  Radio frequency interference (RFI) is the principal factor limiting the
sensitivities of radio telescopes, particularly at frequencies below 1 GHz. I
present a conceptually new approach to mitigation of RFI in interferometric
data. This has been used to develop a software tool (RfiX) to remove RFI from
observations using the Giant Metrewave Radio Telescope, India. However, the
concept can be used to excise RFI in any interferometer. Briefly, the
fringe-stopped correlator output of an interferometer baseline oscillates with
the fringe-stop period in the presence of RFI. RfiX works by identifying such a
pattern and subtracting it from the data. It is perhaps the only purely
software technique which can salvage the true visibility value from
RFI-corrupted data. It neither requires high-speed hardware nor real-time
processing and works best on normal correlator output integrated for 1-10s. It
complements other mitigation schemes with its different approach and the regime
it addresses. Its ability to work with data integrated over many seconds gives
it an advantage while excising weak, persistent RFI unlike most other
techniques which use high-speed sampling to localise RFI in time-frequency
plane. RfiX is also different in that it does not require RFI-free data to
identify corrupted sections. Some results from the application of RfiX is
presented including an image at 240 MHz with a Peak/noise ratio of 43000, the
highest till date at wavelengths >1m.
