Abstract: The range from which transponder replies can be received at a virtual secondary surveillance radar of the type described in U.S. Pat. No. 4,115,771 is extended by replacing the omnidirectional antenna previously utilized to receive replies from transponders within the range of the SSR with a stationary array of a plurality of directive antennas arranged in a circle and having radiation patterns pointing in different directions from the center of and covering a respective sector of the circle, and sequentially switching on the antenna of the array covering the sector in which the rotating beam of the associated SSR is then pointing.

Abstract: The range from which a collision avoidance system at an Own station can receive SSR interrogations and reply messages from Other stations, is extended by utilizing P2 pulses for timing TOA measurement in the event P1-P3 pulse pairs are unavailable from the scanning main beam or its side lobes. The amplitude of the P2 pulses in the SLS radiation pattern being greater than the P1-P3 side lobes of the main beam over an angular sector of about 60.degree. centered on the main beam can insure reception of P2 pulses at much greater ranges than P3 pulses contained in the main beam side lobes can be reliably received. Using P2 timing, interlaced Mode A and Mode C reply messages contained in a main beam burst reply sequence are separated into two "families" of TOAs, the Mode C (altitude) TOAs always being longer than the Mode A TOAs by 13 .mu.sec. A "true" TOA is obtained by subtracting an appropriate time period from the TOA of each family, from which identity, altitude and range information is readily derived.

Abstract: The operation of a positive finding collision avoidance system at an Own station that determines Own's and Others' positions from SSR interrogations and replies thereto is initialized by briefly transmitting interrogations from Own station and receiving non-garbled replies thereto from Other stations to determine their direct ranges from Own. The range data is used in a trigonometric computation to obtain Own's and such Others' initial positions. Interference with the normal operation of the standard ATCRBS is limited to a minimum by transmitting from Own very briefly and only when necessary. The distance between pairs of SSRs is also computed.

Abstract: A position-finding collision avoidance system (CAS) at an Own station within the service areas of at least two identified SSRs at known locations derives differential azimuth (A), differential time of arrival (T), identity and altitude data regarding one or more transponder-equipped Other station from standard ATCRBS interrogations and replies. These data are used to compute the positions of Own and Other stations for display at the Own station.

Abstract: A position finding and collision avoidance system derives, at an Own station within the service area of an identified SSr at a known location, differential azimuth (A), differential time of arrival (T), identity and altitude data regarding any transponder-equipped Other station or stations within a predeterminable region surrounding Own station, from standard ATCRBS interrogations and replies. These data are used to compute the positions of Other stations for display at Own station.

Abstract: A simple active/passive proximity warning system for an Own station that uses differential time of arrival values, direct time of arrival values, and differential altitude values determined from 1030 MHz interrogations from ground-based SSRs and Own station and 1090 MHz transponder replies from Other stations to identify threats is disclosed. The invention combines a passive detection system and an active detection system into an integrated system that overcomes the limitations present when either type of system is used by itself. Generally speaking, the passive system identifies threats and produces a threat alert when the differential altitude value and the largest time of arrival value associated with an Other station are below predetermined values. A preferred embodiment of the active system monitors the environment for SSR density, classifies the environment into a plurality of "cases", and automatically selects in response thereto one of a plurality of control modes.

Abstract: A threat warning collision avoidance system at an Own station that produces pseudo range data, identity and, when available, differential altitude data from standard ATCRBS interrogations and Other station's replies, rejecting the identity and data from Other stations that are outside a proximity space surrounding Own station, and producing a threat alert signal and displaying the identity and associated data of any Other station entering said proximity space.

Abstract: A virtual secondary surveillance radar (SSR) is provided at any desired location within the coverage area of an actual SSR by receiving the interrogations from the actual SSR and the transponder replies thereto. The received signals are processed to yield information as to the positions of the transponders with respect to the desired location, simulating the performance of an actual SSR at that location.

Abstract: Proximity indication and evaluation of mobile vehicles, using the signals emitted by the hundreds of existing secondary radar ground stations and the nearly 100,000 cooperating transponders on the vehicles, to detect intrusion in a monitored proximity volume and to determine slant range and/or relative bearing to the intruder.

Abstract: A collision-predictive CAS at a protected station or vehicle that produces TAU data relating to differential azimuth and/or TOA from standard ATCRBS interrogations and replies, and selects the largest closing TAU signal of a threatening Other station to provide reliable warning while minimizing false alarms. Similarly processed differential altitude information provides a further criterion. Multiple radar environments enable production of additional sets of TAU values and increased discrimination against false alarms.

Abstract: A method and apparatus for determining the distance to a secondary surveillance radar (SSR) station from an own transponder station when another transponder station lies within a monitored region surrounding the own station. The distance information is utilized to modify the operating mode of a proximity indicating system at the own station.

Abstract: Proximity indication and evaluation for aircraft, using only the signals emitted by secondary surveillance radar and cooperating transponders, to detect intrusion in a monitored proximity volume and determine slant range and relative bearing to the intruder.