A SIMPLE PULSE RADAR SYSTEM

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The best way to explain radar is to imagine standing on one side of a canyon, and shouting in the direction of the distant wall of the canyon. After a few moments, an echo will come back. The length of time it takes an echo to come back is directly related to how far away the distant canyon wall is. Double the distance, and the length of time doubles as well. Given that the speed of sound is about 1,200 KPH (745 MPH) at sea level, then timing the echo with a stopwatch will give the distance to the remote canyon wall. If it takes four seconds for the echo to come back, then since sound travels about 330 meters (1,080 feet) in a second, the distance is about 660 meters (2,160 feet).

Radar uses exactly the same principle, but it times echoes of radio or microwave pulses and not sound. Like a wireless telegraphy set, a simple radar has a transmitter and a receiver, with the transmitter sending out pulses, short bursts, of EM radiation and the receiver picking them up.

In the case of the radar, the receiver is picking up echoes from a distant target, with the echoes timed to determine the distance to the target. Early radars simply used an oscilloscope to perform the timing, with the detected return signal fed into the oscilloscope as a “video” signal, and showing up as a peak or “blip” on the display.

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An oscilloscope measures an electrical signal on an electronic beam that moves or “sweeps” from one side of a display to the other at a certain rate. The rate is determined by a “timebase” circuit in the oscilloscope. For example, the sweep rate might push the sweep from one side of the display to the other in a millisecond (thousandth of a second). If the display were marked into ten intervals, that would mean the sweep would pass through each interval in 0.1 milliseconds. While this would be shorter than the human eye could follow, the sweep is normally generated repeatedly, allowing the eye to see it.

Since EM radiation propagates at 300,000,000 meters per second, or 300,000 meters per millisecond, then each 0.1 millisecond interval would correspond to 30,000 meters, or 30 kilometers (18.6 miles). If the sweep on the scope is “triggered” to start when the radar transmitter sends out the radio pulse, and the sweep displays a blip on the sixth interval on the display, then the pulse has traveled a total of 180 kilometers (112 miles). Since this is the round-trip distance for the pulse, that means that the target is 90 kilometers (56 miles) away. The trigger signal provides synchronization, so it can be regarded as a type of “synch” or “sync” signal. The sweep is called a “range sweep” and the output of the display is called a “range trace”.

The display scheme described here is known as an “A scope”, and allows the user to determine the range to a target. A simple representation of an A-scope is shown below, along with a graph plotting the travel of the pulse with respect to time:

The amplitude of the return also gives some indication of the size of the target, though the relation between return amplitude and target size is not straightforward, as discussed later.

* It would also be nice to know what the direction to the target is, in terms of its “altitude (vertical direction)” and “azimuth (left to right direction)”.

This is a bit trickier to describe, but no more complicated in the end. Some early radars, like the famous British “Chain Home” sets that helped win the Battle of Britain, simply transmitted radio waves from high towers in a flood over their field of view, and used a directional receiver antenna to determine the direction of the echo. Chain Home actually used a scheme where the power of the echo was compared at separated receiver antennas to give the direction, which astoundingly actually worked reasonably well. Other such “floodlight” radars used directional receiver antennas that could be steered to identify the direction of the echo. Incidentally, a radar that uses receive and transmit antennas sited in different locations is known as a “bistatic”, or in the more general case “multistatic”, radar.

Floodlight radars were quickly abandoned. They spread their radio energy over a wide area, meaning that any echo was faint and so range was limited. The next step was to make a radar with a steerable transmitter antenna. For example, two directional antennas, one for the transmitter and the other for the receiver, could be ganged together on a steerable mount and pointed like a searchlight, an arrangement that is sometimes called “quasi-monostatic”. The transmitter antenna generated a narrow beam, and if the beam hit a target, an echo would be picked up by the receiving antenna on the same mount. The direction of the antennas naturally gave the direction to the target, at least to an accuracy limited by the width in degrees of the beam, while the distance to the target was given by the trace on the A-scope.

Of course, it was realized early on that it would be more economical and less physically cumbersome to use one antenna for both transmit and receive instead of separate antennas; it was possible to do so in theory because a radar transmits a pulse and then waits for an echo, meaning it doesn’t transmit and receive at the same time. The problem in practice was that the receiver was designed to listen for a faint echo, while the transmitter was designed to send out a powerful pulse. If the receiver was directly linked to the transmitter when a pulse was sent out, the transmit pulse would fry the receiver.

