AFREF First Computation: Processing and Analysis
of GNSS Data
The meeting of the Committee on Development Information,
Science and Technology (CODIST)
Economic Commission for Africa
Addis Ababa, Ethiopia
May 2, 2011
By
Elias Lewi
Institute of Geophysics Space Science and Astronomy (IGSSA)
Addis Ababa University (AAU)
ETHIOPIA
1
The internal Structure of IGSSA
Advisory Board
IGSSA Director
Departments (5)
Applied
Geophysics
Atmospheric
& Oceanic
Sciences
Geomatics
Geodesy &
Gravimetery
Centres (2) Administration
Seismology,
Earthquake
&
Geotechnical
Engineering
Space
Sciences
•Director office
•Academic
commission
•Registrar
Remote Observatories of Departments
Instrumentation
Centre
Computational
Centre
Personnel
Finance
Audit
General
service
Purchasing
Store
Top right:
IGSSA’s old
building
Top Left: The Inauguration of the
Geophysical Observatory November
1957 by H.I.M. Haile Selasie I
BottomLeft:
Lidge Kassa W/Mariam, 1st
president of AAU signing guest
book
Shown on the left is Prof. Pierr
Gouin, 1st director of the
Geophysical Observatory
Some of the geodetic work in the Geophysical Observatory
The late Ato Alemu
Mamo
during
a
Geodimeter survey in
1990 GC.
GPS survey in the
Dobi Graben, 1992 GC
A Geodemeter Network by the
Observatory (1969 GC)
Compared to the demand the present day continuously
measuring GNSS stations in Ethiopia are negligible
These 16 GPS stations and possible
future additional stations will play
pivotal role in:
1)
2)
3)
GNSS data from the 12 stations run by IGSSA is
available in the internet to the international
scientific community and even better to
national organizations
In the realization of the
African
Reference
Frame
(AFREF)
numerous infrastructure work
related to development
diverse fields such as:
Research in geodesy
Research in Space science
Research in atmospheric
science
Earth rotation
air- and auto-navigation
Geodynamic
Geography
Defence etc.
International GNSS Stations (IGS) network
ADIS
http://igscb.jpl.nasa.gov/networ
k/site/adis.html
ADIS Station at IGSSA
The station ADIS, at IGSSA, is one of IGS
stations in Africa and it will play pivotal
role in the definition of the African
Reference Frame (AFREF)
Available Ntrip Real-Time GNSS Streams
It is not only one of the rare station in Africa, but also one of the few
that can broadcast data live. The data is available to all, in the
internet after a 4 second latency period.
Results from the GNSS stations
in Ethiopia already proved that
the stations can be used as high
quality reference stations not
only nationally but regionally
and internationally.
IGSSA and the airborne gravity survey of Ethiopia
The aircraft used for the survey
was from Abyssinia flight, which
is an Ethiopian private company
Consortium of international and national
Institutions participated in the survey are
•University of Edinburgh
•Danish Space Centre
•IGSSA
•Ethiopian Mapping Agency
•Ethiopian Ministry of Defence
•Ethiopian Geological Survey
Dr. Addisu Hunegnaw during the
Airborne gravity survey.
The computational facilities at the computational center of IGSSA
Air-conditioning
system
Computational
server “Tekle”
with 128
processors
Data Server
“Kagnew” with
20 TB
memories
Computational
server
“Tatek” with
24 processors
A modern, multi-purpose computational facility and Lecture
room at IGSSA
Goals and Objectives
•
The main Objective of AFREF is the establishment, maintenance and
enhancement of a three dimensional African geodetic reference
frame.
•
The data that emanates from different geodetic stations should be
properly archived, analysed and made accessible for geodetic and
non-geodetic applications
•
Necessary to use receiver and software independent formats.
•
Therefore the main aim of the data analysis centres should be to
provide well monitored products with sufficient accuracy to the enduser, without interruption in receiver and software independent
format.
Goals and Objectives cont.
• It should also target the easy transformation and integration of the
AFREF to other international reference frame
• To accommodate this objective it is necessary to develop appropriate
standards and encourage continent wide data analysis center to adhere
to this standard and conventions.
• On the other hand, these standards should be accommodative enough
to allow the usage of different receivers and software.
Goals and Objectives cont.
• The expected products from the analysis center among others are:

The three dimensional coordinates of reference stations with the
associated accuracies. Here the vertical coordinate will be a
geometric, height until the development of a common orthometric
height system in Africa.

The velocity field of the different stations, with the associated
accuracies. These might need to wait for some few years to get
enough data from some stations.

The determination of a common geoid, at a later stage.
Scope of activity
•The responsibility of these centers is to receive and process primarily,
but not limited, GNSS data from the data centers to produce high
accuracy coordinates and velocities (other products might be included
in the future).
•It is not expected from each processing center to provide all possible
products, i.e., a center may focus on particular objectives of the AFREF.
Proposed processing Scheme
•
Different centres, based on the availability of software, might use
different internationally accepted processing software such as
GAMIT, BERNESE or GIPSY
•
Data should be acquired using Receiver INdependent Exchange
(RINEX) format from AFREF data centres
•
Daily solutions should be computed at each data analysis centre in
a daily basis
•
Specific daily products should be exported to compute the final
weekly solution
•
Products should be updated each week for weekly solutions
•
Final product should be submitted to a selected evaluation and
consolidation centre in the continent using SINEX (Software
INdependent EXchange) format, to produce a unique AFREF
solution. In the future as the computational capacity of many member
nations are strengthened, final products might be released from different
regional centres
Model of processing GNSS data
(AFREF Data Analysis Center)
(Adopted from EUREF Local Analysis Center)
Proposed major processing criteria
•The proposed geodetic datum is ITRF 2005
•Ambiguities should be resolved using QIF strategy
•Elevation cut-off should be set to 50
•Ocean loading using FES2004
•Based on the availability, a minimum of 18 selected common sites
should be fixed for all data processing centres
Call for expression of interest
The AFREF stirring committee calls for expression of interest from any
African Institution to serve as a data Analysis Center independently or
in collaboration with any other international Institution working in the
area. Certain non-African organizations that might already be working
in the continent with the same or similar aim can also express their
interest in collaboration with an African Institution.
Call for participation is currently open, though not limited, for a GNSS
data analysis to a broad range of organizations. The scope of expression
to participate in the AFREF data analysis center may encompass any
degree of support that can be made available by the applicant
institution.
It is expected that the institutions that express their interest are ready to
adhere to the major guideline for a data analysis center, to assure the
quality of the products and their consistency and homogeneity with the
ITRF.
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