Radio Beacon Transponders

INTRODUCTION
The site discusses the principles of airborne radio navigation, whereby the pilot can determine the aircraft's position by triangulation using two ADF or VOR stations, or by polar coordination using VOR bearing and DME distance information. Upon request, the pilot can relay the aircraft's position and altitude to the Air Traffic Control (ATC) center by means of the VHF communication system. Although this method is effective, it is not an optimum solution in high-traffic areas where the controller must be constantly informed of the exact position of all aircraft at all times within the controlled airspace(Though there is no such higher density of traffic in our region). For this reason, the ATC center uses a ground-based radar surveillance system to automatically monitor the location of all aircraft within the control area without cluttering up the radio communication channels. With this information constantly displayed on the ATC radar scope, the controller is able to make timely decisions regarding handing over aircraft to the approach or departure control center, vectoring aircraft to avoid collision courses, maintaining safe altitude separation between aircraft, and locating and directing aircraft that are lost.

The ground-based ATC radar system consists of a primary surveillance radar (PSR) and secondary surveillance radar (SSR). The PSR locates and tracks aircraft within the control area by transmitting a beam of energy which is reflected from the aircraft and returned to the PSR antenna. The SSR transmits interrogation signals to the airborne radio beacon Transponder. Upon receiving the interrogation, the Transponder sends a coded reply signal back to the SSR system. Data received from the PSR and SSR are used in conjunction to develop the total air traffic situation display on the controller's radar scope. This enables the controller to identify Transponder-equipped aircraft in addition to determining the range and direction of all aircraft within the control area.

PRINCIPLES OF ATC RADAR SURVEILLANCE SYSTEM OPERATION
There are two types of radar systems installed each ATC ground station. The first, called the Primary Surveillance Radar, operates on the principle of sending a narrow beam of energy, which is reflected from the aircraft under surveillance, and measuring its distance by noting the time lapse between the radar pulse transmission and its received echo. The second, called the Secondary Surveillance Radar, operates on the coded reply sent from the airborne radio beacon Transponder in response to an interrogation sent from the ground station. The Radar station at Piduruthalagala(Sri Lanka), the PSR and SSR antennas are co-located and scan synchronized, and both radars are used in conjunction to develop the total air traffic situation display on a single CRT radar scope, called the Plan Position Indicator (PPI). The Radar station at Katunayaka(Sri Lanka) is a PSR type and rotate at a speed of 15 revolution per minute while Radar at Piduruthalagala at a speed of 12 rev. per minutes. The reason for this difference in revolution speed is that, as aircrafts approaches close to the airport the sky gets congested and needs constant updates of the positions of the air movements.


ATC PSR/SSR System

The PSR sends out radio waves in a very narrow beam. The ground antenna is made to rotate so that the position of the narrow beam of energy can be directed. When the directed beam strikes an object or target, some of it is reflected back to the radar antenna. This reflected signal is detected and processed to provide a display (indicated by a bright "blip') on the ATC PPI, which shows the location of the target (i.e., aircraft).




Approach Radar antenna

The PSR system works well in low traffic areas; however, as the air traffic increases in a given area, the PPI display becomes cluttered-and specific targets may become difficult to distinguish from one another. Also, since the energy of a radiated RF signal is attenuated as the square of the distance it travels, the resulting weaker radar returns are accompanied by noise which tends to obscure the displayed target. Targets may also be lost due to ground clutter from terrain and precipitation unless a Moving Target Indicator (MTI) circuit is employed to detect and display only moving objects. Finally, the PSR has the distinct disadvantage In that the operator has no way of knowing the altitude of the aircraft unless the pilot reports It. All of the problems associated with the PSR system have been addressed with the introduction of Air Traffic Control Radio Beacon System (ATCRBS).

The ATCRBS incorporates the use of the Secondary Surveillance Radar in conjunction with the airborne radio beacon Transponder. The SSR was developed from the military Identification-Friend-or-Foe (IFF) system, in which an airborne radio beacon Transponder responds to ground radar interrogations on one frequency by transmitting, coded replies on another frequency. The coded replies, displayed as short lines on the PPI, allow the controllers to identify the various targets by having each one send back a different coded reply.

