Secondary surveillance radar

A secondary radar is a radar that operates with the active targets and therefore can operate at low power. In contrast to the primary radar, in which the target only a (passive ) reflection takes place, an interrogator ( interrogator ) sends a data signal which is active from the transponder replies with an "answer" in the SSR. This answer may additional information such as altitude or friend -or-foe recognition included.

Application find secondary radars mainly in aeronautics, but other applications have been developed over time. They are used for radar surveillance, mainly in transport. Distant space probes such as Voyager 1 can only be located by secondary radar. Following the same principle, but over very short distances to work RFID.

• 3.1 Standard Mark X 3.1.1 Query Format
• 3.1.2 Response telegram
• 3.1.3

• 3.2.3 Transmission Protocols 3.2.3.1 uplink
• 3.2.3.2 Downlink

• 4.1 secondary radar in automotive engineering

Development

The secondary radar method has evolved from a military system whose original mission was the distinction between friend and enemy on the radar screen. During the Second World War there was an urgent need to distinguish between their own and opposing target character on the radar screen for the English air defense. Therefore, it was of Frederic Calland Williams and later knighted for his services Bertram Vivian Bowden developed a system, which later became known as Identification Friend Foe (IFF ).

The first systems (IFF Mark I and Mark II) have only received the transmit pulse of the radar device and emitted amplified again. The transponder was turned on only on request. In radar no technical enhancements were necessary: ​​The private plane was shown twice on the radar screen. ( A consequence of the internal signal-to- maturity in the transponder. ) Only from the system IFF Mark III was the answer on a separate frequency band, then at 157-187 MHz, sent. From this point, a special receiver in the radar device was needed.

As of 1943, developed the IFF Mark V as a joint Anglo- American project in the United States Naval Research Laboratory under the auspices of the relocated to Washington Doctor Bowden and adjusted under the name United Nations Beacon ( UNB ) for series production. This IFF system was already working in the frequency band 950-1150 MHz, which is the same frequency band as the modern IFF / SIF. A further development of this principle was already called Mark X. The letter X was here for the time being as a sign for an unknown version number. This system was very simple structure and worked on 12 different channels with a frequency spacing of 17 MHz. This Mark X (IFF ) was still able to transfer any individual identification of an aircraft.

It was not until the development to the system Mark X (SIF ), which is now assigned importance to the character X Roman numeral ten and the abbreviation for SIF Selective Identification Feature, has a pulse-coded response allows an individual identification. Based on this version was by the International Civil Aviation Organization ( ICAO) in 1953, the international standard Mark X formulates and defines with many extensions to the year 2008 into it as the basis for the civil use of the secondary radar in air traffic control, while the purely military use of the standard Mark X more and more lost its importance.

In the same period a secondary radar was developed as an identification device in the former Soviet Union as well. Here Kremni was under the system name broadcast on a frequency in the UHF range, a three - and sometimes four -digit pulse pattern received by the transponder, in the presence of a valid encoding ( only three pulses have a valid query, the fourth pulse is at deception ) with a low frequency modulated and sent out again. These low- frequency represented the ID and was realized by twelve pluggable code filter. These different filters were changed in the armed forces of the Warsaw Pact after a specified centrally secret pseudo -random pattern at a distance of two to six hours. From Russia's Pacific coast to Europe to Cuba this code filters are changed at the same time regardless of the local time zone. A civilian use was not originally intended, but since the civil aviation was organized by the state and all civil aircraft should be used in the event of mobilization as a military transport, all civilian aircraft were equipped with this transponder.

The military use of the secondary radar method is still an important task, but the code size of the IFF / SIF for an imitation safe Freund/Feind- identifier is far too low. Therefore, it was later for purely military applications, known as IFF Mode 4 and Mode 5, integrated with crypto computers encrypted data transmission method in the SIF.

From the military terminology of the names of the Mark- derived systems. You combine multiple Identifizierungsmodi together in a name and were in the NATO Standardization Agreement 4193 ( STANAG 4193 Part I - VI) described.

