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Every radar question in physics, from speed guns to weather radar, uses one idea: send out a radio pulse, time the echo, and halve the round trip. Robert Watson-Watt, a Scottish physicist, showed in 1935 that this idea could find aircraft long before anyone could see or hear them. This post sets out what he and his team did, using IEEE history pages and the Royal Society's memoir of his life, works through the numbers, and ends with what you can borrow from his way of working.
Who was Robert Watson-Watt?
The Royal Society's biographical memoir by J. A. Ratcliffe gives his dates as 13 April 1892 to 5 December 1973. The IEEE's Engineering and Technology History Wiki biography records that he was born in Aberdeenshire, studied electrical engineering at University College, Dundee, and became an assistant to the professor of physics, working on wireless telegraphy.
During and after the First World War he worked at a meteorological station that used radio to locate thunderstorms. In 1927 it merged into the Radio Research Station, where he and his colleagues used cathode ray tubes to monitor the atmosphere and measure its height. The same ETHW biography credits him with coining the word "ionosphere". That work on radio pulses bouncing off a layer high above the ground turned out to be the training for radar.
What did the 1935 memorandum propose?
According to ETHW's page on Watson-Watt proposing radar, on 12 February 1935 he sent a memorandum titled "Detection of aircraft by radio methods", which, the page notes, Hanbury Brown called "the birth certificate of radar". The ETHW biography says it estimated the best wavelength for the signal and predicted that short pulses could measure the range of a target.
The physics in it was not exotic. A 1999 paper by B. A. Austin, Precursors to Radar: the Watson-Watt memorandum and the Daventry experiment, rebuilds the reasoning and shows that he reached his conclusions "using nothing more than simple calculations".
What happened at Daventry on 26 February 1935?
David L. Boslaugh's IEEE history Radar and the Fighter Directors tells the story. Watson-Watt gave his colleague Arnold Wilkins a couple of weeks to set up a cheap demonstration. Wilkins had worked out that the best wavelength would be about equal to a bomber's wingspan, around 25 m, but he had no transmitter powerful enough. So he used the BBC's shortwave station at Daventry, which broadcast at about 49 m.
On the morning of 26 February 1935 a Handley Page Heyford bomber flew back and forth over Wilkins's van. Inside were Wilkins, Watson-Watt and A. P. Rowe of the Air Ministry. As the bomber approached, the green spot on the cathode ray tube rose into a vertical line: a reflected radio wave. Rowe reported that night, and Hugh Dowding, briefed the next day, released £10,000 to start the project.
How does radar measure distance?
The team then moved to Orfordness on the Suffolk coast. Boslaugh's history says they adapted Watson-Watt's ionosphere equipment, which worked at a wavelength of about 50 m and sent pulses about 100 times a second. Range was measured from the time a pulse took to reach the aircraft and come back. By the end of July 1935 they were tracking aircraft out to sixty kilometres, and in December the Treasury gave £60,000 for five stations, the start of the Chain Home network.
The pulse travels out a distance d and back again, so it covers 2d in time t:
2d = ct, so d = ct/2
Worked example, using the figures above and c = 3.0 × 10^8 m/s:
Frequency of a 50 m wave: f = c/λ = (3.0 × 10^8) ÷ 50 = 6.0 × 10^6 Hz, or 6 MHz.
Echo time for an aircraft 60 km away: t = 2d/c = (2 × 60,000) ÷ (3.0 × 10^8) = 4.0 × 10^-4 s, which is 400 microseconds.
Time between pulses at 100 per second: T = 1/100 = 0.01 s.
Farthest echo that returns before the next pulse: d = cT/2 = (3.0 × 10^8 × 0.01) ÷ 2 = 1.5 × 10^6 m, or 1,500 km. That is far beyond 60 km, so each echo comes back long before the next pulse leaves.
What mistakes do students make with radar problems?
Forgetting the 2. Using d = ct gives twice the true distance. The time measured is always the round trip.
Mixing units. 400 microseconds is 4.0 × 10^-4 s, not 4.0 × 10^-3 s. Write the power of ten before you multiply.
Using the wrong speed. Radio waves travel at c. Sonar uses the same halving idea but with the speed of sound in water.
Why do modern radars use microwaves?
Watson-Watt's 50 m waves were long. The current NCERT Class 12 chapter Electromagnetic Waves (2026-27 reprint) says microwaves, with frequencies in the gigahertz range, are suitable for radar in aircraft navigation because of their short wavelengths. At 3 GHz, λ = (3.0 × 10^8) ÷ (3.0 × 10^9) = 0.1 m, which is 500 times shorter than 50 m. Our guide to the electromagnetic spectrum covers each band.
If a pulse going out and an echo coming back is hard to picture, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read.
What can a student take from Watson-Watt's way of working?
Estimate first. Austin's paper shows the memorandum rested on simple calculations. Before a long problem, make a rough estimate so you know what a sensible answer looks like.
Test the cheapest version. Daventry used a borrowed transmitter and a van. Check an idea with the simplest case before building the full solution.
Reuse what you already know. Pulse timing from ionosphere work became aircraft ranging. Old chapters often hold the method for new ones.
Share the credit. Wilkins did key calculations and built the demonstration. Good science is usually a team effort.
Try it now: a radar echo returns after 0.2 milliseconds. Find the distance, then check that you used the factor of 2.
