The major limitation of a precise local tie is the surveyor’s ability to measure the internal geodetic instrument offsets. For example, for a GNSS/GPS-VLBI co-location, the local tie vector consists of the sum of the following three components: (1) the connection from the GNSS/GPS external reference point to the VLBI external reference point; (2) the VLBI internal offsets; and (3) the GNSS/GPS internal offsets. Segment (1) is the tie between the physically accessible points (or markers) that surveyors would normally measure. Segment (2) is the sum of all effects internal to the VLBI observing and data analysis systems that can introduce biases between the point referenced by VLBI data analysts and the external physical reference point used by surveyors. These include any sort of physical deformation of the VLBI antenna structure (due to temperature or the instrument’s own weight), especially those that cannot be distinguished from true height displacements or tropospheric refraction effects. Segment (3) is the sum of all effects internal to the GNSS/GPS observing and data analysis systems that can introduce biases between the point referenced by GNSS/GPS data analysts and the external physical reference point used by surveyors. These include direction-dependent errors in the signal propagation model due to antenna or radome effects and near-field long-wavelength multipath biases. The estimated uncertainty for each segment is probably no better than 1–2 millimeters; consequently, the overall error would be at best 3 millimeters for the local tie. Similar uncertainties apply to ties with SLR and DORIS. Consequently, technological innovation is needed to improve the ground-based methods for determining local ties and to regularly monitor the ties for changes.
Although terrestrial techniques might be limited by the uncertainty of measuring instruments’ internal offsets, dedicated space missions could provide a prime opportunity for future innovation in this domain. One such space mission currently being proposed by NASA’s Jet Propulsion Laboratory (JPL) is GRASP (Geodetic Reference Antenna in Space). GRASP is a proposed micro-satellite mission dedicated to the enhancement of all the geodetic techniques, with potential to improve the definition of the ITRF, its densification, and its accessibility. GRASP proposes to co-locate VLBI, GNSS/GPS, SLR, and DORIS sensors on a well-calibrated spacecraft (for which internal offsets are measured very accurately), to establish precise, stable ties between the key geodetic techniques used to define and disseminate the ITRF. GRASP also offers a potential solution to another difficult problem—the consistent calibration of the myriad antennas used to transmit and receive the signals of existing and future GNSS/GPS infrastructure. Improving GNSS/GPS signal modeling will benefit all precision applications of these systems. For example, simulations at JPL indicate that GRASP would improve by a factor of three the accuracy of orbit determination of GNSS/GPS satellites, of GNSS/GPS positioning, and of GNSS/GPS-based ITRF determination.
To achieve the GGOS program goals and support future high-precision geodetic science, the ITRF needs to be robust and stable over many decades. Future scientific objectives drive a target accuracy of 0.1 millimeters per year in the realization of the origin of the ITRF relative to the center of mass of the Earth system (geocenter stability) and 0.02 parts per billion per year (0.1 millimeters per year) in scale stability. Achieving this goal will require improving the geographical distribution of the geodetic techniques, especially SLR and VLBI (GPS and DORIS are already well-distributed), as well as continued investment in the analysis of the data generated by those networks. For example, geocenter stability depends on accurate dynamic modeling and observations of the SLR satellites. Scale stability can be improved by minimizing ranging biases for SLR and better modeling of tropospheric refraction and antenna deformation for VLBI.
The anticipated increase in GNSS/GPS satellites over the next decade suggests the strong potential for GNSS/GPS to contribute to both geocenter and scale stability, but a combination of SLR and VLBI will also continue to be required. Currently, VLBI provides the only stable