• Mark X or MKX (pronounced "mark ten" ) includes the modes 1, 2, 3 / A;
• Mark XA or MkXA includes the modes 1, 2, 3 / A, C;
• Mark XII or MkXII comprises the modes 1, 2, 3 / A, C, 4;
• Mark XII -A or -A MkXII comprises the modes 1, 2, 3 / A, C, 4, 5;
• Mark XII -A / S or -AS MkXII includes the modes 1, 2, 3 / A, C, 4, 5, S

The terms and Discrete Address Beacon Super Beacon System ( DABS ) still be used as a former designations for the Mode S - procedure in older literature.

Operation

The secondary radar principle is a measuring method with runtime measurement which, in contrast to conventional radar technology not reflected on the target energy, so the passive echo of a target works, but with the on board of the target an active response unit (transponder) is located. Respond to a secondary radar, the active targets to a received radar signal by transmitting a response to the same or a different frequency. In the system used in civil aviation to search on the frequency of 1030 MHz and the response to the transmitted frequency 1090 MHz.

To this end, the radar pulse is received with an antenna and triggers the emission of a characteristic " echoes " on the same antenna. This response can be a modulation characteristic or a data packet. In the simplest case, this is the delayed radar pulse itself, with the first systems for friend - foe identification, a double point has been written on the radar screen - a point from a passive reflection signal and behind another ( delayed ) from the secondary radar.

Both systems have different advantages and disadvantages on the basis of different principles. The main advantage of the secondary radar compared to the primary radar is its significantly greater range and the ability to identify the target. With the primary radar are secure information about the direction, height and distance of goals and won completely independent of the target. A secondary radar provides additional information, such as identification, identification, and also the amount of goals. However, the employees of the target is required for this. If this cooperation, for example because the transponder is faulty, the secondary radar is not able to work and this flying object is not recognized. Therefore, most secondary radars operate in combination with a primary radar.

Block diagram

The process consists of two units: the interrogator ( interrogator ) and the responder ( transponder ). In aviation, the query devices are partially ground stations, some (especially fighter planes ) can be scaffolded in an aircraft a transponder interrogator like.

The interrogator sends depending on the type of modulation ( the so-called mode ) encrypted, for example, pulses with different query. These pulses are received and evaluated by the transponder. Depending on the content of the query, a response is generated, encrypted and transmitted again.

Due to the delay measurement between transmission pulses and the response message, the distance between interrogator and responder can be calculated. Due to the problems caused by the transponder decoding and encoding delays this distance calculation is only correct if this additional delay time is known.

Range calculation

Through active participation of the target can drastically reduce the required transmit power for the same coverage can be achieved because at primary radar free-space loss is received with the round trip in the radar equation, because of the way back one at secondary radar only with the outward journey as an independent radio link. In contrast to the primary radar, in which the range is marked by the fourth root in the radar equation, the secondary radar, the range is calculated by a function with a square root. As a guide, it can be assumed here a factor of 1000. This results in a significantly simpler, smaller and above all cheaper stations. The usual practice in the performance of a secondary radar transmit pulse is between 250 and a maximum of 2000 watts. In the same order of magnitude, the transmission power of the transponder.

At the same time, the receiver can be sensitive, because the benefits of active responses are higher than those of the passive echoes. In a primary radar receiver sensitivity achieved with good radar receivers values ​​of -110 dBm ... -120 dBm. In a secondary radar values ​​are around -65 dBm Pe = optimal to have sufficient sensitivity and to be sufficiently resistant to interference simultaneously. The range is according to the formula:

R = range Ps = transmit power Pe = received power G = gain of the transmitting antenna Ge = Receiving antenna gain λ = wavelength (in this case about 29 cm)

Calculated. Since the secondary radar, such as the primary radar, the transmit and receive antennas (and thus inserted into the formula antenna gains ), are the same both in the scan path and on the response pathway, in this case the only difference due to the different wavelengths is at 1030 MHz ( scan path ) and 1090 given Mhz ( response pathway ). The transponder can operate by the slightly higher transmission frequency with a slightly lower transmission power, since both antennas have an effective active surface for the reply frequency.

