RF & Microwave

It was May 24, 1844 when Samuel Morse transmitted his famous telegraph message “What hath God wrought” from Washington to Baltimore. Twenty years later, the U.S. Military Telegraph Corps had trained 1,200 operators and strung 4,000 miles of telegraph wire, which increased to over 15,000 miles by the end of the Civil War. While long-distance communication proved a significant advantage for the Union armies, it also opened the door for wiretapping. It was these early experiences that demonstrated the impact of surveillance and set the foundations of electronic warfare (EW).

Over the last century, electronic warfare has had an increasing role in shaping the outcomes of conflicts across the globe; however, few people appreciate its significance and fewer still understand the technology. In this first post of our electronic warfare blog series, we present a brief history of the technology behind electronic warfare. Just as older cars are more intuitive to repair, the early EW systems are easier to understand.

While wire-tapping was used during the Civil War, it wasn’t until the 20th Century that the field of electronic warfare began to mature. By the start of World War I, the need to for rapid communication over long distances became even more critical—leading to significant advances in the emerging field of signal intelligence. Immediately following the declaration of war, the British severed Germany’s undersea cables, forcing them to rely on telegraph and radio—both vulnerable to interception. To protect the content of the transmissions, Germany began expanding on its cryptography capabilities.

During World War II, the use of the electromagnetic spectrum played an even larger role. It was quickly discovered that by flying bombing runs at night, the bomber crews were protected from anti-aircraft fire. However, locating targets at night was no easy feat.

The Lorenz System

Prior to the start of the war, Germany had invested in commercial RF systems to support blind landings at airports with reduced visibility. Called the Lorenz System, it operated by switching a signal between two antenna elements—one pointed slightly more towards the left and the other towards the right. Instead of equal pulse lengths on each antenna element, the switch sent the signal to the right element for a longer period of time—creating a long pulse on the right antenna and a short pulse on the left. As the plane approached the runway, the pilots would hear short tones if they were too far to the left and long tones if they were too far to the right. When they were properly aligned, they would receive both signals and hear a continuous tone.

During the war, this system was modified to use large, high-directivity antennas to transmit long-range, narrow beams. Two systems were built such that the beams could be steered to intersect directly over the target. By following one beam, the pilots listened for the second signal to know when they were over the target and timed the release of bombs. This simple system drastically increased the effectiveness of the night raids over England and made the development of a system to counter the beams a top priority.

Upon discovery of the German system, the British developed a method to interfere with the beams. Using high power transmitters, the British would broadcast the same long-tone pulse signal used by the German system. When this signal was superimposed on the same frequencies, the German aircraft would never hear the steady tone and would be unable to simply follow the beam to their target. Other methods of jamming the German beams involved the use of a BBC transmitter to broadcast a steady tone on the same frequency. This CW signal filled in the breaks between pulses rendering the German system unusable.

As the British began their bombing campaigns over Germany, they too needed a method to locate targets at night. Their approach was a similar system that used two transmitters; each broadcasting a train of pulses. By measuring the time difference between received pulses, the pilots were able to navigate. However, this system was also susceptible to jamming.

The Emergence of Radar

In addition to the jamming of their navigational aids, the British bombers faced a new threat—German fighter pilots that were able to track the British planes using radar. One type of radar encountered by the British was a land-based early warning system that alerted the Germans to an approaching attack and also provided details such as the number of aircraft. Through intercepted radio communications and direct raids on radar installations, the British were able to learn the details of these systems—such as the operational frequencies—that enabled them to develop the technology to combat them.

Instead of simply jamming the radar, the allies developed a system that would receive the radar signals, amplify them, and re-transmit them to the radar receiver. These additional signals were perceived by the radar system as reflections from additional aircraft. Employing this technology, a single aircraft could function as a decoy and pull resources away from other areas. However, these early systems were dependent on the radar frequency, and by using multiple radars with different frequencies, it became much more challenging to deceive them.

To respond to the radars that operated over a wider band of frequencies, the Allies developed a jamming system that would transmit noise in various frequencies across the radar bands. This was effective until the Germans started using additional frequencies for the radar. Instead of jamming the radar itself, the allies discovered they could jam the communication signals between the radar operators and the fighter pilots. By sweeping a receiver over a broad frequency range, the British were able to determine the specific frequency that the Germans were using to communicate then transmit noise on that frequency.

