in the second alternative method the third method may use a technique for using the relative timings of the occurrences of quadrature nulls for determining direction that is analogous to using the timings of quadrature nulls for determining transponder motion as described in u s pat no 5 510 795 as the phase of the lo reference signal is swept quadrature null conditions are observed at the outputs of the three homodyne receivers at different times since the inserted phase on the lo signal corresponding to a 180 degree phase shift corresponds to the change in phase between quadrature null conditions on one channel comparison of the timings of quadrature null conditions on the three channels provides a measurement of the relative phase between the channels for example if the phase of the lo signal is swept from 0 to 180 degrees and the difference in timing between quadrature nulls between antennas a and b corresponds to u00bc of the sweep time the phase difference between the signals received by antennas a and b is 45 degrees 180 4 once the phase is known the direction to the transponder is calculated by using the equation this method does not require linear amplifiers a d converters or phase meters the added complexity of a phase shifter over the foregoing methods provides an actual value of the angle u03b8 in addition to the direction from the reader in a fourth alternative method of the present invention the transmit receive properties and equations are.