Communications: directive radio wave systems and devices (e.g. – Air traffic control – Secondary surveilance radar
Reexamination Certificate
2001-05-15
2002-10-29
Tarcza, Thomas H. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Air traffic control
Secondary surveilance radar
C342S036000
Reexamination Certificate
active
06473027
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to techniques for using secondary surveillance radar to identify and determine the location of a target such as an aircraft. More particularly, this invention relates to techniques for distinguishing real targets from reflected targets and for generating a map of all radar reflector objects in the secondary surveillance radar region.
An air traffic control radar system typically includes a primary surveillance radar system and a secondary surveillance radar (SSR) system. Both systems can determine the range and direction of an aircraft from the radar installation. A secondary surveillance radar system, however, can also identify each aircraft using a specific code reported by that aircraft.
The primary and secondary radar systems can be either collected to operate together, or they may operate autonomously. The primary surveillance radar system uses a primary antenna mounted on a tower to transmit electromagnetic waves. The primary antenna rotates continuously to scan a selected surveillance region. These electromagnetic waves are then reflected or “bounced back” from an object (such as an aircraft). This reflected signal is then displayed as a “target” on the air traffic controller's radarscope. The primary surveillance radar system measures the time required for a radar echo from the aircraft to return to the primary radar antenna. The primary surveillance radar system also measures the direction and height of the echo from the aircraft to the primary radar antenna. Secondary surveillance radar was originated in WWII to add the capability of distinguishing friendly aircraft from enemy aircraft by assigning a unique identifier code to the friendly aircraft. The system was initially intended to distinguish between enemy and friend but has evolved such that the term “identify friend or foe” (IFF) commonly refers to all modes of SSR operation, including civil and foreign aircraft use.
The secondary surveillance radar system, also known as beacon radar, uses a secondary radar antenna. In most installation when the two radar systems are co-located, this secondary antenna is attached to the primary radar antenna. However, the SSR system can operate in an autonomous installation where the SSR system is used for the radar surveillance task. The SSR antenna is used to transmit the interrogation calls and to receive the aircraft data. Military and commercial aircraft have transponders that automatically respond to a signal from the secondary surveillance radar interrogation with an identification code and altitude. The code is a predetermined message in response to a predefined interrogation signal. Before an aircraft begins a flight, it receives a transponder code from an air traffic controller. Normally only one code will be assigned for the entire flight. These codes are sometimes called mode codes. The range to the target is calculated from the time delay between the interrogation and the response time. Thus the SSR system provides for friendly aircraft, all the data that primary radar can provide, and more.
There are five major modes of operation and one sub-mode currently in use in the United States. Mode 1 is a nonsecure low cost method used by ships to track aircraft and other ships. Mode 2 is used by aircraft to make carrier-controlled approaches to ships during inclement weather. Mode 3 is the standard system used by military and commercial aircraft to relay their positions to ground controllers throughout the world for air traffic control (ATC). Mode 4 is used for secure encrypted IFF. Mode “C” is the altitude encoder. Mode S is a new IFF procedure for both military and civilian air traffic control that includes transmission of other data in addition to the mode code. The non-secure codes are manually set by the pilot but assigned by the air traffic controller.
A secondary surveillance radar system includes three main components: an interrogator, a transponder and a radarscope. In an air traffic control radar system, the interrogator, a ground based radar beacon transmitter-receiver, scans in synchronism with the primary radar and transmits discrete radio signals that repetitiously request all transponders on a selected mode to reply. The replies received are then mixed with the primary returns, and both are displayed on the same radarscope.
The transponder on an aircraft has an omni-directional antenna so that it can receive and reply to a radar signal from any direction. The transponder receives the signals from the interrogator and selectively replies with a specific pulse group (code) only to those interrogations being received on the mode to which the transponder is set. These replies are independent of primary radar returns, which are received from the target “skin” return. The replies processed by the SSR interrogator for display are sometimes called “plots.” The radarscope used by the controller displays returns from both the primary radar system and the secondary radar system. These returns are what the controller refers to in the control and separation of air traffic.
It is known that the secondary surveillance radar (SSR) suffers from a target reflection problem where a single target may be reported in several directions during one antenna scan. Only one position is the correct one for the target, and the others are “phantom” images that confuse the radar operator. Ground objects that act as electromagnetic “mirrors” reflect the electromagnetic wave to the target and back to the SSR system generate these reflections. These reflector objects can be comprised of any electrically conductive material located in the proximity of the radar site (buildings, hangars, metallic fences, etc.). The problem is much more significant in an SSR system than in primary radar. The SSR transponder generates a high signal level that is not sufficiently attenuated by the interrogator one-way receiving antenna. The primary radar skin return is much weaker, attenuated faster as a function of radar range and is attenuated by the two-way antenna beam (versus one-way antenna beam of the SSR system). In some typical test conducted the number of SSR false reports can be as high as 30% of the total target reports.
The false target is generated when the SSR directional radar antenna is pointed at a reflector object rather than to the real target. The interrogator signal is reflected from the reflector object that acts as a mirror, toward the real target. The transponder in the target emits signals in all directions including the direction of the ground reflector. This signal is now reflected back from the same reflector back toward the SSR system resulting in a false target reported at the direction of the ground reflector. As a result, a target may appear on the radar screen in all azimuths where ground reflectors exist. To make the situation more complicated, unlike in primary radar systems where the ground reflectors are mapped by the radar surveillance, they are not visible by the SSR system, which responds only to active target code reports.
Although current SSR systems contain processes to reduce the number of false target reflections, the final results are not satisfactory. Receiver gain reduction at shorter range, Gain Time Control (GTC), may reduce the number of false targets at short ranges (at the expense of height coverage at those ranges). There is a false target rejection algorithm that requires complete mapping of all reflectors in the surveillance area including their electromagnetic properties. This is a very time-consuming task, with limited accuracy and will not provide a solution for the case where reflectors are dynamically changed (car on the road, new structures built or reflection conditions change due to changes in electromagnetic properties). An automatic technique that rejects all false targets and required no prior knowledge of the reflectors in the surveillance area is presented in this invention.
SUMMARY OF THE INVENTION
One aspect of the present invention identifies and rejects all secondary survei
Andrea Brian
Lynn & Lynn
Northrop Grumman Corporation
Tarcza Thomas H.
LandOfFree
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