Along-track Spatial Resolution Research Articles

New estimates of pan-Arctic sea ice–atmosphere neutral drag coefficients from ICESat-2 elevation data

Abstract. The effect that sea ice topography has on the momentum transfer between ice and atmosphere is not fully quantified due to the vast extent of the Arctic and limitations of current measurement techniques. Here we present a method to estimate pan-Arctic momentum transfer via a parameterization that links sea ice–atmosphere form drag coefficients with surface feature height and spacing. We measure these sea ice surface feature parameters using the Ice, Cloud and land Elevation Satellite-2 (ICESat-2). Though ICESat-2 is unable to resolve as well as airborne surveys, it has a higher along-track spatial resolution than other contemporary altimeter satellites. As some narrow obstacles are effectively smoothed out by the ICESat-2 ATL07 spatial resolution, we use near-coincident high-resolution Airborne Topographic Mapper (ATM) elevation data from NASA's Operation IceBridge (OIB) mission to scale up the regional ICESat-2 drag estimates. By also incorporating drag due to open water, floe edges and sea ice skin drag, we produced a time series of average total pan-Arctic neutral atmospheric drag coefficient estimates from November 2018 to May 2022. Here we have observed its temporal evolution to be unique and not directly tied to sea ice extent. By also mapping 3-month aggregates for the years 2019, 2020 and 2021 for better regional analysis, we found the thick multiyear ice area directly north of the Canadian Archipelago and Greenland to be consistently above 2.0×10-3, while most of the multiyear ice portion of the Arctic is typically around ∼1.5×10-3.

Relationship Between Altimetric Quality and Along-Track Spatial Resolution for iGNSS-R Sea Surface Altimetry: Example for the Airborne Experiment

The altimetric quality and the along-track spatial resolution are the critical parameters to characterize the performance of interferometric global navigation satellite systems reflectometry (iGNSS-R) sea surface altimetry, which is closely related to each other through signal processing time. Among them, the quality of sea surface height (SSH) measurement includes precision and accuracy. In order to obtain higher altimetric quality in the observation area, a longer signal processing time is needed, which will lead to the loss of spatial resolution along the track. In contrast, higher along-track spatial resolution requires more intensive sampling, leading to unsatisfactory altimetric quality. In this study, taking the airborne iGNSS-R observation data as an example, the relationship between the altimetric quality and the along-track spatial resolution is analyzed from the perspectives of precision and accuracy. The results indicate that the reduction in the along-track spatial resolution will improve the altimetric quality. The accuracy range is 0.28–0.73 m, and the precision range is 0.24–0.65 m. However, this change is not linear, and the degree of altimetric quality improvement will decrease as the along-track spatial resolution worsens. The research results in this paper can provide a scientific reference for the configuration of parameters for future spaceborne iGNSS-R altimetry missions.

Diverse evolution of mountains and hummocks on Titan as observed by the Cassini RADAR altimeter

Mountain formation and evolution on Titan is poorly understood, due in part to a lack of high-resolution topographic data. By applying advanced processing techniques, we are able to increase the along-track spatial resolution of the Cassini RADAR altimeter by up to a factor of ten, enabling more detailed analysis. A survey of mountainous and hummocky terrain reveals a unique, characteristic waveform shape. By modeling reflections from different landscapes, we are able to show that these waveforms contain sub-resolution topographical information. We found that mountain elevation on Titan varies greatly over short distances, and evidence suggests that many mountain ranges on Titan have been eroded down to the plains level even within a single high-resolution radar footprint.

Assessment of Cryosat-2 and SARAL/AltiKa altimetry for measuring inland water and coastal sea level variations: A case study on Tibetan Plateau lake and Taiwan Coast

Satellite altimetry has been proven as an effective technology to accurately measure water level, ice elevation, and flat land surface changes since the 1990s. To overcome limitations of pulse-limited altimetry, new altimetric missions such as Cryosat-2 and Satellite with ARgos and AltiKa (SARAL/AltiKa), have been designed to have higher along-track spatial resolution to measure more accurately inland water levels for small water bodies, and coastal sea level changes. In this study, we evaluate the performance of Cryosat-2 low-resolution (LRM) and SARin modes and SARAL/AltiKa Ka-band data on two connected lakes in central Tibetan Plateau, and in the coastal region of Taiwan. Results are compared with in situ tide gauge data in Taiwan and altimetric lake level time series from the CNES Hydroweb database. Our results show that water level change trends observed by Cryosat-2 20-Hz retracked observations, the SARAL/AltiKa 40-Hz Ice-1 retracked data, and the Hydroweb measurements are consistent with the estimated water level trend of ∼0.30 m/y, during 2011–2017, and 2013–2015, for the Tibetan Migriggyangzham Co and Dorsoidong Co, respectively. For the coastal region, the performance of SARAL/AltiKa is better than that of Cryosat-2 LRM data in Taiwan. This finding demonstrates the superiority of the Ka-band over Ku-band radar altimetry.

