Currently the precision of SLR varies from better than a centimeter
for the best instruments to a few centimeters for a subset of
systems operated by international partners. Possible systematic
measurement errors arise from atmospheric propagation and system
hardware effects. Estimates of these errors are in the 0.5 to 1.5 cm
range [ Degnan, 1985,1993]. Various error sources including
non-linearities in the tracking electronics as a function of
signal strength, errors in the distance to the calibration targets,
together with remaining spurious effects, are current system
limitations. Nevertheless, 
1 centimeter absolute accuracy for the best
SLR systems is achieved. Within the next few years, millimeter
level accuracies may be realized with two-color systems under
development [ Degnan, 1993] and other ongoing upgrades described in
Eichinger et al., [1992].
Lasers provide the most accurate and unambiguous range measurements
for orbit positioning on an observation-by-observation basis.
SLR-based geodesy has benefited from two recent achievements. The
most important is improvements in laser tracking hardware
[4]
allowing for
the centimeter level accuracy described previously. Second is the
expansion of the global SLR network, which, together with improved
system accuracies, has enabled the laser data to contribute directly
to improving orbit force models. The development of ancillary
force, environmental, and measurement models is closely coupled with
the improvement in the gravity field and has enabled the exploitation of
SLR data near their noise level. All these factors play an important
role in the more accurate modeling of SLR measurements within
orbit determination solutions.
These advances are verifiable through the reduction in residual variance seen directly in SLR orbit solutions. Nerem et al., [1994a], Tapley et al., [1994b] and Lerch et al., [1993a, 1993b] all compared the level of SLR data fits as geopotential modeling improvements are made. Lerch et al., [1993c] demonstrated that errors in the gravity models can be effectively calibrated via subset geopotential solutions by predicting the change in SLR residual variance with that predicted by solution covariances projected into the space of the SLR data. Understanding the physical basis for the remaining SLR signal within orbit solutions has driven a considerable enterprise to evaluate, validate and where warranted, incorporate a whole host of small orbit forces within current solutions.
The coincident maturation of SLR, GPS, and DORIS tracking
capabilities provides a unique opportunity.
[4]
With unprecedented
tracking overlap on several missions (e.g., T/P supports tracking
from SLR/DORIS/
[4]
GPS/Tracking and Data Relay Satellite System
[4]
(TDRSS); two
GPS satellites, GPS-35 and 36, carry laser retroreflectors),
independent orbit comparisons have been used to assess and better
understand the capabilities of each network. Improved reduction
strategies for each system's observations have also resulted. By
acquiring observations concurrently from common sites, and through
the simultaneous tracking of common satellites, unification of
networks within the International Terrestrial Reference Frame (ITRF)
has progressed significantly [ Boucher et al., 1992], with SLR tracking
of the Lageos satellites providing the principal connection to
the geocenter. All of these comparisons demonstrate that
excellent agreement across technologies for site positions (
1 cm) and
their velocities (
1-3 mm/yr) has been achieved. Himwich et al., [1993]
and Ray et al., [1991] compared SLR and Very Long Baseline
Interferometry (VLBI) station positioning; Watkins et al., [1994]
extended the comparison to site velocities. Heflin et al., [1993]
presented GPS site position and velocity comparisons with VLBI.
Pavlis [1994] evaluated orbits determined using the SLR data acquired
on GPS-35 which will be used to further unify the SLR and GPS
networks within a common reference frame.