GPS Meteorology
USER REQUIREMENTS
Version 0.1 (1st draft)
January 7, 2000
Prepared by
S.J.M. Barlag (KNMI)
GPS Meterorology URD
Revision 0.1
COST716/URD/0.1
1 Introduction
1.1 Purpose of the Document
This document is a first attempt to summarise the user requirements of the European
meteorological and climate user communities with regard to meteorological data derived
from the Global Positioning System (GPS) signals received on ground. These user
requirements are gathered under the COST716 action, work package 3, meteorological
and climate applications. This document serves as a reference for further updating and
revision. The document is in the first place intended for members of the COST716
Project Team, who can use it to check available GPS network products against the
evolving User Requirements and thus make any adjustments to meet them. This
document also serves as an interface between the COST716 participants and the
representatives of the GPS networking communities. The applicability of this document
terminates by the end of the COST716 project.
The requirements stated in this document apply to the expected mode of operation of
GPS ground based networks, including operations of receiver equipment, network
connections between receiver stations and regional processing centres. The expected
operation mode of the network is characterised by the following:
 No satellite or ground network anomaly impacts on the on-ground processing
 The intra-network data flow and data production operate at the planned capacity and
efficiency
 ……..
1.2 Applicable and Reference Documents
1.2.1 Applicable Documents
The following list contains documents with a direct bearing on the contents of this
document.
[AD.1] COST716 proposal
[AD.2]
[AD.3]
1.2.2 Reference Documents
The following documents provide some supplementary or background information, and
could be helpful in conjunction with this document.
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[RD.1] CBS Working Group on Satellites, Second Session, 15–19 April 1996,
Final Report. WMO, 1996
[RD.2] Preliminary Statement of Guidance How Well Satellite Capabilities Meet WMO
User Requirements in Several Application Areas, Technical Document,
WMO/TD No. 913
[RD.3] Simulated 1D-variational assimilation of ground based GPS measurements of
total zenith delay, M.Higgins, UKMO Forecasting Research, Technical Report
No. 285
1.3 Acronyms and Abbreviations
CBS
EC
EUREF
GCM
GLONASS
GNSS
GPS
GTS
HIRLAM
IGS
IWV
NASA
NMS
NWP
POD
PWV
TBC
TBD
TZD
UTC
Commission for Basis Systems (WMO)
European Community
General Circulation Model
Globalnaya Navigatsionnaya Sputnikovaya Sistema (GLObal Navigation
Satellite System, Russia)
Global Navigation Satellite Systems (generic name for GPS +
GLONASS)
Global Positioning System (USA)
Global Telecommunication System
High Resolution Limited Area Model
International GPS Service
Integrated Water Vapour
National Aeronautics and Space Administration (USA)
National Meteorological Service
Numerical Weather Prediction
Precise Orbit Determination
Precipitable Water Vapour
To Be Confirmed
To Be Determined
Total Zenith Delay
Universal Time Co-ordinated (in practice same as Greenwich Mean Time)
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WMO
WWW
WWW
Revision 0.1
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World Meteorological Organisation
World Weather Watch (WMO)
World Wide Web, component of the Internet data access system
1.4 Identification of Requirements
The requirements in this document are uniquely identified as follows:
XXX-nnnn
where XXX represents the requirements group identifier (products) and nnn is the
requirement number. The following group identifiers are used:
TOP
HGM
RTV
OLV
Top level generic, covering all products and services.
Hourly GPS Meteorology product
near-Real Time Validation product
Off-Line Validation product
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2 General Description
Basic user requirements for ground-based GPS meteorological products have been
gathered from WMO documents summarising the needs of several user communities for
meteorological products [RD.1]. Feedback from some COST participants has been
obtained, while that from other participants is solicited through this document. Also the
characteristics of geodetic reference networks (IGS, EUREF) will be considered. In the
future the user requirements will be refined with the help of COST User Workshops.
2.1 Characteristics of GPS ground based network systems
Ground-based GPS receivers can provide continuous information on integrated water
vapour at a site. Several networks of GPS receiver stations exist world-wide. The main
objective of the IGS network is …. Other ground based networks exist for regional
geodetic survey purposes (e.g. EUREF). These provide for the best possible information
on GPS receiver positions through differential processing of GPS signal information
received at two or more stations. [reference]. Tropospheric parameters in the form of
total zenith delays (TZD), also called zenith path delays (ZPD), are derived on a routine
basis by the stations in these networks as corrections to the pseudo range.
