Blarel and Legresy, proceedings of the ESA-CNES symposium on altimetry Venice, 2012.
INVESTIGATIONS ON THE ENVISAT RA2 DOPPLER SLOPE CORRECTION
FOR ICE SHEETS.
Fabien BLAREL and Benoit LEGRESY
LEGOS/CTOH, 14 Avenue E. Belin, 31400 Toulouse, FRANCE.
[email protected], http://ctoh.legos.obs-mip.fr/
ABSTRACT:
The Centre for the Topography of Oceans and the
Hydrosphere (CTOH), the Altimeter Data Service of the
LEGOS laboratory validates ENVISAT RA2 altimetry,
in particular over Antarctic and Greenland. We
investigate the stability and reliability of different
corrections for the altimetric measurements. With the
previous processing, the ICE Validation chain of the
CTOH detects some problems with the Doppler slope
correction over the Cryosphere. Here we present
validation of the latest re-processing (v2.1) of
ENVISAT data over the cryosphere and show the
improvements. We observe the limitation of this reprocessed doppler slope correction, especially near the
coast and over other land surfaces. We investigate a
solution and suggest an alternative algorithm which uses
directly the altimetric range measurement rather than
any digital elevation model (DEM).
1.
OBSERVATIONS OF THE DOPPLER SLOPE
CORRECTION
The Doppler slope correction is quite important in radar
altimetry over continental surfaces especially in the
sloppy areas. This correction is made to correct the
Doppler Effect produced by the distance rate from the
radar altimeter to the ground target during its revolution.
In practice, this correction is computed in the ENVISAT
GDR by using different Digital Earth Model (DEM)
over only Antarctica (Radarsat Antarctic Mapping
Project DEM Version 2) and Greenland (Bamber DEM)
continents.
For this study, we use two releases of GDR dataset. The
old one: The ENVISAT RA-2 GDR data from cycle 9 to
94 (October 2002 to October 2010). The new one: the
re-processed ENVISAT RA-2 GDR data (V2.1) from
cycles 6 to 94 (June 2002 to October 2010). These
datasets cover the same period until the orbit change in
October 2010. The re-processed data v2.1 was released
in by the end of 2011. In this study we work on the
Doppler slope correction from these two datasets over
Antarctica to compare them and to check their
improvement.
We validate both datasets by using the Ice Validation
Chain developed by the CTOH in order to assess the
altimetric data over the cryosphere [2]. This validation
is base on crossover difference statistic where each
ICE2 parameters [4] and corrections are assessed. We
use these various outputs available to monitor the
behaviour of this Doppler slope correction in time, in
space and as a function the surface slope.
At large scale (figure 1-a), the surface height rate
presents a global shape which looks like to the both
Doppler slope corrections taking into account the sign
difference. It gives a global check of the Doppler slope
correction according to the effective surface height rate
seen by the radar altimeter. We note in addition that
there is a phase difference in the 20-30km scale between
the two correction releases.
Figure 1-b and 1-c are plots at closer scale respectively
inside and on the coast of the Antarctic continent. For
the new dataset, the Doppler slope correction (blue dot
line) appears coherent to the surface rate within the
continent (figure 1-b). And we can see that for each
bump of the surface height rate we get a trough of the
correction. The Doppler slope correction follows nicely
the surface height variations observed by the altimeter.
In coastal areas (figure 1-c) the correction loses its
coherence to the surface height rate. The correction is
no more consistent with the altimeter observation.
For the old dataset (red dot line), the Doppler slope
correction does not show a clear consistence as the
observed surface height rate. The Figure 1-b zooms on a
small part of the profile to better show the phase
difference between the old and new dataset (blue dot
line) seen figure 1-a. The phase difference can be
evaluated close to the 30 km between the two
corrections releases.
