SPECIAL
STUDY GROUP 1.180:
"GPS AS AN ATMOSPHERIC
REMOTE SENSING TOOL"
Introduction
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.
Members
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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