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Essay: Internship report – ASELSAN

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  • Published: 15 November 2019*
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Last summer I have completed my internship at ASELSAN Identification Friend or Foe and Data Link System Engineering Management department. My internship report consists company and department description, and how identification of friend or foe/Secondary surveillance radar works and how system designed for this purposes.

2.Company Description

ASELSAN is the largest defence electronics company in Turkey.It was founded in 1975 by Turkish Armed Forces Foundation to producing electronic devices for national army. In that days ASELSAN aims to satisfying communication tool requirements of Turkish Army.At the present time it is 58th defence electronics company in the world and together with satisfying Turkish Army requirements it exports its products to many countries.[1]

As the largest defence electronics company in Turkey, ASELSAN produce electronic devices in many areas like military communication systems, cryptography and information security systems, electro optic systems, air and missile systems, weapon systems, radar systems, transportation systems, energy systems etc. When producing these high level technologies ASELSAN aims to contribute to the technological independence of Turkey.

3. Identification of Friend or Foe – Secondary Surveillance Radar

3.1 Introduction to Secondary Surveillance Radar

Radar is an acronym for radio detection and ranging. Firstly, primary radar uses high power pulse of radio frequency energy for detecting objects.The high power beams transmitted from a rotating antenna.Primary radar can detect any object which has reflecting surface.The reflecting signals come to primary radar again.So the signals don’t have any information (except direction and distance) about the object.On the other hand secondary surveillance radar requires a special receiver-transmitter on the aircraft. It uses signals with different frequencies for ground to air and air to ground communication. So in primary radar signal path length is twice as secondary surveillance radar. Since secondary surveillance radar signal path is shorter, the path loss is much lower than primary radar. Because of this, secondary surveillance radar ground antenna, transmits low power beams. Also since aircraft has a special receiver-transmitter, its reply can have information about aircraft identity, altitude, route etc.

3.2 History

When the radar was invented is not known exactly. In 1885, a patent was filed by Thomas Edison. The patent is about using of radio for avoiding collision at sea. Similar applications of radio waves were also used independently by some scientists. Identification of Friend or Foe system firstly used in World War II. Identification Friend or Foe systems are ancestor of Secondary Surveillance Radar. In the World War II, aircrafts were used to much, and had very important roles. United Kingdom firstly used Identification Friend or Foe system for detecting which country aircrafts belong to. The system is similar to Radar but it is using two separate signal in different frequency band. After the war, this system was used for civil aviation because it gives much data about aircraft like SSR ID number, flight number, height etc. It can be solution for increasing aircraft traffic. Since many aircrafts travel between two different countries even two different continents, there should be some international standards about civil use of Secondary Surveillance Radar.

3.3 Principles of Operation

In the most common style of Secondary Surveillance Radar, the ground station emits radio frequency with 1030 Mhz, pointing to aircraft’s receiver which will be called as transponder. The radio frequency emitted by ground antenna will be called interrogation and the ground antenna will be called interrogator. So interrogation is used for ground to air communication. Transponder detects interrogation signal and emits a reply signal with frequency equal to 1090 Mhz. After then this reply signal is detected by the ground station and processed by a plot extractor. On the contrary primary radar, secondary surveillance radar uses two different frequencies.

3.3.1 Antenna Charasterictics

Ground antenna uses two principal beams, these are interrogate beam and control beam. Interrogate beam has high gain and narrow main lobe, control beam is broader and has lower peak gain than interrogate beam. Main characteristics of these two beams are that, control beam is greater than interrogate beam everywhere except main lobe. From the picture it can be more easily understood. Benefits of two principal beams will be explained in next parts.

3.3.2 Interrogation Signals

With two beams, the aircraft transponder can detect the signal is coming from main lobe or side lobe. If it detects the signal is coming from side lobes, it will not reply. Therefore, it will inform ground station about is azimuth angle more precisely.Mainly the interrogation signal has three pulses. Pulse specifications like pulse durations, amplitudes, spacings are determined by international standards. P1-P3 pulse spacing determines the interrogation mode.The transponder decides whether the signal  comes from main lobe or side lobe by comparing amplitude of P1 and P2 because P1 amplitude is greater than P2 in interrogate beam. In control beam P2 amplitude is greater than P1.

3.3.3 Reply Signals

Reply signals are emitted from aircraft transponders. In reply signal format the F1 and F2 pulses are always used and spacing between them are determined by international standards and it is 20.3 μs. Between F1 and F2 there are 13 pulses available. One of them the X pulse is not used in many modes for controlling mechanism. Other 12 pulses can carry information. With 12 pulses 212=4096 permutation occurs.

Main working principles of IFF and also Secondary surveillance radar are briefly described. The interrogation and reply modes and system problems will be explained in next parts.

3.4 Characteristics of Equipments

3.4.1 Ground Antenna Characterictics

Ground antenna characterictics can be divided into two subclasses.There are horizontal and vertical characteristics.Horizontal characteristics of ground antenna determines its accuracy of the aircraft bearing measurement, sensitivity of ability to resolve different aircrafts at similar ranges and minimizing of replies interrogated by sidelobes.Vertical characteristics of ground antenna provides eliminating the reply signals reflecting from ground.Also generally ground antenna mounted on primary radar.So backlobe of ground antenna is also important, it should minimizes it is rear emission to not affect primary radar.Also the radome consists radar to protect it from environmental effects like rain,snow,cold.Radome architecture also important because since it is protecting radar from environmental effects, also it should allow radio signals to pass.

3.4.2 Interrogator Characteristics

As described above, interrogation signals have two beams.Main interrogation beam consists two pulses P1 and P3, control beam consists P2. P1 P3 pulse spacing is determines the type of interrogation.Firstly, let explain modes in secondary surveillance radar and their feautures.

