97 
APPLICATIONS OF GEOSAT ALTIMETER DATA IN THE NORTH-EAST PACIFIC 
J. F. R. Gower, and S. Tabata, 
Institute of Ocean Sciences, P.O. Box 6000, Sidney, BC, V8L 4B2, Canada 
Tel: (604) 356-6558, Fax: (604) 356-6479, Telex: 049-7281 
ABSTRACT 
The Geosat altimeter has successfully mapped sea surface height, ice, waves and winds 
during the 5 years of its life (1984-1989). We present two examples of the use of this data in 
the north-east Pacific. Consistent sea surface height anomalies with space scales of order 
100-300km, are observed at 17 day intervals. Anomaly amplitudes are in the range 10 to 30 
cm, with anomaly displacement velocities of about 1 cm/s. These altimetry results are 
compared with the more precise, but less frequent data from ship hydrographic and CTD 
measurements. Examples are also shown of wave-height maps derived from Geosat data. 
The spatial coverage pattern from a single satellite is insufficient to follow the pattern of 
waveheight growth and decay induced by the global surface wind field, but gives valuable 
statistical data on the long term average wave climate. 
1 GEOSAT ALTIMETER DATA 
A satellite altimeter measures the travel time of pulses 
emitted vertically downwards, and reflected back to the 
satellite by an area of the earth’s surface typically a few 
kilometers across. Accurate knowledge of the satellite’s 
position and of the pulses’ propagation speed, allows 
measurement of the absolute height of the earth’s surface. 
Over the oceans, the mean flatness of the sea surface on 
kilometer scales, and the symmetry of height offsets 
introduced by surface waves, allows a mean range to be 
determined with a precision of a few centimeters. 
Such a precision gives important oceanographic data on 
ocean circulation and on smaller scale motions, since the 
geostrophic balance of these water movements on a rotating 
earth requires changes in the mean ocean surface height. 
These height variations are in the range 10 to 30cm for 
mesoscale eddies, and up to 100cm at the edges of intense 
western boundary currents. Horizontal scale of the height 
changes are of the order of 100km. Oceanic applications of 
satellite altimeter sea surface height data have been 
demonstrated by Cheney et al. (1983), Fu and Chelton 
(1985) and Tai et al. (1989). 
The US Navy Geosat satellite was designed to provide 
classified information on the shape of the geoid. This has 
not been made available to the non-military research 
community. After completion of these measurements, the 
satellite orbit was altered to duplicate that of the Seasat 
mission from which similar data had already been made 
available in 1978. After November 1986, Geosat was held to 
within 1km of this nominal orbit and provided a sequence of 
global altimetry data which has been widely distributed. 
Some results from this mission were recently reported in a 
special journal issue introduced by Douglas and Cheney 
(1990). 
This is the so-called Exact Repeat Mission (ERM) which 
began in November 1986, in which Geosat repeated its orbit 
pattern every 17.05 days. In this period, 244 satellite orbits 
covered an oblique grid with an equator crossing separation 
of 164km. This coverage pattern is defined by ascending 
node longitudes 1.05°E + 360n/244 for n = 1 to 244, orbit 
inclination of 108.04° and an orbital period of 6037.55s. 
2 OBSERVATIONS IN THE N.E. PACIFIC 
The present study area is shown in Figure 1. Eddies were 
tracked in the two areas outlined by rectangles. 
Waveheights were studied in the full area 0 to 60°N, 90 to 
180°W. For the eddy tracking use was made of ascending 
orbits which crossed this area with a mean spacing of about 
90km as shown. These correspond to n = 141 to 173 in the 
formula for ascending node longitudes given above. Similar 
descending orbits also crossed this area approximately 
orthogonally to those shown. However, much of the data 
from descending orbits were unusable due to attitude 
instability of the satellite after crossing arctic Canada, and 
information from them is not considered here. 
Figure 1. Map of the study area showing North America and 
the Hawaiian islands, the positions of Geosat 
ascending orbit tracks for n = 141 to 173 and 
rectangles outlining areas where eddies were 
tracked, west of California and in the Gulf of 
Alaska.