From the range formula especially the necessary transmission power of the interrogator is to be calculated. The transponder must always be at full power ( less than 2 kW) answer, because the transponder is not known, the distance of the interrogator. Only in the interrogator is known up to what range the IFF / SIF information can be drawn at all. Therefore, the coverage formula be changed according to the transmission power of the interrogator:

The term in the square here is the free-space loss at a constant wavelength as a function of distance. The losses L can in internal losses of the antenna to the plot extractor ( a total of about -3.5 dB ) and external losses by the diagram form (see -3 dB limits ) of the antenna, interference in the presence of reflections ( with an average of -4 dB) or the influence of a radome (about -0.2 dB) are divided. Under very unfavorable conditions, the internal and external losses total can reach up to -9 dB.

With Inclusive of the antenna gains and the losses can be accepted by the sender to the receiver for a range of 150 nautical miles ( = 278 km ) attenuation of 122 dB. The receiver sensitivity of the transponder is at least -65 dBm. The transmitter must for this distance therefore a pulse power of

Muster. A larger transmission power than 500 watts for the maximum displayable distance of 150 nautical miles has no more influence on the range and only causes an increase in the mutual interference from Fruit. The transmitters of the secondary radar devices are usually designed in air traffic control for a pulse power of 2000 watts, but can be reduced in 3dB steps in the transmission power. When the primary radar, for example, has only a limited range, then an increase in the transmit power of the secondary radar is counterproductive.

Operating organization

The secondary radar method is applied in the civil and military aviation with each other compatible systems:

• Civil: " SIF " for Selective Identification Feature and " SSR " for Secondary Surveillance Radar
• Military: " IFF " for Identification Friend or Foe - Friend or Foe

Secondary radar provides additional information about an aircraft which is or can determine the quality of a primary radar not not in aviation. It is a cooperative process, that is:

• The aircraft must participate in the process and
• The individual steps of the process must be standardized so that aircraft and ground station understand each other.

Standard Mark X

By ICAO mandatory standards are defined that govern the secondary radar method. One such standard is as Mark X (pronounced Mark th ) known. In this standard, the classic mode and code are defined. The purpose of this query is an identification of the aircraft and the query of the additional information altitude.

Query format

The query is often referred to as fashion. The question is something like: " Who are you? " The transponder in the aircraft responds with a transponder code. Alternatively, can also be queried "How high can you fly? " (This question arose from the fact that most radars previously were only 2D radars. ) The aircraft responds with another code. Fashion and code always belong together, because if the question is not known, the geantwortete numerical value is not unique. The mode is transmitted coded by the short distance between two transmitter pulses, and has the following meanings:

This mode only supports 32 different codes (though also 4096 codes are technically possible ). Normally, transmitted by these codes information about application, task and type. Is hardly used in peacetime.

The Mode 2 contains 4096 different codes for military purposes (such as Mode A ). Normally, these codes are transmitted by an individual of the aircraft ( military ID) code.

The answer to the Mode A (code ) is a four-digit number (octal 0000-7777, three bits or pulses per point BCD coded ) for the identification of the aircraft. It is entered by the pilot on the transponder directly or on the remote control unit. (Was not forget this as an individual code, but enough for this job now no longer. )

Altitude of the aircraft in 25 -foot increments (formerly in 100 -foot increments). This value is determined by a barometric altimeter which all aircraft in the world has the same default (ICAO Standard Atmosphere ). The value is transmitted technically similar to the Mode 3 / A, but not directly encoded in octal, but. Means Gillham code The range of values ​​includes information from -1000 to 127,000 feet.

Wherein the requests are sent generally to 1030 MHz, in this method, a pulse P2 is always still integrated. This is the side lobe suppression and causes only transponder from the main direction of the answer to the query.

Response telegram

The response telegram is 20.3 microseconds long at all modes previously mentioned, and is transmitted in 1090 MHz of frequency. In this response, a larger tolerance range must be accepted because, for example, at high altitudes in extremely cold temperatures, frequency-determining components of the transponder have larger deviations from the setpoint.

