fig1 is a block diagram of a tcas system . shown in fig1 are tcas directional antenna 10 , tcas omnidirectional antenna 11 , and tcas computer unit 12 which includes receiver 12a , transmitter 12b , and processor 12c . also shown are aural annunciator 13 , traffic advisory ( ta ) display 14 , and resolution advisory displays 15 . transponder 16 is comprised of transponder unit 16a , control panel 16b , and transponder antennas 16c and 16d . the tcas and transponder operate together to function as a collision avoidance system . those skilled in the art understand that this is merely illustrative of a typical tcas . many other configurations are possible such as replacing omnidirectional antenna 11 with a directional antenna . the operation of tcas and each component are well known in the art and need not be described in detail . an excellent discussion of tcas can be found in &# 34 ; introduction to tcas ii &# 34 ; referenced above . the present invention is primarily related to modifying receiver 12a and processor 12c to achieve extended useful range . fig2 is a block diagram of the invention . tcas radio frequency signals are received at antenna 10 . tcas radio frequency signals ( e . g . mode s squitters and replies ) are pulsed radio signals transmitted at 1090 +/- 3 mhz . radio signals are input to rf amplifier 21 . rf amplifier 21 is a conventional low noise amplifier ( lna ). rf amplifier 21 establishes the receiver primary noise factor ( nf ) performance . rf amplifier 21 drives conventional passive mixer 23 . mixer 23 produces a 60 mhz intermediate frequency ( if ) by mixing the 1090 mhz input signal with 1030 mhz local oscillator input 22 . primary if amplifier 24 provides gain at 60 mhz to the output of mixer 23 and maintains the nf established by rf amplifier 21 . first mux switch 25 switches the if signal from if amplifier 24 to either normal filter 26 or narrow filter 27 as directed by range control signal 29 . range control 29 directs the if signal to normal filter 26 for normal range operation and directs the if signal to narrow filter 27 for extended range operation . normal filter 26 and narrow filter 27 are conventional bandpass filters having bandwidths of approximately 8 mhz and 4 mhz respectively . second mux switch 28 switches the appropriate if signal from filters 26 and 27 as directed by range control signal 29 . range control signal 29 preferably is generated by processor 12c but may also be controlled by the pilot . the if signal output from second mux 28 is processed in a conventional manner and communicated to processor 12c for analysis as needed . fig3 shows the preferred embodiment of the invention . rf amplifier 21 , mixer 23 , and if amplifier 24 function identically as in fig2 . primary if filter 30 is a fixed tuned 60 mhz filter which limits the noise bandwidth applied to secondary if amplifier 31 . primary if filter 30 has a bandwidth of 10 mhz . secondary if amplifier 31 is a log / if amplifier / detector which outputs a first logarithmic video current pulse 31a and two amplified linear 60 mhz if signals 32 . one amplified linear if signal 32a is filtered by normal bandwidth filter 33 and the other amplified linear if signal 32b is filtered by narrow bandwidth filter 34 . normal bandwidth filter 33 has a bandwidth of 8 mhz and narrow bandwidth filter 34 has a bandwidth of 4 mhz . the outputs of filters 33 and 34 are applied to mux 35 which switches either the output of normal filter 33 or narrow filter 34 to third if amplifier 36 . mux 35 is a single pole / double throw ( spdt ) intermediate frequency mux and is controlled by range control signal 29 . switching time is within ten nanoseconds . third if amplifier 36 outputs a second logarithmic video current pulse 36a and two amplified linear 60 mhz if outputs 36b . the if outputs 36b are not used in the invention . both secondary if amplifier 31 and third if amplifier 36 are identical and have differential input connections but are not used in that configuration . current pulses 31a and 36a are summed through resistors 37a and 37b to provide more than 60 db of usable dynamic range . summed video pulse 38 is communicated to tcas processor 12c for analysis and actions as required . in the preferred embodiment , bearing of an intruder at extended range is provided by the directional antenna . however , the only range information provided is the knowledge that the intruder is greater than a certain distance away . in an alternate embodiment , mode s squitters include gps coordinates . in this embodiment , bearing and range of an intruder are determined by comparing intruder gps coordinates with the tcas equipped aircraft gps coordinates . fig4 shows an alternate embodiment of the invention which uses an active filter to replace several components of fig3 . in this embodiment antenna 10 , rf amplifier 21 , local oscillator 22 , mixer 23 , and if amplifier 24 are identical to the preferred embodiment in fig3 . however , the dual filters and mux &# 39 ; s of fig3 are replaced by an active filter 40 . active filter 40 is controlled by range control 29 to switch between wide bandwidth and narrow bandwidth functionality . a drawback of using an active filter is the increased cost . receiver range sensitivity is a function of signal to noise ratio ( s / n ). when s equals n , tangential receiver sensitivity is established . it is described by the equation : with the establishment that all factors remain constant except b , noise n ( i . e . e 2 ) is reduced in proportion to bandwidth while s remains constant . therefore , when filter bandwidth is halved , noise power is reduced by the power ratio or decibel reduction of : because signal ( s ) remains constant and noise ( n ) is reduced , a 3 db improvement in s / n at near tangential sensitivity is established . this translates into a linear range improvement factor of 1 . 41 since the range increases proportionally to the square root of the power or s / n improvement . the method of the invention follows from the apparatus . a tcas having a dual bandwidth receiver as described above is provided . tcas radio signals are filtered , for a first time period , through the normal bandwidth filter and not through the narrow bandwidth filter so that all transponder spontaneous transmissions and replies are received and processed by the tcas system . tcas radio signals are filtered , for a second time period , through the narrow bandwidth filter such that the signal to noise ratio is improved for desired transponder spontaneous transmissions and the effective range of said tcas system is increased . the narrow bandwidth filter is selectively switched into and out of the receive path in accordance with a predetermined algorithm such that as air traffic density increases the second time period is reduced . this description has been for descriptive purposes only and is not intended to limit the scope of the invention . those skilled in the art recognize numerous alternate embodiments of the invention which deviate from the described embodiment but still perform the same work in substantially the same way to achieve substantially the same result and are therefore equivalent to the invention . it is clear from the foregoing that the present invention represents a new and useful method and apparatus for extending the range of tcas .