Using networks of ground-based GPS receivers it is possible to observe the integrated water vapour (IWV) and the total electron content (TEC) of the Earth's atmosphere. While at first these parameters were considered a mere nuisance, it is nowadays considered to be an important signal for atmospheric sciences.

Water vapour is one of the most important constituents of the atmosphere. It plays a crucial role in many atmospheric processes covering a wide range of temporal and spatial scales. Furthermore, it is also the most important greenhouse gas and highly variable. Climate research and monitoring, as well as operational weather forecasting, need accurate and sufficiently dense and frequent sampling of the water vapour, to which existing GPS networks could contribute significantly. In order to be of any use for operational weather forecasting, firstly GPS networks must be able to provide integrated water vapour in near real-time (NRT) (with a typical delay of one hour), and secondly GPS observations must be assimilated into Numerical Weather Prediction (NWP) models.

Dual-frequency GPS receivers enable the estimation of total electron content (TEC) along a given satellite-receiver signal path. By combining observations from regional and global networks of continuously operating dual-frequency receivers, parameters describing the spatial and temporal distribution of total electron content can be derived. Such observations of TEC, available globally on a near real-time basis, allow an excellent opportunity for monitoring ionospheric signatures associated with space weather. For example, the development of ionospheric storms can be observed in global patterns of TEC, while small-scale irregularities in electron density (associated with scintillation) can be observed in short-term variations of TEC and/or spectral analysis of GPS phase observations. The website of the Special Study Group 1.180 is http://www.gmat.unsw.edu.au/snap/gps/iag_section1/ssg1180.htm.


Objectives of the SSG 1.180

The focus of the SSG is to explore the issues related to the derivation of water vapour and TEC in NRT using GPS, the assimilation of GPS water vapour data into weather forecasting models, use of GPS water vapour data for climate applications and the integration of GPS-derived TEC estimates and scintillation indices into space weather applications. The main objectives of the special study group are:

Identify key signatures observed in GPS-derived estimates of TEC, as associated with phenomena such as ionospheric and geomagnetic storms, scintillation, travelling ionospheric disturbances, magnetospheric substorms and auroral activity.

Assess methods to quantify the level and nature of ionospheric activity, based on TEC estimates.

Explore key issues related to the feasibility of integrating TEC estimates, and TEC-based indices, into space weather forecasting and nowcasting - such issues include real-time requirements, and the temporal and spatial resolution necessary for reliable detection and prediction of ionospheric phenomena.

Identify key problems in GPS-derived integrated water vapour, as associated with phenomena related to the near field of the antenna, such as multipath and phase centre variations, and local weather (gradients, mapping to the vertical), reprocessing and archiving of data, in relation to climate applications.

Explore key issues related to the assimilation of GPS-derived integrated water vapour observations into NWP models - such issues include real-time requirements, temporal and spatial resolution, slant or vertical delays, temporal and spatial correlation and quality insurance issues.

Assess the potential impact of tropospheric tomography using GPS-estimated slant delays.

The activities of the SSG consist of compiling a database of relevant literature and research groups, and to facilitate discussions of key issues though email between members, and describe the products of the research through periodic progress reports. Due to the large number of meetings, sessions and symposia in relation to the work of the SSG it was not necessary to organise a special working group meeting.



 Hans van der Marel (Co-chair, THE NETHERLANDS), Susan Skone (Co-chair, CANADA), Helen Baker (UK), Michael Bevis (USA), Steven Businger (USA), Galina Dick (GERMANY), Mark Falvey (NEW ZEALAND), Manuel Hernandez-Pajares (SPAIN), Per Hoeg (DENMARK), Tetsuya Iwabuchi (JAPAN), Mark Knight (AUSTRALIA), Tony Mannucci (USA), Christian Rocken (USA), Akinori Saito (JAPAN), Peter Stewart (CANADA), Rene Warnant (BELGIUM).


