Huseyin Duman

Physical accessibility constitutes a fundamental dimension of spatial justice, ensuring that all individuals can equally benefit from the built environment. This study aims to evaluate and quantify the accessibility challenges faced by wheelchair users within the Sivas Cumhuriyet University campus and to propose a model that objectively identifies spatial inequalities. For this purpose, the intra-campus pedestrian network was modelled using Python, and shortest-path analyses were conducted via Dijkstra’s algorithm for 1,000 randomly selected origin–destination pairs. These routes were further simulated within a multi-agent system (MAS) environment to estimate travel times and compare mobility performance between wheelchair users and able-bodied individuals. This study provides one of the first quantitative frameworks to integrate Dijkstra-based shortest-path computation and MAS-driven simulation for assessing wheelchair accessibility in outdoor environments. The resulting data were used to develop a numerical accessibility scoring system that expresses spatial disadvantage as an accessibility coefficient. The findings revealed that 85.8% of the routes were completely inaccessible for wheelchair users and that, where access was possible, travel distances were on average 8.5 times longer than those of non-disabled individuals. By establishing a reproducible and data-driven framework, the study connects the aim of promoting spatial equity with quantifiable outcomes, thereby providing a decision-support tool for campus redesign and urban accessibility planning. These findings provide a scalable analytical framework for promoting spatial equity, offering practical guidance for policymakers and urban planners seeking to improve accessibility in built environments.

The Yedisu Segment is one of the most significant seismic gaps of the North Anatolian Fault Zone (NAFZ), following the segments located near Istanbul. This segment is approximately 80 km long and has remained seismically quiet for about 242 years. It has the potential to generate an earthquake magnitude of greater than Mw 7. The Tercan and Nazımiye dextral faults are located in the vicinity of the Yedisu Segment and exhibit similar strikes of approximately N70–75°W. In this study, slip rates and locking depths of the Yedisu Segment and its neighboring Tercan and Nazımiye faults are estimated by inverting GNSS-derived Eurasia-fixed velocity fields using elastic half-space dislocation models. Velocities from Turkish Real-Time Kinematic GNSS Network (CORS-TR, including a few older stations), Turkish National Fundamental GPS Network (TNFGN), and previously published regional GNSS networks are used, and the inversions are performed using fault-parallel velocity components while accounting for differences in fault strike. Two different forms of elastic half-space dislocation model are tested for the Yedisu Segment: (i) symmetric and (ii) asymmetric fault-slip behavior. The symmetric model yields a Yedisu slip rate of approximately 18.7 mm/yr, while the Nazımiye and Tercan faults contribute only minor deformation to the regional velocity field. The asymmetric model conversely discloses a difference between the south- and north-side of the Yedisu Segment, with slip rates of about 6.5 and 11.9 mm/yr, respectively. The fault slip rate asymmetry correspondes to a ratio of 1.83. The asymmetric model explicits a significantly better fit to the GNSS velocity field than the symmetric approach. Assuming a long-term average slip rate of 18.4 mm/yr, the Yedisu Segment has accumulated approximately 4.45 m of slip deficit over the past 242 years, consistent with the potential for a large, destructive earthquake. These results indicate that the Yedisu seismic gap is highlighting its critical importance for seismic hazard assessment in eastern Türkiye. This research is supported by the TÜBİTAK project No. 124Y204.Keywords: North Anatolian Fault, Yedisu Segment, GNSS, Fault slip asymmetry, Seismic gap

