AuScope Geospatial at RSES

Geospatial at RSES

Publications

Introduction

In November 2006, the Australian Federal Government announced $15.8M in funding for geospatial research infrastructure through the National Collaborative Research Infrastructure Strategy (NCRIS) as part of the AuScope capability area. In addition, significant co-investment was forthcoming from universities and State/Territory and Federal government departments. The national geospatial infrastructure to be acquired includes absolute and tidal gravimeters, radio telescopes to be used for geodetic Very Long Baseline Interferometry (VLBI), a continent-wide network of continuously operating Global Navigation Satellite System (GNSS) receivers, Satellite Laser Ranging (SLR) instruments, as well as the associated operating costs for all these components. Funding is provided until the end of June 2011.

RSES is involved directly in the gravity and satellite laser ranging components of AuScope Geospatial, receiving funding for both infrastructure acquisition and operating costs.



Satellite Laser Ranging

Satellite Laser Ranging (SLR) is a space-geodetic technique that allows us to measure the distance between a ground-based laser and satellites orbiting the Earth. The general principle is that short pulses of laser light are fired from the lasers, reflect off special reflectors fitted to the satellites and return to the ground laser. By measuring the time it takes the light to travel to and from the satellite, the distance between the ground laser and the satellite can be calculated.

SLR is used to estimate the orbits of many satellites including satellites equipped with altimeters that measure, for example, the height of the Earth's oceans and polar ice sheets. It is therefore of great importance to estimate as accurately as possible the orbits of these satellites.

The satellite laser ranging (SLR) component of AuScope Geospatial encompasses two components: the upgrade of the existing SLR facility at Mt Stromlo to a system with more power, and the testing of a mobile SLR system. RSES played a significant role in the latter component, using AuScope funding to employ a SLR observer/analyst as well as contributing to the field deployment costs.

The Burnie field campaign

Around 40km offshore from Burnie is the location of what is known as a satellite altimetry cross-over site - a site where the ascending and descending ground paths of the ocean-measuring altimeter satellites cross and therefore, a site where multiple measurements of the sea surface height are made by the altimeters. This site has been used previously by researchers at the University of Tasmania as the only southern hemisphere calibration site for satellite altimetry missions [Watson et al, 2006].

We chose this location to test the accuracy and performance of a mobile SLR system and a 5-month observing field campaign was undertaken from December 2007 to April 2008. Concurrently with the SLR observations, the following measurements were made:

  • GPS observations at a site co-located with the mobile SLR instrument;
  • tide gauge observations of relative sea level at the nearby harbour;
  • GPS buoy observations to estimate directly the ocean surface offshore at the altimeter cross-over location;
  • ocean bottom pressure measurements, using infrastructure provided by the IMOS capability area.

Map showing altimeter ground tracks and the locations of the tide gauge, SLR and GPS sites in the Burnie region.

The French Transportable Laser Ranging System

The French Transportable Laser Ranging System (FTLRS) is .... Under a collaborative arrangement with French scientists, the instrument was sent to Burnie, Tasmania, for a 5-month period to observe directly the distance from the ground to a number of satellites as they passed overhead. Details of the FTLRS can be found here. RSES took part in the field observation for the Australian-French joint campaign of Jason-1 calibration and validation, operating the French Transportable Laser Ranging System (FTLRS) on a daily basis from early Dec 2007 to mid April in Burnie, Tasmania. The City of Burnie is located right under the trajectory of Jason-1’s 88th descending pass and is one of the best calibration sites in the southern hemisphere. About 3km away in the east from the Burnie Port, a continuous operating GPS reference station is located at the Round Hill Light House. This station is remotely operated by the Geoscience Australia based in Canberra, sampling at every 30 second. Upon requests, the MicroZ GPS receiver can be switched to observe at 1Hz rate for kinematic operations such as GPS buoy deployment.

