Investigations on the ENVISAT RA2 Doppler slope correction ... · ctoh.legos.obs-mip.fr ABSTRACT :...
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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 re-processed 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). Papua New Guinea Antarctica East Russia Australia Investigations on the ENVISAT RA2 Doppler slope correction for ice sheets F. Blarel and B. Legresy ESA-CNES Symposium, 20 Years of Progress in Radar Altimetry, 24-29 September 2012, Venice, Italy LEGOS/CTOH, 14 Avenue E. Belin, 31400 Toulouse, FRANCE. [email protected], http://ctoh.legos.obs-mip.fr/ • (a) Both corrections show a consistent behaviour with the surface height rate. We note there is a small phase difference between the two correction releases. • (b) At finer scale (~100 km) and in the Antarctica interior, the correction from the old GDR (red) is not coherent with the surface height rate observed by the radar altimeter. Whereas the correction from the new data (blue) is improved and shows a clear consistency with the surface height rate. We see that for each bump in the surface height rate we get a trough in the correction (grey arrows). The new Doppler slope correction follows nicely the surface height variations observed by the altimeter. We evaluate the phase difference between the two correction releases to be around 20-30 km (dotted arrows). • (c) In the area close to the coast, we note that neither correction shows any coherence with the surface height rate observed by the radar altimeter. OBSERVATIONS OF THE DOPPLER SLOPE CORRECTION FROM THE OLD GDR AND NEW GDR (V2.1) 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 b c THE DOPPLER SLOPE CORRECTION CALCULATED FROM RANGE An alternative method: 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 the range contains the two same contributions. By applying the range rate to the relationship detailed in [1] we get a Doppler range correction which is the sum of the two corrections: By subtracting the Doppler correction due to the orbit (DORIS) supplied with the GDR data we get the Doppler slope correction: To take into account the radar echo foot print size on the ground, we estimate the range rate by interpolating the range along track segments of 10 km [4]. The plots on the left illustrate the Doppler range correction (red) over a half earth revolution. ku δh + ku δh = ku δh Slope Doppler Orbit Doppler Doppler _ _ _Range ku δh ku δh = ku δh Orbit Doppler R Doppler Slope Doppler _ ange _ _ - An example for track 834 cycle 9: From Antarctica to the North Pole. Top: The Doppler correction due to the orbit rate (GDR, blue dots) and the Doppler correction calculated using the range rate (red dots). Middle: the newly calculated Doppler slope correction. Bottom: the surface height alongtrack b c b c b c Along track data from pass 834 cycle 9 over Antarctica. Top left (a-1): Doppler slope correction from old GDR in red, from new GDR in blue and from our re-computing using the range rate (green). Below (a-2): the difference between the re-computed and the old GDR in red and the difference between the re-computed and the new GDR in blue. Right (b) and (c) zoom of (a-1). a a-1 We use two releases of the GDR dataset: • The old : The ENVISAT RA-2 GDR data from cycle 9 to 94 (October 2002 to October 2010). • The new : 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 up until the orbit change in October 2010. • The plots above show the Doppler slope correction from the new GDR (V2.1) is better phased with the surface rate observed by the radar altimeter. • In the old GDR, the Doppler slope correction is shifted by around 20-30 km, due to DEM problems. • Both corrections need to be improved in the coastal areas. • The range is a good estimator of the surface rate as an alternative method to calculate the Doppler slope correction. THE DOPPLER SLOPE CORRECTION IMPROVEMENT a-2 • The alternate method to compute the Doppler slope correction confirms the good agreement with the new GDR for the interior of the Antarctic continent (shift less than ~ 2 km) • In the coastal and mountain areas, there is no agreement between the new GDR and our method. • This correction depends strongly on the accuracy of the surface slope which is variable in the coastal areas but it also depends on 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. RESULTS REFERENCES 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. 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. • 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. • The CTOH plans to evaluate this alternative method for hydrology needs • This correction will be available when validated on the CTOH web site (http://ctoh.legos.obs-mip.fr/) The Doppler slope correction from the new GDR (V2.1) using DEM is only available over Greenland and Antarctica The Doppler slope correction from our alternative method using direct radar altimeter range measurements is available over all continental surfaces • The CTOH also disseminates the Quality Assessment on the cryosphere of re-processed ENVISAT RA-2 GDR data (V2.1). Currently, a validation chain is already developed specifically to the cryosphere (ICE validation). Validation reports are generated describing the quality of the cycles. They include figures, maps, and tables of parameters and corrections validated for each cycle and details of the anomalies detected. The products of that validation are available on Antarctica and Greenland. Validation table of the ENVISAT mission over Antarctica : This table gives the state validation of the number of the track as function of the cycle of the mission. This figure is also available as ascii file and could be integrated to track selection procedure of a processing program. Histogram of RMS of Surface height as function of slope over Antarctica Surface height (m) Surface height anomaly (m) over Antarctica for all cycles An example of validation output for surface height at crossover for Antarctica and Greenland
Transcript of Investigations on the ENVISAT RA2 Doppler slope correction ... · ctoh.legos.obs-mip.fr ABSTRACT :...
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 cryosphereand show the improvements. We observe the limitation of this re-processed 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).
