Unlike most pilots, I've been listening to the aviation frequencies since I was 10 years old. And I've listened to and studied quite a lot of signals in my "other life" as a licensed amateur and commercial radio operator. Few of the people I've shared the flight deck with are concerned or have a clue as to what is in the background when the regular aeronautical traffic is not transmitting. I actually pay attention to who or what is in the background. Perhaps it is that instinct developed long ago when I would stay up throughout an evening chasing DX on the HF bands, or trolling the military air band for elusive satellite downlinks. Whatever the reason, I listen for the unusual - and noticed something in the spring of 2013.
It go got my attention one day, while I was flying near Hong Kong, going northbound across the border into the mainland. We were using an ATC frequency near 119 MHz. In between routine calls from aircraft or ATC, I could hear a weaker signal breaking the squelch at a regular rate of several seconds. It was an odd whining sound, and something quite broadband in nature. Our radios tune in 12.5 kHz increments, and the signal covered at least a couple of channels. Other times, I could hear a different wideband signal come on with a low buzzing component. It did NOT sound like a cable TV leak on the aero frequencies. Nor did it resemble a harmonic or spur from a poorly adjusted FM transmitter somewhere below my aircraft.
After noticing the signals many more times, they showed a pattern: appearing mostly on the coastline along the Taiwan Strait and certain inland areas featuring restricted airspace. There is one area, well inland, that I call "no man's land" due to a lack of civilian airports, that had the mystery signals. No man's land is also an area where our GPS accuracy would degrade from the usual 0.2 or 0.3 nautical miles to a large 0.7 or 0.8 nautical miles. Considering the where and what of the environment, plus the resemblance of the signals to known radars, the logical conclusion is that the aeronautical mobile radio band is also host to a frequency agile VHF radar system designed to detect large stealthy aircraft.
The Zuhai Air Show in 2014 shed some light on the new Chinese radar system. On display was the JY-26 VHF / UHF phased array radar, said to be capable of detecting F-22 and B-2 aircraft. One thing odd about the system photographed is that the array seems small. I believe that there is another phased array system, not on display, which can effectively work at a 2.6 meter wavelength. That would match my personal observations on the aeronautical mobile communications band. Such an antenna would resemble a flat billboard 50 to 100 percent larger than the JY-26 radar antenna. It doesn't have to be monstrous like the American PAVE PAWS radar system, but should be larger in order to produce a usefully narrow and intense beam.
|Chinese Anti-Stealth Radar 2013-09-11-0217 UTC|
|Chinese Anti-Stealth Radar 2013-09-24-0228 UTC|
|Chinese Anti-Stealth Radar 2013-11-03-0851 UTC|
|Chinese Anti-Stealth Radar 2013-11-25-0918 UTC|
There may be two separate radars operating on the same frequencies, or perhaps one radar runing in two modes. If similar to older radars, the lower frequency buzz would be the long range search mode. Higher pitched signals would correspond to a short range mode which would much more accurately track the movement of a target. To a fast computer system, the aircraft would be moving in slow motion and cover several meters between pulses. The radar sends a signal, waits for the returns from objects. If the radar needs 6.66 microseconds per kilometer distance from the transmit antenna, then it must wait almost 23 microseconds to get returns from 150 km away. Reflections from stealth aircraft may be well below the background noise at those ranges unless very high power and tightly controlled directional antenna arrays are used. After getting the return signals, signal processors analyse things like phase and frequency to determine each object's motion and refine its position.
The radars may be smart enough to use quiet parts of the aero band. A lot of communication between ATC and area flights would interfere with operation of the system. Even a spread spectrum / frequency hopping / chirped radar would suffer if too many strong voice transmissions are in its passband.
Why is there no mention of these systems on the internet? There are references to "VHF radar" but nothing specific about these new systems operating on frequencies allocated to the aeronautical mobile service.
Stealth aircraft do two things to defeat radar: reflect and absorb. Look at a picture of the F-22 fighter aircraft, and note that, when viewed from the front, it has few surfaces that will reflect a signal directly backwards. Classic reflective points have been sharpened and reshaped in order to scatter radar at oblique angles and return very little energy to the sender. Stealth aircraft have very little bare metal visible. Surfaces are covered with radar absorbing paint, which is most effective for microwave signals.
