Processing methodology for the ITS_LIVE Sentinel-1 ice velocity products
Authors: Lei Yang , Gardner Alex S. , Agram Piyush 
Published: November 2022

Abstract. The NASA MEaSUREs Inter-mission Time Series of Land Ice
Velocity and Elevation (ITS_LIVE) project seeks to accelerate
understanding of critical glaciers and ice sheet processes by providing
researchers with global, low-latency, comprehensive and state of the art
records of surface velocities and elevations as observed from space. Here we describe the image-pair ice velocity product and processing methodology for
ESA Sentinel-1 radar data. We demonstrate improvements to the core
processing algorithm for dense offset tracking, “autoRIFT”, that provide finer
resolution (120 m instead of the previous 240 m used for version 1) and
higher accuracy (20 % to 50 % improvement) data products with
significantly enhanced computational efficiency (>2 orders of
magnitude) when compared to earlier versions and the state of the art
“dense ampcor” routine in the JPL ISCE software. In particular, the
disparity filter is upgraded for handling finer grid resolution with
overlapping search chip sizes, and the oversampling ratio in the subpixel
cross-correlation estimation is adaptively determined for Sentinel-1 data by
matching the precision of the measured displacement based on the search chip
size used. A novel calibration is applied to the data to correct for
Sentinel-1A/B subswath and full-swath dependent geolocation biases caused by
systematic issues with the instruments. Sentinel-1 C-band images are
affected by variations in the total electron content of the ionosphere that
results in large velocity errors in the azimuth (along-track) direction. To
reduce these effects, slant range (line of sight or LOS) velocities are used
and accompanied by LOS parameters that support map coordinate (x/y) velocity
inversion from ascending and descending slant range offset measurements, as
derived from two image pairs. After the proposed correction of ionosphere
errors, the uncertainties in velocities are reduced by 9 %–61 %. We further
validate the ITS_LIVE Version 2 Sentinel-1 image-pair
products, with 6-year time series composed of thousands of epochs, over
three typical test sites covering the globe: the Jakobshavn Isbræ
Glacier of Greenland, Pine Island Glacier of the Antarctic, and Malaspina
Glacier of Alaska. By comparing with other similar products (PROMICE, FAU,
and MEaSUREs Annual Antarctic Ice Velocity Map products), as well as other
ITS_LIVE version 2 products from Landsat-8 and Sentinel-2
data, we find an overall variation between products around 100 m yr−1 over
fast-flowing glacier outlets, where both mean velocity and variation are on
the order of  km yr−1, and increases up to 300–500 m yr−1 (3 %–6 %) for the
fastest Jakobshavn Isbræ Glacier. The velocity magnitude uncertainty of
the ITS_LIVE Sentinel-1 products is calculated to be
uniformly distributed around 60 m yr−1 for the three test regions
investigated. The described product and methods comprise the MEaSUREs
ITS_LIVE Sentinel-1 Image-Pair Glacier and Ice Sheet Surface
Velocities: version 2 (DOI: https://doi.org/10.5067/0506KQLS6512, Lei et al., 2022).