Analysis of Retrackers’ Performances and Water Level Retrieval over the Ebro River Basin Using Sentinel-3

Satellite altimeters have been used to monitor river and reservoir water levels, from which water storage estimates can be derived. Inland water altimetry can, therefore, play an important role in continental water resource management. Traditionally, satellite altimeters were designed to monitor homogeneous surfaces such as oceans or ice sheets, resulting in poor performance over small inland water bodies due to the contribution from land contamination in the returned waveforms. The advent of synthetic aperture radar (SAR) altimetry (with its improved along-track spatial resolution) has enabled the measurement of inland water levels with a better accuracy and an increased spatial resolution. This study aimed to retrieve water levels from Level-1B Sentinel-3 data with focus on the minimization of the land contamination over small- to middle-sized water bodies (130 m to 4.5 km), where continuous clean waveforms rarely exist. Three specialized algorithms or retrackers, together with a new waveform portion selection method, were evaluated to minimize land contamination in the waveforms and to select the nadir return associated with the water body being overflown. The waveform portion selection method, with consideration of the Digital Elevation Model (DEM), was used to fit the multipeak waveforms that arise when overflying the continental water bodies, exploiting a subwaveform-based approach to pick up the one corresponding to the nadir. The performances of the proposed waveform portion selection method with three retrackers, namely, the threshold retracker, Offset Center of Gravity (OCOG) retracker and two-step SAR physical-based retracker, were compared. No significant difference was found in the results of the three retrackers. However, waveform portion selection using DEM information great improved the results. Time series of water levels were retrieved for water bodies in the Ebro River basin (Spain). The results show good agreement with in situ measurements from the Ebro Reservoir (approximately 1.8 km wide) and Ribarroja Reservoir (approximately 400 m wide), with unbiased root-mean-square errors (RMSEs) down to 0.28 m and 0.16 m, respectively, depending on the retracker.

Data Processing and Analysis Approach to Retrieve Carbon Dioxide Weighted-Column Mixing Ratio and 2-<inline-formula> <tex-math notation="LaTeX">$\mu$ </tex-math> </inline-formula>m Reflectance With an Airborne Laser Absorption Spectrometer

We describe the data processing and analysis algorithms used for high-precision retrievals of CO2 weighted-column mixing ratio and 2- $\mu \text{m}$ surface reflectance from the Carbon Dioxide Laser Absorption Spectrometer (CO2LAS). The CO2LAS at the Jet Propulsion Laboratory, Pasadena, CA, USA, is one of the instruments designed to demonstrate capabilities needed for the NASA Active Sensing over Nights, Days, and Seasons mission concept. The integrated path differential absorption technique is used in CO2 retrieval. The along-track spatial resolution for the airborne measurements described here ranges from 10 m to 1 km. Our approach employs heterodyne detection of two laser signals reflected off earth’s surface. Frequency domain processing enables return signal peak detection and high-fidelity power estimation. We compensate for range to ground variations using a digital elevation model and emphasize the importance of reflectance weighting in time averaging of the surface elevation. Quality control filters are applied, as well as a statistical methodology to filter laser speckle fluctuations. Reflectance is also retrieved on the scale of a few meters of ground track. Our data processing workflow and example retrievals are presented.

Height Precision of SAR Altimeter and Conventional Radar Altimeter Based on Flight Experimental Data

Compared with conventional radar altimeter (CRA), synthetic aperture radar altimeter (SARAL) is a new generation radar altimeter with better height precision and along-track spatial resolution. As using synthetic aperture technique, SARAL data processing and waveform model are very different from CRA. To verify the performance and the data processing algorithm, an airborne experimental system of SARAL is developed. The experimental system was installed in Y-12 aircraft, and some valid data was obtained in October 2014. The echo data with SAR mode is obtained in the experiment and the echo data of CRA mode is extracted from SAR mode data. The two modes have the same observation targets and error sources and, then, their height measurement precisions are comparable. The results indicate that the range measurement precision of SARAL is improved about 1 times. The difference of the significant wave heights from the SAR mode data and CRA mode data is negligible.

