Signal Intelligence, as the name suggests, is a general definition given to intelligence operations performed on signals. Although it has had different uses in the military for many years, it can be considered as a concept that emerged after the Second World War.

In order to understand the phenomenon of Signal Intelligence, it is necessary to master the concepts such as Signal, Frequency, Electromagnetic field. After all these concepts, it is essential to have detailed information about each of the broadcast types such as GSM, Radio, Wi-Fi.

What is done in Signal Intelligence?

This is quite a large area. The facilities established for the purpose of listening to radios, which were military communication systems in the past, have spread over a large area as a result of the development of today’s wireless technologies. We can call Signal Intelligence the interpretation of signals in many different areas such as GSM, Satellite, Digital Radios, TV Broadcasts, Wi-Fi. Electronic Intelligence and Signal Intelligence are often confused. Due to today’s technologies, Electronic Intelligence mainly operates on the Internet. Signal Intelligence is a phenomenon that aims to study all radio signals in the world. For this reason, the Internet’s wireless data transmission paths also fall within the field of Signal Intelligence. Signal Intelligence, which works with very difficult, complex and large data, is considered one of the most difficult professions in the world. Currently, there is very little human resource trained in this field in the world. Likewise, in terms of hardware and software, it offers opportunities as a very untouched area in the world. This area, which was only active in the military field in the past, has now started to appear with civilian uses. Secret services, law enforcement, and public institutions use Signal Intelligence solutions according to their task execution areas.

What are the Tools Used in Signal Intelligence?

Sometimes even a simple radio receiver can provide important information for Signal Intelligence, while millions of dollars of software and hardware are also used in this field.
Antennas: Different antennas are used for each different broadcast type. Although the Z generation does not remember much, in the past, there were antennas on the roof of every house for terrestrial broadcasts and they were turned towards the transmitter in the city where they were located. These antennas were Yagi type antennas designed to receive broadcasts in the UHV/VHF (Ultra High Frequency – Vera High Frequency) bands.


You may have noticed that much smaller antennas are used for GSM, Wi-Fi and Bluetooth receivers. This is because these broadcasts broadcast at very high frequencies. For example, while Wi-Fi broadcasts on 2.4GHz and 5.4GHz frequencies, GSM broadcasts in ranges such as 900MHz-1800MHz-2600MHz, and Bluetooth also uses 2.4GHz frequencies. The higher the frequency, the smaller the height of the signal in terms of measure, that is, it is inversely proportional. That’s why the antennas used in your mobile phones are of very low dimensions and are usually mounted on the phone’s case. However, the antenna length of a military long-distance radio can exceed 30 meters. While it is possible to broadcast to much longer distances with large wavelength and low frequency, short distance broadcasts can be made with high frequency and low wavelength. For example, the coverage area of the antenna in a base station broadcasting 4G can be considered as a maximum of two kilometers.

Radio Receivers: The cornerstone of Signal Intelligence is receivers. They are devices that process and demodulate the signals coming from the antenna, that is, make sense of them. The satellite receiver in your home is a kind of radio receiver device. They pass the signals coming from the satellite antenna through different modulations and encryption processes and are used to output the television as an image. Radio receivers also perform the desired function by using different modulation and encryption techniques according to the function they will perform. We can give an example of handheld radios used almost everywhere, they are used to convert the signal coming from the antenna into sound and at the same time, they are transmitters that convert the sound into analog or digital data and transmit it with the help of antenna.

Encryption Software/Hardware: You must have heard the concept of Encrypted or Encrypted radio. Encrypting audio data with hardware or software built into a conventional handheld radio is important for security. The hardware and software that perform these encryption processes are very important in both the military and civilian sectors. Decryption of encrypted transmissions in Signal Intelligence can sometimes take months. After all the radio signals in the air are received in a recording medium, it may take very short periods of time, sometimes months, for the raw data to become meaningful data by applying the necessary password cracking processes.

Signal Analysis Software: These softwares work together with the signal receiving hardware to enable you to analyze all the signals in the air. By focusing on the frequency you specify, they help you see all the broadcasts visually and hear the sounds in the broadcast. The software called Gqrx included in the SigintOS Linux Distribution does just that. You can analyze all the frequencies supported by your signal receiver device with the help of this software.

GSM Intercepting Systems (Intercepter): These are hardware and software that enable the recording of conversations by listening to 2G GSM signals in the air and passing them through the necessary encryption processes. After performing the necessary operations on SMS and voice data, they enable conversations to be listened to. These systems can only work on 2G. It is very difficult to use in advanced technologies such as 4G. The reason for this is that 4G and advanced technologies have high level encryption and frequency changing feature by taking advantage of the possibilities offered by broadband.

IMSI Catcher: These are systems for catching IMSI numbers. They work just like a GSM listening device, but their job is only to find out which phones are connected in that base station. They enable finding IMSI numbers of mobile phones in the vicinity without any eavesdropping.

Basic concepts

What is the signal?

Signal is basically telling a situation or information by means of signs. For example, when you want to turn to the right with your car, the light you turn on and off by raising the handle on the steering wheel is a signal, the flashing of the yellow lamp on the right side of the car is a signal that you will turn right. The fact that makes signal intelligence meaningful is the information that these signals carry, and this information is determined by Radio waves formed in the electromagnetic field. For example, a radio station first converts the music it wants to transmit to its listeners from digital to analog data, and then, with the help of an antenna, it spreads this data to the whole atmosphere as electromagnetic waves with FM modulation. Here, we call the electromagnetic waves in a field with a certain bandwidth, which are converted to this FM modulation, as Radio Signal.

What is Frequency?

The number of repetitions per second of a continuous process is called frequency. For example; If you perform one applause per second while you applaud the actors at the end of a theater play, you will be applauding at a frequency of 1 Hertz. If you had ten claps per second, it would have a frequency of 10 Hertz. Frequency is indicated as Hertz, and its magnitudes are shown as KiloHertz, Megahertz and Gigahertz.

What is Modulation?

You may think that the telephone is the most important invention in the world, but if the concept we call modulation had not been discovered, neither telephone nor TV broadcasts could have been created. To put it in the shortest sentence, modulation specifies how a signal is generated and how it will be processed. In the image below, you can see the Signal, FM and AM modulations.

As seen in the animation, in AM modulation the data is transmitted with the size of the signal, while in FM modulation it is transmitted with the frequency method, that is, a different amount of data is created in each second.

Why Use Different Modulations as AM and FM?

In the simplest terms, these two different modulations are used because of the distance the signal will reach and the data differences it will carry. A radio station needs to transmit data in stereo and in high quality in order to be able to listen to music broadcasts in a better quality. For this reason, it has to use Frequency modulation, ie FM, in order to send more data with Signals. However, in military communication systems, less large data is used. For example, for communication with a ship or submarine thousands of kilometers away, very low frequency AM broadcasts are made because AM signals can reach much longer distances, but the amount of data they can carry will be less than FM. This is why it is possible to send signals from Turkey to the United States with AM modulation with a well-tuned antenna and a high-power transmitter.