Objective
1.INTRODUCTION
Several groups are independently investigating the potential of deriving meteorological and climatological-atmospheric data from the detailed analysis of signals received by ground-based precision GPS receivers.
The accurate determination of geographical position involves the interrogation by ground-based equipment of signals broadcast by (at least 4) satellites; least squares estimate of position is obtained from the transmission times.
GPS daily mean coordinate accuracy is now sub-cm in plan and -1-3 cm in height, where a baseline of a few hundred km is used. Most of the additional height error is due to the atmospheric propagation errors.
Ionospheric effects are frequency-dependent, and can be handled by combining two GPS frequencies, L1 about 1 575 MHz and L2 about 1 228 MHz, in a way to eliminate the first order effect.
The troposphere refracts and retards the signal. The delay depends upon the integrated refractivity, which depends upon the densities of dry air and water vapour. The tropospheric zenith delay (TZD, i.e. accumulated along a path through the atmosphere oriented in the zenith direction) appears as the sum of two components:
The zenith hydrostatic delay, which accounts for 90% of the tropospheric error, and can be modelled accurately using the surface pressure and mean temperature, and the hydrostatic equation;
the zenith tropospheric water delay (ZWD) caused by atmospheric water vapour, which is allowed for by using either ground based Water Vapour Radiometers (WVPs) or radiosoundings.
The converse problem is to use known coordinates to calculate the delays as the unknown. That is, the TZD is derived, and the hydrostatic delay removed to obtain the zenith wet delay (ZWD).
Then IWV (the integrated water vapour) = ZWD/k, where k is dimensionless and weighted by mean temperature, 1 kg/m2 of IWV = 1 mm IPWV, the integrated precipitable water vapour (also PWV), and 1 mm PWV approximates 6,5 mm of ZWD.
The process is considered to be accurate to about 1 mm PWV. There are absolute (using networked sensors) and relative (using individual, "secondary" sensors with a reference site instrumented with a water vapour radiometer) techniques.
The system is considered to have a number of advantages as an observational instrument: it is
Portable
Relatively cheap
All-weather
Continuous in operation
Consistent world-wide
Needs few calibration checks.
A typical application of data assimilation could be where NWP models (especially mesoscale and limited area) provide profiles, but there is a great need to calibrate to absolute integrated quantities. The IWV can also be used to throw light on frontal and other synoptic features.
There are technological problems to be solved, which would feature in any COST action investigating the feasibility of a ground-based GPS network. One is the acquisition of data in real time (e.g. for weather forecasting as opposed to climatological work), notably of orbital data and adjustments. There are problems concerning data links and storage, and the automatic processing of the data. The way in which the information is utilized in data assimilation schemes would have to be determined.
There is an IGS (International GPS Service for Geodynamics) Global Data Centre (Europe) at the Institut Geographique National (IGN), in Paris, and IGS Regional (Europe) Data Centre run by the Institute for Applied Geodesy (IfAG) in Frankfurt, and two IGS (European) Analysis Centres at CODE (Centre for Orbit Determination Europe) run by Astronomical Institut - University of Bern (AIUB) in Bern and Geo Forschungs Zentrum (GPZ) in Potsdam. All of these can provide valuable data for use within this COST Action.
It is known that experimental networks exist in various countries, and some of these are indicated in section 6.
2.OBJECTIVES OF THE ACTION
Objectives
The primary objective is:
Assessment of the operational potential on an International Scale of the exploitation of a Ground Based GPS system to provide near real time observations for Numerical Weather Prediction and Climate Applications.
Secondary objectives are:
Development and demonstration of a prototype ground-based GPS system on an international scale
Validation and performance verification of the prototype system
Development and demonstration of a data exploitation scheme for NWP and analysis of data exploitation techniques needed for climatic applications
Requirements for operational implementation ground-based GPS system on an international scale.
3.CONTENT OF THE ACTION
The Action will be divided into four projects. An early task of the management committee will be to refine the content of the projects, establish the links between them and establish the overall timetable. The projects are:
State of the art and Product Requirement
Demonstration
Applications
Planning for the operational phase.
4.TIMETABLE
While the action is formally planned to last 5 years, in practice delays in establishing the management committee and the project group of experts leads to the more realistic 4-year working time
5.ORGANIZATION MANAGEMENT AND RESPONSIBILITIES
A Management Committee would be set up following the appropriate number of Signatories to the Action Memorandum of Understanding. A chairman would be elected and instructed to draft an outline Action plan based upon the items specified in Section 3 (The content of the Action). This plan would be approved by the Action Management Committee and submitted to the Technical Committee for Meteorology.
The Action would include cooperation between European meteorological organizations and research institutes.
The Action will consist of four projects. The Management Committee will elect coordinators for each project. The scope and content of the projects will be reviewed and if necessary amended by the Management Committee during the first year, in the light of the inventories of existing methods and the current interests of participation in the Action.
The Action Management Committee would report annually to the COST Senior Officials through the Technical Committee for Meteorology, but would provide short verbal or written reports to each meeting of the Technical Committee. The Management Committee will meet at least twice per year.
Two workshops including all projects are foreseen during this Action. The projects may have their specific workshops as seen appropriate. A detailed Final Report would be written based on a series of technical reports produced by projects throughout the period of the Action.
6.ECONOMIC DIMENSION OF THE ACTION
The table below shows the typical level of activity for ground-based GPS receiving systems. Most of the equipment has been put in place for Geodetic proposes. However several countries are also looking at data exploitation for climate and meteorological purposes and the staff column represents the expected level of activity. Typical equipment costs are about ECU 35 000 per installed receiver plus an annual cost including communications of ECU 10 000. Staff costs are assumed to be ECU 5 000/mm this gives an investment of about ECU 3 million and annual cost of about ECU 1,6 million.
Programme(s)
Topic(s)
Call for proposal
Data not availableFunding Scheme
Data not availableCoordinator
Sweden