Abstract: The measurement accuracy of the groud scatterometer is discussed in this article. Various factors -which can give rise to measurement error of this instrument are analysed. And it is pointed out that the stability of the following radar para-meters is very important for ensuring the measurement accuracy:transmitter power, gain of the receiver, wavelength, pointing corner of the beam, sidelobe levels, and nonlinear sweeping frequency. A method used to define the error which is caused by nonlinear sweeping frequency is developed.