The TZD can be decomposed in two parts: the hydrostatic delay, due to the dry
components of the atmosphere, and a wet delay due to the dipole moment of the water
vapour in the atmosphere. [reference] The hydrostatic delay can be approximated using
an estimate or an exact measurement of the surface pressure. The wet delay can then be
determined by subtraction of the dry delay from the total delay. The error of the hence
determined wet delay is about 4 mm on a measurement of magnitude 2-3 m, thus less
than 1%. The total water vapour content (IWV or PWV) along the zenith path, equal to
the integrated specific humidity in the column, can be derived from the wet delay by
using the mean atmospheric temperature. An estimate of this mean temperature can be
made using an empirical formula containing the surface temperature. Used in this way,
co-incident measurements of surface pressure and temperature yield values of column
specific humidity with accuracy of order 4 mm on a total content of order 40 mm for
temperate regions.
[Here, further discussion of the operation modes of the ground based equipment]
The GPS ground based receiver equipment will …
The sampling rate of the carrier phase, code phase, signal amplitude and navigation
measurements is … 50 Hz.?.
[Here, further discussion of the operation modes of the processing centres]
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2.2 Generic User Requirements for Meteorological Humidity Data
The COST716 action has identified several classes of users, as noted in [RD.1]. For the
purposes of the GPS meteorological network development, we present user requirements
for just two major classes of users – operational meteorology (NWP) and climate.
2.2.1 Operational Meteorology
The User Requirements for operational meteorology presented in this document have
been derived from documents and information presented on the Web site of the World
Meteorological Organisation (WMO).
A summary of the requirements is shown in Table 1. It should be stressed that these
requirements are generic, and independent of any particular observing system.
Horizontal Domain
Horizontal Resolution1)
Time Resolution2)
Absolute Accuracy3)
Timeliness
Column Specific Humidity
Global / Regional
50-100 km / 10-50 km
1-6 hrs / 1-2 hrs
5000 g/m2 / 1000 g/m2
1-3 hrs / <1 hr
Table 1. Generic User Requirements for Operational Meteorology
Notes:
1) Horizontal resolution in this context means the average horizontal sampling
separation distance, and is related to typical NWP model grid spacing.
2) Time resolution is the effective sampling interval at the same location.
3) Column Specific Humidity is usually expressed in g/m2 or alternatively as the
equivalent height of the water column in mm (1000 g/m2 = 1 mm).
The table explicitly includes users requiring data for regional an/or mesoscale NWP
models. In general, the requirements are very similar except that horizontal resolution
better than 50km and timeliness better than 1 hour are preferred.
We should note here that User Requirements do not represent a hard cut-off value of
'good/no good'; rather there is often a broad range of acceptability. Where a low tolerance
is given, data may have marginal use (impact); data exceeding a high tolerance may be
over-specified, as the additional 'quality' cannot be exploited by the application.
2.2.2 Climate
Table 2 illustrates in a similar way the user requirements for the climate community, in
particular those for climate monitoring and prediction where trends in past and future are
analysed. The requirements for 'climatology' are very similar, except that long-term
means are taken and stable, low biases are demanded. For many of the requirements, if
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the requirements for operational meteorology are met, those for climate applications will
be too.
Column Specific Humidity
Global
1° x 1° to
2.5° x 2.5°
> 10 years
4/day -> 1/hr
< 1000 g/m2
200 - 400 g/m2 /decade
1-2 months
Horizontal Domain
Horizontal Resolution
Time Domain
Time Resolution
Absolute Accuracy
Long Term Stability
Timeliness
Table 2. Generic User Requirements for Climate Monitoring and Prediction
2.3 User Requirements for GPS Meteorology Network Products
From the above generic requirements for total column humidity data, it is clear that the
primary goal of the GPS Meteorology Networks should be to provide geophysical (Level
2) products, which meet these. However, from [RD.3] is clear that non-geophysical
(Level 1b) products may be a preferred deliverable of the networks for many users.
2.3.1 Operational Meteorology
The main difference from Table 1 is the introduction of total zenith delay. The horizontal
resolution reflects the expected spacing of global and regional networks.
Horizontal Domain
Horizontal Resolution
Time Resolution
Absolute Accuracy
Timeliness
Column Specific Humidity
Global / Regional
< 300 km / < 100 km
1-6 hrs / 1-2 hrs
5000 g/m2 / 1000 g/m2
1-3 hrs / <1 hr
Total Zenith Delay
Global / Regional
< 300 km / < 100 km
1-6 hrs / 1-2 hrs
5 mm / 1 mm
1-3 hrs / <1 hr
Table 3 Generic GPS Meteorology Network Requirements for Operational Meteorology
2.3.2 Climate
Climate users have expressed no direct interest in using zenith delays; they would either
use time- and spaced-averaged individual water vapour columns, or use 3D analysed
fields, which have assimilated GPS ground-based meteorology data via NWP systems.
