GPS Data

GPS (Global Positioning System) can determine positions on the Earth's surface to millimetre precision. High-precision GPS solutions reveal the speed and direction in which our country is being deformed.

Trimble NetRS GPS Receiver.

The GPS data are available via anonymous ftp from ftp.geonet.org.nz/gps. Raw GPS data in RINEX format are in the rinex directory, and time series of GPS station positions are in the solutions directory. The GPS mark codes may be viewed on the network maps, and the site locations and operational dates may be found in DELTA.

DELTA also contains information (metadata) on equipment changes at each site; some of this information is not available to external users, but much of it can be accessed by using GeoNet's GPS web services.

The RINEX data and associated metadata are the primary GPS data supplied by GeoNet. The time series are provided for indicative purposes only, and should not be used for scientific purposes without expert advice. For additional information on the preparation of the time series, see GPS processing notes.

File formats

RINEX

A description of RINEX file format may be found at The University of Berne.

The RINEX data on our ftp site is in Hatanaka-compressed format, with a filename extension like .YYd.Z, where YY is the 2-digit year. Tools for converting Hatanaka-compressed RINEX to standard RINEX (filename extension .YYo) can be found at ftp://terras.gsi.go.jp/software/RNXCMP/.

Solutions

The .plt files are formatted thus:

• The first column is epoch seconds - the number of seconds since January 1, 1970, 00:00:00 GMT.
• The second column is defined:
  • For stations in the LINZ PositioNZ network, the values are millimetres relative to the official NZGD2000 position of the GPS station;
  • For other New Zealand stations, the values may either be relative to the NZGD2000 position of the GPS station, or relative to the first point in the time series (a long-term aim is to have all values relative to the NZGD2000 position);
  • For stations outside New Zealand, which do not have NZGD2000 coordinates, the values are relative to the ITRF2000 position evaluated at 00:00 UT 1 January 2000.
• The third column is the uncertainty in millimetres (if applicable).

The values in the main solutions directory are the output of the daily processing, which includes orientation of each daily solution into the IG00b reference frame. Some “outlier” data points, which are far different to the neighbouring data points, are removed from the plots using an automated procedure. In the event that a real ground-deformation episode is currently underway, this sometimes means that the data points at the end of the time series, which should show the deformation, have been deleted as outliers.

The values in the regional-filtered directory (below the solutions directory) have been processed to remove a “noise” signal that is more-or-less common to all GPS sites in New Zealand. This “common-mode” signal may be due to the use of slightly erroneous satellite orbits, to regional-scale or global-scale mass redistributions (in the ocean, atmosphere or groundwater), to the use of non-optimal models in the daily processing, and perhaps to other causes.