10. I used your data with my reduction software and my positions are 30 meters (100 feet, 100 m, etc.) different from yours. What is wrong with your position ?

This problem may result from several causes but the chance the NGS CORS position is seriously in error is very small. First, you must ensure that you are comparing NAD 83 positions with NAD 83 positions or the equivalent. Different datums can introduce over 100 meters in position differences, e.g. NAD 27 to NAD 83. If the datums are the same, are you trying to compare an uncorrected point position (yours) with an NGS published position? An uncorrected point position can be in error by as much as 100 meters (horizontally when SA was on) 95% of the time due to Department of Defense signal dithering. If that is not the problem, have you checked to ensure that the reference position you think you are using in your computations is actually being used by the reduction software? The positions in the RINEX files are ONLY APPROXIMATE and may be tens of meters or more in error. You must enter into your reduction software an accurate position from the appropriate coordinate file. There are other possible problems, but these are the most obvious and most common.