on resolving the local oscillator drift induced phase ramps in asar and ers 1/2 interferometric data
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Systematic analysis of 18 yrs of ERS 1/2 data (~5.000 products)
Data Analysis: ERS 1/2
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On resolving the Local Oscillator drift induced phase ramps in ASAR and ERS 1/2 interferometric data
Petar Marinkovic(1), Yngvar Larsen(2)
(1) PPO.Labs, The Netherlands(2) Northern Research Institute Tromsø (NORUT), Norway
Why this study - InSAR perspective?
Why this study - SAR perspective?
Motivation
...systematic increase of the geo-localization error in range direction of ~0.04 pixel/year for ASAR.
Systematic effects observed in both InSAR and GeoLocalization
Methodology
InSAR: Carrier Frequency Wavelength
Geo-localization: Range Sampling Rate Slant Range Time
Local Oscilator (LO) is the common denominator
PRF / RSR / “Slant Range Time” share
common LO frequency of ERS1/2 and ASAR
LO Drift RSR Drift
PRFSWSTSWLTXPL
Systematic analysis of 10 yrs of ASAR data (~10.000 products)
Data Analysis & Correction Model: ENVISAT ASAR
modelingof the drift
2002 2004 2006 2008 2010 2012-5
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Drift Correction Model Residual Model - Latitude Dependency
Summary & Practical Considerations
How to deal with ERS 1/2 data?
To reprocess or not to reprocess?
No deterministic drift model. If you really want to fix for the drift, the only option is to reorder L0 data and estimate the actual PRF from timetags. However, the clock precision of ERS is 1/256 s, so you need a fairly long strip to get an accurate estimate.
ENVISAT: if a relatively small number of scenes in stack (<20) and long duration of timeseries (>2-3 years), in a case of non-linear deformation it would be wise to reprocess. APS and other error sources might not be average out.
ERS 1/2: similar to ENVISAT, although the effect is random, and apparently no drift in time (mean PRF = nominal PRF). Thus the effect will average out for sufficient number of scenes.
....this is good news - quick fix with the provided model!
...the decay is possible to describe analytically almost without residues (RMS residual range error about 2cm!). ASAR LO is really stable (compared to ERS), just a "little bit" of drift over time.
...Geolocalization significantly improved, from 3m to 1dm in LOS in case of Envisat.
...quality of data has been underestimated → ASAR is a perfect SAR machine.
...end of story!
Data: ENVISAT ASARLocation: Inshallah, AlgeriaScale: Full SceneBtemp: 7yrs
Model also 'blind' validated by different teams!
Interferogram with &
without correction model
Model Validation
...a systematic phase ramp in range of ~15 mm/yr has been reported in ASAR interferometric time series displacement results. The ramp is observed by all groups and methods in all data stacks.
...usually ad-hoc removed by polynomial fitting, although there were unsuccessful attempts to physically explain it (e.g., relate it to the orbit error).
...this ramp hampers an analysis of the long-wavelength deformation signal.
...hard to explain phase ramps were also observed in ERS1/2 results.
The actual PRF estimated from Level-0 data. For details see 'Mohr and Madsen' work on the timing errors (2001).