The SLR site was built some kilometres away from the Burnie tidal gauge, in the campus of the Burnie TAFE. A precise spirit levelling was carried out to connect the SLR mark to the Tasmania State Controls and therefore the tidal gauge. The FTLRS was set up on the SLR site in Dec, 2007, for details of the station and the instrument, see http://ilrs.gsfc.nasa.gov/stations/sitelist/BURF_general.html. A Leica 500 GPS receiver system was collocated at the Burnie SLR site, observing and logging twice a minute through out the campaign. The receiver can be pre-programmed to have observation sessions at 1Hz rate. The FTLRS performed well in Burnie in the campaign, which can be evidenced from the Global Performance Report Card evaluated and published by the ILRS http://ilrs.gsfc.nasa.gov/stations/site_info/global_report_cards/perf_2008q1_wLLR.html.

During the campaign, RSES helped to carry out 4 GPS buoy deployments when Jason-1 passed overhead. Each time, two GPS buoy systems were deployed to a selected site in Bass Strait where a mooring system is located in the water (Watson et al, 2003), and three GPS receivers were set up to operate in Stanley, Rocky Cape, and Table Cape respectively. This, together with the two GPS receivers at the FTLRS station and Round Hill, formed a robust control network which is capable to accurately determine the time series of the geodetic heights of the GPS buoys.


The FTLRS in operation at Burnie. Photo: C. Kidd

Results

The SLR observations taken by the FTLRS at Burnie are currently being analysed at RSES using the GINS software [Biancale et al. ????].

Jason, we need to populate this with information about the SLR analysis already done by Francis etc at Grasse, as well as perhaps links to their site(s)?



Gravity

Precise measurement of gravity contributes to the analysis of the time dependent variation of the gravity field near the surface of earth and the study of global change processes.

The NCRIS funding for gravity provides for:

  • The purchase of a precision absolute gravimeter
  • The purchase of one or more precision relative gravimeters
  • The establishment of a building at Mount Stromlo for gravimeter calibrations and intercomparisons
  • The operation of these instruments and the existing Super Conducting gravimeter at Mount Stromlo through to July 2011.

The fieldwork program is being organised through RSES in close collaboration with Nick Dando from Geoscience Australia. It incorporates a regular observation program at a network of short-term observations at fundamental absolute gravity sites throughout Australia as well as longer-term (ie 3-6 month) deployments of the tide meter(s) at specific sites) deployments of the tide meter(s) at specific sites


The FG5 absolute gravimeter operating at Mt Stromlo

Absolute Gravity

A Micro-g LaCoste FG5 absolute gravimeter was purchased and delivered to the ANU in March 2008.

Micro-g LaCoste provided 4 days of training in the setup and operation of the FG5 gravimeter at the ANU during April. Familiarization with the instrument continued with measurements being made at the Mount Stromlo absolute gravity station in the basement of the Commonwealth Solar Observatory building.

Measurements have been completed at existing absolute gravity stations located at the Mount Stromlo Seismic facility vault, National Measurement Institute in Sydney, the University of Western Sydney Werrington, and the Canberra Deep Space Communications Complex at Tidbinbilla near Canberra.

An observation program is being developed to repeat observations at existing absolute gravity sites within Australia. As additional sites are developed by AUScope they will be included in the observation program.

The gravity data from these observations will be processed producing a precise set of time series values for each site.

Relative Gravity

A precise superconducting gravimeter has been in operation since 1997 in the basement of the Commonwealth Solar Observatory building at Mount Stromlo (see details here). The installation has been a collaborative project between the Geodynamics group in the Research School of Earth Sciences and the Japanese National Astronomical Observatory, Mizusawa. Ownership and full operating responsibility for this instrument has now been transferred to the ANU.

This is the most precise instrument available for measuring changes in the Earth.s gravity field. It is essentially a laboratory instrument and not well suited for relocation to other measurement sites. Its prime purpose is to study tidal deformation of the Earth and the structure of the Earth.s interior from measurements over extended periods of time.

A Micro-g Lacoste gPhone portable earth tide relative gravimeter was purchased and delivered to the ANU in June 2008. This instrument has been operating adjacent to the Mount Stromlo superconducting gravimeter since delivery. Familiarization and validation measurements are being carried before deployment to the first remote measurement site late in 2008. The purpose of this instrument is for the study of earth and ocean loading tides at the instrument measurement sites.


Geospatial Publications

The following publications involving RSES staff have utilised the AuScope Geospatial infrastructure:

1

The University Component of the AuScope Geospatial Team, 2008. New geodetic infrastructure for Australia, J. Spatial Sci. , in press