Fig. 1. Tracks of ERS-l Geodetic Phase north of 30
o
S, GeosatiGM south of 30
0
S and
stacked profiles (thick lines) from ERS-l (35-day repeat), GeosatlERM (l7-day repeat)
and Topex (lO-day repeat).
We have recently published a study of the along-track resolution capabilities of stacked
Geosat, ERS-l and Topex altimeters [Yale et al., 1995] using the coherence method
developed by Brammer [1979] and Marks and Sailor [1986]. An overview of the results is
given in Table 1 and Figure 2. Geophysical Data Records were edited, differentiated, low
pass filtered, and resampled at 5 Hz. All available data were then loaded into 3dimensional files where repeat profiles were aligned along-track. The coherence versus
wave number between pairs of repeat profiles (and stacked repeat profiles) was used to
estimate along-track resolution. Two areas were initially selected for coherence analysis:
the equatorial Atlantic (Area 1 in Table 1), a region with high tectonic signal and low
oceanographic noise; and the South Pacific (Area 2 in Table 1), a region with low tectonic
signal and high oceanographic variability. In all cases, along-track resolution is better in
the equatorial Atlantic than in the South Pacific; Geosat and Topex data resolve shorter
wavelengths than ERS-l. To determine the resolution improvement gained by stacking,
the first half of available repeat profiles were averaged independently from the second half
prior to the coherence analysis. Global maps of along-track resolution show considerable
geographic variation. On average globally, the along-track resolution (0.5 coherence) of
Geosat and Topex stacks are approximately the same (24 km) while the resolution of ERS1 stacks are slightly worse (30 km) . However, when equal numbers of repeat cycles are
stacked, all three altimeters have about the same resolution limit of 28-30 km.
T bl 1 S
a e . ummaryo fAl
T kR I· E·
ong- rac
eso utlOn stlmates
AREAl
AREA 2
Global Average
cycle stack stack cycle stack stack cycle stack stack
8
31
8
31
8
31
Geosat
33
26
20
52
38
27
38
29
24
ERS-l
38
26
-
50
33
-
43
30
-
Topex
34
24
19
43
31
23
37
28
22
13
o
S, GeosatiGM south of 30
0
S and
stacked profiles (thick lines) from ERS-l (35-day repeat), GeosatlERM (l7-day repeat)
and Topex (lO-day repeat).
We have recently published a study of the along-track resolution capabilities of stacked
Geosat, ERS-l and Topex altimeters [Yale et al., 1995] using the coherence method
developed by Brammer [1979] and Marks and Sailor [1986]. An overview of the results is
given in Table 1 and Figure 2. Geophysical Data Records were edited, differentiated, low
pass filtered, and resampled at 5 Hz. All available data were then loaded into 3dimensional files where repeat profiles were aligned along-track. The coherence versus
wave number between pairs of repeat profiles (and stacked repeat profiles) was used to
estimate along-track resolution. Two areas were initially selected for coherence analysis:
the equatorial Atlantic (Area 1 in Table 1), a region with high tectonic signal and low
oceanographic noise; and the South Pacific (Area 2 in Table 1), a region with low tectonic
signal and high oceanographic variability. In all cases, along-track resolution is better in
the equatorial Atlantic than in the South Pacific; Geosat and Topex data resolve shorter
wavelengths than ERS-l. To determine the resolution improvement gained by stacking,
the first half of available repeat profiles were averaged independently from the second half
prior to the coherence analysis. Global maps of along-track resolution show considerable
geographic variation. On average globally, the along-track resolution (0.5 coherence) of
Geosat and Topex stacks are approximately the same (24 km) while the resolution of ERS1 stacks are slightly worse (30 km) . However, when equal numbers of repeat cycles are
stacked, all three altimeters have about the same resolution limit of 28-30 km.
T bl 1 S
a e . ummaryo fAl
T kR I· E·
ong- rac
eso utlOn stlmates
AREAl
AREA 2
Global Average
cycle stack stack cycle stack stack cycle stack stack
8
31
8
31
8
31
Geosat
33
26
20
52
38
27
38
29
24
ERS-l
38
26
-
50
33
-
43
30
-
Topex
34
24
19
43
31
23
37
28
22
13