Actively jamming the radar is another means of avoiding detection. Jammers are designed to exploit weaknesses in the radar, and make it useless by creating numerous false targets, burying targets in noise, decreasing radar accuracy, etc. Often the stealth aircraft are accompanied by electronic warfare planes tasked with detecting and jamming an adversary's radar systems.
A prime reason to invade the VHF Aeronautical Communications band with radar signals is that it is the best frequency range to detect aircraft designed to have low observability on radar. Radar absorbing paints are less effective at VHF, so it is less difficult to get usable reflections from a pulse of radar energy. Also, aircraft structures can resonate and reflect strongly at specific frequencies, as would a half-wavelength dipole.
Operating at VHF, an anti-stealth radar has an advantage over microwave systems. Although modern stealth aircraft are indeed designed with consideration to resonances and limitations of absorbative paints, their incredibly low observability is not so low at longer radar wavelengths. There is also the matter of airmass disturbance due to the passage of a heavy aircraft, which is somewhat detectable by radar (due to momentary changes in the air's refractive index).
Multilateration is the practice of deriving a signal's point of origin by measuring the difference in its arrival time at several locations and solving an equation for the distance from each receiver to the origin. Since light moves at a speed of 300 meters per microsecond, each microsecond's difference in arrival time means the origin is closer to one receiver by 300 meters. For each pair of receivers, there is a hyperbolic line along which the signal must have originated. Combine the lines from four or five pairs of receivers and you have a small volume of space where a signal was radiated (or where a radar signal was reflected).
Since a stealth aircraft achieves some of its low observability through deflecting radar signals away fro the transmitter, an effective countermeasure would be the deployment of many remote receiver sites, at oblique angles, to detect the reflected signals. Imagine how sunlight is scattered by a prism or finely cut and polished crystal. Reflections go off to the sides, above, and below such a crystal held up in sunlight. With accurate measurements of the signals' arrival time at each receiver, it is possible to compute the position of the aircraft. This multilateration technique requires very, very accurate timing and quite a lot of computing power, but it is possible for a technologically advanced adversary (or a less advanced adversary able to buy the system).
Multilateration works quite well for strong signals and is a common technique used for tracking trasponder equipped vehicles and aircraft. Weak radar signals present some challenges in fragility and flexibility. The receivers must be sensitive, but resistant to overload from strong signals and especially jammers. The networking among the receivers and central control site must be stable and broad banded enough to handle multiple large data streams. VHF phased array antennas are not small, and would take up a large amount of space around an area defended by anti-stealth radar. Such antennas would be about the size of a roadside billboard, easily detectable by satellite reconnaissance, and among the first things destroyed in an armed conflict.
Multilateration is a concept simple to understand, but not so easy to implement in a hostile environment. Radars would have to transmit VERY strong signals to get consistent results in a noisy radio spectrum. Radar signals are not always simple pulses pinging the sky. Some are necessarily chirped or swept up or down in frequency. Some are continuous waves, and others are obscured by noise by computer algorithms to hide them from detection and reduce the effects of jamming. Doing multilateration with these requires a lot of computing power for signal processing.
Considering the current philosophy of "anti-access / area denial" held by China's military in its ongoing efforts to control the waters off its shores, it is no surprise that these new VHF radars are appearing. I have also heard similar signals near the coastlines of Vietnam and Singapore, and suspect that they have purchased the same system from either China or Russia. Noting the rapid increase in noise on the air-band from these systems, a conflict is brewing over the seas east of mainland China. They see stealth as a threat, and intend to enforce the doctrine of anti-access / area denial.
Interestingly, the defense industry in the USA has been developing new equipment and advanced tactics. For one thing, there are terrain hugging flight profiles for the B2 to use while attacking targets in areas saturated with anti-stealth radar. Hypersonic missile technology is advancing at a moderate pace, to strike targets so quickly that radar detection is useless.
For every measure, militaries develop countermeasures. Technologically advanced powers have developed countermeasures against stealth aircraft, while the USA, Israel, and few others, have continued refinement of low observable technology against these new countermeasures. Why the buldup of stealth and anti-stealth? Our world is polarizing into two adversarial hemispheres. One seeking to live under democratic governance and freedom, under the rule of law. The other one accepting life under authoritarianism, tyranny, and the rule of thuggery. It reflects the mindsets of much of the populations: one asserting its dominion over government; the other allowing government to assert its dominion over people. Indeed, these are interesting times.