Target speed is measured directly by measurement of the Doppler frequency shift. The Doppler effect is a phenomenon that is regularly experienced even in everyday life. For example when a police siren is heard in the distance the tone changes and rises until the police car drives past and the tone starts to fall again. For radar systems, the Doppler effect causes moving objects to shift the frequency of reflected radio waves based on the speed of the object.
A Doppler shift is seen for objects moving radially, that is, directly toward or away from the radar. Doppler measurement is effective at detecting moving targets and ignoring targets that don't move, which is particularly important for ground surveillance radar where many reflections are seen from stationary targets. Radar antennas typically have a narrow field of view that is scanned across a wider area.
When a target is seen the direction in which the antenna is pointing corresponds to the direction of the target. In principle this is like using a telescope to determine the bearing of distant objects. There are many possible antenna methods that can be used, with choice being determined by required size, weight, power and cost. The simplest method is to physically rotate the antenna.
When the radar sees the target echo, the direction of the antenna directly corresponds to the direction of the target. Rotating antennas do have moving parts that can wear, however with clever engineering and use of very small and light materials, the expected lifetime can be extremely long. Some radars have fixed antennas that are steered electronically, a so-called phased array, although this is often much more expensive than simple physical rotation. Another method uses two or more antennas to mathematically calculate the angle of arrival by comparing two or more echo signals.
This method is cheaper than a phased array, but has limitations such as inability to distinguish multiple targets at the same distance and lower sensitivity. There are three main factors that determine the maximum range: The radar must receive sufficient echo energy to be able to make a detection.
The radar must have a direct line-of-sight to the target. Limitations in the receiver circuitry may limit the range that can be measured instrumented range. Where the echo energy is the limiting factor, the radar will have different specified ranges for targets of different types, with higher RCS targets being able to be detected further away.
Where the limitation is due to the instrumented range the maximum range specification will apply equally to all sized targets. Since microwaves do not bend round corners the target can only be seen if there are no large objects in the way. Mounting the radar higher up allows it to look over objects that are in the way. Generally it is not possible to increase the transmitted power to attempt to increase the detection range, as the radio regulations are very strict to enforce a maximum power level.
This rule is in place to prevent other radio users being negatively affected by interference caused by high power transmissions. When multiple radars of the same type are used in close proximity there must be a method to avoid them causing mutual interference.
A common technique is to synchronise equipments to a common clock then slightly offset each radar from its neighbours so they do not transmit on the same frequency at the same time. Another technique is to simply use different operating frequencies that do not overlap at all.
Although this doesn't require synchronisation, it severely limits the number of equipments that may be located nearby and is extremely wasteful of the allocated frequency spectrum.
Clutter refers to sources of unwanted echoes generated by objects that reflect radio waves. Clutter is caused by reflections from the ground. Any radar that detects targets on, or close to, the ground will see more clutter than radars that look upwards into the air, especially if the clutter moves. In the ideal case, the ground would be a very flat concrete expanse with a target located in the middle.
Unfortunately this is rare. Often there are fixed objects such as cars, posts, walls and fences that all contribute to the background clutter levels. Fixed clutter can mask the presence of a target by reflecting the radio waves before they can reflect off the target. Radar signal processing tries to ignore fixed clutter either by filtering objects with no Doppler shift or by comparing the current scan to previous scans to identify fixed objects.
Even so, large fixed objects such as tall fences or buildings generate high clutter levels that make it difficult to detect much smaller targets that are next to the clutter due to an effect called scintillation where there are small changes in the echo amplitude of the large object.
Consider a large building with RCS of 10, square metres exhibiting 0. This presents a background RCS variation of 10 square metres that is easily enough to swamp the RCS of a walking human 1 sq. Orientating the radar so the building RCS is reduced will improve the situation. Moving clutter, such as long grass, bushes, trees and water is very difficult to mitigate using signal processing. Moving clutter generates a Doppler shift and varies from scan to scan so cannot be distinguished easily from real targets.
