<< coefficients are periodically updated - the most recent update actually was done mid-2010 and covers the next 10 years. >>
According to the NGDC site, models need to be revised at least every 5 years. A new IGRF model is adopted every 5 years.
"new observations must continually be made and models generated to accurately represent the magnetic field as it is."
This is because the changes are not incremental and there may be local magnetic anomalies or disturbances.
So when the GPS
dies, the best source of mag var is the local chart, provided that it is up to date, if not, it's a good idea to have a copy of Chart 42 (Magnetic Variation) for the current
epoch. The errors of plotting with the wrong mag var can be surprisingly big.
Hopefully the alert navigator will pay attention to the local variation from the GPS before it dies, at the very least to see how much it differs from the chart.