addition to observational capabilities from the polar 
platforms, the Eos program will be supplemented by 
moderate-cost Earth Probe missions, each of which will be 
characterized by a choice of orbit and spacecraft design 
capable of achieving a particular objective. 
CANADIAN PLANS FOR Eos 
Long-term Plans 
There are currently a number of research initiatives within 
various Canadian Government Departments aimed at 
addressing the role of satellite observations to give a better 
understanding of the Canada’s northern environment 
including the land and ice components, and the surrounding 
oceanic areas. Two major initiatives are particularly worth 
mentioning: 
(a) As an associate member of the European Space 
Agency, Canada is a contributor to the ERS-1 mission. 
Access to the ERS-1 data is a major reason for Canada’s 
participation in the ERS-1 program. National requirements 
for ERS-1 have been derived from a number of 
requirements based on the extension of remote sensing 
programs within various government departments (notably 
Canada Centre for Remote Sensing (CCRS), Canada Centre 
for Surveying (CCS) and the Geological Survey of Canada; 
Atmospheric Environment Service (AES); and Fisheries 
and Oceans), and interested industry and university groups. 
The national objectives represent four major areas: oceans, 
land resources, geology and ice, as well as specific 
scientific and application priorities. Overall, Canada’s 
highest priority is for regular guaranteed acquisition of 
SAR imagery, as well as ERS-1 Radar Altimeter, 
Scatterometer and Along Track Scanning Radiometer 
sensor data. Canada plans to operate two ground stations 
for acquisition of ERS-1 SAR image data, as well as global 
low bit rate data such as altimetry. These data and fast 
delivery products will be distributed to Canadian users 
using national data networks. 
(b) In a similar cooperative effort, an Eos interdisciplinary 
program for use of a Cryospheric System (CRYSYS) to 
monitor global change in Canada has recently completed its 
definition phase (McNutt et al., 1988). At the time of this 
writing, pending final NASA approval, CRYSYS will soon 
enter its ten year-long execution phase (Simard et al., 
1990). There are several objectives for this work which 
follow from the preparatory work for ERS-1 and Canada’s 
own RADARSAT satellite programme; namely: 
• to continue to develop and implement algorithms 
necessary to extract geophysical information related to 
the cryosphere, to validate the results of regional 
models, and to use the regional information to initialize 
and validate polar models; 
• to maintain an extensive array of field sites in critical 
areas; 
• to access information from other related databases, 
especially in Canada, so that a complete record of 
information (including historical, operational and future 
products) on cryospheric processes can be obtained; and • 
• to use this information in models to examine the effects 
of global change as manifested in cryogenic processes, 
Figure 1 - Satellite Altimetry Mode Geometry 
and to deliver this information for use, via the Eos Data 
Information System (EosDIS), in the verification of 
global climate models. 
Current Developments in Satellite Altimetry 
Satellite altimetry is a technique that uses a microwave 
radar pulse sent from a satellite orbiting at a height of 
approximately 1000 km (cf. Figure 1). This pulse interacts 
with the sea surface and part of the incident radiation 
reflects back to the satellite. The two-way travel time of the 
pulse reflected from the ocean (or ice) surface is measured 
with great precision, the equivalent of a few centimeters 
(< 8 cm for the ERS-1 altimeter) in one-way path length. 
From these observations, near-surface wind and significant 
wave height can be determined from the power and the 
shape of the returned signal. The marine geoid is recovered 
by carefully editing the sea surface height data, and 
correcting for time dependent phenomena such as ocean 
currents, tides, and seasonal variations. In an inverse 
procedure, the latter can also be extracted and studied. 
Sea ice can be a problem when using satellite altimeter data 
to recover precise geoid and gravity field information. 
Nevertheless, special algorithms which utilize the altimeter 
signal’s scattering signatures can yield information about 
the presence of ice within the altimeter’s footprint, as well 
as infer the position of ice shelf edges, ice-sheet averaged 
surface gradients and thickness. During the ERS-1 mission, 
large portions of the arctic areas overflown by ERS-1 will 
be ice-free during late summer and early autumn. This 
means the Canadian experiments scheduled then will 
provide for the first time large amounts of much anticipated 
altimetry data in the near-polar regions. 
The altimetry measurements for oceanographic purposes, 
useful for monitoring time-variable oceanic events of 
interest to global environmental monitoring, pose quite 
stringent requirements. For instance, measurements of 
time-variable surface currents require observations with a 
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