Identification friend or foe (IFF) system
A method and apparatus for reducing transponder responses to reflected signals utilizes data in an interrogator message to confirm that if two interrogations are received within a predetermined period of time and if the two interrogator signals correspond to a side lobe followed by a reflected main beam signal, the presence of multipath reflection can be correctly identified and the reply to the main beam interrogation can be suppressed.
1. A transponder comprising:
a linear receiver configured to receive one or more interrogator signals;
an analog-to-digital converter (ADC) having an input configured to receive an analog signal from said linear receiver and having an output at which a digital signal is provided; and
a digital processor configured to receive digital signals provided thereto from said ADC and to suppress transmission of a reply signal if:
two interrogator signals are received within a predetermined period of time;
a first interrogator signal of the two interrogator signals corresponds to a side lobe interrogator signal and a second interrogator signal of the two interrogator signals corresponds to a main beam signal; and
a first interrogator signal and the second interrogator signal both include the same waveform code.
2. The transponder of claim 1 wherein the predetermined period of time is in the range of about 1 microsecond to about 14 microseconds.
3. A transponder comprising:
(a) a receiver to receive one or more interrogator signals; and
(b) a signal processor to suppress transmission of a transponder reply signal in response to two interrogator signals being received within a predetermined period of time with a first received interrogator signal corresponding to a side lobe interrogator signal and a second received interrogator signal corresponding to a main beam signal if the first received interrogator signal and the second received interrogator signal both include the same wafeform code.
4. The transponder of claim 3 wherein said signal processor comprises:
a detector for determining if two interrogator signals are received within a predetermined period of time;
a detector for determining if a first received interrogator signal of the two interrogator signals corresponds to a side lobe interrogator signal and a second received interrogator signal of two interrogator signals corresponds to a main beam signal;
a decoder to decode a first waveform code of the first received interrogator signal and a second waveform code of the second received interrgator signal; and
a comparator to compare the first waveform code and the second waveform code to determine whether they are the same waveform code.
5. A method for determining whether a transponder should reply to an interrogator signal, the method comprising:
(a) determining if two interrogator signals are received at the transponder within a predetermined period of time;
(b) determining if a first received interrogator signal of the two interrogator signals corresponds to a side lobe interrogator signal and a second received interrogator signal of the two interrogator signals corresponds to a main beam interrogator signal; and
(c) determining if the first received interrogator signal and the second received interrogator signal include the same wavecode.
6. The method of claim 5 further comprising:
suppressing transmission of a reply signal if:
the two interrogator signals are received at the transponder within the predetermined period of time;
the first received interrogator signal corresponds to a side lobe interrogator signal and the second received interrogator signal corresponds to a main beam interrogator signal; and
the first received interrogator signal and the second received interrogator signal include the same waveform code.
7. The method of claim 5 , wherein:
determining if the first received interrogator signal and the second received interrogator signal include the same waveform code comprises:
despreading and decoding a first waveform code in the first received interrogator signal;
despreading and decoding a second waveform code in the second received interrogator signal; and
comparing the first waveform code and the second waveform code.
8. The method of claim 5 , wherein:
determining if two interrogator signals are received at the transponder within a predetermined period of time includes determining if two Mode-5 Identification Friend or Foe interrogator signals are received.