The response message consists of 2 to 15 pulses with a pulse duration of 0.45 microseconds per (± 0.1 microseconds ). The two framing pulses F1 and F2 at a distance of 20.3 microseconds must be at least present for the recipient of these pulses are recognized as a valid answer. Between the frame pulses there is an interval of 1.45 microseconds total of 13 positions for the encoder pulses. Of these, only a maximum of 12 used for the transmission of the desired information in an octal code in the mode A and C. The three spaces must not be occupied by pulses, otherwise some decoders interpret the entire response as a disturbance and thus discard. However, the response message does not contain any information about the fashion. The decoder of the secondary radar always assumes that the received response to the queried as the last fashion fit.

The pulses between the frame pulses contain the code that contains depends on the query mode, the desired information as an octal number. By the number of 12 possible pulses, the range of values ​​of the unique information is limited to 409 610.

The SPI pulse (special position identification pulse) is the " squawk ident", according to manual press the pilot of the " IDENT " button in the transponder control panel, placed 4.35 microseconds ( three grid intervals) after the frame pulse F2. This example blinks DERD radar screens at the head symbol of the SSR target on.

The interleaving of the pulses, and the space in the center is included for historical reasons. The precursor of this method had only the possibility to transmit two octal numbers. Later, the additional octal numbers were placed in the spaces between the old answer for compatibility reasons.

Display

In the simplest case, another, usually somewhat thinner target mark is displayed on the radar screen behind the target character of the primary radar. The distance corresponded to the beginning of the additional delay in the transponder. Later he could be adjusted at the viewing device. The numerical values ​​of the identification code and the amount of information must be read on an additional display of light emitting diodes on the secondary radar or at a remote indicator next to the primary radar screen. Some radars can display this additional information on the screen itself as a figure.

Modern digital radars can merge the information from the secondary radar to the target character of the primary radar. Here, both the radar information will be processed in a plot extractor according to a digital data word, which are then correlated in the radar data processor to a data record. The display is so on the digital screen of the primary radar. Since the signal delay times now vary considerably even in the ground station, the antenna of the secondary radar equipment shall be mounted at a small angle offset on the primary radar antenna.

Standard Mark XII (Mode S)

Another defined by the ICAO binding standard ( Mark XII - ie Mark twelve ) is also referred to as Mode S ( Mode selective ). This new standardization was necessary because the previous systems were passed through the rise in air traffic capacity limits. Reasons were: Exceeding the maximum number of workable objectives, false radar echoes (eg generated by ACAS queries FRUIT ), limited azimuth resolution. This resulted in several instances that airplanes were misrepresented or not on the radar screen ( "lost targets" ). Another point was that the range of values ​​for Mode 3 / A (with only 4096 different codes ) was too small.

In the Mode S transponder everyone has a hard-coded individual address. The standard stipulates that no longer respond to all transponders in a query, but only those that are explicitly referenced by their address. Thus, the number of response signals is massively reduced. In addition, the response signal containing the address information, so that it can be uniquely assigned to each aircraft and FRUIT is excluded. This standard was established by the ICAO and is prescribed for aircraft registrations.

" The individual SSR Mode S address should be one of 16,777,214 possible 24-bit addressing, which is allocated by ICAO, or a state or other authorized general registration authority. "

The previous identification system in Mode 3 / A has. Employed by the four-digit octal code only one set of values ​​of 4,096 different identity codes Therefore, this code had to be allocated dynamically, that is, when flying through different zones of responsibility the aircraft one each new identification code was assigned. This has caused on the radar screen yet the likelihood of confusion.

The Mode S system can be defined as the basic protocol for communication between the transponder in the aircraft and the secondary radar on the ground. In addition to the identification code will be assigned for an aircraft individually ( similar to a license plate number ), much more information about the current flight status on the secondary radar can be transferred. The military Mode 4 can not be evaluated for civilian devices because the responses coded and the contents therefore classified ( classified / secret ) is.