Error analysis of a new planar electrostatic gravity gradiometer for airborne surveys

Moving-base gravity gradiometry has proven to be a convenient method to determine the Earth’s gravity field. The ESA mission GOCE (Gravity field and steady-state Ocean Circulation Explorer) has enabled to map the Earth gravity field and its gradients with a resolution of 80 km, leading to significant advances in physical oceanography and solid Earth physics. At smaller scales, airborne gravity gradiometry has been increasingly used during the past decade in mineral and hydrocarbon exploration. In both cases the sensitivity of gradiometers to the short wavelengths of the gravity field is of crucial interest. Here, we quantify and characterize the error on the gravity gradients estimated from measurements performed with a new instrument concept, called GREMLIT, for typical airborne conditions. GREMLIT is an ultra-sensitive planar gravitational gradiometer which consists in a planar acceleration gradiometer together with 3 gyroscopes. To conduct this error analysis, a simulation of a realistic airborne survey with GREMLIT is carried out. We first simulate realistic GREMLIT synthetic data, taking into account the acceleration gradiometer and gyroscope noises and biases and the variation of orientation of the measurement reference frame. Then, we estimate the gravity gradients from these data. Special attention is paid to the processing of the gyroscopes measurements whose accuracy is not commensurate with the ultra-sensitive gradiometer. We propose a method to calibrate the gyroscopes biases with a precision of the order $$10^{-8}$$ rad/s. In order to transform the tensor from the measurement frame to the local geodetic frame, we estimate the error induced when replacing the non-measured elements of the gravity gradient tensor by an a priori model. With the appropriate smoothing, we show that it is possible to achieve a precision better than 2E for an along-track spatial resolution of 2 km.

GNSS-R Altimeter Based on Doppler Multi-looking

Measuring ocean mesoscale variability is one of the main objectives of next generation satellite altimeters. Current radar altimeters make observations only at the nadir sub-satellite ground track, which is not sufficient to sample the ocean surface with the required spatial and temporal sampling. The GNSS-R concept has been proposed as an alternative observation system in order to overcome this limitation, since it allows performing altimetry along several points simultaneously over a very wide swath. Latest proposed GNSS-R altimeter configurations allow measuring sea height with an accuracy of few decimeters over spatial scales of 50–100 km, by means of a single-pass. This paper proposes an innovative processing and retracking concept for GNSS-R altimeters based on the acquisition of the full delay-Doppler map (DDM), which allows to acquire multiple waveforms at different Doppler frequencies, whose footprints are located outside the typical pulse-limited region. The proposed processing adapts the Synthetic Aperture Radar (SAR) delay-Doppler concept of spaceborne radar altimeters for use in a GNSS-R system. This processing yields additional multi-look with respect to conventional GNSS-R concepts and translates into an improvement of the altimetry performance estimated to be at least 25%–30%, and even higher, depending on the wanted along-track spatial resolution. The proposed processing can also provide measurements with high spatial resolution at best possible performance, and more generally, offers various possibilities for optimal trade-off between spatial-resolution and height estimation accuracy.

Evaluating the use of high-frequency radar coastal currents to correct satellite altimetry

[1] Coastal altimeter waveforms may differ from the ones in the open ocean, either from rapid changes in the sea state or the presence of land within the satellite altimeter footprint. The optimal retracking method for an individual track may turn out to be a combination of several retrackers and may depend on the sea state. The coastal high-frequency radar (HFR) ocean surface currents, hourly interpolated with a resolution up to 2 km and an offshore range up to 150 km, are evaluated to validate the altimeter sea surface height (SSH) measurements. A method to derive HFR SSH mapped, with a varying spatial-scale optimal interpolation, from the HFR velocities has been implemented. Evaluated mainly in the regions farther than 25 km off the U.S. West Coast, the HFR SSH shows good agreement with Jason-1–2 altimetry products over the years 2008 and 2009. Three Jason-2 PISTACH retrackers and one generic open ocean retracker have been analyzed using the traditional 1 Hz sampling rate. Nearshore, an experimental reprocessing of the 20 Hz range measurements is also tested to check for a gain in along-track spatial resolution. Referencing to the HFR SSH indicate the need to have several retrackers available, even over the continental shelf, with Ice3 fitting better during Bloom events and MLE-4 (or Red3) for high sea states. These studies demonstrate the value of HFR as a potential tool to correct coastal altimeter SSH, refine their spatial resolution and provide some insight into the altimeter behavior as a function of ocean conditions.