Activities of the SSG1.180

TEC Estimation and Monitoring
Networks of permanent GPS receivers are an excellent tool to compute the Total Electron Content (TEC) of the ionosphere. The International GPS Service (IGS) has set up an Ionospheric Pilot Project in June 1998, involving several International Associate Analysis Centers (CODE, EMR (NRCAN), ESA, JPL, UPC). Estimates of TEC are available on a daily basis in the form of IONEX files. Special campaigns were organised during the solar eclipse in August 2000 and during the solar maximum in 2001 involving high-rate (1ssec) observations of many GPS receivers.

The precise determination of TEC in real-time is important for DGPS and GPS-RTK applications with the closest reference station at several hundred kilometres. Several improvements of ionospheric models with GPS have been made involving tomographic and adaptative approaches.

A real-time ionospheric TEC model for the Australian region, based on a network of semi-codeless receivers extending from Northern Australia to the Antarctic, has been developed by the Ionospheric Prediction Services (IPS) in Australia. The purpose of this work is to provide broadcast corrections for single-frequency users as part of a proposed Wide Area DGPS system. More recent work has involved the use of GPS to monitor ionospheric disturbances during magnetic storm events, for ionospheric TEC and scintillation monitoring in low, mid and high (Southern) latitudes, including the Antarctic, and the use of GPS to measure the Earth's plasmasphere.

In Canada an ionospheric warning and alert system for Canadian Coast Guard DGPS users was developed.


Ionospheric Scintillation Monitoring and Effects of Scintillations on GPS

The Australian Defence Science & Technology Organisation (DSTO) has been developing models of the effects of ionospheric scintillations on GPS with the intention of quantifying losses in navigational accuracy and acquisition performance. The scintillation model they use is essentially a stochastic model in which the amplitude and phase distribution functions are assumed to be Nakagami-m and Gaussian respectively, and the power spectral densities are assumed to follow an inverse power-law relationship. This is based on measurements taken from numerous sources, in particular the Defense Nuclear Agency Wideband satellite experiment from the 1970s. It is also consistent with the Wide Band Scintillation Model, WBMOD, which was developed by Northwest Research Associates and enables key scintillation parameters such S4  and sf  to be predicted. By linking WBMOD with the receiver performance models, predictions can be made about the likely impact of scintillations on a GPS receiver at a given time and location under a specified set of solar and geomagnetic conditions. In parallel with this work it has been attempted to validate the WBMOD model for the Northern Australia / South East Asia region using a network of Ionospheric Scintillation Monitoring receivers (ISMs which provide S4  and sf  measurements etc.) and semi-codeless NovAtel Millennium receivers (used to measure TEC). These receivers have been in place for several years in locations close to both the magnetic equator and the crests of the equatorial anomaly in Indonesia, Malaysia and Papua New Guinea. This work has compared WBMOD predictions with regional measurements of the diurnal, seasonal and solar cycle variations in S4  and sf. Various groups within these countries have been actively involved with DSTO in this effort.

A high latitude scintillation monitoring network has also been established for Northern Canada.


GPS Radio Occultation Measurements

GPS and LEO satellites are used to carry out radio occultation studies of the ionosphere and for ionospheric tomography to reveal vertical density profiles.

The GeoForschungsZentrum (GFZ) has commenced, together with other research centres of German Helmholtz Society, a new strategic project GASP ("GPS Atmosphere Sounding") using ground-based and space-based GPS techniques for applications in numerical weather predictions, climate research and space weather monitoring. One of the two sub-projects of GASP focuses on water vapour estimation, and temperature and pressure profiles from radio occultation measurements.

Development of a 6-satellite constellation for GPS occultation and space weather measurements (COSMIC) has commenced. An occultation data analysis centre is being developed at UCAR (COSMIC Data Analysis and Archive Center), for the processing of data from COSMIC and other occultation missions.


