an error in the north alignment will shift the azimuth data and a timing error will cause the data to be delayed or time tagged at a wrong time there are also various errors in axis or sensor tilting that can be important for sensors with low measurement noise such as axis tilt north east antenna roll as well as antenna pitch the axis tilt errors may cause data to appear to high or low in north south and east west directions respectively an antenna pitch error will make data appear at a too high or too low elevation and an antenna roll error will shift data clockwise or counterclockwise at high elevations mode c data will give incorrect altitudes as they relate to a standard air pressure the selection of the bias parameters may of course be different e g the sensor position bias parameters may be expressed in a coordinate system other than east north for each sensor s a vector of bias parameters b s is introduced whose components are the parameters e g range offset etc to be estimated the vectors corresponding to the different sensors are not necessarily of the same type and may subsequently have different sizes and different components it is convenient to stack all the b s vectors into one large bias vector b b 1 b 2 u22ee 1 a.