1. The International Terrestrial Reference Frame (ITRF) needs to be made more robust and stable over multi-decadal time scales.

1
Geodetic Observations - Recommendations

• 1. The International Terrestrial Reference Frame
  (ITRF) needs to be made more robust and stable
  over multi-decadal time scales.
• target accuracy is 0.1 mm/yr in the realization
  of the center of mass of the entire Earth system
  (geocenter stability),
• 0.01 ppb in scale stability, 0.6 mm/yr
• This requires strong connections between the
  reference frames of the various geodetic systems

2
Geodetic Observations - Recommendations

• The strong connections between the reference
  frames of the various geodetic systems require
• a. Implement more sites of every technique with a
  good (even) global distribution
• b. Collocate different techniques
• c. Sustain and evolve support for the scientific
  services (i.e., IERS, IGS, IVS, ILRS, IDS),
• d. Place laser retro-reflectors on all future
  GNSS satellites,
• e. Support Global Geodetic Observing System
  (GGOS) as the new paradigm
• f. Conduct research into understanding biases
• g. Conduct research towards improving
  tropospheric delay models.

3
Geodetic Observations - Recommendations

• 3. Ensure long term stability and consistency in
  the measurements of sea level by altimetry and of
  mass by GRACE-type missions,
• (a) analogues of current altimeter and gravity
  missions be continued indefinitely with
  sufficient overlap between missions, and with no
  gaps between missions
• (b) a world vertical datum be established
• 4. Tide gauges be monitored for height variations
  using continuous GPS systems installed directly
  at the tide gauge
• - additional tide gauges, co-located with GPS, be
  installed in areas with poor tide gauge coverage.
• 5. Research and development of comprehensive
  Earth models that can assimilate all geodetic
  data types (including tide gauge data) that are
  relevant to determining sea level change.
• At the next level, the models should be
  integrated with terrestrial hydrological models,
  cryospheric models, and ocean/atmospheric
  circulation models.
• 6. In order to achieve the goal of determining
  sea level changes at the level of 0.1 mm/yr,
  support for the recommended systems and research
  activities must be brought up to a level capable
  of meeting that goal, and the support must at
  least be sustained for decades.