  We present the results of 16 years of monitoring stellar orbits around the
massive black hole in center of the Milky Way using high resolution NIR
techniques. This work refines our previous analysis mainly by greatly improving
the definition of the coordinate system, which reaches a long-term astrometric
accuracy of 300 microarcsecond, and by investigating in detail the individual
systematic error contributions. The combination of a long time baseline and the
excellent astrometric accuracy of adaptive optics data allow us to determine
orbits of 28 stars, including the star S2, which has completed a full
revolution since our monitoring began. Our main results are: all stellar orbits
are fit extremely well by a single point mass potential to within the
astrometric uncertainties, which are now 6 times better than in previous
studies. The central object mass is (4.31 +- 0.06|stat +- 0.36|R0) * 10^6 M_sun
where the fractional statistical error of 1.5 percent is nearly independent
from R0 and the main uncertainty is due to the uncertainty in R0. Our current
best estimate for the distance to the Galactic Center is R0 = 8.33 +- 0.35 kpc.
The dominant errors in this value is systematic. The mass scales with distance
as (3.95 +- 0.06) * 10^6 M_sun * (R0/8kpc)^2.19. The orientations of orbital
angular momenta for stars in the central arcsecond are random. We identify six
of the stars with orbital solutions as late type stars, and six early-type
stars as members of the clockwise rotating disk system, as was previously
proposed. We constrain the extended dark mass enclosed between the pericenter
and apocenter of S2 at less than 0.066, at the 99% confidence level, of the
mass of Sgr A*. This is two orders of magnitudes larger than what one would
expect from other theoretical and observational estimates.