Use of Ground-Based GPS for Numerical Weather Prediction (NWP) and Climate Research Applications

Requirements for the use of ground-based GPS for Numerical Weather Prediction (NWP) have been formulated by the European COST-716 project "Exploitation of Ground Based GPS for NWP and Climate Applications". The upper limit for the latency of the GPS data is 1h 45m. Also, it has been established that it is best to use Zenith Total Delays in NWP applications, without converting to Integrated Water Vapour first. It is expected that GPS may improve the forecast of precipitation under certain conditions.

To gather experience with a NRT system, and to assess the quality of tropospheric estimates in the framework of the GASP project, a small test network of ten GPS receivers was installed by the GFZ at the synoptic sites of the DWD in 1999. The NRT network established for the test campaign has been expanded by existing German DGPS sites (SAPOS network) and by an additional 12 GFZ GPS receivers installed at the synoptic sites of the DWD during the year 2000. The total number of sites in the analysis is presently 70, with an expected increase to about 90 sites. A new analysis strategy has been developed to make possible the automatic operation of 100 and more stations, a ZTD estimation interval of 15 minutes, as well as the estimation of gradients.

New Zealand has an operational system in which estimates of PW are obtained with a delay of 1-3 hours (http://www.gns.cri.nz/earthact/crustal/precip/gpspw.html). The website also shows radiosonde and global weather model PW for comparison. The use of GPS PW in mesoscale models was found to positively influence rainfall simulation during a storm observed during SALPEX'96 (Southern ALPS EXperiment).

Several groups have started investigating true real-time water vapour determination. A network of over 100 GPS stations, and the real-time analysis facility for these data to generate PW, called the SuomiNet, is currently being established.


GPS Water Vapour Tomography and Slant-Delay Estimation

UCAR has initiated the development of ground-based GPS slant measurement techniques to obtain refractivity profile and signal bending information from a mobile platform. In Oklahoma a dense 25-site GPS network for water vapour tomography is operating (ARM-Tomography).


List of Meetings Relevant to the SSG 1.180

XXII General Assembly IUGG, July 18-30, 1999 Birmingham, UK (HM, GD)

COST 716 Workshop, Soria Moria, Oslo, 10-12 July 2000. (HM)

COST 716 Management Committee and Working Group Meetings. (HM, GD)

ION-GPS'99, Nashville, USA, September 1999. (HP, MK)

GPS'99, Tsukuba, Japan, October 1999. (HP)

PLANS 2000, San Diego, USA, March 2000. (HP)

EGS'2000, Nice, France, April 2000. (HP)

IRI workshop 2000, Warsaw, Poland, July 2000. (HP)

ION-GPS'2000, Salt Lake, USA, September 2000. (HP)

AMS meeting Albuquerque Jan 2001 (CR). Special session on GPS slant and Special session on SuomiNet

COSPAR meeting, Green Bay, Taiwan, Sept. 27-29 2001 (CR). Special meeting on COSMIC mission.

URSI meeting Boulder, CO, Jan 2001 (CR). Special Session on GPS remote sensing.

IAIN World congress, San Diego, June 2000. (MK)

ION National Technical Meeting, Anaheim, USA, January 2000.

URSI National Radio Science Meeting, Boulder, USA, January, 2000.

S-RAMP Conference (Solar-Terrestrial Energy Program for Space Weather), Sapporo, Japan, October, 2000.

Fall Meeting of the American Geophysical Union, San Francisco, California, December, 2000.

EGS'2001, Nice, France, March 2001. (HM,HP) Special session on GPS Meteorology.

GNSS'2001, Seville, Spain, May 2001.

Beacon Satellite Symposium 2001, Boston, USA, June 2001.

IEEE AP-S International Symposium and USNC/URSI National Radio Science Meeting, Boston, Massachusetts, July 8-13, 2001.

ION meeting SLC, Sept. 2001 (Session on GPS meteorology.

IAG Scientific Meeting, September 2001


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