We present results from the application of a global photometric calibration ( GPC ) procedure to calibration data from the first 2 years of The Two Micron All Sky Survey ( 2MASS ) .
The GPC algorithm uses photometry of both primary standards and moderately bright ‘ tracer ’ stars in 35 2MASS calibration fields .
During the first two years of the Survey , each standard was observed on approximately 50 nights , with about 900 individual measurements .
Based on the photometry of primary standard stars and secondary tracer stars and under the assumption that the nightly zeropoint drift is linear , GPC ties together all calibration fields and all survey nights simultaneously , producing a globally optimized solution .
Calibration solutions for the Northern and Southern hemisphere observatories are found separately , and are tested for global consistency based on common fields near the celestial equator .

Several results from the GPC are presented , including establishing candidate secondary standards , monitoring of near-infrared atmospheric extinction coefficients , and verification of global validity of the standards .
The solution gives long-term averages of the atmospheric extinction coefficients , A _ { J } = 0.096 , A _ { H } = 0.026 , A _ { K _ { s } } = 0.066 ( North ) and A _ { J } = 0.092 , A _ { H } = 0.031 , A _ { K _ { s } } = 0.065 ( South ) , with formal error of 0.001 .
The residuals show small seasonal variations , most likely due to changing atmospheric content of water vapor .
Extension of the GPC to \sim 100 field stars in each of the 35 calibration fields yields a catalog of more than two thousand photometric standards ranging from 10 ^ { th } to 14 ^ { th } magnitude , with photometry that is globally consistent to \sim 1 \% .