Continued Technology Development

This back-and-forth cycle of inventing new ways to use the electromagnetic spectrum and developing the means to counter these new technologies continued through World War II and the Cold War. Even in the early days it was not sufficient to just have the best technology—in order to stay ahead, the technology required constant updates. Instead of deploying a system that could operate independently for a decade, EW systems required consistent modification to address emerging threats.

Now, over a century and a half after that famous telegraph message, the invisible battle over control of the electromagnetic spectrum continues. The ability to communicate, track objects with radar, and to use GNSS to navigate have become critical to success on the battlefield. Additionally, a major advantage is achieved by disrupting an adversary’s ability to communicate, use radar and use GNSS. With today’s environment of rapid technology growth—such as compact GaN, high speed processing and AI—the battle for EW superiority is at its fasted pace yet.

When one country develops a new radar system, its adversary starts working on a Jammer. In order to mitigate the effects of the Jammer, the radar developer then must design a system that protects the radar from those effects.

This invisible battle over control of the electromagnetic spectrum is critical to success on the battlefield and is the topic of the subsequent posts. However, understanding the technology to jam and deceive radar requires an understanding of the radar systems.

We’re all familiar with the applications of radar—that yellow warning light on your mirror telling you someone is in your blind spot, police radar monitoring your speed, images on the news showing the path of a storm. However, for the purpose of understanding electronic warfare, we’ll look at the types of radar in three main groups.

Three Types of Radar Systems

Search radars scan over long ranges and wide angles to locate targets. For example, early warning systems can detect distant threats to provide adequate time for a response. These types of radar systems often operate at lower frequencies since range is prioritised over resolution. Because range is so critical, these systems are sometimes installed on-board aircraft to increase the line-of-sight to the horizon.

Tracking radar systems accurately measure a small number of targets—often just one. By operating at a higher frequency, the resolution is increased, providing improved accuracy. Additionally, instead of scanning a large area, they focus on a much smaller region in order to continuously monitor their target. Fire-control radars employ this type of system.

Imaging radars employ a variety of techniques to analyse an area to create an image. For example, airborne synthetic aperture radar (SAR) can be used to create images of landscapes. The below image is of the Teide Volcano in the Canary Islands and was created using a space-based synthetic aperture radar on-board the Space Shuttle Endeavour.

Radar Basics

Now that we’ve broken down types of radar by their application, we’ll move on to describing the details behind their operation.

At its core, radar technology is simple. The transmitter sends a signal towards the target. Some energy from this pulse reflects off of the target and is received by the radar system. Since the radar signals propagate at the speed of light, measuring the elapsed time between sending the original signal and receiving the reflection allows for a calculation of the distance. By confining the radar signal to a narrow beam, the location of the source of the reflection is known.

Doppler Radar

The approach above provides the distance to the target and the angle of the target relative to the radar system. However, what if we also need to understand how the target is moving? To do this, we use Doppler radar.

If you aren’t familiar with the Doppler effect, it explains why the pitch of a siren appears to change right as the ambulance drives past. In terms of radar, the frequency of the reflected wave is modified as a function of the reflector’s velocity. In practice, the velocity of the target is determined by analysing the phase shift in the received radar signal. Measuring this phase shift requires the transmitted radar pulses to have the same phase offset. While creating coherent pulses is a challenge, it enables a much more capable radar system.

In addition to determining the target’s velocity, Doppler radar can filter out reflections from objects with low velocities. In the case of an airborne Doppler radar, this technique is able to discard many of the reflections from the ground. Referred to as “ground clutter rejection”, this technique increases the radar’s ability to easily identify other airborne targets—especially those flying low near the source of the unwanted ground reflections.

Range And Resolution

Next, we’ll describe two key performance metrics of a radar system—range and resolution—and discuss some common techniques to improve them.

Range

To understand how to optimise the range of a radar system, we first need to review some of the factors that determine the range. Depending on the gain of the antenna, a percentage of the transmitted power is incident on the target, and depending on the target’s radar cross section, some of that power is reflected back to the radar antenna. As the range is increased, less and less power returns to the radar receiver. When the received power of the signal is close to the noise floor, the range of the radar has been exceeded.

