Figure 2 - Typical GEOS AT 5-day ground 
track coverage 
precision of a few centimeters along an exactly recurring 
satellite track (cf. Figure 2). Measurements of time- 
averaged currents, such as those produced by the general 
ocean circulation, require even more exact repeating. 
Particularly, they require a knowledge of the geoid over 
wavelengths of 200 to 10,000 km, of the measurement of 
the height of the satellite relative to the Earth’s centre of 
mass, and of the height of the altimeter above the 
instantaneous sea surface, all with an accuracy of less than 
5 to 10 cm. Fortunately, technological developments in 
radar altimeter instrumentation have come a long way since 
the early GEOS-3 and SEASAT altimeters, as shown by the 
improved measurement precision of the GEOSAT altimeter 
and the projected precision of the future TOPEX-type 
altimeters (cf. Figure 3). The uncertainties in the 
knowledge of the geoid and of the satellite’s geocentric 
altitude variations, gained independently, to a level of 
accuracy comparable to the resolution of the altimetry 
measurements, remain as important unresolved problems. 
For the ERS-1 mission, precise laser ranging tracking and 
the experimental PRARE system will be used to provide 
this orbit information. In the future, the satellites of the 
Global Positioning System (GPS) and a few globally 
distributed fiducial tracking sites operating in concert with 
GPS receivers on the altimeter-carrying satellites will 
provide the required sub-decimeter accuracy in the 
determination of the satellite’s orbit. 
Within the framework of the aforementioned initiatives, the 
Geodetic Survey Division, Canada Centre for Surveying 
(CCS) works closely with other agencies in Canada and 
USA to develop and test algorithms to extract information 
from satellite altimetry measurements to monitor oceanic 
variability and to measure surface elevations of the ice- 
covered regions (which in itself is perhaps the most 
meaningful parameter to measure to monitor volumetric 
change of the ice sheets). 
To date, satellite altimetry data from the GEOSAT Exact 
Repeat Mission (ERM) have contributed to the 
improvement of our knowledge of the Canadian marine 
geoid. Operational software is being developed to carry out 
the pre-processing and crossover adjustment of GEOSAT 
data in order to obtain a precise approximation of the sea 
surface heights globally or, in regional solutions, for 
instance, of the North Atlantic and North Pacific oceans, 
Hudson Bay, etc. The methodologies used currently make 
use of altimetry data to: 
• determine the time-averaged long wavelength sea 
surface topography over periods of a year or more; 
• detect the large scale fluctuations of the world’s oceans 
over periods of a few weeks to several months; and 
• estimate the annual cycle in the sea level variations. 
Figure 4 illustrates the present GEOSAT data processing 
flow at CCS. The primary data include GEOSAT 
Geophysical Data Records (GDR) tapes received from the 
U.S. National Oceanic and Atmospheric Administration 
(NOAA) [Cheney et al., 1987] through the Canadian 
Marine Environmental Data Service (MEDS), Ottawa. 
GDRs include merged altimeter and timing data, 
atmospheric corrections data, and orbital information 
(accurate to 3-4 m level, which is relatively large compared 
with the meter-level precision of the SEASAT orbits) as 
generated for the GEOSAT operational needs by the U.S. 
Navy Astronautics Group using a GEM-10 gravity field 
model, Doppler tracking data from only the continental 
U.S. and Hawaiian tracking sites, and other auxiliary 
parameters. Improved GEOSAT satellite ephemerides are 
received (with a few months delay) from NASA’s Goddard 
Space Flight Centre (GSFC). They are generated using 
additional Doppler tracking data from TRANET tracking 
stations overseen by the French Space Agency (CNES), the 
Geological Survey of Canada (at Priddis, Alberta and 
Ottawa, Ontario) and the Royal Observatory of Belgium, 
and the GEM-T1 or GEM-T2 gravity field model (Marsh et 
Figure 3 - The evolution of satellite altimeter 
instrumentation precision 
84 
Figure 4 - GEOSAT Data Processing Flow at CCS