Originally Posted by JohnBurke

The easiest way to set the bottom of your beam at a range is to set the next highest range, so say, 160 miles. The 80 mile arc will be in the middle of the display, or half-way up the display. Tilt your beam down until your see tickles of ground return near the 80 mile line, and you've got the bottom of your beam at eighty miles.

If you think of this like your car headlight, which projects from a single point on your vehicle, looking forward, the light projects out in a cone-shape, pointy end of the cone at the light, extending outward and expanding away from your vehicle. Radar works in a similar fashion, so you may envision your car headlight projecting a beam forward, and the point where it first directly illuminates the ground is the bottom of the headlight beam. Likewise for your radar. From that point, moving forward, or down range, you're looking down to the ground, through anything in the air.

Objects that aren't in the beam must be progressively lower, the closer you get to the bottom of the beam, so if you're set the tilt to paint ground returns at 80 miles, what's beyond it is in the beam, what's just shy of 80 miles can be a little above ground and not get painted, and what's directly under you can be nearly as tall as you are and not get painted ("painted" meaning you see a return on the radar, or in other words, the object is reflective of the radar, and it's in the beam). Beyond the beam, objects from the surface to a much higher altitude, can be detected. Because when you're in cruise, most weather comes from the surface and goes up, starting looking at the ground, upward, is a wise choice. Precip will be falling, storms building upward, etc. It's a good place to detect storms, and the most reflective (wet) parts of the storms will be the lowest parts, like looking for the roots of a tree to know there's a tree above it. You'll paint a lot of the tree, or the storm, as well.

Where this becomes useful is as you move forward, the object that you painted beyond 80 miles, may be tall enough to stay in your beam, or it may have been a shorter object which will eventually not be tall enough to be directly illuminated; it will appear to weaken and then disappear, as you approach it. This suggests weather well below youIf the weather continues to be clear and well defined, and especially if it grows at say, 40 miles, it's time to be thinking about which way you'll be deviating based on surrounding activity, winds, etc...try to go upwind, but try not to go between other cells that are close or building, etc. This is where you have to look at the total picture and begin to form a plan.

I see a lot of radar users who throw their range display out to 320 (or farther) and tilt the radar up, or use a 1 or 0 or -1 setting for tilt (the tilt numbers aren't actually telling you where true zero is...that's something you have to establish on your own). This is false confidence, and while it may illuminate something of marginal use, it's not providing what they may think it is. As distance increases, so does attenuation, such that the information that comes back needs re-interpretation, or it may be deceptive. Zero, incidentally, is a value that assumes the center of the beam at the horizon. On your display, the radar is a two dimensional wedge that lays flat, extending away from your radome, so many degrees left or right, but it also extends up and down, too, and it has a bottom edge and an upper edge, of the cone, and one can envision an imaginary middle of the cone...that's where the tilt number comes in. Despite gyro stabilization and other neat tricks, the nature of radar installation, and aircraft pitch and attitude, and other factors, the true zero of the radar beam usually doesn't coincide with the number you see on your display in the cockpit.

Knowing cone or beam width and angle and distance and finding true zero are elements of making an accurate determination of storm tops and bases...something that's not really taught any more, and which takes some time to go over (and depends on what you're flying and the radar installed, dish or plate size, etc). Mostly we use radar to get the picture of what's out there and go around it, without getting too fancy, and that's generally a good plan. My personal use in cruise over long distances when I'm not examining something specific is to park the beam lower edge at 80, and watch to see what appears...and then begin tilting and scanning and looking, as well as frequently ranging in and out, during the flight. Combined with as much preflight information as you can get, it's another tool for avoidance, rather than penetration, and that should be emphasized. Today pilots are not given adequate training to analyze storms, and I don't think they necessarily should be: our emphasis is rightfully on avoiding storms and convective activity.

Sometimes we find ourselves in places with a lot of activity, and at that point, we are looking to get around or through, even if it's an escape path. In such a case, the single biggest thing we need to understand is attenuation. Your car headlight has only so much light to throw out there. It can get diffused and not go as far, or absorbed, or reflected back, such as driving in fog, or near road signs, or obstacles, and it may not illuminate as far down the road as you might hope. You could see an object in the headlights, and think you've got the big picture, but you really don't, because too much light is being reflected back. You come up on some road construction, and all the reflective signs throw a lot of light back in your face, so much so that you don't see the deer or moose standing just beyond, or in the construction. In radar terms, this is a "radar shadow," where some of the reflective (moist) weather has reflected or absorbed or diffused so much of our radar energy that we don't see what's laying beyond, and it looks clear, or black, on our display. Any time you see a clear area just beyond what you're painting, suspect that there's something you're not seeing in that clear area. If the plan is to squeeze through that gap then zig back on course into that clear area, don't be surprised that you've fallen into a trap and that worse weather lays beyond...hidden by what you were just avoiding. Attenuation.

Another issue that's not discussed much at all is that radar units not are used infrequently. This might be the case with an aircraft that isn't pulled out of the hangar often (corporate airplane, for example), or may be the case with an aircraft equipped with more than one radar unit). Not so true of newer radars, but very true of older radars, the equipment itself, specifically the magnatron, requires frequent use or it looses efficiency; some older radars might require use within a month or might need component replacement. Few seem aware of this today; energizing of those components affects the efficiency and viability of those components, and if they sit for too long, they won't tell you what you think they should be telling you. Older weather radar in particular is very much a use-it-or-lose-it proposition.

Another issue that I've seen has been radome transmissibility; radomes are usually a honeycomb material, which means they're fiberglass layers over an expanded matrix that acts as a filler or stiffener or thickener of the radome, usually shaped like honecomb cells in a beehive. These cells are hollow and if pin holes develop in the radar, the honecomb holds moisture, which is reflective to radar. If you start seeing lobes or lines appear on your radar, or inverse lobes, seen as streaks of color away from the radar beam center, or streaks of dark, these may be signs of moisture in the radome, and should be reported to maintenance; they can hide airborne returns, or reflect high levels of energy back to the radar system; one is deceptive, the other potentially harmful, but requires maintenance. This can occur as a result of deterioration, lightning strikes, physical damage, inadequate repair work, etc, and should be noted when using the radar. Remember, the guys in the hangar don't operate it, and don't see what you see. The only chance to catch such stuff may be when you're using it operationally. Be as detailed as you can in your description.