Finally, we show that the DEM used for first release of
data set is strongly shifted to the effective radar
altimeter observation. It also shows that the new dataset
supplies an improved Doppler slope correction in
Antarctica. The new DEM, used to calculate it, seems to
have a better resolution and to be better defined within
the continent. However in the coastal and relief areas,
where the ice cap motion is strong, this correction
remains definitely bad or missing. This examination
shows that this corrections could not be warranted to
study these particularly areas.
Blarel and Legresy, proceedings of the ESA-CNES symposium on altimetry Venice, 2012.
a
c
b
Figure 1: Doppler slope correction (top) from the old GDR (red dot line) and from the new GDR (blue dot line)
along track. Below the surface height rate (middle) estimated from the surface height (down). (a) At large scale
(~2000 km) both corrections have a consistent behaviour with the surface height rate: for each bump of the
surface height rate we get a trough of the correction. We also note there is a little phase difference between the
two correction releases.(b) At closer scale (~100 km) and inside the Antarctica, we note that the correction from
the old GDR does not show any coherence with the surface height rate observed by the radar altimeter. While the
correction from new data is better improved and let appear a clear consistent with the surface height rate (grey
arrows). We can evaluate the phase difference between the two correction releases to around 30 km (dotted
arrows). (c) At the same scale (~100 km) and in this area close to the coast, we note that the both correction do
not show any coherence with the surface height rate observed by the radar altimeter. The correction has to be
again improved in the coastal areas.
Based on this examination, we see that the surface
height (or the range) observed by the radar altimeter is
good enough to estimate a range rate coherent to the
new DEM of the re-processed data (blue line, figure 1b). In the following part, we investigate the possibility
to calculate this correction from the actually observed
range rate in order to compare it to the others. We
evaluate this alternative approach in those areas which
are not enough improved by the DEM.
In a second part, we recall the principle of the Doppler
Effect for the radar altimetry measurement. In the third
part, we present its computation. And we conclude by
comparing this correction to the two others (original and
re-processed data).
2.
DOPPLER EFFECT
Doppler Effect is the relative change of frequency of the
echo radar produced by the relative displacement of the
satellite and the target on the earth surface. It appears
that the echo shifts forward or backward from its
position. In the ENVISAT Handbook [1] the
relationship to correct this effect on the radar echo is
given by:
δhku = −
C0
∗ Sat_Alt_Rate (1)
λku Chirp_SL
Where δhku is the Doppler correction, C 0 light velocity,
λku radar
wavelength,
Chirp
Chirp_SL
and Sat_Alt_Rate the satellite altitude rate.
Slope
The Chirp Slope is defined by:
Chirp_SL =
Bandwidthku (Mode ) (2)
pulse_duration
Where Bandwidth ku (Mode ) is the bandwidth value
(320MHz, 80MHz or 20MHz) given by the flag mode
ku_chirp_id_flag in the GDR data and pulse_duration :
20 µs.
3.
DOPPLER CORRECTIONS
In radar altimetry and in particularly over continental
surfaces, there are two phenomena which produce
Doppler Effect: orbit rate and surface slope (inducing
surface height rate).
Blarel and Legresy, proceedings of the ESA-CNES symposium on altimetry Venice, 2012.
3.1. Orbite Rate
The spacecraft revolution is not exactly centred to the
mass center of the earth and the satellite orbit is
perturbed by various phenomena like gravitation
variation or friction, which move the satellite from its
orbit. The vertical component of this displacement
generates a Doppler effect in the echo radar. This
displacement is evaluated by the DORIS system which
controls the orbit revolution of the spacecraft and is
supplied in the ENVISAT GDR data under the
parameter named: instant_alt_rate. Thanks to this
information, it is possible to check the relationship eq 1
to calculate the Doppler correction. The Figure 2 shows
the Doppler correction from the GDR in one hand and
re-calculated using the relationship eq 1 on other hand.
They are exactly equal and both show outliers dots over
continental areas. They are due to the bandwidth
switching of the on board tracker to keep the radar echo
in the receiving time window. This error appears
because the process generates incompatibility data in
the mixed of bandwidth modes [1]. They can easily be
edited from the dataset by using the flag
error_flag_chirp_id_flags in the GDR data [1].
