## 1. Introduction

[2] It is well established that the conjugate beam technique developed by *Vincent and Reid* [1983] is an unbiased estimator of momentum flux of wind fluctuations under the assumptions that the second order statistics of the wind field are horizontally homogeneous. The technique has been used by various workers to estimate momentum flux of wind fluctuations [*Fritts and Vincent*, 1987; *Worthington and Thomas*, 1996; *Chang et al.*, 1997; *Fritts et al.*, 2006]. But it is only recently that attention has been paid to estimate the errors associated with the method. The effect of geophysical noise in the measurement of momentum and heat flux has been addressed by several workers [*Tao and Gardner*, 1995; *Thorsen et al.*, 1997; *Gardner and Yang*, 1998; *Riggin et al.*, 2004; *Dutta et al.*, 2005]. *Kudeki and Franke* [1998] showed that the dominant contribution to the uncertainty of momentum flux estimates scales as the geometric mean of the horizontal and vertical wind fluctuation variances and to obtain statistically significant measurements of momentum flux, long integration times are necessary. *Riggin et al.* [2004] estimated the momentum flux of short period wind fluctuations together with the estimation uncertainties using tropospheric and stratospheric wind data obtained at Jicamarca. They observed day-to-day variability of the flux profiles and felt that stationarity assumption could be violated during integration periods approaching or exceeding a day. *Dutta et al.* [2005] used wind data collected by Gadanki MST radar to estimate momentum flux of high frequency wind perturbations. A careful error analysis showed that the statistical estimation error becomes almost irreducible after about 15–16 hours of integration and that the stationarity assumption was not violated during this period.

[3] *Thorsen et al.* [2000] extended the work of *Kudeki and Franke* [1998] by including the influence of measurement noise and finite spatial correlation of the wind fluctuation on the statistical error in the dual coplanar beam technique. They derived a generalized form for the variance of the Vincent and Reid momentum flux estimator. Assuming a functional form for the spatial correlation and using typical values for the temporal and spatial correlation length scales, they showed that there is an optimal beam separation angle that minimizes the statistical estimation error and for typical mesopause parameters this beam separation is approximately ±13°.

[4] It is observed that there has been no study to simultaneously examine the effect of temporal and spatial (beam separation) stationarity on the statistical estimation error. The present work is motivated by the theoretical framework of *Thorsen et al.* [2000] and aims at examining the dependence of estimation error of momentum flux of gravity waves on radar beam angles and time of integration in the troposphere. The paper also brings out the importance of incorporating correction factors in the radial winds measured by MST radars, which get affected due to the anisotropy of VHF radio wave scatterers in the atmosphere.