Evaluation of runway surface conditions at Vienna airport using Sentinel SAR interferometry

The present paper proposes an advanced observation technology for the airport runway monitoring service (RMS) based on the complementary use of interferometric SAR data from Sentinel-1-A/B satellites, hydrometeorological observations and ground geotechnical surveys. We investigate the suitability and technical feasibility of using satellite dual-sense persistent scatterer interferometry for non-invasive remote control and long-term monitoring of runway surface conditions at Vienna International Airport. The research focuses on the detection, measurement and interpretation of runway surface distress, pavement deformation and contamination, separately in the touchdown, midpoint, and rollout thirds of the Vienna airstrips on a weekly to monthly time scale, regardless of subjective judgement. We identify system requirements, observation targets, essential conditions and opportunities for the combination of space and ground data in close contact with airport operators and experts on site. Preoperative developments towards practical implementation of the RMS service prototype for planning airport maintenance operations are discussed. The remote sensing technique of persistent scatterer interferometry (PS-InSAR) using Sentinel-1 SAR data is well suited for geotechnical monitoring of pavement distress at airports. However, strict accuracy requirements and usually a small number of permanent scatterers found on runways and taxiways due to the relatively low backscatter of microwaves and random fluctuations in local weather are the main limiting factors for the practical application of PS-InSAR at airports. It appears that we have solved the latter problem on the list using the iterative remove-restore approach to SAR data analysis and weather correction referred to as “better-half” technique, which, unlike other known stratagems, takes into account changes in the state of the target surface in addition to tropospheric corrections. Two Sentinel-1 SAR image sequences with precise orbital data for the periods of 2014-2017 (94 IW scenes) and 2019-2020 (118 IW scenes) were ranked and rearranged according to the hourly precipitation amount and the condition (dry, wet, moist, frozen, snow-covered etc.) of the runway surface recorded at airport weather station No. 110360. These rearranged “all-weather” image sequences were divided into two equally long “dry” and “wet” parts, each including the same master scene, and processed separately using the standard procedure for persistent scatterer interferometric analysis implemented in the SARscape software package. The threshold for interferometric coherence values was set at 0.85. The number of persistent scatterers found on runways and taxiways is 1233 for bad-weather data, 1663 for all-weather data and 3228 PSs for the image time series obtained under good (dry) weather. The resulting displacement time series show a fairly stable state of the runways with an average displacement rate of about 2.5 mm / a around the value zero. In some places and dates, however, we detected outliers with magnitudes up to 9 mm/a that exceed three times the standard deviation criterion. In the “all-weather” displacement time series the outliers constitute approx. 8% of all measurements while in the “better-half” displacement time series the number of outliers is radically reduced. In good-weather displacement time series there are still some outliers with magnitudes exceeding the limit of 6 mm specified for medium-severity distress on runways (see Appendix). Such outliers were subjected to further causal statistical analysis to determine whether they were due to weather-related errors or runway distress. The average magnitude of weather-related errors in the study airfield is between 2 and 4 mm/a, and rainy weather is responsible for 50% of all outliers. Some outliers identified in both the original and the better-half displacement signal, were related to wet haze events or the wet and iced surface of runways. For example, between 2014 and 2017, 28 events of wet haze caused 12 outliers. 29% of all outliers could not be associated with hydrometeorological effects. In close proximity to each suspicious outlier, we have determined several conjugate persistent scatterers, which are scatterers that can be identified as the same target in better and worse halves of SAR datasets. Then we measured the difference in the range coordinates of conjugate “dry” and “wet” PSs with subpixel accuracy. On runways and taxiways, the magnitude of relative range shifts varies between 0.1 and 0.75 pixel and depends on the incidence angle of the radiation. Usually, conjugate “dry” and “wet” PSs are shifted in opposite directions from the position of the reference PS, which is determined with the all-weather dataset. Systematic relative range shifts indicate weather impacts. Wet air has a lower density and refractivity than dry air and causes straightening and shortening of the radar propagation path in the troposphere. The relative range shift of wet PCs is thus towards the satellite (negative). In the deformation maps derived from “wet” data most of the PSs and ground surface appear to be uplifting. In our products they are marked with cyan and blue points. Rapidly varying shift values indicate local deformations on the runway surface. We have accounted for varying incidence angle and mapped relative shifts in the range over the entire airfield on a scale of 20.000. The resulting displacement maps were rasterized with a pixel size of 10 m and laid over optical images from WorldView-3 to identify PSs and to analyse the PS phase variation (see Appendix). The somewhat patchy appearance of the displacement maps is explained by the limited number and the irregular distribution of the conjugate PSs detected on and near the runway surface. The main maximum of the range shift was found near reference point no. 3 on the old tow route with numerous cracks on the surface. Several extremes were found between the runways and in the area of their intersection. Other local maxima coincided with optically recognizable rubber deposits from aircraft tires that accumulated in the take-off, brake-and-turn and touchdown zones of airport runways. In addition, in the south-western part of the airfield, where a third runway is to be built, we discovered a small area with a land subsidence of 1 mm/a. Two outliers at reference points 3 and 2, which were detected on August 1 and 7, 2020, were interpreted as thermal displacements of separate slabs due to rapid daytime changes in surface temperature of 12 ° C and 14 ° C, respectively. The expansion joints built into the runways largely absorb the expansion or compression of separate concrete slabs. However, the proper functioning of the joints is sometimes disrupted by the fact that solid materials fall into the joint space. In hot weather, this leads to a local upward movement of the slab edges or to splinters near the joint. Such emergencies must be reported and repaired immediately because of severe damage potential to aircraft. The resulting deformations of concrete slabs, both horizontal and vertical, are large enough to be recorded and measured with Sentinel PS-InSAR. We conclude that the satellite interferometric RMS controller is useful and feasible. The research was carried out under the contract ID. 881043 (ICARUS) with the Austrian Research Promotion Agency. Sentinel-1 SAR data was downloaded from the Copernicus Open Access Hub at https://scihub.copernicus.eu/ .