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Conventional methods in horizontal drilling processes incorporate magnetic surveying techniques for determining the position and attitude of the bottom-hole assembly (BHA). This results in an increased weight of the drilling assembly, higher cost due to the use of nonmagnetic drill collars necessary for shielding the magnetometers, and significant errors in the position of the drilling bit. A novel inertial navigation system (INS)-based technique has been previously proposed as an alternative to magnetometer-based downhole surveying. Previous studies have shown theoretically that an adaptive-filter-based in-drilling alignment (IDA) fine alignment method successfully limits the error growth associated with INS. This study aims at examining IDA's practical feasibility and, specifically, its ability to estimate the azimuth angle. Experimental testing of the IDA method was conducted under laboratory conditions with an apparatus that can easily be adopted for downhole conditions. The experimental results demonstrate that the IDA-estimated azimuth is more precise, compared with the one estimated by conventional magnetic surveying systems. The high accuracy and implementation simplicity of the proposed INS-based surveying system render it a preferred method for future horizontal drilling operations.