incorrect ground track. Station declination is only updated  when a NAVAID is site checked by the governing authority  that controls it, so it is often different than the current  magnetic variation for that location. Using an onboard  means of determining variation usually entails coding  some sort of earth model into the avionics memory. Since  magnetic variation for a given location changes predictably  over time, this model may only be correct for one time in  the lifecycle of the avionics. This means that if the intended  lifecycle of a GPS unit were 20 years, the point at which the  variation model might be correct would be when the GPS  unit was 10 years old. The discrepancy would be greatest  when the unit was new, and again near the end of its life  span.  Another issue that can cause slight differences between  charted course values and those in the database  occurs when a terminal procedure is coded using  magnetic variation of record. When approaches or other  procedures are designed, the designers use specific rules  to apply variation to a given procedure. Some controlling  government agencies may elect to use the epoch year  variation of an airport to define entire procedures at that  airport. This may result in course discrepancies between  the charted value and the value calculated using the actual  variations from the database.  Pilots should be aware that the length of the airborne  navigation database revision cycle could cause  discrepancies between aeronautical charts and information  derived from the database. One important difference  between aeronautical charts and databases is the length  of cutoff time. Cutoff refers to the length of time between  the last day that changes can be made in the revision, and  the date the information becomes effective. Aeronautical  charts typically have a cutoff date of 10 days prior to the  effective date of the charts.  6-17  Issues Related To Revision Cycle 