GNSS Time Series Notes

Ground Deformation Parameters

Daily solutions are available as data extracts and plots using FITS data services.

Output from FITS Downloaded Data and Plots

For each site, 3 CSV files (named site_[e/n/u].csv) are available, containing the difference with respect to the East, North and Up (NEU, WGS84 ellipsoid) reference position:

• the first column is the date-time of the observation in ISO8601 format, UTC time zone;
• the second column is the difference (dN, dE, dU) from NEU reference position (in millimetres);
• the third column is the dN, dE, dU standard deviation (in millimetres).

These three plots show the east, north and up components of GPS station position, evaluated once per day.

The use of these plots is to show the variation in station position with time. The plots are a derived product of the basic RINEX GPS data provided by GeoNet, and are for indicative purposes only. Any scientific interpretation of the time series plots should only be done in consultation with someone with appropriate expertise.

In the absence of nearby earthquakes or slow-deformation events, these should be nearly straight lines. Variations about a straight line show the uncertainty (or noise) in the measurement of station position from a single day of GPS data. Large departures from a straight line usually indicate some problem in the data processing; these problems are likely to be fixed if you view the plots again in a few days or weeks.

The values in the plots are in the ITRF2008 reference frame, but with a constant value subtracted from each time series so that its mean value is zero. This means that the absolute values in the time series do not relate directly to a position on Earth, but that the slopes of the lines do give velocities relative to ITRF2008.

The plots are updated with new information after each day’s daily processing of the GPS network is complete, which is presently about 20 hours after the end of each UT day.

Quality Control Parameters

avgMP1 and avgMP2

MP1 and MP2 are linear combinations of the code (pseudorange) and phase measurements made by the GPS receiver. Unusual variations in MP1 and MP2, particularly sudden jumps or trends, may indicate data quality problems at the GPS station.

MP1 is a linear combination of P1 (or C/A pseudorange), L1, and L2. MP2 is a linear combination of P2, L1, and L2.

MP1 and MP2 can be calculated throughout the GPS session. In this case, the non-constant part of both combinations is dominated by the respective pseudorange multipath, with very small contributions from the phase multipath.

The values plotted by GeoNet are not the record of MP1 and MP2 throughout the GPS session, but are their RMS values averaged over the length of the session (usually 1 hour for near real time (nrt) data, or 24 hours for daily data). Hence the names avgMP1 and avgMP2.

The values of avgMP1 and avgMP2 are typically a few tenths of a metre. The values are generally different for different receiver types, and may be different for different firmware versions in the same receiver.

The use of avgMP1 and avgMP2 is as a quality control tool to assess whether anything has changed in the environment of the GPS site. If the receiver, firmware and antenna have remained the same, avgMP1 and avgMP2 should remain fairly constant. If the values change it is likely that something has changed at the site. Perhaps the antenna has become partially covered in snow? Perhaps there has been a partial failure of the receiver or antenna? Perhaps a tree has grown up, or a building erected, in the vicinity of the antenna?

The derivations of MP1 and MP2 are summarised in a 1999 article: "TEQC: The Multi-Purpose Toolkit for GPS/GLONASS Data" in GPS Solutions, Vol. 3, No. 1, pp. 42-49, 1999 (published by John Wiley & Sons, Inc.).

GNSS Processing

Before making any scientific interpretation of GPS time series data, one should have a full understanding of at least the following items:

• how the data are collected;
• equipment or other changes at a site that may affect the data;
• the data processing;
• any filtering or other adjustment of the time series.

GeoNet provides site metadata, including equipment changes, in the DELTA database and as IGS (International GNSS Service) format sitelogs and XML files on the GeoNet data site.

GeoNet makes every attempt to ensure that the information in DELTA is correct and transcribed accurately to the RINEX headers, and that the same information is used during GPS processing.

GPS daily processing using GAMIT/GLOBK software

The GPS processing is done using the GAMIT/GLOBK software developed and maintained by the Department of Earth Atmospheric and Planetary Sciences of the Massachusetts Institute of Technology (MIT). Without going into detail, some of our processing strategies are as follows:

• Solutions calculated each day for data 1, 3, 30 and 90 days behind UTC time;
• Orbits are held fixed and not solved for;
• IGS final orbits for data 30 and 90 days behind;
• IGS rapid orbits for data 3 days behind;
• IGS ultrarapid orbits for data 1 day behind;
• Earth Orientation parameters from IERS Bulletin A values (USNO and USND for most recent solutions);
• Ambiguity resolution by wide-lane and narrow-lane L1 and L2 (LC, ionosphere free) combination;
• Absolute IGS phase center corrections for satellite and receiver antennas from igs08_wwww.atx file (wwww = GPS week of the last file change), provided by MIT;
• Ocean loading from FES2004 grid ocean tide model;
• Atmospheric delay estimated using the "global mapping function" (GMF) [Bohem et al (2006)];
• Atmospheric loading not applied;
• Depending on the increasing size of the network in time, a loosely-constrained solution is done as increasing numbers of clusters, each of less than about 30 stations and 6 tie sites, that are then combined during the reference frame realisation;
• Solution is aligned to ITRF2008, using a 6-parameter Helmert transformation (translation and rotation) onto available and good quality IGS sites in Australia, the south-west Pacific and the Antarctic.

The processing methodology is kept consistent throughout the time series. For further details please refer to the GAMIT/GLOBK documentation.