Papua New Guinea
AntarcticaEast Russia
Australia
Investigations on the ENVISAT RA2 Doppler slope correction for ice sheetsF. Blarel and B. Legresy
ESA-CNES Symposium, 20 Years of Progress in Radar A ltimetry, 24-29 September 2012, Venice, Italy
LEGOS/CTOH, 14 Avenue E. Belin, 31400 Toulouse, [email protected], http://ctoh.legos.obs-mip.fr/
• (a) Both corrections show a consistent behaviour with the surface height rate. We note there is a small phase difference between the two correction releases.
• (b) At finer scale (~100 km) and in the Antarctica interior, the correction from the old GDR (red) is not coherent with the surface height rate observed by the radar altimeter. Whereas the correction from the new data (blue) is improved and shows a clear consistency with the surface height rate. We see that for each bump in the surface height rate we get a trough in the correction (grey arrows). The new Doppler slope correction follows nicely the surface height variations observed by the altimeter. We evaluate the phase difference between the two correction releases to be around 20-30 km (dotted arrows).
• (c) In the area close to the coast, we note that neither correction shows any coherence with the surface height rate observed by the radar altimeter.
OBSERVATIONS OF THE DOPPLER SLOPE CORRECTION FROM T HE OLD GDR AND NEW GDR (V2.1)
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 b c
THE DOPPLER SLOPE CORRECTION CALCULATED FROM RANGE
An alternative method: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 the range contains the two same contributions. By applying the range rate to the relationship detailed in [1] we get a Doppler range correction which is the sum of the two corrections:
By subtracting the Doppler correction due to the orbit (DORIS) supplied with the GDR data we get the Doppler slope correction:
To take into account the radar echo foot print size on the ground, we estimate the range rate by interpolating the range along track segments of 10 km [4].
The plots on the left illustrate the Doppler range correction (red)over a half earth revolution.
kuδh+kuδh=kuδh SlopeDopplerOrbitDopplerDoppler ___Range
kuδhkuδh=kuδh OrbitDopplerRDopplerSlopeDoppler _ange__ −
An example for track 834 cycle 9: From Antarctica to the North Pole. Top: The Doppler correction due to the orbit rate (GDR, blue dots) and the Doppler correction calculated using the range rate (red dots). Middle: the newly calculated Doppler slope correction. Bottom: the surface height alongtrack
b
c
b
cb
c
Along track data from pass 834 cycle 9 over Antarctica. Top left (a-1): Doppler slope correction from old GDR in red, from new GDR in blue and from our re-computing using the range rate (green). Below (a-2): the difference between the re-computed and the old GDR in red and the difference between the re-computed and the new GDR in blue. Right (b) and (c) zoom of (a-1).
a
a-1
We use two releases of the GDR dataset: • The old : The ENVISAT RA-2 GDR data from cycle 9 to 94 (October 2002 to October 2010). • The new : 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 up until the orbit change in October 2010.
• The plots above show the Doppler slope correction f rom the new GDR (V2.1) is better phased with the surface rate observed by the radar altimeter.
• In the old GDR, the Doppler slope correction is shi fted by around 20-30 km, due to DEM problems.
• Both corrections need to be improved in the coastal areas.
• The range is a good estimator of the surface rate a s an alternative method to calculate the Doppler slope correction.
THE DOPPLER SLOPE CORRECTION IMPROVEMENT
a-2
• The alternate method to compute the Doppler slope c orrection confirms the good agreement with the new GDR for the interio r of the Antarctic continent (shift less than ~ 2 km)
• In the coastal and mountain areas, there is no agre ement between the new GDR and our method.
• This correction depends strongly on the accuracy of the surface slope which is variable in the coastal areas but it also depends on the real surface observed by the altimeter.
• In larger slope areas, the echo return is not neces sarily 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 ob served by the radar altimeter is inconsistent with the surface rate int erpolated along the nadir track on the DEM. In these cases, the Doppler Effec t then appears more reliable from the range measurement. This needs to be further investigated with the re-location method [3] and cl early evaluated.
RESULTS
REFERENCES
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.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.
• Our alternative method has the advantage of being v alid directly for all the continents and to be comp uted (where the range is available) without any external DEM and any shift problem with the tracks.
• The CTOH plans to evaluate this alternative method for hydrology needs
• This correction will be available when validated on the CTOH web site (http://ctoh.legos.obs-mip.fr/)
The Doppler slope correction from the new GDR (V2.1) using DEM is only available over Greenland and Antarctica
The Doppler slope correction from our alternative method using direct radar altimeter range measurements is available over all continental surfaces
• The CTOH also disseminates the Quality Assessment on the cryosphere of re-processed ENVISAT RA-2 GDR data (V2.1). Currently, a validation chain is already developed specifically to the cryosphere (ICE validation). Validation reports are generated describing the quality of the cycles. They include figures, maps, and tables of parameters and corrections validated for each cycle and details of the anomalies detected. The products of that validation are available on Antarctica and Greenland.
Validation table of the ENVISAT mission over Antarctica : This table gives the state validation of the number of the track as function of the cycle of the mission. This figure is also available as ascii file and could be integrated to track selection procedure of a processing program.
Histogram of RMS of Surface height as function of slope over
Antarctica
Surface height (m)
Surface height anomaly (m) over Antarctica for all cycles
An example of validation output for surface height at crossover for Antarctica and Greenland
