Radar Energy and Power Tutorial
[Honeywell Direct-To, FMS Quarterly Update and Newsletter, December 2012, page 4.
- There is a subtle but distinct difference between energy and power. Power is a constant, especially with the fixed power transmitters we have in our radar systems. You may have a 2 kilowatt (KW), 5KW or 10KW transmitter on your radar. Energy = Power X Time, or in English, the amount of energy transmitted depends on how long the power is on. But then you already know that because you get an electric bill each month. Let’s do an experiment with you and two of your neighbors.
- You have a 300-watt light bulb that you leave on for one hour. One neighbor has a 150-watt light bulb he leaves on for two hours. Your other
neighbor has a 150-watt light bulb that he turns on for an hour, turns off for an hour, and then turns back on for an hour. When you get your
electric bills you are all billed for the same amount of energy - 300 watt/hours.
- Well, there you have it – you’ve just learned everything you need to know about radar energy. Let’s look at it a slightly different way. Here are our light bulbs again. We have a 300-watt light bulb that we keep turning on for an hour, then off for an hour, and repeating.
- Of course, in our industry, we have names for all of this. You’ve probably heard these referred to as peak power (P), pulse width ( ), and PRF (T), or pulse repetition frequency. Peak power is our 300-watt or 75-watt light bulb. Leaving it on for one or two hours is the equivalent of pulse width and turning it on and off every other hour is PRF, or pulse repetition frequency.
- I can create the same amount of energy by transmitting a 10KW pulse for 1 microsecond (uSec), or a 100W pulse for 100uSec. So you’re probably wondering why would you pay for a radar with a 10KW transmitter when you could buy a 100W transmitter and create the same amount of energy. The answer is how we intend to use that energy and the engineering trade-offs involved. The time it takes for a radar signal to travel one mile, hit a target and return is called a radar mile and is approximately 12.36uSec. When the radar is transmitting it can’t receive, so we say we are blind while we are transmitting. For our 10KW transmitter with a 1uSec transmit pulse, we are blind for about 1/12th of a mile. But for our 100W transmitter with a 100uSec pulse we are blind for over 8 miles. Now that might not be as big an issue for weather avoidance but it certainly would be if you were trying to find and land on an oil rig in the Gulf of Mexico. A more important reason that you want to use a narrower pulse width is resolution. The width of your pulse becomes your measurement instrument. Notice I didn’t say ruler or yardstick; this is because those measurement devices have markings on them so you can measure less than a foot or yard. You could measure a table and say it is 2 feet, 3¾ inches. With a 100uSec transmit pulse you have a stick 8 miles long with no other markings on it. Everything you measure is either 8 miles, 16 miles, 24 miles, etc. So every storm is at least 8 miles wide and if two storms are separated by less than 8 miles they will look like one storm cell. Even within a storm cell, all the returned energy is averaged over that 8 miles, so you have very poor resolution within a cell.
- The other way we said that you could create energy is by transmitting more often. A higher pulse repetition frequency will create more energy but we still have an engineering trade-off. Let’s say we use our 100W transmitter with a 1uSec pulse width and transmit a pulse every 125uSec. We have a low power transmitter and a narrow pulse for good resolution. But if you look at the diagram below, you can see it limits our range to about 10nm. Remember that one radar mile is about 12.36uSec so 10nm is about 123.6uSec. In the real world we are even further limited because of a phenomenon we call “second time around.” The upside-down red V after the second pulse represents a weather return. The problem is it’s hard to tell if the return is a return at a long distance from the first transmit pulse, or at a short range from the second pulse. So we are limited by how close we can transmit together.
- Those of you old enough to remember the old radars that used klystrons and traveling wave tubes know that these radars could transmit at 65,000 to 75,000 watts with a very narrow pulse. And that is ideal for both good resolution and long range detection. The problem is these radars weren’t able to do Doppler, so you would lose turbulence and windshear detection capability. The life of these radars was also measured in just hundreds of hours. With modern radars we have found creative ways to deliver similar or better performance to the older radar systems. Our transmitters have fixed peak power but we can, and do, vary the other parameters. Most radar specifications list at least two different pulse widths. At short ranges where we don’t need as much energy, we can transmit a narrow pulse, providing better resolution. At longer ranges where we need more energy, but not as good resolution, we can use a longer pulsewidth.
- We have also improved antenna designs, and the newer radars are much more sensitive. We often say the older radars shout louder and the newer radars listen better. Say you have an elderly parent who has a hearing problem. To make them hear you could shout at them, or you could provide them with a hearing aid so they can listen better. Listening better is how the newer radars use the energy we transmit. Another important radar concept is signal-to-noise ratio. It’s a bit difficult to explain signal processing in a short article but now that the presidential election is over with we can use it to explain this concept. I moved from Washington state to Arizona four years ago. Washington state was primarily Democratic whereas Arizona is mainly Republican. If you were to take a poll in either of these states the outcome would be obvious. So pollsters try to separate the signal from the noise by taking a random sample around the United States. If they were to poll 100 people in each state they would have a result with a large margin of error (noise). If they sampled 10,000 people in every state they would have a better result as more noise has been filtered out.
- Signal processing works much the same way. Every time we transmit a pulse of energy we get a sample. Let’s say our radar is transmitting at 800 PRF, or 800 pulses per second. Our antenna is moving while we transmit, so for any given point on the display, we might get about 10 samples. We have a very weak radar return at 40nm and normal atmospheric noise in between. Let’s give the weak weather return a value of +1. Our signal, although weak, is always there, whereas noise is random. Sometimes it could be +1, sometimes 0, and sometimes -1. If we pick two points, one where there is signal and one where there is just noise and add them up, we would get:
Signal: 1+1+1+1+1+1+1+1+1+1 = 10 The average is 10/10 = 1
Noise: 1+0+1-1+0+1-1+1+0+1 = 3 The average is 3/10 = 0.3
- So the signal adds up, and so does the noise, but not as quickly. Inside the radar we set up a threshold of 0.7. Anything 0.7 or greater is shown
as green on the display; anything less than 0.7 is shown as black. So now you understand how we use energy effectively and filter out noise.
A typical aircraft weather radar system, the G450 shown here, makes the mechanics of antenna aim and energy transparent to the pilot. More on that below, under Radar Reflection Characteristics.
Radar Signal Flow
The purpose of airborne weather radar is to find the distance and direction of weather in relation to the aircraft, as well as to discern a few of the target's characteristics. The radar transmitter sends a measur ed pulse of energy in a specific direction. The target reflects some of that energy back. The radar times the round-trip distance of the signal to determine distance, the amplitude of the return to determine strength, and the direction of the pulse going out to determine direction. The energy of the pulse has a direct relation to the reflection.
Radar Reflection Characteristics
The energy level of the radar signal going out directly impacts the energy of the reflection coming back in. Too little energy will not be reflected at all; too much energy risks going right through the target or skewing the reflection intensity. Getting it just right so that the information display to the pilot is all a matter of gain and calibration.