Global Navigation Satellite Systems (GNSS) can observe a variety of surface deformations on Earth, including periodic oscillations at different frequencies. An example of such phenomena is ocean tide loadings (OTL), which result from the redistribution of water mass. The Global Positioning System (GPS) exhibits orbital geometry that causes its revisit and orbital periods to coincide with the diurnal and semi-diurnal luni-solar declination constituents, known as K1 and K2, respectively. Consequently, the system faces challenges in accurately estimating these periodic oscillations due to its orbital artifacts. This study aims to quantify the extent to which GPS orbital artifacts introduce periodic signals into the K1 and K2 constituents by utilizing the Galileo system and determining the most suitable positioning approach. A dataset from the International GNSS Service (IGS), spanning 40 days in 2024 and covering six stations, was analyzed. Coordinates were estimated using both kinematic positioning every 5 minutes and a 6-hour static precise point positioning (PPP) mode with a 3-hour shift. The power spectra for the east, north, and up components indicated that, on average, the GPS system contributes 52.8% to the K1 constituents and 66.3% to the K2 constituents. Despite expectations that the diurnal K1 and semi-diurnal K2 tidal constituents would be more prominent in the power spectra of the GPS comparing to that of natural signature or of other navigation system (Galileo for this study), the diurnal K1 tidal constituent appeared weak in the kinematic mode power spectra for the GPS system. These findings validate that the overlapped-static PPP mode is a more appropriate approach for estimating these periodic deformations.

This study examines the Vertical Total Electron Content (VTEC) estimation performance of multi-instruments on a global scale during different ionospheric conditions. For this purpose, GNSS-based VTEC data from Global Ionosphere Maps (GIMs), COSMIC (F7/C2)—Feng–Yun 3C (FY3C) radio occultation (RO) VTEC, SWARM–VTEC, and JASON–VTEC were utilized. VTEC assessments were conducted on three distinct days: geomagnetic active (17 March 2015), solar active (22 December 2021), and quiet (11 December 2021). The VTEC values of COSMIC/FY3C RO, SWARM, and JASON were compared with data retrieved from GIMs. According to the results, COSMIC RO–VTEC is more consistent with GIM–VTEC on a quiet day (the mean of the differences is 4.38 TECU), while the mean of FY3C RO–GIM differences is 7.33 TECU on a geomagnetic active day. The range of VTEC differences between JASON and GIM is relatively smaller on a quiet day, and the mean of differences on active/quiet days is less than 6 TECU. Besides the daily comparison, long-term results (1 January–31 December 2015) were also analyzed by considering active and quiet periods. Results show that Root Mean Square Error (RMSE) values of COSMIC RO, FY3C RO, SWARM, and JASON are 5.02 TECU, 6.81 TECU, 16.25 TECU, and 5.53 TECU for the quiet period, and 5.21 TECU, 7.07 TECU, 17.48 TECU, and 5.90 TECU for the active period, respectively. The accuracy of each data source was affected by solar/geomagnetic activities. The deviation of SWARM–VTEC is relatively greater. The main reason for the significant differences in SWARM–GIM results is the atmospheric measurement range of SWARM satellites (460 km–20,200 km (SWARM A, C) and 520 km–20,200 km (SWARM B), which do not contain a significant part of the ionosphere in terms of VTEC estimation.

Thanks to the development of the real-time kinematic (RTK) algorithm and the emerging Global Navigation Satellite System (GNSS), especially for Galileo and BeiDou-3, reliable positioning accuracy for medium and long-baseline RTK became possible globally. Moreover, with the development of the GNSS receiver hardware, baseline length limitations due to radio-based communications are removed thanks to internet-based communication. In this work, single-baseline RTK, incorporated partial ambiguity resolution with troposphere and ionosphere weighting, using GPS (G), Galileo (E), BeiDou-3 (C3) and multi-GNSS (GE and GEC3), is conducted with real GNSS data of EUREF Permanent GNSS network under three different cutoff angles (10°, 20°, and 30°) for six different lengths of baselines (~50, ~150, ~250, ~350, ~450, and ~550 km). The results show that the multi-GNSS RTK solution significantly contributed to the positioning accuracy and convergence time of the single-system RTK solutions. Based on the results, non-available epoch-wise solutions for the high-degree cutoff angles are more obvious for the single-system RTK, whereas multi-GNSS solutions provide 100% solutions for each cutoff angle and baseline. The results indicate that instantaneous and a few epochs single-epoch ambiguity resolution is feasible for 50, 150, 250 and 350 km baseline lengths for multi-GNSS RTK. Based on the positioning results, horizontal–vertical positioning improvements of multi-GNSS RTK (GEC3) compared with the single-system GPS RTK are found as 50%–37%, 40%–35%, 55%–47%, 53%–54%, 57%–49% and 57%–49% for 50, 150, 250, 350, 450 and 550 km, respectively, under a 10° cutoff angle. For 20° and 30° cutoff angles, the accuracy improvements are much higher. The convergence time improvements (n/e/u) of multi-GNSS RTK (GEC3) compared with the single-system GPS RTK are found as 86/92/75%, 77/67/72%, 75/77/83%, 53/56/52%, 69/49/62%, and 52/45/39% for 50, 150, 250, 350, 450 and 550 km, respectively, under a 10° cutoff angle.

