The SDR rates are synthetic rates derived to provide a neutral means of monitoring currency fluctuations in order to revise, if necessary, local currency fares and charges. Every currency in the ICH database is expressed as an SDR base rate. Every new rate expressed in SDR is compared to the base rate and if there is a variation exceeding 3% during 20 consecutive days, the system will calculate a new base rate. SDR rates are reviewed on the 15th and the last day of every month.