Radar systems will have a higher detection threshold in areas where there is lots of moving clutter to avoid excessive false alarms. The most effective way to improve the performance is to remove or reduce the moving objects that generate the clutter.
The basic job of the antenna is to emit radio waves when fed with an electrical signal. Antennas are reciprocal, that is, they work just as well in reverse, so the antenna also captures radio waves and emits an electrical signal. Radar may use a single antenna that is shared by transmitter and receiver or may have two antennas, one that transmits, another that receives.
Usually pulse radars share a single antenna and FMCW radars use two. These elevation slices comprise a volume coverage pattern or VCP. Once the radar sweeps through all elevation slices a volume scan is complete. In precipitation mode, the WSRD completes a volume scan every minutes depending upon which VCP is in effect, providing an updated 3-dimensional look at the atmosphere around the radar site.
The radar continuously scans the atmosphere by completing volume coverage patterns VCP. Within these two operating states there are several VCPs the NWS forecasters can utilize to help analyze the atmosphere around the radar.
These different VCPs have varying numbers of elevation tilts and rotation speeds of the radar itself. Each VCP therefore can provide a different perspective of the atmosphere. The scanning begins with 0. As it completes that elevation scan the radar is tilted another degree with the center line of the beam now at 1. Please Contact Us. Please try another search. Multiple locations were found. Please select one of the following:. Location Help. News Headlines. Customize Your Weather.
Privacy Policy. Current Hazards. Imagine that the car is standing still, it is exactly 1 mile away from you and it toots its horn for exactly one minute. The sound waves from the horn will propagate from the car toward you at a rate of mph. What you will hear is a six-second delay while the sound travels 1 mile at mph followed by exactly one minute's worth of sound.
Now let's say that the car is moving toward you at 60 mph. It starts from a mile away and toots it's horn for exactly one minute. You will still hear the six-second delay. However, the sound will only play for 54 seconds. That's because the car will be right next to you after one minute, and the sound at the end of the minute gets to you instantaneously.
The car from the driver's perspective is still blaring its horn for one minute. Because the car is moving, however, the minute's worth of sound gets packed into 54 seconds from your perspective. The same number of sound waves are packed into a smaller amount of time.
Therefore, their frequency is increased, and the horn's tone sounds higher to you. As the car passes you and moves away, the process is reversed and the sound expands to fill more time. Therefore, the tone is lower.
You can combine echo and doppler shift in the following way. Say you send out a loud sound toward a car moving toward you. Some of the sound waves will bounce off the car an echo. Because the car is moving toward you, however, the sound waves will be compressed. Therefore, the sound of the echo will have a higher pitch than the original sound you sent. If you measure the pitch of the echo, you can determine how fast the car is going.
While we're here on the topic of sound and motion, we can also understand sonic booms. Say the car was moving toward you at exactly the speed of sound -- mph or so. The car is blowing its horn. The sound waves generated by the horn cannot go any faster than the speed of sound, so both the car and the horn are coming at you at mph, so all of the sound coming from the car "stacks up. At exactly the same moment the car arrives, so does all of its sound and it is LOUD!
That is a sonic boom. The same phenomenon happens when a boat travels through water faster than waves travel through the water waves in a lake move at a speed of perhaps 5 mph -- all waves travel through their medium at a fixed speed.
The waves that the boat generates "stack up" and form the V-shaped bow wave wake that you see behind the boat. The bow wave is really a sonic boom of sorts. It is the stacked-up combination of all of the waves the boat has generated. The wake forms a V shape, and the angle of the V is controlled by the speed of the boat. We have seen that the echo of a sound can be used to determine how far away something is, and we have also seen that we can use the Doppler shift of the echo to determine how fast something is going.
It is therefore possible to create a "sound radar," and that is exactly what sonar is. Submarines and boats use sonar all the time. You could use the same principles with sound in the air, but sound in the air has a couple of problems:.
Radar therefore uses radio waves instead of sound. Radio waves travel far, are invisible to humans and are easy to detect even when they are faint.
0コメント