8 Things You Might Not Understand About Airborne Weather Radar – Part 2
8 Things You Might Not Understand About Airborne Weather Radar – Part 2
In the previous Radar Corner article, we started looking at why Honeywell developed the RDR-4000 and RDR-7000 weather radar systems. There was a short 8-question quiz; we provided the answers and the reasoning behind the first four quiz questions and showed you the results. This month we’ll explain the answers to the final four questions.
The Quiz Answers
5. For radars with an 18-inch antenna the beam is 5.6 degrees wide, and the radar will show targets at their true color level if part of the target is inside the beam. FALSE
6. At cruise altitudes (>FL310), radar targets, displayed as green at short range should be avoided. TRUE
7. If I am climbing (or descending) at a three-degree flight path angle, I should set my tilt angle to 3 degrees to see weather along my flight path. FALSE
8. With the weather radar you are currently using, do you know the range at which it is no longer calibrated, and returns are not displayed at their true levels? If yes, what is the range? 60-100nm
Questions 5 through 8 deal with reflectivity. The radar sends out a pulse of energy in space and measures how much is returned – that’s reflectivity. The heavier that it’s raining the more energy is returned and we represent that as different color levels on the display. If only it were that simple. Unfortunately, objects like buildings, parking lots, and the ground itself reflect radar energy extremely well, hence why you get ground returns.
Before we explain the answers let’s take a moment to understand the target that we are trying to detect. Figure 1 below illustrates the general model of a storm cell. Reflectivity increases from the ground to just above the freezing level and then drops off very rapidly. The bright band, or area of maximum reflectivity, is right around the freezing level.
Figure 1 – Simple model of storm cell reflectivity
The bright band is formed by melting hail or hailstones coated with a sheet of water which makes them look like extremely large raindrops to the radar. Wet hail and wet snow are excellent reflectors of radar energy. Unfortunately, dry hail, dry snow, and ice crystals only reflect about 3% of the energy of a raindrop. Therefore, reflectivity drops off very rapidly with altitude (Figure 2).
Figure 2 – Relative reflectivity
Figure 3 shows the cross section of a storm cell. The reflectivity values and colors are shown in Table 1 below:
Table 1 – Reflectivity & Color Level
The bright band (60dBz) is in the center. Even though this cell had enough energy to lift moisture over 39,000 ft into the atmosphere, it is only showing 20 dBz (light green) reflectivity at the top. The arrows indicate the wind speed and direction in the cell. The longest arrows are at the top indicating they have the strongest updraft velocities even though they would only show as light green on the display.
Figure 3 – Cross section of a storm cell with reflectivity and wind flow
By now you’re probably starting to visualize the answers to some of the quiz questions. For question #5, over half of our test group provided an incorrect answer (Figure 4). Over 1/3rd got question #6 wrong but almost three quarters of the group got #7 correct. Question #8 was admittedly the hardest of the 8 and over 88% either did not know or provided the wrong answer.
Figure 4 – Results of questions 5-8
Let’s start by looking at Question #5 – “For radars with an 18-inch antenna the beam is 5.6 degrees wide, and the radar will show targets at their true color level if part of the target is inside the beam.” It really doesn’t matter what size the beam is so we can ignore that. Remember from previous articles that the radar beam is like a flashlight beam with the energy strongest at the center and tapering off. In Figure 5 tilt is set so that the most intense part of the beam is pointed at the most intense part of the cell. If we were to raise the tilt by just two degrees, we would lose over half of the reflectivity because the center of the beam isn’t pointed at the bright band. Raising the beam just another degree would remove the bright band from the weather radar beamwidth and we lose 93% of the signal return.
To measure the true reflectivity, you can’t just get some part of the cell. For this reason, we break radar operation into detection, analysis and avoidance. Use the cruise ground park technique to detect cells. This isn’t going to show you the maximum reflectivity unless the screen is filled with ground return. When you need to analyze a cell tilt down to see the maximum reflectivity. Then tilt up to measure the vertical extent of the cell.
Figure 5 – Proper Tilt Setting
With the RDR-4000 & RDR-7000 we take multiple scans and show you the maximum reflectivity at any given point on the display (composite image) while removing ground returns and compare it to the aircraft’s flight path. The altitude slices allow you to easily measure the height of a cell without doing any math.
For question #6 let’s refer to Figure 3 where there is lower reflectivity at the top of the cell. The radar beam is very narrow at close range and intersects the cell at high altitude where there are mainly less reflective frozen storm tops (Figures 6 & 7). The figure below is from a B777 delivery flight. The aircraft was at FL370 with the tilt set to detect the lower, more reflective portion of the cell 80 nm ahead. There is green and yellow reflectivity shown.
