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GNSS data products, post processing and real time services and their applications in the context of

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a set of core AFREF stations (the 1st order network) ... Review closures and adjust baseline network ... Do it yourself or... – PowerPoint PPT presentation

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Title: GNSS data products, post processing and real time services and their applications in the context of


1
GNSS data products, post processing and real time
services and their applications in the context
of AFREF
  • AFREF Technical Workshop
  • 10-13 July 2006
  • UCT
  • Ludwig Combrinck

2
AFREF Products
  • Products that do not necessarily require real
    time data
  • Coordinates (the MAIN product)
  • Coordinate velocities
  • Meta information
  • Products that do require real time data
  • Differential correction services (DGPS and RTK)
    perhaps an AFRICA DGPS based on cell phone
    /internet technology
  • Total electron content maps
  • Integrated water vapour maps for operational
    weather forecasting
  • For crustal motion (Earthquakes, volcano
    eruptions)
  • Early warning systems (e.g. Indian Ocean Tsunami
    Early Warning System)

3
How to get to products
  • Need to define
  • a starting epoch for the AFREF datum (e.g 1
    January 2008) and the AFREF specific geodetic
    datum (the set of parameters that describe the
    size and orientation of the earth etc.)
  • a set of core AFREF stations (the 1st order
    network)
  • a processing strategy to obtain ITRF coordinates
    for the core stations
  • A procedure/centre to maintain the core AFREF
    stations (positions, velocities, data integrity,
    metafiles etc.)
  • A set of guidelines for users of AFREF to enable
    them to use the products efficiently and
    accurately (AFREF cookbook)

4
And then
  • Strategies within the various SG depts to utilise
    the AFREF products
  • Campaign surveys
  • AFREF densification (2nd order network)
  • Network adjustments/transformations

5
Baseline Solutions/Network Position Determinations
  • 1st order network
  • A processing strategy needs to be decided upon
    before processing data
  • For instance, a selection of global IGS core
    stations must be selected and included in a
    global solution to compute AFREF ITRF positions
    and
  • careful thought must be given to the weighting of
    stations
  • 2nd order network
  • GPS baseline solutions are generated through an
    iterative process.
  • From approximate values of the positions occupied
    and observation data, theoretical values for the
    observation period are developed.
  • Observed values are compared to computed values,
    and an improved set of positions occupied is
    obtained using least-squares minimisation
    procedures and equations modeling potential error
    sources.
  • Observed baseline data are also evaluated over a
    loop or network of baselines to ascertain the
    reliability of the individual baselines.

6
Typical flow of process to measure baselines from
AFREF core station/s to local new Datum point
  • Download AFREF core station data precise
    ephemeris logfile from AFREF data centre
  • Download baseline data from receivers (fixed
    stations and campaign data)
  • Download and prepare all other necessary input
    data such as station heights, antenna models
  • Make changes and edit raw baseline data (e,g,
    reject short observational periods etc)
  • Use AFREF core station as fixed with its
    position as at AFREF Datum epoch and process all
    baselines
  • Review, inspect, and evaluate adequacy of
    baseline reduction results
  • Make changes and reject bad baselines
  • Reprocess baselines and re-evaluate results
  • Note/Designate independent and trivial baselines
  • Review closures and adjust baseline network
  • End Product a set of ITRF coordinates relative
    to AFREF Datum (say 1 January 2008).

7
How to do the processing (or with what)
  • On relatively short baselines, up to several
    hundreds of kilometres, commercial, off the shelf
    software (Trimble, Ashtech etc.) will probably be
    fine
  • However, for longer baselines, say 500 km , one
    should rather use software that adequately
    incorporates earth tide modelling and other
    effects on position
  • It will depend on your processing and network
    occupation/reoccupation strategy (e.g.
    selfcentring plates on trig beacons) and levels
    of accuracy required of your new network

8
Unless GIPSY (JPL software) is the processing
software, carrier phase differencing will be
used (GAMIT etc.)
The results of the Triple Difference baseline
solution can then be input back into the Double
Difference equations in order to resolve, or
"fix," the integers in the Double Difference
solution. Fixing the integers in a Double
Difference solution constrains the integer
ambiguity N to a whole number of cycles, and
is the preferred baseline solution
9
Commercial Software (Trimble, Ashtech etc.)
  • Baseline processing software is now fairly
    automatic and user-friendly.
  • Allows a processing strategy to be set up
  • Most software automatically performs all the
    differencing operations needed to solve for
    integer ambiguities
  • Displays the resultant baseline vectors along
    with adjustment and accuracy
  • Generate reports and statistics that can be used
    to evaluate the results.

10
Non-commercial scientific software (including
Bernese)
  • BERNESE, developed by the University of Bern,
    GAMIT developed by MIT, GEODYN by NASA, GIPSY by
    JPL, TOPAS, Germany, GPSOBS by ESA, TEXGAP by
    Univ. Texas, GEPHARD by GFZ, Germany, etc.
  • Are not very user friendly
  • Could need advanced knowledge of operating
    system, Fortran compiler etc.
  • Mostly need knowledge of C-shell scripts
  • Good background in processing strategies and most
    of the time, experience
  • So, bottom line for this is, a steep learning
    curve and time invested, but is worthwhile

11
Solution Acceptance Criteria
Evaluation of results is important, as they
vary and you may have to query (or reject)
measurements
12
Residual Plots typically the L1 phase residual
error is plotted
Residual plots depict the data quality of the
individual satellite signals, typically vary
around 5 mm from the mean. Deviations exceeding
15 mm are suspect. Multipath problems may show
up on the residual plot as a sinusoidal wave
over time and is best minimised by good site
selection, choke ring antennas, and long session
times which allow a change in satellite geometry.

13
Do it yourself or
  • We could look towards establishing a processing
    centre for AFREF which would take your data,
    process it and give you the AFREF coordinates
  • It depends on what is realistically achievable in
    each country within the time frame you have set
    yourself

14
Real time products
Could be added to global WADGPS systems such as
Starfire (accuracy about 35 cm horizontal)
15
Starfire reference network
Source NavCom Technology Inc.
16
Could be used for
17
and share the vision..
18
Africa sparse contribution at this time
19
SourceStreaming Real-Time IGS Data and Products
Using NTRIP, Georg Weber IGS Workshop, May 08-11,
2005, Darmstadt, Germany
20
In terms of AFREF
  • The sky is the limit
  • But we have to get beyond the talking point. and
    start doing something. soon!

The End
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