GNSS-R Derived Centimetric Sea Topography: An Airborne Experiment Demonstration

The results of two airborne experiments performed to test the precision and the relative accuracy of the conventional Global Navigation Satellite Systems Reflectometry (GNSS-R) technique employing only the C/A code are presented. The first and the second experiments demonstrate, respectively, a 17 cm precision for a 500 m flight altitude with a 8 km along-track spatial resolution, and a 6 cm precision for a 3000 m flight altitude with a 6.6 km along-track spatial resolution. In both, the Relative Mean Dynamic Topography (RMDT) is compared with results derived from traditional radar altimetry provided by Jason-2. The Root Mean Square (RMS) of the RMDT difference between both measurement systems is 48 cm for the first flight, and 198 cm for the second flight. During the second flight, the feasibility of the proposed technique to measure the sea slopes is demonstrated by superposing over the aircraft ground track the measured sea surface height with the geoid undulations, which are about 1 meter.

Direct observations of PMC local time variations by Aura OMI

The Ozone Monitoring Instrument (OMI) on the Aura satellite obtains unique measurements for polar mesospheric cloud (PMC) analysis. Its wide cross-track viewing swath and high along-track spatial resolution makes it possible to directly evaluate PMC occurrence frequency and brightness variations between 65° and 85° latitude as a function of local time over a 12–14h continuous period. OMI PMC local time variations are closely coupled to concurrent variations in measurement scattering angle, so that ice phase function effects must be considered when interpreting the observations. Two different phase functions corresponding to bright and faint clouds are examined in this analysis. OMI observations show maximum frequency and albedo values at 8–10h local time in the Northern Hemisphere, with decreasing amplitude at higher latitudes. Southern Hemisphere values reach a minimum at 18–20h LT. Larger variations are seen in Northern Hemisphere data. No statistically significant longitudinal dependence was seen.

Application of retracked satellite altimetry for inland hydrologic studies

We explored the application of satellite radar altimetry for the monitoring of small inland bodies of water and hydrologic studies using a water-detection algorithm, optimally retracked TOPEX/POSEIDON data at 10-Hz sampling, and investigated the use of radar backscatter to improve land cover classification. The procedure was demonstrated over Manitoba and south-western (SW) Ontario, and the Amazon River Basin study regions. Compared with an L-band synthetic aperture radar data generated water-land cover mask, the water-detection algorithm detected more water points over the Amazon basin. High correlation of 0.98 between the retracked 10-Hz altimetry and the gauge measurements in Manitoba confirmed that the retracked TOPEX data are more accurate than the non-retracked data, and with higher along-track spatial resolution by virtue of its higher sampling at 10 Hz.

Synthetic aperture laser optical feedback imaging using galvanometric scanning

We describe a new one-dimensional synthetic aperture imaging laser radar (ladar) using the resonant sensitivity of a microchip laser to frequency-shifted optical feedback and galvanometric scanning of the target under investigation. In our experiment, the laser is both the source and the detector, providing optical amplification with self-aligned heterodyne detection. By using galvanometric scanning, we achieve an along-track spatial resolution better than the diffraction limit.

An Optimal Filter for Geostrophic Mesoscale Currents from Along-Track Satellite Altimetry

Abstract An optimal difference operator is derived for smoothing along-track sea surface height measurements from satellite altimeters and computing geostrophic surface currents at or below ocean mesoscale wavelengths with second-order approximation accuracy. The difference operator minimizes the white noise of the height measurements to find a compact difference formula for the sea surface height derivative and a residual noise estimate for the surface geostrophic velocity. This compact, optimal operator has fine along-track spatial resolution ideal for studies in areas of shallow water, near landfall, and at high latitudes where the first-baroclinic Rossby radius of deformation becomes small. Accurate extraequatorial geostrophic velocities are determined by varying the filter window to maintain accuracy and resolution, which will benefit global mesoscale studies from the tandem Jason-1 and TOPEX/Poseidon missions and, particularly, the proposed wide-swath altimetry mission.