Proceedings of the Bulgarian Academy of Sciences <p>Founded in 1948 by academician Georgy Nadjakov, "Comptes rendus de l’Académie bulgare des Sciences" is also known as "Доклади на БАН","Доклады Болгарской академии наук" and "Proceedings of the Bulgarian Academy of Sciences".</p> <p>If applicable, the name of the journal should be abbreviated as follows: C. R. Acad. Bulg. Sci. (according to ISO.)</p> <p><big><strong>Related subjects:</strong></big></p> <p class="condensed">Astronomy, Mathematics, Physics, Chemistry, Biology, Geology, Geophysics, Space Sciences, Engineering Sciences, Physical Geography, Medicine, Agricultural Sciences.</p> <p class="condensed">The manuscripts should contain only unpublished data in the respective scientific field. They can further be expanded to be published elsewhere. All manuscripts are subject to peer review and copy editing.</p> <p><strong><big>2002-2021 website:</big></strong></p> <p>The site with the archive of <em>Comptes rendus de l’Académie bulgare des Sciences</em> - 2002 to 2021 is <a href="" target="_blank" rel="noopener">still accessible at that address.</a></p> „Prof. Marin Drinov“ Academic Publishing House en-US Proceedings of the Bulgarian Academy of Sciences 1310-1331 <p>Copyright (c) 2022 Proceedings of the Bulgarian Academy of Sciences</p> <p>Copyright is subject to the protection of the <em>Bulgarian Copyright and Associated Rights Act</em>. The copyright holder of all articles on this site is Proceedings of the Bulgarian Academy of Sciences. If you want to reuse any part of the content, please, contact us.</p> On Weak Projection Invariant Extending Modules <p><span style="font-size: 0.875rem; font-family: 'Noto Sans', 'Noto Kufi Arabic', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen-Sans, Ubuntu, Cantarell, 'Helvetica Neue', sans-serif;">We introduce the notion of weak π-extending modules which is a proper generalization of π-extending and weak CS-modules. Several characterizations and connections between weak π-extending modules and related concepts are obtained. Direct sums and direct summand properties are also provided. Moreover, we investigate when the former class has an indecomposable decomposition and exchange properties.</span></p> Yeliz Kara Copyright (c) 2022 Proceedings of the Bulgarian Academy of Sciences 2022-06-30 2022-06-30 75 6 785 794 10.7546/CRABS.2022.06.01 New Computation Method of Minimum Dwell Time for the Schur Stability of Discrete-time Linear Systems <p>We are interested in Schur stability of discrete-time switched linear systems in this paper. For this purpose, we have obtained an upper bound of the solution of discrete-time switched linear systems depending on Schur stability parameter. Also, we have obtained dwell and average dwell time which satisfies the Schur stability of the system. The method given in this paper is based on Schur stability parameter.</p> Ahmet Duman Gülnur Kızlkan Copyright (c) 2022 Proceedings of the Bulgarian Academy of Sciences 2022-06-30 2022-06-30 75 6 795 803 10.7546/CRABS.2022.06.02 Involvement of Polyamines in Physiological Reactions of Herbicide-treated Wheat Seedlings Subjected to Drought and Waterlogging Stress <p>Polyamines are plant growth regulators, which take part in plant growth and development, as well as in the physiological responses to diverse biotic and abiotic stresses. Drought and waterlogging are environmental stress factors that disturb normal plant growth. In our study, we determined the content of polyamines spermine, spermidine, and putrescine in young wheat seedlings (<em>Triticum aestivum</em> L., cv. Sadovo-1) pretreated with herbicide Serrate<sup>®</sup> (Syngenta) and subjected for 7 days to drought or waterlogging. We found that when applied alone the herbicide caused some decrease in polyamine levels but it was not substantial as compared to drought and waterlogging stresses. Obvious reduction of polyamine content was caused by both stress factors when applied alone or in combination with the herbicide. The decrease was more significant in drought-stressed seedlings than in waterlogged. When plants were transferred to normal irrigation regime the polyamine concentrations in drought-stressed plants tended to increase. The waterlogging stress continued to reduce polyamine content even during the recovery period. These data correlate with the growth parameters (fresh weight, height of shoots) indicating the involvement of polyamines in the physiological responses of herbicide-primed wheat seedlings under stress conditions.