Intelligence and Communication

When technology and intelligence are mixed together it always inspire our imagination.

The history of the communication intelligence (COMINT) starts in the early days of the radio era. From the early days of the radio transmission the humans aspiration was to identify the location of a transmitter or to just enable the listener the chance to listen to others chats even from the simple need to satisfy the curiosity for peeking. The importance of the radio intelligence was demonstrated during the first WW in the brutal battle between the German Eighth Army was led by General von Hindenburg, and his Chief of Staff, Major General Ludendorff and the Russian first army. The Germen were out number however an intercepted massage between the Russian headquarter and the field Russian IV Army Corps revealed upcoming attack and enabled the Germans the chance to attack the Russian flank and to achieve fast battle achievement.

The communication intelligence experienced huge progress during the years and it enables the intelligence agencies the ability to receive battlefield awareness of the deployed enemy units. The available technology enables directional of arrival identification in addition to the voice/data information used in the traditional COMINT systems.?The most common phrase is Direction Finding and it refers to the use of specialized methodologies to determine the physical location of a source of RF energy.??

Three things may be variate during waves propagation: amplitude, frequency and phase/time. Each Direction Finding methodology can leverage one of the variations to allocate the source/origin of the energy or bearing of arrival. Bearings is the direction of a target and signal bearing can point of the direction but cannot witness the distance information. However, if we able to take bearings from several different locations we can plot them together and identify the most probable location of the target (the process definition is triangulation). The triangulation is a simple mathematical calculation and the accuracy of the triangulation is based on the accuracy of the bearing determination and in most of the cases three or four bearings may gain good location accuracy.?

?The simplest RF direction finding system consists of a directive antenna and a single receiver that implements the amplitude methodology.?In this methodology, the antenna points to different directions while the receiver indicates the received signal strength. When the antenna aims to the RF source, the received signal has the highest amplitude and the signal amplitude will decrease when the antenna aims to other direction. The accuracy of this technique depends on the width of the antenna beam. A narrow beam improves the accuracy but increases the time spent scanning all possible directions.?Too narrow beam may miss the target especially with intermittent transmission sources.?The main advantage of the technique is the simplicity and the cost.

Another technique based amplitude methodology is the Watson-Watt - amplitude comparison DF system that was developed shortly after the first WW (named after Sir Robert Alexander Watson-Watt 1892-1973). The technique is based on antenna named Adcock antenna which is four equally spaced vertical elements arranged in pairs.?This connectivity creates two figure eight lobes and the antenna patterns yields a unique set of magnitudes for every direction. To clarify I have listed the horizontal elements (blue color) as East-West axis and the green elements as the North-South axis. When a signal is captured by the North-South bearing (A) it will be sensed strongly by the North-South and almost none at the East-West elements. The opposite is for a signal (C) captured strongly by the East-West element and almost none by the North-South. We can then configure the amplitude value for each of the azimuth bearing reception amplitudes.?

An example is the US coast guard "Rescue 21" system that uses over 225 Watson-Watt DF sites to radio locate across over 40,000 miles of coastline for search and rescue ships in distress.??

The second methodology is based on a Doppler shift affect named by Christian Doppler (1803-1853). The Doppler DF system uses a single receiver connected to an OMNI directional antenna that is physically rotated on the circumference of a circle.?As the antenna moves toward the radio source, Doppler shift will increase the received frequency and the received frequency will decrease as the antenna moves away from the source.?The change in frequency is used to determine the direction of the radio source and to determine a bearing.

The modern approach is to successively sample each antennas in a circular array of antennas removing the need for any moving parts.?This is referred to as Pseudo-Doppler DF.

One of the most common example usage of the Doppler methodology is a system use to track stolen car that is mounted on the roof of a police car to enable successful chase to capture the car thief. Similar system can be found on the belly of the police helicopter for the same usage.?

Correlative interferometry is a technique calculates bearing based on phase differences at multiple co-located antenna elements. In most of the cases there is a usage of odd number of antennas arranged in a circular pattern. When a signal is received by the antenna array in a certain degree it would be phased offset from each of the rest of the antennas. The system record this offset for each of the antennas and creates a phase offset table. The process repeats itself for each of the reference angle until all 360 degree in azimuth is finalized.?The result is a table incorporates each of the phases may appear in each of the azimuth degree. When a signal is received in the system the system measures the phase offset for each of the antennas and verifies its match for each of the pre-defined phase offset table.???

Time Difference of arrival (TDOA) is based on three or more receivers at different locations that receive a signal. In most cases the distances between the transmitter and the receivers has different length. Thus, there is a difference/deviation between the time of arrival at different receives. The time differences can be present as hyperbola which crosses the transmitter location. The transmitter will be in the intersection of the hyperbola line.

For better explanation, If both receivers located in the same distance from the transmitter the time difference will be zero and the time differential will be equal for all the measurements . On the other hand, the hyperbola will be curved if the difference time of arrival is different and none-zero.

An example of TDOA is a location of a mobile device in a cellular network.?The mobile device is usually covered by multiple base-stations and the time difference of arrival technique can gain accurate location.?

The most successful DF techniques are the hybrid techniques that collaborates both Correlative interferometry technique and the time difference of arrival.??