Air Search Radar

Before I delve into the Aegis system, I want to talk a little about air search radar. You’ll need a bit of fundamental understanding of it to understand why Aegis was a leap forward from its predecessors.

Radar stands for Radio Detection and Ranging. The radar set sends a pulse of radio frequency energy out, stops transmitting, and waits for any return signal. After a given time, whether any signals have returned or not, it transmits again.  When pulses are reflected back, by measuring the time elapsed from transmission, and dividing by 2, and knowing the speed of light, the range can be determined. Knowing what direction the antenna was pointing at when the pulse was transmitted gives the bearing.

These pulses are very short, and the “pulse repetition frequency” may be as high as several thousand per second. Note, this frequency is how many times a second the radar transmits. It is not the frequency of the radar energy transmitted.

We’ve all seen countless war movies or airline disaster movies where the tense radar operator is hunched over a scope watching for blips on the screen.  That round top-down scope is what is known as a PPI, or Plan Position Indicator. Generally, such a radar is what is known as 2D, or Two Dimensional. That is, it indicated the range and bearing from the radar. But in air search, knowing the altitude of the target is also critical.

Early 2D radars used a  parabolic antenna. The antenna itself is not the transmitting element. Rather, the RF energy from the transmitter is fed through a waveguide. This waveguide is a hollow steel tube that channels the energy to a feedhorn. The feedhorn is positioned shortly in front of the antenna and directed toward the antenna.  By shaping the parabolic antenna, the actual shape of the radar beam could be managed. Upon leaving the feedhorn, the RF energy would be reflected off the face of the antenna, and out toward the target. Energy reflected from the target would strike the antenna, and if you remember your high school math (I don’t!) be reflected to the feedhorn, where it would go back down the waveguide, to the amplifiers and receivers and eventually converted to display on the PPI.

The radar beam, known as a lobe, on these 2D radars is very narrow in azimuth. But it is very wide in elevation, so that targets at both low and high altitude can be detected.

So we’ve managed to determine the range and bearing of a target. But how to determine its altitude?

Well, the first radars to address this were known as height-finders. They were basically the same radar and antenna mounted perpendicular to the 2D set.  Pointed along the line of bearing determined by the 2D set, the heightfinder antenna would nod up and down, while projecting a lobe that was wide in azimuth, but very fine in elevation. By measuring the elevation angle, and the range to the target, the ship could, with a little help from Pythagoras, determine the altitude of the target.

There were two problems with heightfinders. First, the heavy antenna mounted high on a ship already near its load limits was bad for the ship stability. Many ships in the fleet simply couldn’t carry such a burden. Second, it was a slow process to slew the heightfinder to the correct bearing, and determine the target altitude. With the ever increasing speeds of aircraft in the 1940s and 1950s, that meant there would be very little reaction time available to any ship. And if there were multiple raids inbound, the ship could well be overwhelmed with targets, what is now called a saturation attack. There had to be a better way.

As electronics improved, and understanding of RF energy likewise improved, and further improvements in signal processing arrived on scene, radar engineers learned they could project a pencil beam radar, on thin both in azimuth and elevation, directly from a waveguide, without reflecting it off a parabolic dish. A sizeable antenna would still be needed to receive the reflected signal, but a fairly precise lobe could be sent. By stacking a series of these waveguide emitters vertically, a series of lobes that scanned sequentially from bottom to top could be sent. Each lobe in this stack had its own frequency. Rather than running several different waveguides through the mast and up to a rotating antenna, instead, a single waveguide was used, but the emitting portion was called a slotted waveguide. Simply put, the RF energy is sent out of very precise slots cut into the antenna. But the slots are of varying sizes. Since slot size is critical to propagation of the signal, only the RF energy of the proper frequency for a desired lobe will propagate, and only through the desired slot. Each slot is also aligned to send its lobe at a specific elevation angle. Thus, a single waveguide is all that is needed.

These slotted waveguide radars could quickly determine not only the range and bearing to a target, but also provided altitude information. While not as precise as a heightfinder, it was sufficient to either control friendly interceptors, or cue onboard fire control radars to the target. These radars are said to be 3D radars, and are sometimes described as scanning mechanically in bearing, and electronically in elevation.  The first 3D radar in US service was the SPS-39, quickly replaced by the SPS-52 and SPS-48. Oddly, of the two, the SPS-52 was the smaller, less powerful and less capable radar, designed to fit on smaller warships such as frigates. It has since been retired from US service. The SPS-48, on the other hand, was designed for larger ships, such as cruisers and aircraft carriers. It has evolved over the last 40 years, with improvements to the transmitter for power and reliability, and in the signal processors to reduce the noise to signal ratio.

http://www.globalsecurity.org/military/systems/ship/systems/images/an-sps48-a.jpg

Some radars, rather than using the slotted waveguide, use the similar stacked lobe concept to provide 3D coverage, but use a phase modulation method to control the elevation of a given lobe of a vertical scan.

These 3D radars are still very useful, and for most jobs, quite sufficient. Indeed, in certain applications, they still outperform their successors. But one problem with them is that the antenna rotates rather slowly. For instance, the SPS-48G(V) rotates at either 7.5 rpm or 15rpm. This gives a “refresh rate” of either 8 or 4 seconds. A lot can happen in 4 seconds.

We’ll take a look to the solution to that problem in our post in Aegis.