3.2. Surface Slope
Over the continental surface the radar perceives the
slope of surfaces similarly surface in motion. This
produces a Doppler effect in the radar echo and has to
be taken into account in the range measurement. In the
GDR data, the Doppler slope correction is calculated by
using a DEM. It is interpolated to get the surface height
rate along the track, and to deduce from the equation
eq.1 the additional Doppler slope correction.
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Antarctica
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Antarctica
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Figure 3: An example with the track 834 cycle 9. Top:
The Doppler correction due to the orbit rate (GDR, blue
dots) and the Doppler correction calculated using the
range rate (red dots). Below: the Doppler slope
correction deduces. Down: the surface height along of
the track.
Figure 2: Top left: Doppler correction due to the orbit
rate. In red dot: the Doppler correction from the GDR
and in black the recalculted Doppler correction. Down
left :the surface height. Below the map of the pass 834.
The plot of the Doppler correction is satisfactory and
checks positively the relationship Eq.1.
Here we chose to use directly the range observed by the
radar altimeter to calculate the Doppler correction. The
range measurement contains both rates: the orbit rate
and the surface slope rate. And the Doppler correction
from range contains the two same contributions. And by
applying the range rate into the relationship eq1, it gives
then a Doppler range correction which is the sum of the
two corrections:
δhDoppler_Range ku = δhDoppler_Orbit ku + δhDoppler_Slopeku (3)
By subtracting the Doppler correction due to the orbit
(DORIS) we get the Doppler slope correction:
δhDoppler _Slope ku = δhDoppler _Range ku − δhDoppler _Orbit ku (4)
Blarel and Legresy, proceedings of the ESA-CNES symposium on altimetry Venice, 2012.
a-1
c
b
b
a-2
c
Figure 4: Along track data from pass 834 cycle 9 over Antarctica. Top left: Doppler slope correction from old
GDR in red, from new GDR in blue and from our re-computing using the range rate (green). Middle left: the
difference between the re-computing and the old GDR in red and the difference between the re-computing and
the new GDR in blue. On the right b and c are zooms of a-1
From this relationship eq. 4 the Doppler correction due
to the orbit rate is supplied by the GDR data.
To take into account the radar echo foot print size on the
ground, we estimate the range rate by interpolating the
range along of segment track of 10 km [3].
The plots (figure 3) illustrate the relationship of the
Doppler range correction (red) over half earth
revolution: From Antarctica to the North Pole for the
track 834 cycle 9. We can see the two Doppler effects:
the large wavelength which is the Doppler correction of
the orbit and the small wavelengths over continental
surfaces which are the Doppler slope correction. Below
the Doppler slope correction, the result of the
subtraction of theses two corrections.
4.
IMPROVEMENT OF THE DOPPLER SLOPE
CORRECTION
We evaluate this new approach over the Antarctica and
we compare it to the old and new GDR dataset. The
along track observation shows in figure 4-a1 that the
Doppler slope re-calculated (green line) has a good
agreement with the Doppler slope correction from the
new GDR (blue line).
At closer scale (figure 4 b) within the continent, we note
that the new GDR (blue) and the re-calculated
correction (green) are very close even if we can still
note a shift between them of around of 2 km. Below
(figure 4 c) on the coast, we see three correction
releases definitely disagree.
Figure 4-a2, it is plotted the difference between our recalculated Doppler slope correction and the old (red)
and new (bleu) GDR dataset. We see that the difference
is smallest with the new GDR than the old GDR but
remains highest on the coast.
The maps, Figure 5, show the difference along of the
track (like the difference plotted in Figure 4-a2)
between the old GDR and the re-calculated correction
(map on the left) and between the new GDR and the recalculated correction (map on the right). It shows that
difference is reduced inside the continent with the new
GDR. On the coast and mountain areas the difference
remains stronger.
