Abstract's details
Seasonal effects on the pitch measurements for Cryosat
CoAuthors
Event: 2015 Ocean Surface Topography Science Team Meeting
Session: Quantifying Errors and Uncertainties in Altimetry data
Presentation type: Type Oral
Contribution: PDF file
Abstract:
This abstract is aimed at describing an analysis of the pitch bias in CryoSat products acquired over the German Bight spanned from 2010 till 2015. The pitch information, as extracted from CryoSat-2 FBR Baseline B products and computed from on board star trackers, has been cross-compared against the pitch as estimated by analysis of the stacks of the SAR single look echoes according to [1].
It is worth recalling here the standard processing chain for a SAR altimeter [1]: firstly an approximately equally spaced set of ground locations on the Earth surface, i.e. surface sample, is identified. A surface sample gathers a stack of single look echoes coming from the processed bursts during the time of visibility. However, in a surface sample stack, each single look echo has been acquired from a different position of the instrument along the orbit and, as a consequence, it results to be scaled in power by the antenna pattern as function of the look angle. According to [2], on a uniformly rough spherical surface, e.g. the ocean, the power of the single look echoes in the stack is modulated by the along-track antenna pattern.
Thus, by fitting the along-track antenna pattern on the power distribution of each stack, it is possible to measure with sufficient accuracy the actual pitch of the satellite directly from the acquired and processed data. It is worth underlining that the pitch retrieved exploiting this method is totally independent of the attitude information annotated in the FBR products.
In the present work, the pitch bias was estimated similarly to [1] building from each SAR stack data a sub-stack of 64 looks and multi-looking 100 consecutive sub-stacks together in order to knock-down the speckle noise. The outcome was that the pitch bias resulted in the combination of a constant term equal to 0.055 deg and a sinusoidal function of the time, that has been addressed to the seasonal solar illumination on the Star Trackers, as shown in the attached figure. By compensating the pitch bias on the pitch from Star Tracker (that is annotated in the FBR products), the residual difference between the two pitch measurements results to have a standard deviation equal to about 2.5 millideg.
Possibly, the analysis will be repeated also on a wider region, as the South-East Pacific Box, in order to investigate any spatial correlation between the two pitch estimations.
[1] M. Scagliola, M. Fornari, and N. Tagliani, “Pitch estimation for cryosat by analysis of stacks of single-look
echoes,” IEEE Geosci. Remote Sens. Letters, vol. PP, no. 99, pp. 1–5, 2015.
[2] Wingham D. J., Phalippou L., Mavrocordatos C. and Wallis D., The Mean Echo and Echo Cross Product From a Beamforming Interferomet-ric Altimeter and Their Application to Elevation Measurement, IEEE Transactions on Geoscience and Remote Sensing, 42 (10), 2305-2323,doi:10.1109/TGRS.2004.834352, 2004.
It is worth recalling here the standard processing chain for a SAR altimeter [1]: firstly an approximately equally spaced set of ground locations on the Earth surface, i.e. surface sample, is identified. A surface sample gathers a stack of single look echoes coming from the processed bursts during the time of visibility. However, in a surface sample stack, each single look echo has been acquired from a different position of the instrument along the orbit and, as a consequence, it results to be scaled in power by the antenna pattern as function of the look angle. According to [2], on a uniformly rough spherical surface, e.g. the ocean, the power of the single look echoes in the stack is modulated by the along-track antenna pattern.
Thus, by fitting the along-track antenna pattern on the power distribution of each stack, it is possible to measure with sufficient accuracy the actual pitch of the satellite directly from the acquired and processed data. It is worth underlining that the pitch retrieved exploiting this method is totally independent of the attitude information annotated in the FBR products.
In the present work, the pitch bias was estimated similarly to [1] building from each SAR stack data a sub-stack of 64 looks and multi-looking 100 consecutive sub-stacks together in order to knock-down the speckle noise. The outcome was that the pitch bias resulted in the combination of a constant term equal to 0.055 deg and a sinusoidal function of the time, that has been addressed to the seasonal solar illumination on the Star Trackers, as shown in the attached figure. By compensating the pitch bias on the pitch from Star Tracker (that is annotated in the FBR products), the residual difference between the two pitch measurements results to have a standard deviation equal to about 2.5 millideg.
Possibly, the analysis will be repeated also on a wider region, as the South-East Pacific Box, in order to investigate any spatial correlation between the two pitch estimations.
[1] M. Scagliola, M. Fornari, and N. Tagliani, “Pitch estimation for cryosat by analysis of stacks of single-look
echoes,” IEEE Geosci. Remote Sens. Letters, vol. PP, no. 99, pp. 1–5, 2015.
[2] Wingham D. J., Phalippou L., Mavrocordatos C. and Wallis D., The Mean Echo and Echo Cross Product From a Beamforming Interferomet-ric Altimeter and Their Application to Elevation Measurement, IEEE Transactions on Geoscience and Remote Sensing, 42 (10), 2305-2323,doi:10.1109/TGRS.2004.834352, 2004.