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

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

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

12. Residual Plots; typically the L1 phase residual error is plotted

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

15. Starfire reference network

16. Could be used for�

17. and share the vision�..

18. Africa sparse contribution at this time

20. In terms of AFREF The sky is the limit But we have to get beyond the talking point�. and start doing something�. soon!