Table 4 reflects the averaging requirements for any realistic network of GPS ground
receiver equipment.
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Column Specific
Humidity
Global
2.5° x 2.5°
> 10 years
1 day
???
Horizontal Domain
Horizontal Sampling1)
Time Domain
Time Resolution
Absolute Accuracy
No. of observations/grid
box/day based on random
accuracy
Long Term Stability
Timeliness
> 40
0.2-0.4 mm /decade
1-2 months
Table 4 Generic GPS Meteorology Requirements for Climate Monitoring and Prediction
Notes:
1) This represents the sampling/averaging box size for atmospheric profiles.
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3 Specific Requirements
3.1 Product Description
The (TBD) processing centres of the GPS meteorology network will provide Column
Specific Humidities, together with supporting data for each (TBD) of the stations in the
network. These will be know as 'Level 2' products. The supporting data include at least
time, location, receiver altitude, pressure and temperature at receiver location (if
measured or else their estimates (TBC)) and quality information. In addition, the Level 2
product will also contain Total Zenith Delays and their error covariances derived from
the network solution (TBC). The product domain will be restricted to the European
(TBC) region.
Level 2 products will be available in near-real time (i.e. within 3 hours of observation
time).
The geographical and temporal coverage of GPS meteorology products will be limited
only by the characteristics of the GPS network and not by the proposed processing
algorithms. Working assumptions are based upon EUREF practices (TBC).
HGM-00xx Hourly GPS Meteorology products shall contain both TBD Level 1b
parameters and all required Level 2 parameters
TOP-00xx Total Zenith Delays shall be given in units of mm (TBC)
TOP-00xx (Surface) Pressures shall be given in units of Pascals (Pa) or 10Pa (TBC by
error propagation calculations)
TOP-00xx Temperatures shall be given in units of degrees Kelvin (K)
TOP-00xx Total Specific Humidities shall be given in units of g/m2 (TBC)
TOP-00xx Receiver altitudes shall be given relative to a reference surface ( WGS84?,
TBD) in units of TBD
TOP-00xx The latitude and longitude of the receiver shall be given in 0.01 degrees
TOP-00xx The observation shall be date- and time-tagged appropriate to the location,
and shall include year, month of year, day of month, hour and minute.
TOP-00xx Quality information shall be provided, both as error estimates for all derived
quantities, and as product Q/C data (flags) (TBD)
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3.2 Validation
3.3 Interfaces
HGM-00xx Hourly GPS Meteorology products shall be available to users via TBD
interfaces
TOP-00xx Archived products shall be available to users via TBD interfaces
3.4 Dissemination
Dissemination concerns the delivery of products to users.
HGM-00xx >90% of Hourly GPS Meteorology products shall be disseminated to users
within 3 hours of observation time.
HGM-00xx Hourly GPS Meteorology products shall be disseminated via GTS (TBC) and
other TBD links
RTS-00xx Hourly GPS Meteorology products shall be disseminated using WMO BUFR
(FM94) encoded formats (TBC)
3.5 Archiving
Archiving concerns the saving of processed products and its subsequent extraction.
Especially climate users have an interest in extracting and reprocessing the data or to
have the archived data reprocessed. To produce useful and consistent time series and to
make any reprocessing feasible each observation should be accompanied by all relevant
meta-data. These meta-data should at least include the type of meteorological station (or
method of estimation) used to produce P and T, the TZD solution from the network
processing and its error covariances, and an error estimate for the humidity product to 1%
level (TBD).
In case reprocessing is required addition of the following information to the archived
products should be considered:
 Ocean tide model
 Building information for multi-path
 Orbit processing (JPL)
 Environment information in general
TOP-00xx The GPS Meteorology products shall be archived on a 2-4 daily basis (TBC)
TOP-00xx The GPS Meteorology products shall be archived in full RINEX format
(TBC)
TOP-00xx The GPS Meteorology products shall be archived with no time expiry limit
(TBC)
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TOP-00xx Archived products shall be capable of extraction, with no degradation to the
original product quality, on user request
3.6 Maintainability
3.7 Portability
Portability refers to the ability of the processing code used at processing centres – at
source level – to be implemented at arbitrary sites, which may, or may not, be the same
computing platforms.
TOP-00xx Software for processing Level 1b to Level 2 GPS Meteorology products shall
use a standard high-level programming language (e.g. C, F90, F77)
TOP-00xx Software shall as far as is practicable use only the standard features of the
programming language, avoiding compiler-specific extensions
TOP-00xx Software shall be capable of being ported to a wide variety of host platforms
without requiring significant modification to source code
TOP-00xx Software shall be coded and documented to agreed standards (TBD)
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