Thanks to the rapidly emerging low-cost dual-frequency GNSS receivers, a feasible alternative for geodetic-grade GNSS receivers became available for some GNSS applications. In this study, the performance of data integrity and quality of a low-cost ZED-F9P u-blox GNSS receiver was investigated by comparing it with a geodetic-grade GNSS receiver. Availability of the epoch and phase/code signal channels, signal-to-noise ratio (SNR), code multipath, and cycle slips were analyzed for the geodetic-grade and low-cost ZED-F9P u-blox GNSS receivers. One month’s data of GPS, GLONASS, and Galileo constellations were analysed using the RINEX files of the receivers. The results showed that the epoch availability of the geodetic-grade and u-blox GNSS receiver is comparable to each other, while the availability of phase/code signal channels of the geodetic-grade receiver is higher than the u-blox receiver. In terms of data quality, SNR values from both receivers are comparable, while the multipath level of the u-blox GNSS receiver is significantly higher than the geodetic-grade one. The results also showed that the number of cycle slips of the u-blox receiver is significantly higher than the geodetic-one.

Depremin meydana gelmesi ile yeryüzünde ani yer değiştirmeler (kosismik yer değiştirme) ve akabinde deprem sonrası rahatlama evresi süreçleri yaşanır. Deprem sonrası rahatlama evresi, yer kabuğunun düşük viskoziteli katmanında ve üst mantoda biriken gerinimin gevşemesi sürecidir. Zamana bağımlı doğrusal olmayan bu geçiş sürecinin yeryüzündeki etkileri Küresel Konumlama Sistemi (Global Positioning System, GPS) ile izlenebilmekte ve GPS zaman serileri ile matematiksel olarak üstel fonksiyonlarla modellenebilmektedir. Modellen(e)meyen deprem sonrası rahatlama evresi matematiksel modelin diğer parametrelerini etkilemektedir. Bu çalışmada, deprem sonrası rahatlama evresi deprem öncesi ve sonrası ölçülerin hem bütünleşik hem de ayrı ayrı değerlendirilmesinin GPS zaman serileri hata karakterine, hız ve kosismik yer değiştirmelere etkileri irdelenmiştir. Deprem sonrası rahatlama zamanı Nelder-Mead yakınsama algoritması ile optimize edilmiş, bu değerler ve ek deterministik büyüklükler ile GPS hata zaman serileri beyaz gürültü (BG) + kırpışma gürültüsü (KG), BG+KG+rasgele yürüyüş gürültüsü (RYG) ve BG+güç-yasası gürültüsü (GYG) stokastik model kombinasyonları ile analiz edilmiştir. Bütünleşik analiz edilen zaman serileri, RYG ya da spektral indeks değeri ortalama -1.25’lere yaklaşan GYG gürültü modelleri ile temsil edilebilmektedir. Aksine, ayrı ayrı analizlerden birleştirilen GPS hata zaman serilerinin tümü BG+KG modeli ile karakterize edilmektedir. Hem kalite ölçütleri hem de GPS hata zaman serilerinin görsel irdelemeleri, ayrı ayrı analiz edilmelerinin doğru bir yaklaşım olduğunu göstermektedir. Buna göre bütünleşik analiz, hız bileşeninde 0.52 mm/yıl’a kadar yanlı kestirime, hız standart sapmalarında %94’e kadar aşırı tahmin edilmesine ve kosismik yer değiştirmelerde 8 cm’ye kadar farklılıklara sebep olabilmektedir.