examples of FMICW and FMCW radars, respectively. Both of these radars operate in the X-Band frequencies. They require little if any maintenance for years.
Pulsed radar works by generating bursts of RF (Radio Frequency) energy with long off times in between. Once a burst, or pulse, of energy is radiated, a timer is started. Since radio waves travel at the speed of light, the range of an object can be determined by logging the time of flight of the signal and dividing it by two for the round trip flight of the pulse.
The more peak power, or pulse amplitude, the greater the range. The longer the pulse in time, the greater range. Sometimes longer pulses are used instead of high peak power, but this greatly limits the use of the technology to sterile environments.
An example of pulsed radar is navigation, shipboard or marine radar. These don’t normally work well for security. These shipboard radars are meant to be used on boats in short bursts of time, like for a day of group of days at a time. For the casual fishing boat, this radar might last over a year without repair. But for security, they suffer from the following:
- brushes needing repair every 4-5 months (failure to do so results in motor failure;
- annual magnetron replacement (transmission stops with its failure);
- rotary joint failure (which can occur from 9 months to 1.5 years);
- typical replacement for drive belts is 1 year or less.
Although the navigation radar is inexpensive, the maintenance cost as a result of the above is very high. And of course, causes significant downtime of the hardware.
For the larger open array antennas (the 3-foot or longer linear beams that rotate), the antennas must be parked (stop rotation) when winds get high. Debris can also collect between the array antenna and the motor and must be cleaned. In addition, these systems have difficulty in changes sea conditions and foul weather. Such conditions cause tremendous false alarms. Since tracking can’t be built in, separate servers must be added to provide the detection and tracking capability. By the time this is added and the maintenance is considered, this type of solution is generally considered unacceptable and expensive for security.
DMT does build navigation radars using pulsed technologies. The Marlin radar, however, works to overcome many of the above mentioned shortcomings. It has no rotary joints, uses dual magnetrons or solid-state amps for longer life, uses belts with an expected life of over 4 years, and has a radome to protect against the elements and high winds. It also has a built-in tracker for security applications. Having said this, the Marlin should only be used for security applications an open area and waterside side application. For higher clutter and foul weather conditions, switch to the IDAR radar.
Pulsed Doppler Radar is the most versatile of radar technologies. The modern day implementation of the technology is the transmission of a burst of low-power pulses, which are then integrated and processed to provide long-range detection and instantaneous measurement of speeds. Processing of Doppler is normally done in one of two methods: (1) the faster of the two methods is the Rummler or Pulse-Pair processing, which computes the average Doppler; (2) the more in-depth and full signature method which is use FFT processing. DMT’s IDAR primarily uses (1) and DMT’s AIMS Fast-Scan uses (2), primarily.
Doppler processing works by monitoring phase changes from the object over time. Phase change can be converted to speed. So Doppler radars measure all speeds in the environment — from the intruder and all other sources of reflection (such as rocks, blowing grass, poles, buildings, swell and breaking waves). These other sources of reflection are lumped normally under the term “clutter.” Clutter speeds and intruder speeds are typically different, so Doppler radar systems can remove the clutter and leave the intruder. There are also a host of other algorithms that can be applied using Doppler.
There is a misconception/myth that Doppler radar cannot see stationary or slow-moving objects. This is untrue, and in fact has been spread by other radar manufacturers that don’t really understand Doppler technology. Doppler radar can not only detect stationary objects, it can actually identify such items far better than non-Doppler technology. However, it is typical for Doppler radar systems to use this powerful ability to remove such items from consideration. For DMT radars, non-moving items are ignored unless than were first detected as a moving track. In such cases, the tracker will continue to track slow moving or stopped objects. After a certain point in time, the object that has stopped will be labeled “Parked.” If the object starts moving again, the original track ID is applied to the object.
Of all technologies, Pulsed Doppler offers the best all-around performance in the greatest variety of sites and conditions. X-Band versions of Doppler radar offer the best performance in all situations. IDAR, MDR and AIMS Fast-Scan are examples of X-Band, Pulse Doppler radars.
Pulse Compression Radar is the least-used technology for radar security solutions. It is the most complex and costly of the approaches. Pulse compression radar transmissions are similar in appearance to FMICW. The technology give great high-resolution images of the environment with low power.
Pulse compression radars have similar problems with near-in clutter as FMCW and are easier to jam. They suffer from long minimum ranges. For instance, a 12 km system may have a 2 km to 6 km minimum range. So for most security applications, it is not a great technology — especially given the cost.
There is a subset of pulse compression radars that are also Doppler radars. These add the Pulsed Doppler capabilities to the pulse compression technology. This is truly a powerful radar solution, but is very expensive. And the same poor performance with near-in clutter, interference and jamming exists. DMT is developing a pulse compression Doppler radar for release in 2013. This system will be for applications requiring detections ranges on the order of 2 km for small objects (people or kayaks).