1. Thomas Herring, IERS ACC, MIT
Improving the standards for IERS products and homogenizing technique- specific standards
Thomas Herring, IERS ACC, MIT

2. Overview
Consider areas where SLR/VLBI/DORIS and GNSS could ensure consistency of models
Examine parts of the geodetic positioning problem
Target observed
Propagation signal
Ground geodetic reference location
7/12/17
UAW 2017

3. Target Observed
VLBI: Extragalactic radio sources although in the future Earth orbiting (and other nearer objects) could be possible
Other services: Earth orbiting vehicles but dominant force model errors tend to be different.
GNSS: Radiation forces (solar, albedo, transmission thrust and thermal re-radiation). Attitude of space critical and not directly reported. Short orbit integrations. 20,000 km
SLR: LAGEOS I/II Exotic, slowly rotating. Longer orbit integrations km
DORIS: Low Earth orbiters. Many different effects. Some are also tracked with SLR (as are some GNSS) and GNSS receivers on some satellites
All share the gravity field but sensitivity very different. Need for consistent time-variable gravity field. GNSS can truncate to very low degree and order.
General relativity effects on orbits (GNSS Central force only and maybe J2).
SLR makes measurements to GNSS and DORIS satellites, and GNSS receivers on some DORIS satellites. More coordinated comparison campaign? Currently falls between the services and could be a GGOS task.
7/12/17
UAW 2017

4. Signal propagation
SLR: Optical not sensitive to ionosphere and low sensitivity to water vapor. Needs updating for changing concentrations of CO2 as it increases.
VLBI (2/8 GHz), GNSS (1.2/1.5 GHz), DORIS (0.4/2 GHz) are microwave systems sensitive to the ionosphere and water vapor concentration.
Commonalities: Hydrostatic delays although the microwave systems do not “worry” about this terms as much as SLR because of sensitivity to water vapor which needs to be estimated.
Unify the non-water vapor dipole refractivity between optical and microwave (e.g., CO2). Evaluation of any new refractivity formulas
Water vapor dipole term: Evaluation of any new work on refractivity and emission.
Higher-order ionospheric corrections. Comparison of methods used by different services.
Relativistic effects on propagation. Near Earth observations should have similar effects compared to extra-terrestrial signal propagation. (Impact on clocks need to be considered as well).
7/12/17
UAW 2017

5. Ground reference point
VLBI and SLR: Tend to be geometric location based on motion of telescopes as they track objects. (Range bias in SLR also has impact; absorbed on clock estimates in VLBI).
DORIS and GPS: Electrical ”phase center” that needs to be calibrated. Major issue addressed at IGS AC workshop particularly with respect to interaction between antenna and environment.
In GNSS: Multiple calibration methods (robotic arm, anechoic chamber) but call for in-situ calibration
For all systems: All ground stations have large motions with respect to ”fixed coordinates” e.g. ~30 cm due to solid Earth tides. Question now is what motions are “well known” and should be removed from coordinates to allow for easier interpretation of geophysical signals.
Tides: Clearly but small residual effects from ocean and atmospheric thermal tides.
Loading effects: Consensus on what should be applied and source (e.g., limited time range of GRACE). Ensure consistency between orbit modeling in positions
Pole tide: Whole session on this topic
7/12/17
UAW 2017

6. Conclusion Discussion: How do we move forward?
Reaching consensus within services is difficult between services even more so.
Reason: Differences in assessments of how large an impact different models have and what deficiencies can be absorbed in “nuisance” parameters
Effort in coding and validating implementations of new models
Effort to make model test data sets and possibly codes easily available to aid in implementation and validation (e.g., codes available for some of the IERS conventional models).
How critical is diversity of models? How do we assess when a model is definitive?
7/12/17
UAW 2017