Airborne Weather Radar - Separating Fact from Fiction
R. Fred Polak | Editor
Today’s modern airborne weather radar systems are lightweight, multicolor digital systems designed to provide flight crew with weather location and analysis. The intent is to detect and avoid storms along the flight path of the aircraft. These weather radar systems have come a long way, showing much more today than when they first arrived on the scene some 40 years ago. Windshear and turbulence detection have been integrated into airborne weather radar systems, along with auto-tilt capabilities and more. These are complex systems, and as a whole are not really understood very well as to what they can and cannot do.
Most modern day airborne weather radar systems are X-band systems that radiate anywhere between 18 watts and 10 kW of power. The X-band is a segment of the microwave radio region of the electromagnetic spectrum. In radar engineering, this frequency range is specified by the Institute of Electrical and Electronics Engineers (IEEE) at between 8.0 and 12.0 GHz. With that said, let’s separate fact from fiction concerning airborne weather radar systems in general.
Fiction: Exposure to high power X-band radiation can lead to sterility.
Fact: Not true! However, all nerve tissue, especially the optic nerve, is very susceptible to X-band radiation and care should be taken to observe all manufacturers and FAA recommended safety procedures when working around a weather radar system that is radiating radio-frequency (RF) energy. Prolonged exposure can lead to glaucoma in years to come. Refer to FAA Advisory Circular AC No. 20-68B, Aug. 8, 1980, Subject: Recommended Radiation Safety Precautions for Ground Operation of Airborne Weather Radar.
Fiction: Weather radar will detect fog, ice crystals and small dry hail. It can also detect other aircraft in flight.
Fact: Weather radar detects moisture. It detects wet hail, rain and wet snow, but not dry hail or dry snow. The larger the water droplets, the stronger the return signal. It cannot detect other aircraft in flight.
Fiction: The weather radar’s energy is reflected by the weather it detects.
Fact: Saying that the RF energy is reflected is an easy way to describe how the weather radar system displays a returned signal, but it is an inaccurate description. What is really taking place is referred to as “dipoling” and “backscattering.”
The mechanism of a radar “return” is as follows:
When a Transverse Electro Magnetic (TEM) wavefront encounters a conductive object (moisture), it induces voltages and currents (incident power) in the object. This is how a receiving antenna works. These voltages and currents cause the object to become a transmitting antenna in turn, and it will re-radiate a small portion of the incident power. (Now it gets technical, so you can skip the next paragraph if you want to.)
If the plane of polarization dimension of a conductive object is equal to the wavelength of the incident wavefront (31mm for most weather radars), the conductive object will resonate and the backscattering will be at maximum. Smaller objects will backscatter, but less than maximum.
Water is a fairly good electrical conductor, so it is readily visible to radar. Although most raindrops are much smaller than 31mm, they still backscatter. (Google: Rayleigh scattering.) Non-conductive objects do not permit currents to flow in them, so they are almost invisible to radar. Ice (i.e., hail) is a very poor electrical conductor, provided it is not coated with water, so hail is largely invisible, especially at high altitudes.
The strongest weather radar returns are from wet snow because the icy snowflakes serve to hold very large drops of liquid water together. Some extremely alarming radar paints can often be seen around the freezing level in stratiform rainstorms.
Fiction: Dings and dents to a flat plate radiator antenna will not degrade the radar’s performance significantly.
Fact: A flat plate radiator should be as its name implies: flat. It should not be concave or convex. Lightly running a straight edge over its surface will show if it is true or not. The smaller the dings and dents and the closer to the outer edge of the antenna, the less they will degrade the radar’s performance. The larger the dings and dents and the closer to the center of the antenna, the more they will degrade the radar’s performance.
One of the most important parts of an airborne weather radar system is not even part of the system! It is the radome — part of the aircraft.
A radome is a structural, weatherproof enclosure that protects a radar antenna. Physically, a radome should be strong enough to withstand the airloads that it will encounter and should be contoured to minimize drag.
