Pip-squeak facts for kids

Pip-squeak was a special radio navigation system used by the British Royal Air Force (RAF) at the start of World War II. It helped ground crews find friendly planes in the sky.

The system worked by having an aircraft's radio send out a short, high-pitched sound, like a "squeak." Special ground stations called "huff-duff" receivers listened for this sound. By using three of these stations, controllers could figure out exactly where a friendly plane was located. This method is called triangulation.

Pip-squeak was very important for fighter aircraft during the Battle of Britain. It was part of the Dowding system, which helped direct planes. At that time, radar systems mostly covered the coast, not inland areas. So, Pip-squeak was crucial for knowing where friendly planes were over land. It also helped tell friendly planes apart from enemy ones. As more radar stations were built, Pip-squeak was slowly replaced by newer systems.

The name "Pip-squeak" came from a popular comic strip called Pip, Squeak and Wilfred. The system was first used with the TR.9D radio. The USAAF (United States Army Air Forces) also used a similar system called RC-96A.

Why Pip-squeak Was Needed

Before Radar Helped Everywhere

In the 1930s, Britain was getting ready for possible air attacks. The Air Defence of Great Britain (ADGB) planned to use acoustic mirrors (giant listening devices) and a network of observers. These observers later became the Royal Observer Corps (ROC). This system was mainly for defending London.

During tests, especially when planes flew high or in cloudy weather, it was hard for observers to keep track of friendly fighters. This made it difficult for control centers to guide the planes to their targets. Finding planes using radio signals seemed like a good solution.

Introducing Huff-duff Stations

Hugh Dowding, who led RAF Fighter Command, made it a top priority to set up high-frequency direction finding (HFDF) stations, also known as "huff-duff." By 1937, he wanted every air sector to have three huff-duff sets. This would allow them to quickly find the location of fighters. Around the same time, a new radio, the TR.9B, was being used.

At first, there weren't enough special screens (called cathode ray tubes) for the huff-duff sets. So, by late 1937, only five sectors had them. But tests in 1938 showed how valuable these direction-finding systems were. Soon, 29 more sets were ordered for all sectors.

How Pip-squeak Worked

In 1938, the Royal Aircraft Establishment developed a new version of the TR.9 radio, called the "D" model. This radio was made specifically to help the huff-duff operators. It could quickly switch between two radio frequencies. One was for talking, and the other was just for sending the special Pip-squeak signal. This meant the signal wouldn't interrupt other radio conversations.

A key part of the system was a special device that made a 1 kHz tone, the "squeak." When this tone was sent through the TR.9D's transmitter, it made a clear sound that huff-duff sets could easily find. An automatic switch was added to turn the radio to the Pip-squeak frequency and send the tone for a short time. Pilots would also hear a quiet version of the tone in their headphones, so they knew it was working.

Newer IFF Systems Arrive

Some early systems to identify friendly planes (called IFF, for identification friend or foe) were tested in 1936. But these didn't work very well. A better system, called a transponder, came out in 1939. However, it only worked with the main coastal radar stations.

The IFF Mark II was a big improvement. It could work with many different types of radars. This system became available in 1940, just as the Battle of Britain began. After the battle, these IFF systems were quickly installed on many RAF planes.

Even with IFF, Pip-squeak was still used for a while. This was because radar didn't cover all inland areas yet. New radars, like the AMES Type 7, started to fill these gaps in 1941. Pip-squeak then became a backup navigation system. If a plane got lost, ground operators could still find it using its voice radio.

How Pip-squeak Was Used

The Plane's Equipment

The Pip-squeak equipment in the plane had two main parts: a device to make the 1 kHz whistle, and a special clock. This clock had electrical contacts that would automatically turn the whistle and the special radio channel on and off. The TR.9D radio, common at the start of the war, had two channels. Pilots would choose the frequencies before a mission. Usually, both the section leader and one other plane in a group had Pip-squeak.

After taking off, squadron leaders would get ready to use their Pip-squeak clocks. They would turn a knob to set a second hand to a specific position. Each section of planes in a squadron had its own starting position: red section at 12 o'clock, yellow at 9 o'clock, blue at 6 o'clock, and green at 3 o'clock.

When the clocks were set, the ground controller would start a countdown: "Synchronize time, 5, 4, 3, 2, 1, mark." At "mark," pilots would turn on their clocks. The second hand would start moving. When it reached the 12 o'clock position, the whistle would automatically turn on. It would turn off just before the 3 o'clock position, broadcasting for 14 seconds each minute. The radio would also switch from voice to the Pip-squeak channel. If a pilot was talking, they would be cut off.

The red section, starting at 12 o'clock, would broadcast immediately. When they stopped 14 seconds later, the yellow section's clock would reach 12 o'clock and start broadcasting, and so on. This way, all four sections could be located within one minute.

Pilots could stop the radio signal from broadcasting while the clock kept running. This allowed them to set up the system early and then turn it off during combat when clear communication was needed. They could turn it back on anytime. Controllers would ask, "Is your Cockerel crowing?" to check if it was on.

There were two main types of Pip-squeak. One had the clock in the cockpit. The other used a remote clock system, where the main clock was near the radio. This main clock was set before the mission for each section. A display in the cockpit showed the time, powered by signals from the main clock. This version had a simple switch to turn the clock on and off.

Plotting on the Ground

Each sector had three huff-duff sets to find the Pip-squeak radios. While two were enough, a third helped prevent errors. The stations were placed about 30 miles (48 km) apart, forming a triangle. One station was at the Sector Control center, and the other two communicated by telephone.

At the Sector Control, a simple system was used to quickly find a plane's location. This was a circular board with a map of the area and compass angles around the edge. Small holes marked the location of the three huff-duff stations. Strings with weights were passed through these holes. When a huff-duff operator reported an angle, a plotter would pull their string to that angle.

With all three reports plotted, the strings would usually meet at a small triangle on the map. This showed the plane's location. Operators knew which section they were tracking by looking at a special clock with section colors. A fourth operator would then tell the main operations room the plane's position. This all had to happen very fast, as they only had 14 seconds to make a plot before the next section broadcasted.

When the location was sent to the operations room, a marker for that section would be updated on the plotting table. Pip-squeak didn't directly identify friendly planes, but it helped operators know which radar or observer reports were tracking their own forces.

Challenges of Pip-squeak

Even though Pip-squeak worked well, it had some problems that led to it being replaced.

First, it used up a radio channel. The TR.9D radio only had two channels. Using one for Pip-squeak left only one for talking. All planes in a squadron shared the same frequencies. This meant squadrons could talk among themselves and to their Sector Operator, but not easily with other squadrons or sectors. Pilots were also constantly interrupted when the system broadcasted. Things got better with newer TR.1388 radios, which had more voice channels, but Pip-squeak still interrupted pilots.

Second, Pip-squeak needed its own separate system of equipment, buildings, staff, and telephone lines. This information was mainly used by the Sector Controls to guide fighters. So, information about friendly planes had to be sent up the chain to higher commands, which added a lot of communication traffic.

Pip-squeak was eventually replaced by the IFF system. IFF was a self-contained device that responded to radar signals from the ground. When a radar signal hit the plane, the IFF transponder sent out its own short radio pulse. This pulse was picked up by the radar station and created a special "secondary" blip on the radar screen, showing it was a friendly plane. This is why modern radar systems are often called secondary radar.

IFF systems were available early on, but they weren't widely used because the first Chain Home radar stations were only along the coast. By 1942, the radar network had greatly improved, especially with ground-controlled interception units. At this point, IFF was used by almost all planes.