  Reaching the thermal noise at low frequencies with the next generation of
instruments (e.g. SKA, LOFAR etc.) is going to be a challenge. It requires the
development of more advanced techniques of calibration compared to those used
from the traditional radio astronomy until now. This revolution has slowly
started, from self-cal, going through field based correction and SPAM up to the
formulation and application of a general Measurement Equation. We will describe
and compare the several approaches of calibration used so far to reduce low
frequency data. We will present some results of a 74 MHz VLA observation in
exceptional ionospheric conditions of the giant radio galaxy 3C326 for which
some of these methods have been successfully applied.