To see better the difference amplitude of the change
between the two maps (Figure 5), we calculate the
Blarel and Legresy, proceedings of the ESA-CNES symposium on altimetry Venice, 2012.
histogram Figure 6. It shows by class of slope the
average of the difference at crossovers between the old
GDR and the re-calculated correction (red bars) and the
average difference at crossover between the new and recalculated correction (blue bars).
radar echo.
Figure 6: Histogram of the Doppler slope difference at
crossover between old GDR and the re-calculated (red)
and between the new GDR and the re-calculated (blue)
over Antarctica. Below: the crossover distribution.
5.
RESULTS
First, our alternative method to calculate this correction
from the range has a good agreement to the new GDR
within the Antarctica continent. The improvement of the
Doppler slope correction form the new GDR over
Antarctica then appears here clearly. The shift of the
DEM observed in the old GDR is now minor (less than
around 2 km) and the resolution seems also better.
Figure 5: Doppler slop correction difference between
the old GDR and re-calculated correction on the top
and between the new GDR and re-calculated correction
on the down.
From the histogram, we see that difference is gradually
larger when the surface slope becomes larger. We also
observe that the difference is reduced when we pass
from the old GDR to the new GDR, over all class of
slope. We note that the reduction is around of 50% for
the flat area and around of 25% for the surface slope of
15 m/km. It thus shows that the DEM used for new
GDR is closer to the surface estate observed by the
radar altimeter. On the coast, the difference remains
high and can be explained by higher divergence
between the two approaches (DEM and range method)
to estimate the location impact on the ground of the
Second, in the coastal and mountain areas, there is no
agreement between the new GDR and our method.
Indeed this correction depends strongly on the accuracy
of the surface slope which is variable in theses areas but
it also depends to the real surface observed by the
altimeter. In larger slope areas, the echo return is not
necessarily located in the satellite’s nadir direction [3].
The altimeter does not “see” exactly what it overflies at
nadir. In this case the surface rate observed by the radar
altimeter is inconsistent with the surface rate
interpolated along the nadir track on the DEM. In these
cases, the Doppler Effect then appears more reliable
from the range measurement. This needs to be further
investigated with the re-location method [3] and clearly
evaluated.
Finally, our alternative method has the advantage of
being valid directly for all the continents and to be
computed (where the range is available) without any
external DEM and any shift problem with the tracks
Blarel and Legresy, proceedings of the ESA-CNES symposium on altimetry Venice, 2012.
(Figure 7 and 8).
The CTOH plans to evaluate this alternative method for
hydrology needs. This correction will be available when
it will be validated on the CTOH web site
(http://ctoh.legos.obs-mip.fr/)
Figure 8: An example of the Doppler slope correction
for descending over the entire continents: on the top
from the new GDR and below from the new approach
using the range (cycle 10).
6.
REFENCES
1. ENVISAT RA2/MWR Product Handbook, Issue 2,
2007. http://envisat.esa.int/dataproducts/
2. F. Blarel, B. Legresy and F. Remy, Validation of
ENVISAT radar altimetry within the OSCAR project,
proceeding ESA Living Planet Symposium, 28 June 2 July 2010, Bergen, Norway.
Figure 7: An example of the Doppler slope correction
for ascending (top) and descending (down) track over
Antarctica re-calculated from our alternative method
(cycle 10).
3. Roemer, S., B. Legresy, M. Horwath and R. Dietrich.
2007. Refined analysis of radar altimetry data applied
to the region of the subglacial Lake Vostok,
Antarctica, Remote Sensing of Environment. 106,
269-284.
4. Legresy B., F. Papa, F. Rémy, G. Vinay, M. Van den
Bosch and O.Z. Zanifé. 2005. ENVISAT Radar
Altimeter measurements over continental surfaces and
ice caps using the Ice2 retracking algorithm, Remote
Sensing of Environment. 95. 150-163.