INTRODUCTION

The term Early Warning, which is also be expressed as EW, relates to a category or Function of Radar and thus should not be confused with the acronym EW when related to Electronic Warfare. If any comparison should be made, it should be to reflect that both Early Warning radar and Electronic Warfare devices have the ability to provide early warning. The first instances of Early Warning radar dates back to the start of WW2 and was used by both the Allies and the Axis.

ALLIED EFFORT

Chainhome (AMES Type 1)

Beginning in 1935, under a very strict veil of secrecy, a new detection capability was invented by the British and was coined as "Radio Directing Finding" or RDF. The initial operational RDF device using this ability was the Chain Home (CH) system. Chain Home was a bistatic Transmitter/Receiver designed to detect aircraft over the southern part of the North Sea. The Chain Home network had the ability to provide azimuth and target heights up to 30,000 feet, with a detection range out to about 100 kilometres. The system operated in the High Frequency portion of the Electromagnetic Spectrum using frequencies of 22.9, 25.3 or 30.7 Megahertz (MHz), which was the extent of their understanding of the spectrum at the time. Chain Home also went by the names AMES (Air Ministry Experimental Station) Type 1 and RDF1.

A Chainhome Station Model

The left hand side of the diagram above shows the basic structure of the transmitter towers, which in many respects is similar to today’s hydro towers. The transmitting masts were set up in line NW to SE, facing out to sea. On the right, the receiving antennas were constructed in a semi rectangular arrangement and at some distance from the transmitter towers. Each tower would carry several groups of receiving dipole aerials at different heights. Typically only one tower would be used at any one time, the signals from the different aerials being used to determine height and direction of the target. Both arrays were fixed and were non-rotating.

Chainhome Station

In operation, RDF operators would report their detected contacts which would then be sent to a networked Filter Room, where they would then be assessed and correlated. If deemed a threat, Fighter Command would then vector their fighter aircraft to intercept and engage them.

Chainhome Operator Workstation (Courtesy IWM CH15332)

Above is Denise Miley at the Chainhome RDF workstation. Her right hand has selected direction or heightfinding and her left hand is ready to register the goniometer settings to the calculator which provides the target input data.

Calculator Lamp Display (Courtesy of Michael Bragg author of RDF1)

The Calculator Lamp display was fitted in the console facing the operator. The left hand side is a block of 7 by 7 letters each representing a 100km square of the Cassini grid. The centre letter is the station reference, in this case M for Bawdsey, Great Bromley, Canewdon and High Street. The four groups of 1-0 digits along the bottom show the vertical and horizontal coordinates. The centre group of lamps indicate height from below 500 feet to over 30,000 feet. The lamps above show the number of aircraft as decided by the operator with F for friendly, DS for a plot stored and ready to display, NH1 to change the aerial to obtain a height, NH2 that no height is available and H that height information is being displayed or in the process of being displayed. The indications above show 3 aircraft at 13, 500 feet at M73, 49.

The Chainhome Reporting Network

Fighter Command Operations Room (Courtesy of IWM C1869)

Operational by 1939, twenty Chain Home systems were installed around Britain providing them a detection ability against aircraft penetrating their airspace reaching out to about 190 Km. The Chainhome network, coupled with decrypted Enigma traffic, played a vital role in winning the Battle of Britain.

Chainhome RDF1 Operating Characteristics
Frequency	Pulse Repetition Frequency	Pulse Width	Scan Time	Range
22.9 MHz	12.5, 25 or 50 Hz	5.0-45.0 usec	0 (Fixed)	190km
25.3 MHz	12.5, 25 or 50 Hz	5.0-45.0 usec	0 (Fixed)	190km
30.7 MHz	12.5, 25 or 50 Hz	5.0-45.0 usec	0 (Fixed)	190km

Chainhome Low (AMES Type 2)

The Chain Home system lacked the ability to detect air targets at close ranges or when aircraft were flying at altitudes less than 5000 feet. To cover off this shortcoming, a gap-filler system called the Chain Home Low or CHL went into operation. Chain Home Low (CHL) operated at 200 MHz and was collocated with most CH systems. Using a newer frequency range of 200 MHz CHL was able to detect aircraft flying at low as 500 feet out to a range of 48 kilometres, with higher aircraft being tracked out to 160 kilometres. Up until the invention of the cavity magnetron in 1940, the 200 MHz frequency was the frequency off choice for both British, Canadian and American Early Warning radar.

Chainhome Low RDF1 Operating Characteristics
Frequency	Pulse Repetition Frequency	Pulse Width	Scan Time	Range
200 MHz	400 Hz	3 usec	60 Sec (Circular)	48-160 Km
200 MHz	400 Hz	3 usec	90 Sec (Circular)	48-160 Km
200 MHz	400 Hz	3 usec	139.8 Sec (Circular)	48-160 Km

Chainhome Low

A more detailed account of this system and the British approach to have the first operational detection and reporting effort can be found in Michael Bragg's book "RDF1" ISBN 0953154408.

AXIS EFFORT

At the start of WW2, and largely unbeknownst to each other, both the Allies and the Axis had radar apparati. This was largely due to each having their own independant understanding of the electromagnetic spectrum at the time and disbelieving that others could be operating elsewhere. The initial German system to be deployed was known as Freya or FuMG 39G. The Freya began testing in 1934 and became operational in 1936. Instead of calling it RDF, it was called FunkMessGeraet which translated into "Radio Measuring Device".

The initial understanding of the Freya device by the British came from the famous an"Oslo Report, which were documents that anonymously arrived in the mailbox of the British Embassy in Norway on November 2, 1939. The report provided details on technology and weapons that were being developed by the Germans. Thinking it was a hoax, the report wasnt taken too seriously. It wasn't until after the war, in 1947, that the true author of the Oslo Report was identified to be a German mathematician and physicist named Hans Ferdinand Mayer.

Nonetheless, it wasnt until Feburary 1940, that the first known electronic intercept of the Freya system was gathered by an enthusiastic employee of Dr. R.V. Jones, named Derek Garrand. To do this, Garrand used a S27 Halicrafters receiver which he set up on the British coastline looking across the English Channel into France. This coupled with previous Engima decrypts, photographic intelligence and prisoner of war interrogations, it started to show more value into the Oslo Report. The Freya system operated at 125 MHz and was produced by the Gema Company.

The Freya was part of an elaborate network of Radar stations located along the "Atlantic Wall" which provided warning and guidance for the co-located Wurzburg Anti-Aircraft radar and 88 mm flak guns. Initial detection ranges on aircraft was out to a range of 60 kilometres. As the war and technology progressed, this range increased to about 120 kilometres. To counter the Freya emissions the British created the Electronic Countermeasure device called Moonshine. Moonshine was the first known countermeasure to be used against radar and the effort to what we now know as Electronic Warfare was born.

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Freya Radar

The use of Early Warning radar continued to evolve by both sides during WW2. After the war, its use proliferated with the technology expanding into Soviet, American and British applications. A standard rule of thumb is that the lower the frequency the greater the detection range.

Today, Early Warning radar can be seen fitted in Air, Land and Sea platforms with nominal frequency ranges ranging from as low as 150 MHz up to 3500 MHz. These applications are used by both military and civilians for long range target detection and warning, and the surveillance of aircraft or other airborne objects.

AN/APY-1/2/3	AN/FPS-117	AN/SPS-49
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EW Intercept Station - Click Image to Enlarge
The intercept station above depicts the IP-480 Analysis screen of the  AN/WLR-1C.  Along with the audio provides an example of what an EW operator may see on their scope when listening to the intercept.