In helicopters as well as business aircraft, the radome for the weather radar system is the nose of the aircraft. The radome is constructed of material that minimally attenuates the weather radar signal transmitted and received by the weather radar antenna. In other words, the radome is designed to be transparent to radar or radio waves at a particular frequency range. Radomes protect the antenna surface from the environment (e.g., wind, rain, ice, sand and ultraviolet rays).
Radomes are expensive to repair or replace and should be treated accordingly. Tears, punctures or cuts in a radome are an invitation for problems to the weather radar’s ability to perform properly, not to mention possible severe damage to other avionics equipment and possibly the airframe. Helicopter and business jet radomes are generally a fiberglass honeycomb composite. Other more exotic materials can be used, but that has more to do with the aircraft’s mission than the weather radar system.
The most frequent damage to radomes is holes in the structure caused by static electrical discharges (lightning). This damage can be seen as large holes that are readily apparent, or pin-size holes that are almost imperceptible. Any hole, regardless of size, is a concern and can cause major damage to a radome since moisture can enter the radome wall and cause internal delamination. If the moisture freezes, more serious damage may occur. If enough moisture collects, the weather radar’s radiation pattern will be distorted and the transmitted signals and return echoes seriously attenuated.
Ram air flowing through a hole can delaminate and break the inner surface of the radome and result in separation of the skins or faces of the material from the core, weakening the radome structure. Other types of damage are characterized as dents and scratches caused by impact with stones, hail and birds, and improper handling of the radome when it is removed from the aircraft for maintenance actions.
Aircraft X-band radomes are precisely engineered and constructed. The slightest change in their physical characteristics can adversely affect the weather radar system’s performance. This can be caused by a faulty repair action or something as mundane the wrong kind of paint. All repairs to radomes, no matter how minor, should return the radome to its original or properly altered condition, both electrically and structurally.
The performance of proper maintenance to aircraft radomes requires special knowledge, skill and techniques, and the use of proper tools and materials. An improper minor repair can eventually lead to an expensive major repair. A radome having undergone major repairs should be tested to ensure that it meets all electrical and structural criteria. The testing of radomes requires test equipment that usually is found only in repair facilities specializing in radome maintenance.
Even minor repairs may affect one or all of the following:
Transmissivity: The ability of a radome to pass radar energy through it.
Reflection: The return or reflection of the outgoing radar energy from the radome back into the antenna and radar receiver.
Diffraction: The bending of the radar energy as it passes through the radome.
Reflection: The inner surface of the radome acts as a reflector and reflects some out-bound energy backwards against the radome bulkhead … then in unpredictable directions, often causing short-range nuisance targets or altitude rings.
These electrical properties, when changed by an improper repair, may cause loss of signal, distortion and displacement of targets, and can clutter the weather radar display to obscure the weather target. The bottom line on performing weather radar radome maintenance is to only allow qualified personnel and qualified facilities do the work. Quality and safety is everyone’s business.
Fiction: A signal generator can be used to adjust the receiver frequency of the radar.
Fact: The problem in ensuring correct tuning of a radar receiver to the same frequency as the transmitter is that any pulse-modulated transmitter transmits a spectrum of frequencies, not one discrete frequency. In a transmitter that is working well, the main spectral lobe is 13 dB stronger than the next strongest lobe. The difficulty lies in correctly aligning the signal generator frequency with the main spectral lobe, and not one of the minor lobes.
If the radar under test provides a pilot frequency output, which can be used to steer the signal generator to the correct frequency, well and good — except that now you are using an uncertified test device to make a critical adjustment.
Some signal generators claim to find the main lobe automatically, but I am cynical about that. The IFR RD300 has an output that shows the difference between the signal generator output and the radar under test. This makes a sort of poor man’s spectrum analyzer and works well.
It is possible to use a spectrum analyzer for this purpose, but the difficulty here is that the very weak signal generator output must be viewed on the same display as the much more powerful transmitter spectrum.
The one way that always works is to use the tried and true, old faithful echo box. The difficulty here is that nobody makes them any more. We are so bound up in our whiz-bang gizmology that we have forgotten the old iron-age tricks. For those of our readers who have never heard of one, an echo box is a high Q mechanically tunable ‘X’-band cavity that is coupled to the system under test. When tuned to the transmitter frequency, a tuning meter shows the best tune, and at the end of the transmitter pulse, it continues to resonate for about 20 μS, and a phantom target will be seen that can be used for adjustment purposes.