Figure 6 – Tilt set to see lower, more reflective part of cell
In Figure 7, we’re now 10nm from the cell. At this range the beam is narrower and intersects the top of the cell. This cell had enough energy to lift moisture over 37,000 ft but is only showing as light green on the display. At night, when you might not be able to visually acquire the cell, the updrafts could certainly cause unexpected turbulence.
Figure 7 – Storm cell reflectivity at high altitude and close range
Question #6 was, “At cruise altitudes (> FL310), radar targets, displayed as green at short range should be avoided,” and this is most definitely TRUE. It is also part of why we developed a 3D radar. Looking at colors only tells us part of the story. With increased reflectivity there is a higher probability of turbulence and hail. But as we just saw, there are conditions where even green can indicate hazardous conditions. The RDR-4000 and -7000 use several methods to alert you of hazardous convective activity. While scanning and collecting data, the system automatically increases gain to help make frozen storm tops more visible. We also provide a composite display of the data which shows the maximum reflectivity for any given point and indicate hazards relevant to your flight path.
Question #7, “If I am climbing (or descending) at a three-degree flight path angle, I should set my tilt angle to 3 degrees to see weather along my flight path” (Figure 9). This is false because we always want to detect and show the maximum reflectivity. In many parts of the country like Denver and Phoenix with arid conditions, rain can evaporate before reaching the ground (virga) and show very little reflectivity despite updrafts and downdrafts existing. You should always set tilt to detect the most reflective part of the cell. Setting it to your departure or arrival flight path might cause you to scan under the maximum reflectivity providing a false presentation of the threat.
The RDR-4000 and -7000 show the maximum reflectivity and compares it to your vertical flight plan or vertical flight path. Some display systems can provide a vertical profile view of weather, terrain and flight path.
Figure 8 – Tilt angle vs flight path angle
Question #8, “With the weather radar you are currently using, do you know the range at which it is no longer calibrated, and returns are not displayed at their true levels? If yes, what is the range”. This was admittedly the toughest question of the quiz and the results verified this with over 88% providing a wrong or “don’t know” as the answer. This isn’t surprising as it depends on the cell strength, antenna size, and other factors. The important point is that you realize that the radar won’t always show the correct colors due to physical limitations. Figure 10 shows our little pulse of energy going out and hitting three identical storm cells at different ranges. All the energy is hitting the cell closest to the aircraft and shows as red. Most of the energy is hitting the second cell and it shows up as yellow. At further ranges the energy is going around the cell and it shows up as green (top row). That wouldn’t be very useful for avoidance because as you got closer to each cell it would turn red. So, every radar has a circuit or software function that maintains color calibration with range and is called Sensitivity Time Control, or STC. It’s very simple and effective. We know how large the beam is growing with range and we increase the gain over time to compensate for it (bottom row). The problem is the beam continues to grow but we only have a limited amount of gain that we can provide. As Scotty would say to Captain Kirk – “Captain, I’m givin it all she’s got”. And the range where we can no longer maintain color calibration is called the STC limit.
Figure 9 – Without STC or Sensitivity Time Control
Figure 11 illustrates this point. The black bars indicate the signal returned from the storm cell. They are strongest close to the aircraft and decrease with range. Overlaid upon these are the different color thresholds for green, yellow, red, and magenta. The storm is above the red threshold out to about 110nm, then the return strength decreases and is shown as yellow, then green, and finally it completely disappears. For this antenna size and storm cell, it remains calibrated out to 110nm.
But when flying you see it the other way around and need to think of it that way. At longer range the cell will eventually show up in green even though it is a red cell. As you get closer and closer, it will turn to yellow and then red. It looks like the cell is growing in intensity, but it’s just getting to a range where the radar can maintain color calibration. Because this is mostly physics, we can improve but not eliminate this with the RDR-4000 and -7000. Because it is a newer system with more sensitive receivers and greater dynamic range, we can extend the range that the radar remains calibrated for any given antenna size.
Figure 10 – Understanding how STC works
That finishes up the answers to the 8 questions. Next time we’ll have a much shorter article and look at how our test group did in the test scenarios. We’ll also provide a brief introduction to the new RDR-7000 radar.
Program Pilot Stephen Hammack supports Honeywell Apex and radar for Flight Technical Services. He can be reached via email at stephen.hammack@Honeywell.com.