The solution to this problem was the “duplexer”, a circuit element that protected the receiver, effectively becoming an open connection while the transmit pulse was being sent, and then closing again immediately afterward so that the receiver could pick up the echo. This was done with certain types of gas-filled tubes, with the output pulse ionizing the gas and making the tube nonconductive, and the tube recovering quickly after the end of the pulse. More sophisticated duplexer schemes would be developed later. The receiver was also generally fitted with a “limiter” circuit that blocked out any signals above a certain power level. This prevented, say, transmissions from another nearby radar from destroying the receiver.

* After this evolution of steps, the result is a simple, workable radar. It has a single, steerable antenna that can be pointed like a searchlight. The antenna repeatedly sends out a radio pulse and picks up any echoes reflected from a target. An A-scope display gives the interval from the time the pulse is sent out and the time the echo is received, allowing the operator to determine the distance to the target.

The transmitter emits pulses on a regular interval, typically a few dozen or a few hundred times a second, with the A scope trace triggered each time the transmitter sends out a pulse to display the receiver output. The number of pulses sent out each second is known as the “pulse repetition rate” or more generally as the “pulse repetition frequency (PRF)”, measured in hertz.

The width of a radar pulse is an important but tricky consideration. The longer the pulse, the more energy sent out, improving sensitivity and increasing range. Unfortunately, the longer the pulse, the harder it is to precisely estimate range. For example, a pulse that last 2 microseconds is 600 meters (2,000 feet) long, and in that case there is no real way to determine the range to an accuracy of better than 600 meters, and there is also no way to track a target that is closer than 600 meters. In addition, a long pulse makes it hard to pick out two targets that are close together, since they show up as a single echo.

PRF is another tricky consideration. The higher the PRF, the more energy is pumped out, again improving sensitivity and range. The problem is that with a simple radar it makes no sense to send out pulses at a rate faster than echoes come back, since if the radar sends a pulse and then gets back an echo from an earlier pulse, the operator is likely to be confused by the “ghost echo”. This is usually not too much of a problem, since a little quick calculation shows that even a PRF of 1,000 gives enough time to get an echo back from 150 kilometers (95 miles) away before the next pulse goes out. However, as mentioned propagation of radar waves can be freakishly affected by atmospheric conditions that create ducting or other unusual phenomena, and sometimes radars can get back echoes from well beyond their design range.

This can be confusing, because a pulse will be sent out and a return will be received very quickly, indicating that the target is close. In reality, the target is distant and the return is from the previous pulse. This is called a “second time around” return. Given a PRF of 1,000, then a target 210 kilometers (130 miles) away will appear to be only 60 kilometers (37 miles) away. Similar confusions could be caused by returns that arrive from long ranges after more than one additional pulse, resulting in “multiple time around” returns. Of course, a simple pulse radar also has “blind ranges” or “blind zones”: if our example radar is trying to spot a target exactly 150, 300, or 450 kilometers away, the return will arrive when the next pulse is being sent out and the radar will never spot it.

To deal with such “range ambiguities”, radars were designed so they could be switched between different PRFs. Switching from one PRF to another would not affect a “first time around” echo, since the delay from pulse output to pulse reception would remain the same, but the switch would make a ghost return from a current pulse jump on the display. Suppose our radar could be switched from a PRF of 1,000 Hz to 1,250 Hz, and is trying to track a target 210 kilometers away. At 1,000 Hz, the maximum range is 150 kilometers and the target appears to be 60 kilometers away, but at 1,250 Hz the maximum range is 120 kilometers (75 miles) and the target return jumps to a perceived range of 90 kilometers (56 miles). The fact that the target range jumps when PRF is changed reveals the range ambiguity; adding perceived range to the maximum range for each PRF setting gives the actual range.

* Incidentally, the power of the transmitter pulse is given as “peak power”, usually in kilowatts or megawatts. This may be an impressive value, but it’s only the power that goes into the pulse itself. Suppose we have a pulse width of 2 microseconds with a peak power of 150 kilowatts. If we have a PRF of 500, then the time from pulse to pulse, or “pulse period”, is 1/500 = 2 milliseconds, or two thousandths of a second. This means that the average power transmitted by our radar is only:

150 kW * ( 2 microseconds / 2 milliseconds) = 0.15 kW = 150 watts

— which is about as much as the power draw of a bright old-fashioned household incandescent light bulb.

* One of the first US early warning radars, the “SCR-27O”, provides a good example of such a simple radar. It was a VHF radar, actually operating at what by modern standards at a low frequency / long wavelength of 100 MHz / 3 meters. It had a steerable rectangular “flyswatter” style dipole array of 4 x 8 dipoles, and featured a pulse width of 10 to 25 microseconds, a PRF of 621 Hz, and a peak power of 100 kW