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Modern Geomatics Technologies and Applications

CΔt
R  (1)
2
Hence the reflected power from the surface which called waveform is the fundamental observation of in SA [4, 22]. However,
in coastal area due to the presence of none-water object and dynamic water waves the altimeter waveform shape deviate from its
standard form and the pregiven gate is no longer valid to define mid-point of leading edge. Consequently, the re-tracking gate
need to be corrected using equation 2.
c
C     (G  G ) (2)
ret
2 R 0
Where Cret is the re-tracking correction,  is the transmitted pulse length, GR is the re-tracked gate and G0 is the pregiven gate[23].
Given the satellite altitude (H) and the essential corrections (ΔC) the SA derived SST is obtainable using Equation 3.
SST=H-(R+ΔC+C ) (3)
ret
Where ΔC stands for a combined correction terms for the influence of the atmosphere and sea state to the SA signal and could
be derived by Equation 4.
ΔC=ΔC +ΔC +ΔC iono +ΔC +ΔC tides +ΔC DAC +ΔC Geoid (4)
dry
SSB
wet
Where ΔC , ΔC wet are the dry and wet tropospheric corrections, ΔC iono is ionospheric correction, ΔC SSB and is the
dry
sea state bias related to pulse interaction with the environment and ΔC tides is the tidal correction including ocean tide, load tide,
pole tide and solid earth tide. ΔC DAC is the Dynamic Atmosphere Correction and ΔC Geoid is the geoidal height correction[24].

 Waveform Re-tracking Approaches in This Study

According to previous studies expressed in introduction, threshold algorithm generally shows better performance
comparing to the other algorithms, therefore threshold 10 to 90 with 10 unit steps have been used in this study. Mathematical
equations this re-trackers is expressed with equations 5 to 8.


N n 2
 p (k)
4
5
A  k 1   1 n (5) p  N 1  P (6)
K
N n
 2 p (k) 5 K=1
4
k 1  1 n
Th  P
Th=P +q(A-P ) (7) , G  (k 1)  k-1 (8)

N
N
R
P  P K-1
K
where A is the amplitude of waveform (using OCOG algorithm), n  1 n  2 4 are the number of gates (bins) affected by
aliasing at the beginning and end of the waveform, N is the total number of bins in the waveform. P N is N is thermal noise,
PK is the waveform power, k is the first gate whose power exceeds the threshold power (Th), q is the threshold value e.g.,
0.1 (10%) and GR is the re-tracked gate is [25].

3..1. Original Waveform Re-tracking

Original waveforms available in Level-1 data (SR_1_SRA) re-tracked using threshold re-tracker has been used as
first approach in this study with equations 5 to 8.

3..2. First Meaningful Sub-Waveform Re-tracking

Costal waveforms are mostly in multi peak form, each peak could be defined as a small waveform itself which is called
meaningful sub-waveform. These meaningful sub-waveforms could be re-tracked by different algorithm to determine best re-

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