The desired code can be manually selected by the pilot on the Transponder control head in Mode W operation, or automatically set by an encoding altimeter or altitude digitizer for reporting the Aircraft's altitude in Mode "C" operation. Since the reply signal from the airborne Transponder is stronger than the reflected PSR signal, it will reinforce the "pip" on the PPI to provide positive aircraft identification.

At the ATC radar ground station, received radar video and antenna azimuth Information signals are relayed from the radar site to the air traffic control center, where the signals are processed and displayed on plan position indicators. Since radar coverage of each site includes a large area in high traffic density areas, several controllers are assigned to various segments of the area covered. Each controller's segment of the area is displayed on his respective PPI. (You cannot observe such situation in Sri Lanka due to limited number of air movements prevail in compared to other countries).

The PPI presents the operator with a map like view of the space surrounding the area covered by the ATC radar antenna. Four dots appear on the PPI; one at the center, and one of each of the three 1 0-mile points out to the edge of the radar scope. These dots rotate, in synchronize with the rotation of the radar antenna, to display concentric circles that indicate range.

The incoming radar video signals are applied to a decoder control before being displayed. By adjusting the decoder to pass only a selected code, Transponders operating on the controller's code will appear as a short arc (blip) on the PPI, and as a bright arc when transmitting a special position identification pulse. Replies from Transponders not transmitting the selected code will be filtered out. "Skin-paint" echoes detected by the primary surveillance radar will be displayed for all aircraft. An illustration of a typical PPI display format is shown in the figure below.


PRINCIPLES OF RADIO BEACON TRANSPONDER OPERATION

As previously mentioned, the ATC radio beacon system incorporates the use of the ground based SSR and the airborne radio beacon Transponder to determine the range and direction of aircraft responding to SSR interrogations. The following section will discuss the operation of the airborne Transponder in regard to receiving these interrogation signals and generating a coded reply signal to be transmitted back to the SSR ground station.

SSR Interrogation

An airborne Transponder transmits a reply signal on a frequency of 1,090 MHz in response to the SSR interrogation which is transmitted on a frequency of 1,030 MHz. Currently, there are two types of SSR interrogations, Mode "K and Mode "C", that may be transmitted by the ATCRBS ground station. The signal characteristics of the Mode A and Mode C interrogations are shown in the figure.


Mode A interrogations are sent to request the specified aircraft identification code. Mode C Is used to request altitude reporting with identification. Mode B is occasionally used in place of Mode A in some countries and Mode D in presently not in use. Each interrogation mode is distinct from the other and is characterized by the spacing between the P3 pulse and the P1 pulse. Regardless of the interrogation mode, all three pulses are 0.8 microsecond wide.

The purpose of the P2 pulse is to allow the Transponder to determine whether the interrogation was received from the main beam or a side lobe of the SSR radiation pattern, as shown in the following figure. A reply to a side-lobe interrogation would give the controller an erroneous indication of the aircraft's Position. For this reason, Side-Lobe Suppression (SLS) is used to inhibit the Transponder's reply in response to a side-lobe interrogation.


Propagation pattern of SSR Interrogation Signal

The three-pulse SLS interrogation method uses a directional radar antenna that transmits a pair of pulses referred to as PI and P3 pulses. As previously mentioned, the time spacing between these pulses determines the mode of operation. Two microseconds after the P1 pulse is transmitted from the directorial antenna, the second pulse, P2, is transmitted from an omnidirectional antenna. The P2 pulse is used as a reference pulse for SLS determination. The signal strength of the omni-directional P2 pulse is just sufficient to provide coverage over the area that side-lobe propagation presents a problem.

Side-lobe interrogation is detected by the airborne Transponder SLS circuitry by comparing the amplitude of the P2 pulse in relation to the PI pulse. When the omnidirectional P2 pulse is equal to or greater than the directional P1 pulse, no reply will be generated. Identification of the side-lobe interrogation is established before the P3 pulse is received. Therefore, the Transponder will be inhibited for a period lasting 35 microseconds, regardless of the interrogation mode. A valid main-lobe interrogation is recognized when the PI pulse is at least 9dB larger than the P2 pulse, as shown in the following figure.


i think its enough for aviation , more information will be very hard and related to electronic engineering field

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