The simple pulse radar described above was actually preceded in the prewar timeframe by simpler systems that consisted of a transmitter sending a signal to a receiver on a continuous basis. For example, the transmitter could be placed on one side of the mouth of a harbor and the receiver placed on the other, with a ship entering the harbor breaking the radio beam and announcing its presence. The transmitter and receiver could also be placed on the same side of the harbor, with the passing ship bouncing the radio beam back to the receiver. These simple “continuous wave (CW)” systems weren’t really radars since they couldn’t realistically give a range to a target, they could only detect that something was there. They are better described as “CW alarm systems”, and they are really something of a footnote in the history of radar.
However, although a simple CW alarm can’t determine range, a direct variation on it can be used to determine velocities. Suppose a continuous radio signal at a given frequency is focused on an aircraft approaching head-on; the velocity of the aircraft will actually increase the frequency of the echo. If the aircraft is going directly away, it will decrease the frequency of the echo return. This is known as the “Doppler shift”. The same effect is observed with sound waves when an aircraft is flying overhead: as it approaches, the sound of the aircraft has a high pitch, in other words a high audio frequency, and when it is going away it has a low pitch, or a low audio frequency.
Derivation of the Doppler shift is a bit beyond this document but can be found in any simple physics text. For a radar, which generally tracks targets moving much slower than the speed of light, the shift in frequency, or “Doppler frequency” is roughly:
doppler_frequency = 2 * target_velocity / radar_wavelength
For example, if:
The radar frequency is 100 MHz, corresponding to a wavelength of 3 meters.
The target velocity is 1,000 KPH, corresponding to 278 meters per second.
— then the Doppler frequency is:
2 * 278 / 3 = 185 Hz
This is the shift up in frequency if the target is approaching and the shift down in frequency if the target is moving away. Incidentally, the Doppler shift for radio emissions transmitted from the target itself is only half this, 92.5 Hz; the value is doubled for radar because the target is moving as the radar arrives, causing a shift, and as the return is reflected, causing a second shift.
It would be possible to imagine a simple continuous-wave radar Doppler velocity meter, consisting of a transmitter unit and a receiver unit. Since this device is only intended to measure velocity, the two units can share the same antenna, with the receiver blocking out the signal of the transmitter with a “band rejection filter” that blocks signals at the transmit frequency but allows all others to pass. The transmitter would send a radio beam at a target. A calibrated dial on the receiver could then be used to adjust a variable filter until the Doppler-shifted echo was received and lit up an indicator light. The velocity could be read off the dial. Of course, this scheme only gives the velocity of the target on a straight-line radial to the detector. At any angle off the radial the perceived velocity is smaller, until at 90 degrees it falls to zero.
A more practical device would feature a receiver that had sets of fixed filters in a range of frequencies above and below the transmitter frequency. Each filter would be connected to a simple circuit that activated a light on the front panel of the receiver, with each light marked with a particular range of velocities. If the Doppler shifted echo passed through a specific filter, it would turn on the appropriate light to give the target velocity. The radars used by police to catch speeders actually operate more or less along such a principle, though their implementation is much more sophisticated and they will actually give speed readings.
The essential point here is that, although timing radio pulses can be used to obtain ranges to targets, it is also useful to obtain information from the Doppler shift to learn about the velocities of targets. The combination of these two approaches in “pulse Doppler” radars is an important theme in modern radars and is discussed later.