Did American radar technology improve over the course of world war 2?

[u/[deleted]]

I know in the beginning of the U.S’s involvement military radar technology was very primitive and this was shown before the attack of Pearl Harbor, when Japanese enemy jets/aircraft appeared undistinguished on the radar blips.

But over the course of the war, did radar technology improve substantially for the U.S. military? How did it compare to other allied military radar performance, such as Britain?

[u/Downtown-Act-590]

Your question is really broad, so I will not answer it in full, but rather focus only on the example you have chosen. We are gonna compare the radar on Opana Point which detected the Japanese planes on the fateful morning when the fleet was attacked with a US radar for the same purpose which was state-of-the-art at the end of WWII.

So, in December 1941 on Opana Point there was a freshly installed SCR-270 early-warning radar. What does early-warning radar mean? It is a radar supposed to detect the enemy at very long distances in order to alert the air defense in time. The operators are not so much interested in the exact position of the enemy. But they want to know whether there is someone out there, roughly where, at what altitude and how many. This is in sharp constrast with other types of radar (e.g. a targeting radar), which would be often used to get precise position information on a known target, but they lack the ability to effectively sweep the entire sky for unknown intruders.

SCR-270 was one of the first radars in the world in its class. It spanned many metres in height and width and it could be rotated around to provide a two-dimensional picture of the situation in the air within more than 200 km radius (you would get the bearing and range of the contacts, but no altitude information)[1]. But the precision would be rather poor. The range readings would have an error of several kilometres and the bearing could be off by more than a degree (which would add another units of kilometres to the position error over the long distances). As a result, you would also have trouble discerning individual aircraft and you would see the Japanese formations only as a large blip with enormous radar echo. It would also struggle to detect smaller targets and even aircraft at a poor angle.

Why was it so bad though? It has mainly to do with the frequency of the radar. SCR-270 operates at 106 MHz which means that it emits waves with a wavelength slightly smaller than 3 meters. This has multiple drawbacks and we will number them and go over each of them. For further study of these principles please refer to [2]

1. Objects smaller than cca. 1/10th of the wavelength are essentially invisible to radar due to physics of scattering. As a result anything smaller than 30 cm will not really show. With small aircraft and poor angle, this can affect the signal.
2. Choice of the frequency limits the bandwith available for the radar pulses which in turn limits their minimum duration. As a result a lower frequency radar has to use a longer pulse. This has poor effects on the radar range resolution and you will also for example not be able to tell apart targets close to each other as their return pulse will be accidentally bunched together. SCR-270 had a pulse width between 10 and 25 microseconds and as a general rule of thumb the range resolution grows by 500 feet per microsecond (150 m) [3]. So you easily see why it was in order of kilometres.
3. The higher the frequency, the narrower will be the beam and as a result the higher angular resolution which means better bearing information (and altitude information if you try to get them as well). The beamwidth of a radar is determined primarily by ratio of its wavelength to the length of its aperture (essentially given by the antenna frontal area). To achieve a fine resolution e.g. smaller than a degree, you would like this ratio to be very small and that is really difficult if your wavelength is 3 meters. Well, at least if you don't want your antenna to be enormous.

In radio everything scales with the wavelength. To get a nice, small radar with high precision, you need a higher frequency. But at the time of the SCR-270 design, the US military didn't have access to a device which would let them emit such high frequency signals.

In area of early-warning and tracking radars, US was not too far behind UK at that point. The British Chain Home system was not vastly better than SCR-270. In comparison to German Würzburg which was operating at higher frequency, it was a bit archaic though.

Allies would not be stuck with meter long wavelengths for long though. In 1940, John Randall and Harry Boot at University of Birmingham developed the cavity magnetron (yes, the same device which you now use to heat up your food in a microwave). This device would allow to build radars with centimetre wavelengths. Suddenly possibilities opened up. Small, very precise radars for aircraft applications are one example which became a reality after the magnetron invention.

Cavity magnetron reached the US a year before Pearl Harbor as part of the so-called Tizard mission [4]. That was a British delegation aiming to share their secret R&D efforts with their US counterparts. Magnetron was an instant sensation and was even reffered to as "the most valuable cargo ever brought to our shores" [5]. It wouldn't be long until works started on a centimetre wavelength early-warning radar in the US.

[u/Downtown-Act-590 → u/Downtown-Act-590]

Answer split into two for technical issues

As time went by war was also bringing new challenges, which were even further reinforcing the need for such devices. Both sides started actively jamming each others radars both mechanically (aluminium dipoles stripes, so called chaff) and electronically (by feeding the opposition radars with noise and false signals). The aluminium dipoles were particularly feared by both sides and vastly increased the importance of narrow beamwidth [4]. That was because the narrow beam could be used to scan only very small vertical portion of the sky where the aircraft were while ignoring the stripes falling below.

In the summer of 1943 the US early-warning centimetre radar was finally ready and became known as the AN/CPS-1 [6]. It ran at 3200 MHz (so rougly 10 cm wavelength) and had pulses shorter than a microsecond (resulting in range resolution below 150 meters). Its narrow beam also allowed for rather precise altitude and bearing calculations giving a full three-dimensional picture of the state in the air. This is in contrast with the wide beam of the SCR-270 which was giving a rough two-dimensional overview and it was really important. It e.g. also allowed to scan for targets very close to the ground which would otherwise be lost in the ground clutter, so it improved the range against low flying targets substantially.

There was one more thing unique about the AN/CPS-1. The beam was no longer steered by the physical rotation of the antenna, but it was a so-called phased array. That is an array of antennas which allows to electronically steer the beam by varying the phase on its individual elements. As such you can steer the beam as the combined wave is reinforced in some directions and suppresed in others. AN/CPS-1 was first such array in the centimetre wavelength and it would pave the path for many such radars to follow.

If there was AN/CPS-1 on Opana Point in December 1941, the operators would most certainly realize that it is not a flight of a few B-17s from the continental US. They would see the formations laid out rather clearly in front of them even at a larger distance. But there wasn't so the Pearl Harbor happened as it did. AN/CPS-1 was sort of an ultimate US early-warning radar in WWII. It saw some action including on D-Day.

So as a conclusion, yes it did improve a lot! While the AN/CPS-1 specifically outperformed some of the older British radars in the area, it definitely can't be said that the US radar technology would outpace the British one in general during WWII though.

[1] SCR-270 on radartutorial

[2] Introduction to Airborne Radar, G.W. Stimson

[3] Electronic Warfare and Radar Systems, Engineering Handbook

[4] Instruments of Darkness: The History of Electronic Warfare, 1939-1945, A. Price

[5] Scientists Against Time, J.P. Baxter

[6] AN/CPS-1 on radartutorial