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IMPROVING GHANA’S GEODETIC
REFERENCE NETWORK FOR SOCIOECONOMIC DEVELOPMENT
Yaw Poku-Gyamfi
CSIR-BRRI
Outline
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Ghana Geodetic Survey before the GRN
Why the GRN
Establishing the GRN (Phase 1)
The benefits
Future of the GRN (Phase 2)
Socio-Economic Benefits
History of Ghana’s Geodesy
• Field-Astronomy observations by F.G Guggisberg in
Accra in June 1904, observation of 15 stars
• Nov-Dec 1904, exchange of telegraphic signals from
Cape Town to determine our Longitude
• Connected to Leigon datum GCS 547
• Ellipsoid of reference was the War Office 1926 Ellipsoid
• In 1977 Ghana attempted using the Clarke 1880
Ellipsoid in order to join most African countries but the
result was not put into the public domain although we
have some monuments in this ellipsoid in the system
CLASSICAL REFERENCE SYSTEMS
• Ghana used the War Office (1926) and Clarke 1880 (Ghana
modified) Ellipsoids
• Parameters defining these ellipsoids were realized using
Classical Geodesy, star observation, derivation of longitude
using telegraphic signals etc.
• Unique parameters for most countries
• Committee on Development Infrastructure (CODI) in July 1999
stated that: ‘National governments are requested to transform
their respective national datums to the worldwide WGS84 and
International Reference System (ITRS) including the
determination of the geoid’. ([2] Establishing a Continental
Reference System in Africa – AFREF, March 2002, Proposal to
ICSU)
• LAP’s decision to establish ITRF in Ghana is therefore a step in
the right direction
LOCAL AND GLOBAL ELLIPSOIDS
Local
Ellipsoid
Global
Ellipsoid
A Unified Continental Geodetic
Reference Frame (AFREF)
Why GRN (1)
• Move away from the classical geodesy to satellite
geodesy
• To utilize the GNSS signal from space which keeps on
improving with the modernizations
• Improvement of receiver technology and the
downward trend of the cost
• To join the global community in a common reference
system to enhance cross-border land related
activities by using the international Reference frames
Why GRN (2)
• Confusion in using a system running on two
ellipsoids as a country
• Will not need any rigorous mathematical
transformations when using the Global
Navigation Satellite Systems (GNSS)
• To enhance Land Administration and delivery as a
nation
• To benefit from the ever increasing applications
of GNSS.
International Terrestrial Reference
System (ITRS)
• ITRS is on the contrary is gaining popularity
due to its consistent improvement with the
increase in the number and quality of the
input observations, processing methods.
• This global geocentric coordinate system is
very similar to WGS84 of GPS, the GTRF of the
future Galileo and the other GNSS‘s.
IMPLEMENTING GRN
• Monumentation
• Observation
• Data Processing
– Long Baselines
– Short Baselines
– Satellite Stations
• Network Adjustment
Composition of the GRN
• Continuously Operating Reference Stations
(CORS)
• Second Order Control Stations
• Control Center
GRN (Phase 1) Coverage
• Covers the Golden Triangle of Ghana
• Three Permanent Stations located in Accra,
Kumasi and Takoradi
• Two Hub Stations at Kade and Assin Fosu
• Fifteen New Reference Stations distrubuted
within and around the GT
• Additional CORS at Bolgatanga, Tamale,
Savelugu, Donkorkrom and Winneba
•
Nationwide GNSS
Reference Network
Design for Ghana
–
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Phase 1
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GRN Data Processing
Establishment of Geocentric Coordinates
• Long Baseline Processing
– Selection of IGS Stations
• Maspalomas (MAS1), Spain and N‘Kolatang Libreville
Gabon (nklg)
– Data processing
• GTCE module of the PrePos GNSS Suite was used as the
Scientific Software
• Considerations
– Station velocity, Antenna Eccentricity, Ocean Loading, Solid
Earth Tides, Satellite Eccentricity, Tropospheric Kalman Filter
etc
Some IGS Stations in Africa
GRN Data Processing – Short Baseline
• Short Baseline Processing (Reference Stn,
tightly constraining the PS and HS)
– SEMIKA module of PrePos GNSS Suite
• Atmospheric correction (Iono and Tropo), cycle slip
detection and repair, satellite eccentricity iono-free LC,
Klobuchar model for Iono correction. Interactive
optimization using 3D viewer
New stations
• Area covered with in Phase 1 of the GRN
2.9 Datum Transformations
Geodetic datum
Minimum set of parameters required
to define location and orientation of
the local system with respect to the
global reference system/frame
09/00
Y'
Z'
εz
εY
b
Datum Transformation
Translation of the origin ∆X, ∆Y,
∆Z
Rotation angles εX, εY , εZ
Scale factor µ
Change in ellipsoid ∆a, ∆f
Z
a
εX
X
X'
Y
DGPS Concept
• Requires 2 receivers a base and a rover
• Base located on an known point
• The set of receivers track the same satellite
vehicles at the same epoch
• Error at the base station is computed and
corrected w.r.t. the known coordinates
• This correction is used to correct the biased
observation of the rover
Assumption:
• Within a limited base length, the correction is
considered identical
Drawback:
• The distance of the rover to the base is usually
less than 10 km, and is even worse in low
latitude and tropical country like Ghana
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DGPS Concept
Problem
Accuracy of single GNSS station for many purposes inadequate
Remedy
• Certain GPS error components are similar for users not separated
• The error components are slowly varying with time
• Usually, the coordinates of the reference station are precisely
known
• Total measurement error at the reference station can be
determined for each satellite
• Taking the measured errors into account leads to an
improvement in accuracy for the user station
• Effective quality control by monitor stations feasible
Improved Solution:
The use of a reference network
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Network RTK concepts
GRN Phase 2
• Phase 2 of the GRN will include
– Covering the whole of Ghana with CORS at
relatively shorter inter station distances
(More that 50 stations have been proposed)
– Some selected areas in the country will have
Active network (Network RTK)
Reference Station Networks
Idea: Sufficient coverage of reference stations for a whole country
Benefit: User will no longer need two GPS receivers, but take advantage of a
common reference station
Situation:
S3
Rover
S2
S1
Problem: Due to their different corrections user will get different navigation
results!
Solution: Network RTK concepts (FKP, MAC and VRS)
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FKP and VRS
Area Network Corrections (FKP)
• Linear error description using a number of reference stations
• Correction for every site within the net provided
Virtual Reference Station (VRS)
• All reference stations of a network are connected to a server
• providing a database of regional corrections
• User sends a approximated position (generated by code
• measurements) to the server
• Correction values are derived for approximate position
Assessment
VRS more accuracte, but requires a bi-directional communication
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Expected Socio-Economic Benefits
Thank You