Usage Characterics P1-P3 Spacing

Mode 1 Military Use Tactical Operations 3 μs

Mode 2 Military Use Military ID and mission 5 μs

Mode 3/A Civil/Military Use Identitiy 8 μs

Mode 4 Military Use Encrypted communication

Mode C Civil/Military Use Altitude 21μs

Mode S Civil Use All above and data-link Different type

Interrogators has mod interlace programmers, because they should ask to aircraft regularly about their ID’s and their altitudes etc.So they sends its interrogations like A,C,1,A,C,1 etc. Also pulse durations and spacing should be proper with standards otherwise transponder can’t detect interrogators message.Also receiver(ground antenna receiver) should be sensitive to detect true reply signals.

3.4.3Sliding Window Plot Extraction

The interrogator detects and processed reply signals with Sliding Window Plot Extraction. Sliding window plot extractions detects reply signals from their F1-F2 pulse spacing because F1 F2 pulse spacing is 20.3 μs in every mode.Plot  extractor firstly detects pulses and then it tries to detect brackets and eliminates false replies. Main problems occurs here because interrogator should detects overlapping replies, replies come from reflecting surfaces, replies interrogated from other interrogator etc.These false replies are that:

FRUIT(False Replies Unsynchronized in Time):These replies occurs when ground antenna receives reply signal which are interrogated from another antenna. For solving this problem plot extractor, saves earlier replies and compare with new ones and determine whether they are synchronized or not.

Garbling: These occurs when two reply signal overlaps. So duration between to F1 pulses are less than 20.3μs.And F1 pulses of second reply also corresponds to A,B,C,D pulses of first signal. In this situation, it is hard to detect the which pulses belong to which signals.

Interleaved Replies: In these situations, duration between to F1 pulses is also less than 20.3μs.But F1 pulse of second signal occurs at spaces between expected pulses of first signal. If plot extractor has a very sensitive clock it is easy to find which pulses belong to which signals.

Phantom Replies: The space between two F1 pulses are greater than 20.3 μs but by looking the pulses the plot extractor can detect one more reply signal. If plot extractor can’t eliminate these replies, it will detect aircraft which doesn’t exist.

Multipath : Some of replies don’t come from aircraft, they are come from ground and other reflection objects like buildings etc. Our plot extractor should also eliminate these signals otherwise it detects aircraft which doesn’t exists.From amplitude of these signals it can detect them.

After plot extractor detects brackets and eliminate false replies. It extracts to codes, so information encoded in reply signals. And then it display this information on screen.

3.4.4 Transponder Characteristics.

Transponders should detect interrogation signals. After they absorbs interrogation signal firstly it compares P2 and P1 pulses and determines to interrogation comes whether from main lobe or side lobe.If interrogations come from side lob transponder will ignore it. Also if transponder decided to reply an interrogation, there is a standard waiting time.F1 should follow P3 3μs.Because plot extractor determines the distance of length from the time between when the interrogation sends and when the reply received.Also it is receiver sensitivity is determined by some international standards.It should recognize the signals which are greater than -71 db.

3.5 System Design

Radar Equation: Differ from the primary radar, secondary surveillance radar consists only one-way path loss.So it is range formula differs from primary radar.The range of secondary surveillance can be given as:

  • Power transmitted from ground antenna
  • Transmitter antenna gain
  • Receiver antenna gain
  • Receiver sensitivity at antenna terminal
  • Other system losses like atmospheric.

If we arrange terms, we can determine how much should receiver sensitivity.

Power transmitted from ground antenna and receiver sensitivity can be arrange by desired range.

Reply and Detection Probabilities: Transponder can’t reply to interrogations in some situations. These are replying another interrogation, suppressing from replying by a sidelobe interrogation etc. For working properly transponder reply probability should be higher than %90. Detection probability is also very important. FRUIT, garbles and phantom replies decreases detection probability. With proper receiver sensitivity, and high clock rates at Sliding Window Plot Extractor, detection probability can increase. With high clock rates at Plot Extractor, it can detect pulses and brackets more sensitive and eliminating false replies become easier.

Transmitter Pulse Repetition Frequency:Antenna turning rates for SSR can vary between 5 to 15. For bearing accuracy and true detection there will be a minimum interrogations for detecting aircrafts.FSAHFKJHASFJKAHSFJKHKSFJLAHLFKASKF

3.5 EMI/EMC Tests

EMI is acronym for Electromagnetic Interference, it means any electromagnetic wave that affects ability of electronic devices.Electromagnetic Compability shortly means a produced device will works properly without affecting other electronic devices in environment and works properly without affecting from other electronic devices. EMI and EMC has too much standards. Since ASELSAN generally products devices which will be used in military, EMI and EMC standards are very important for their products. Generally United States Military Standards are used. MIL STD 461G- CE 101 and CE 102 and MIL STD 810G will be briefly explained.

MIL STD 461G- CE101 : Conducted emission, Audio Frequency Currents Power Leads test. In the test the power applied with the frequency from 30 Hz to 10 kHz for power leads. Conducted emissions on power leads shouldn’t exceed the predetermined values to work compatibly with other electronic devices.Test procedure, equipments and requirements are explained explicitly in documents.

MIL STD 461G- CE102 : Conducted emission, Radio Frequency Potentials, Power Leads test. It is very similar to CE101, the only difference is power applied with the frequency from 10kHz to 10 mHz.

MIL STD 810G:It is briefly environmental engineering considerations test.These tests are like pressure, temperature, rain, humidity, sand, dust, vibration etc.Test procedure, equipments and requirements are explained explicityly in documents. In some of the test the product should work in ongoing conditions. And some of the tests it should be work after hard conditions.

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