I suppose that if I was running a radar shop nowadays and couldn’t get an echo box, I would rig a fixed flat plate antenna on the roof, pointed at some handy building or mountain, and use that for receiver adjustment.
Other Weather Radar Considerations
Ground mapping: In the helicopter environment, the ground map function starts to have greater importance as compared to fixed wing aircraft. After all, helicopters tend to fly slower and lower and usually don’t encounter the really nasty high altitude weather phenomena.
When viewing weather, great resolution is not really required, as weather targets tend to be several miles across. Of course, this is less true if the radar uses a turbulence mode, as turbulent cells tend to be small, so solid-state transmitters are acceptable. However, if a ground map function is to be useful, it must have reasonably good resolution, or it will be unable to show ground features as anything better than a mush. Range resolution is determined by the transmitter pulse width, and the azimuth resolution is determined by the antenna size.
A pulse width of 1μS will permit two objects about 600 feet apart in range to be shown as two separate objects.
An 18-inch diameter antenna will permit two objects separated by six degrees in azimuth to be shown as two separate objects.
The ground map functions on various radars range from a simple change of color scheme to radical changes in transmitter parameters and receiver processing protocols.
Solid state transmitter vs. Magnetron: Many of the newer weather radars use solid state transmitters. The advantage of this is that they are more reliable than magnetrons, but they have some severe downsides.
Solid-state transmitters are limited to low power output … on the order of 100 watts or less, so in order to get the necessary energy on the target, very long pulses are used to permit the use of very narrow bandwidth receivers. However, the long pulses result in poor range resolution.
Another downside is that when they do go wrong, they are expensive to fix.
Beacons: Some radars are designed to work with small ground beacons, as well as showing ground targets and weather. These beacons are about the size of a loaf of bread, although they can be a lot smaller, and work in a similar fashion to an ATC transponder, but at X-band frequencies. In order to use this function, the radar has to transmit on a special frequency (9375 MHz), output a specific pulse width (2.35 μS) and must have sufficient power to trigger the beacon, which eliminates solid state transmitters.
Waveguides: All X-band radars use waveguides to some extent or other. They are usually quite robust, but any breaks or cracks destroy them. Waveguide joints come in two forms: plain face-to-face or choke joints.
Plain joints are inefficient, unless the two faces make intimate contact all round the periphery of the waveguide orifice. Choke joints are very forgiving, and do not rely on actual contact. Choke joints can be recognized by the deep groove around the orifice.
Three-box systems: Many weather radars today have the transmitter, receiver and antenna all in one radome mounted unit, with resultant data being displayed on either a dedicated weather radar indicator or on an Electronic Flight Instrument System (EFIS) display. However, larger and more powerful radars have the transmitter and receiver located in a remote T/R unit with a waveguide connection to the antenna.
Attenuation: This is a term for the loss of signal strength as the radar attempts to look through a weather formation to see the weather behind it. The problem is that the outbound energy from the transmitter has to pass through the first target, losing energy to resistive losses and backscatter, and then get back-scattered by the distant target, then pass through the near target again on its way back to the radar.
Turbulence: Many weather radars today include a “turb” mode which extracts turbulence data from weather targets. There are two methods of achieving this: Doppler and Scintillation.
Doppler: If a raindrop is moving towards or away from the radar, the frequency of its back-scattered signal will be shifted up or down. If a target exhibits a large number of such shifts, it is reasonable to assume that it is turbulent.
Scintillation: Any returned signal from a weather formation is, in fact, the vector sum of many millions of signals from many millions of water drops. As these drops are all at different ranges, the vectors will add or subtract from each other. If the drops are moving around violently due to turbulence, the resultant vector sum will be changing rapidly. This rapid change and the speed at which it changes can be used to indicate turbulence.
I hope that this has helped to clarify what is fact and what is fiction with regard to airborne weather radar systems. Like any other system on the aircraft, the more you know about it, the better and more easily you can maintain it.