Simple continuous wave detectors and radars were discussed in the previous chapter. During the war, “proximity fuzes” were developed for anti-aircraft gun shells, allowing the shells to be triggered when they passed within a lethal radius of a target. Coupled with radar-guided automatic tracking, the proximity fuze helped boost the lethality of antiaircraft guns by a large factor. Such fuzes could be thought of as CW radars, but that’s stretching the definition of the term “radar”: they simply generated a continuous radio signal and at close ranges, the “near field”, the presence of a target would “load down” the oscillator generating the signal and changing its frequency of oscillation, which triggered the fuze. The term “proximity detector” seems more suitable.
True CW radars were used in World War II and continue to be used. The most significant application is for guiding anti-aircraft missiles. Some early surface-to-air missiles (SAMs) used two pulse radars, one to track the target and the other to track the missile, with course corrections provided to the missile under radio control. This was a bit inefficient, and it proved simpler to put a radar receiver in the missile. One of early approaches was to have the missile carry a radar receiver that would stay on the track of a narrow continuous beam generated by the targeting radar. This scheme was known as “semi-active beam-riding (SABR)” or just “beam riding” and was used on early versions of the pioneering American “Sparrow” air-to-air missile (AAM).
Beam riding was quickly replaced by a more effective scheme, “semi-active radar homing (SARH)”, in which the missile homed in on radar reflections off the target. Later Sparrow variants used SARH. The launch platform provided continuous-wave illumination for the receiver in the missile, with the launch platform and the missile forming in effect a continuous-wave bistatic radar system. Since the missile had no need to calculate range, the CW scheme was perfectly satisfactory.
* Incidentally, the problem with semi-active AAMs was that the launch aircraft had to keep the target “illuminated” with its radar while the missile was on the way, meaning the aircraft had little freedom to maneuver. The obvious answer was to develop a complete radar guidance system, with transmitter, receiver, and homing circuits, to fit into the AAM. An aircraft could launch an AAM with such an “active seeker” and then take evasive action, or in other words such a missile had a “fire and forget” capability.
That was difficult for the technology of the 1950s. The BOMARC SAM of the late 1950s and early 1960s did have an active seeker, but it was a big machine, essentially a pilotless supersonic aircraft that could carry a (small) nuclear warhead. Many modern SAMs and AAMs, such as the US AIM-120 AMRAAM, the Sparrow’s much-improved descendant, do have such a fire and forget capability. The AMRAAM is targeted using the launch platform’s radar but cruises to the target area using an autopilot system, then performs the “terminal attack” using the active seeker.
* Another simple radar developed during the war was the “radar altimeter”. This was just a pulse ranging radar that could tell how far an aircraft was above ground, bouncing a pulse off the terrain and timing the round-trip from the aircraft and back. However, these simple radar altimeters were only really suited for high-altitude operation. Low altitudes demanded a high PRF, which would have made them very susceptible to ghost echoes. The answer to the problem was a frequency modulated continuous (FM-CW) wave radar.
The idea is basically simple. Imagine a radar that transmits continuously in a cycle, but instead of transmitting at a single frequency, it transmits in a continuously increasing “ramp” of frequencies during each cycle. A receiver synchronized to the transmitter cycle and using a separate antenna can then pick up this continuous signal and determine the frequency of the return to get the range.
There’s a catch to FM-CW radar, though it turns out to yield benefits. As mentioned in the previous chapter, a simple CW radar can be used to measure target speed through the Doppler shift. Returns to an FM-CW radar also end up being affected by the Doppler shift, which creates an ambiguity. Suppose the FM-CW radar isn’t moving and it’s generating a ramp of rising frequencies. If it transmits a particular frequency at a certain time, then when it receives an echo after a specific delay time with the same frequency there is no ambiguity. The round-trip time of the signal is just the measured delay, and the range is easy to calculate.
However, if the FM-CW radar is moving ahead, as it might be expected to if it’s installed in an aircraft and pointed forward to the ground, then the Doppler shift will drive up the frequency of the return. If the return signal comes back with a particular frequency, it’s actually a Doppler-shifted return from a transmission at a lower frequency, and the range is actually greater than would be expected from the delay time between transmission and reception of the same frequency. Of course, if the FM-CW radar was moving backward the return would be shifted down in frequency and the range would be shorter, but aircraft do not fly backwards in normal operation.
The way around this is to generate the ramp of frequencies up and down in a triangular fashion. To show how this works, let’s assume again that the FM-CW radar is stationary and compare the frequency-versus-time plot for transmit and receive, along with a plot of the difference between the two:
As the illustration shows, for a stationary FM-CW radar the return signal tracks the transmit signal perfectly, returning with a fixed delay. Notice that at any single time on the plot there is a constant difference between the transmit and receive frequencies, except for the short window between the time the transmit signal changes direction and receive signal follows.
Now let’s put the FM-CW radar into forward motion and create the same plot. The return from stationary target is rendered in light gray to provide a reference.
The Doppler shift creates a distinctive offset between the transmit signal and the receive signal. This is because on the rising half of the ramp the transmit frequency is increasing and the increased Doppler-shifted return signal is “catching up” with the changing transmit signal, but on the falling half of the ramp the transmit signal is decreasing and the increased Doppler-shifted receive signal is “lagging behind” the changing transmit signal.
This means the difference between the transmit and receive frequencies is small on the rising half of the ramp, and large on the falling half of the ramp. An FM-CW radar can use this difference to determine both range and speed. Since the Doppler-shifted component is subtracted on the rising half of the ramp and added on the falling half of the ramp, the range is given by the average difference of the two cycles. This average can be subtracted from the difference in the second half of the cycle to give the Doppler-shifted velocity component, given as “fd” in the diagram.
An FM-CW radar, then, can be used to obtain both altitude and groundspeed. The radar unit is fixed in the nose of the aircraft and pointed down at an angle, with the altitude and speed indicators scaled to compensate. For example, if the FM-CW radar was pointed down at 45 degrees, the altitude scaling factor would be 1/SQRT(2) = 0.7071 and the speed scaling factor would be SQRT(2) = 1.414. Of course, nonlevel terrain would affect readings. Such FM-CW radars are often called “Doppler navigation” or “CW Doppler” radars.
Since there is some inaccuracy in determining the frequency of the return, ranging accuracy is improved if the frequency ramp is steep, since it gives a larger frequency change for a given range, and the inaccuracy of the measurement of the frequency becomes less important. However, if an FM-CW radar is used for ranging a moving target, a rapidly changing frequency ramp becomes a liability, since the returns from ground clutter at different ranges will be over a very wide band that will hide any Doppler shift from a moving target. This means that FM-CW radars used for tracking moving targets must use a slow ramp, resulting in low ranging accuracy.
FM-CW radars could be used with SARH missiles since such a weapon could home in on the reflections from the target, and so the ranging inaccuracy wasn’t any real problem. The target was generally out of ground clutter anyway. The well-known US Raytheon Hawk SAM initially used such a scheme. SARH is a declining technology as active radar seekers take over for SAM and AAM guidance, but FM-CW still remains practical and useful for navigation purposes.