Obviously, increasing the transmitted power also increases the received power. One way to accomplish this is through the use of a GaN-based power amplifier. This high-power technology supports increased output power in a small form factor. A less obvious approach is to integrate multiple pulses. Since the noise is roughly random in nature, averaging multiple measurements improves the signal-to-noise ratio and therefore increases the radar’s range.

Resolution

As described above, tracking radars need to accurately locate the target with high resolution. Operating at a high frequency is one way to improve the resolution of the radar. Another approach is shaping the waveform, using techniques such as pulse compression. Linear frequency modulation, commonly referred to as “chip”, is one type of pulse compression. While the details are beyond the scope of this article, they result in the radar receiving narrower pulses, which improve the resolution of the radar.

What’s Next

While much more can be said regarding the functioning of radar systems, this brief introduction provides the necessary background for the subsequent posts. In the next article we present a brief overview of electronic support, followed by posts on the other two main elements of electronic warfare—electronic attack and electronic protection. In terms of radar, electronic support involves locating an adversary’s radar system, electronic attack involves degrading an adversary’s radar system, and electronic protection involves protecting a radar system from the effects of an adversary’s use of electronic attack.

“They have a missile-lock on us!” is a phrase we’ve heard countless times in movies and is usually a sign that a radar-guided missile is incoming. Ever wonder how the aircraft’s systems detect this type of threat? In this post, we’ll discuss how a radar warning receiver provides information on an adversary’s radar, as well as some general information on electronic support. Before we get into the details, I recommend reviewing the two previous posts for a brief background of the history of electronic warfare and an overview of radar.

What Is Electronic Support?

Electronic support (ES) is the set of technologies and methods designed to receive and analyse an adversary’s transmissions of electromagnetic signals. This includes locating the sources of radar signals as well as identifying the adversary’s communication signals.

There is crossover between ES and signal intelligence (SIGINT), but the key difference is that ES is more tactical while SIGINT is more strategic. For example, while an ES system might identify an adversary’s communication signal so it can be jammed, a SIGINT system will intercept the transmission for longer-term strategic planning. Additionally, electronic support is less concerned with the content of the signal and instead is focused on the technical details of the transmission itself.

While both ES and SIGINT are critical, this article focuses on electronic support and its objective of improving situational awareness.

Radar Warning Receivers

The purpose of a Radar Warning Receiver (RWR) is to detect and analyse radar signals in order to provide actionable information. For example, a radar detector in a car will detect police radar to notify the driver that if they don’t slow down, they might get pulled over.

However, the below discussion will focus on airborne RWR systems. These systems detect radar signals using wide-band antennas placed in key locations on the aircraft. Amplifiers located near the antennas boost the signal before it’s sent to either a down-converter or directly digitised. The system then analyses the signals and compares them to a threat library to determine information such as the type of radar, the direction to the radar and an estimate of the distance.

Consider an aircraft flying through a hostile area. Suddenly, an alarm goes off indicating the detection of a ground-based search radar. The pilot adjusts course to avoid the radar site but then sees a second alarm indicating the detection of a targeting radar from a ground-to-air missile. Using this information, the pilot is able to take actions to evade the missiles.

The Battle For Best Noise Figure

Looking back at the above example, we find that by increasing the range of the RWR, we give the pilot the maximum amount of time to respond to the threat. In the case of a search radar, the RWR ideally detects that threat before the aircraft is discovered.

Detecting a radar signal requires the signal to be greater than the noise floor. Similarly to radar, one of the first approaches to increasing the range is to improve the noise figure. This is why the low noise amplifiers are placed near the antennas—by reducing the cable length before the amplifiers, the loss is reduced thereby improving the noise figure. This clearly illustrates the back-and-forth competition between radar and electronic warfare. The radar system designer tries to maximise the radar range, and the EW system designer works to optimise the RWR range. Since technology is continually improving, the most successful systems are the ones that can quickly be updated to incorporate the latest technological advancements.

In this competition between radar and radar warning receiver, the RWR designer is faced with an additional challenge. While the radar system designer knows the operational frequency of the radar, the RWR must have sufficient bandwidth to detect multiple types of radar. This is a key concept that differentiates electronic warfare systems from most other technologies and one that we will see in all types of EW system design. For example, as mentioned in the previous post, search radars operate at lower frequencies and targeting radars operate at higher frequencies. In order to accurately detect these different threats, the EW receiver must have a very broad bandwidth, which is why it’s common to see multi-octave electronic warfare tuners and converters.