A significant advantage of the system is that the base stations can share the requested information over a network. This reduces the polling frequency and thus mutual interference by Fruit. This assumes, however, that if the selective interrogation of aircraft is carried out, the response is associated with a primary echo and a further prompt is not displayed. The query must be backwards compatible again. An old Mark X transponder must not be confused by the Mode S queries. Therefore, these old transponder commands are to be queried, but the protocol a fourth pulse ( P4) is appended to the blocks a Mode S transponder for these queries. The old transponders do not know this momentum and ignore him.

The interrogation of a mode- S -capable ground station are roughly classified into two types:

Transmission protocols

Uplink

Besides the actual Mode S interrogation must by the ground stations and the Mark- X system compatible pulse patterns are sent that are shown in the following table:

The transmission protocol (query from the ground station up to the plane ) within the P6 pulse a pulse modulation with differential phase shift keying ( DPSK ) is exploited in the so-called uplink path. Since the P2 pulse is then used to lock the old transponder, the sidelobe suppression must be solved differently. A fifth pulse is emitted as before the P2 pulse via an antenna with omnidirectional characteristic. This pulse overlaps the synchronous bit in the pulse P6. P5 is the pulse is too large, the transponder can not decode the pulse P6. The P6 pulse can contain either 56 or 112 bits.

Technically, it is also possible to send via the uplink information to aircraft, such as weather data, information about the approaching aircraft ( TIS traffic information service, in the U.S. ), flight clearances, etc. These possibilities are not yet exploited in the EU. A total of 25 different queries with Mark -XII are possible; likewise, there are 25 individual answers. "

Downlink

On the downlink path (answer from the aircraft to a requester ) is unfavorable, also the differential phase shift keying ( DPSK ) to use. Transmission reliability is adversely affected by extreme temperature fluctuations to which a transponder is exposed at high altitudes. Therefore, a less sensitive modulation Pulse position modulation (PPM ) is used. Whenever a falling edge is detected at the response graph stored as a time frame synchronous clock, then there is a logic " 1". With a rising edge is detected a logical "0". Also on the response pathway are transmitted as either 56 or 112 bits.

Squitter Mode

A special feature of Mode S capable transponder is called the squitter Mode, in which the transponder regardless of a query, and at regular intervals, for example, GPS location and identification as a broadcast sends (ADS -B Automatic Dependent Surveillance - Broadcast ). However, the support of this mode is not mandatory in Germany. Also, not all Mode S transponder -enabled technically able to send such a message.

This mode allows you to build a simple connected to a computer via a USB interface receiver at the frequency of 1090 MHz, a virtual radar system that can represent the commercial flights within about 40 km on the computer screen in real time with a small rod antenna. By combining many such small receiving stations over a network complete representations of the movements in the airspace are possible.

Equipment requirements

In Germany, a transponder is required:

• For IFR flights (§ 3 FSAV )
• For VFR flights in the air spaces C and D ( not control zone ) (§ 4 FSAV )
• For VFR flights in airspace with prescribed transponder circuit ( Transponder Mandatory Zone - TMZ ) (§ 4 FSAV )
• For VFR flights at night in controlled airspace (§ 4 FSAV )
• For VFR flights with motor-powered aircraft above 5000 ft above sea level or above an altitude of 3500 ft above ground level (§ 4 FSAV )

Other applications

In addition to aviation, which certainly represents the largest user of radar technology, radar systems are ( both primary and secondary) used in aviation foreign areas. Among many other applications, only the transponder are referred to as a secondary radar, wherein a range determination is performed by means of a transit time measurement.

In shipping, a radar beacon operates on a similar principle, the transponder is attached to the navaid.

Secondary radar in automotive engineering

Modern cars use a secondary radar to detect the approach of a radio key to the motor vehicle. In this case, in addition to the propagation time measurement of the signal is performed, a data transmission that identifies the key relative to the vehicle. Only after successful identification and a short distance from the key to the vehicle, the locking system the doors will be unlocked. An additional inner - outer space - detection ensures that the drive can be started only when the key is inside the vehicle. The removal of the vehicle does not lead to re- lock the doors, it must be triggered manually ( controlled).