</p> Dessislava Todorova Zornitsa Katerova Ljudmila Dimitrova Iskren Sergiev Copyright (c) 2022 Proceedings of the Bulgarian Academy of Sciences 2022-06-30 2022-06-30 75 6 923 932 10.7546/CRABS.2022.06.17 Effect of Zinc Oxide Nanoparticles on Shelf Life of Commonly Used Vegetables <p>Most of the harvested fruits and vegetables cannot be stored in natural conditions for satisfactory shelf life duration due to their perishable nature. Applications of nanotechnology to fruits and vegetables shelf life extension may overcome the shortcomings of traditional preservation technologies because nanotechnology has many unique properties. The aim of this study was to examine the effect of zinc oxide nanoparticles (ZnO NPs) synthesized from <em>Nigella sativa</em> extract on shelf life of cucumber and tomato. Nanoparticles were characterized by using UV-Vis spectrophotometry and particle size analyzer. Vegetables were soaked in tap water, distilled water and treated with different concentrations of ZnO NPs (0.05, 0.1, 0.2 and 0.5 g/100 mL). The effect of ZnO NPs on vegetables quality factor like external appearance, weight loss measurement, carotenoid content and total phenolic content was observed. The result showed that ZnO NPs reduced weight loss, delayed disease development and maintained firmness to extend shelf life of vegetables.&nbsp;</p> Sumera Javad Nimrah Azam Nadia Ghaffar Khajista Jabeen Asma Ahmad Copyright (c) 2022 Proceedings of the Bulgarian Academy of Sciences 2022-06-30 2022-06-30 75 6 933 942 10.7546/CRABS.2022.06.18 IWV Measurement and Modelling with GNSS and NWP (WRF) over Northwest Bulgaria <p>In recent decades, the increase in the number and intensity of high-impact weather events, like intense precipitation, hail and thunderstorms has been an indisputable trend worldwide. In Bulgaria they are common in the summer months and cause large economic losses. As a part of the project “Balkan-Mediterranean real-time severe weather service”, since February 2020, the Sofia University Global Navigation Satellite System (GNSS) Analysis Center processes in operational near-real time mode (NRT) stations in Bulgaria. In this study the Integrated Water Vapour (IWV) for four stations in Northwest Bulgaria was calculated using the Weather Research and Forecasting (WRF) model and the results are compared with IWV derived from the GNSS-meteorology concept. The results obtained lead to the conclusion that the model IWV is higher than the one derived from GNSS.</p> Martin Slavchev Tzvetan Simeonov Keranka Vassileva Guergana Guerova Copyright (c) 2022 Proceedings of the Bulgarian Academy of Sciences 2022-06-30 2022-06-30 75 6 845 852 10.7546/CRABS.2022.06.08 Alkaloids from Pandanus amaryllifolius and Pandanus tectorius from Vietnam and Their Anti-inflammatory Properties <p>Phytochemical investigation of the alkaloid composition in <em>Pandanus amaryllifolius</em> Roxb. and <em>Pandanus tectorius</em> Parkinson was carried out. Nine pandane alkaloids were isolated and determined. The alkaloids pandamarilactonine B, N-acetylnorpandamarilactonine A and pandamarilactonine G were found for the first time in <em>P. tectorius</em>. The total alkaloid mixtures and alkaloid fractions of the species were evaluated for their anti-inflammatory activity using in vitro experimental models. </p> Tsvetelina Doncheva Nadezhda Kostova Radostina Toshkovska Stefan Philipov Nam Vu Dat Nguyen Trung Nguyen Giang Do Hau Dang Copyright (c) 2022 Proceedings of the Bulgarian Academy of Sciences 2022-06-30 2022-06-30 75 6 812 820 10.7546/CRABS.2022.06.04 Verification of Ion Density in Magnetosheath Using THEMIS Satellite Plasma Measurements <p>The objective of investigation is to trace the changes of the ion density in the magnetosheath from the magnetopause to the bow shock. The theoretical interpretation is based on the magnetosheath- magnetosphere model, where the boundaries – bow shock and magnetopause, are self-consistently determined. The parameters in the magnetosheath are described by the gas-dynamic theory, while a variant of the Tsyganenko model is used in the magnetosphere, modified to compute the magnetic field in an area with arbitrarily chosen boundary. Data of the WIND satellites are used as solar wind monitoring. Good description is obtained for the considered crossing of the satellite THEMIS (Time History of Events and Macroscale Interactions during Substorms) of both the trend and ion density values of the magnetosheath. </p> Polya Dobreva Olga Nitcheva Monio Kartalev Copyright (c) 2022 Proceedings of the Bulgarian Academy of Sciences 2022-06-30 2022-06-30 75 6 853 860 10.7546/CRABS.2022.06.09 Effect of Silver Water Produced by an Engineered Device Designed to Food Industry <p>Hygienic norms in the food and beverages production systems should be permanently monitored to prevent contamination with pathogens which could cause diseases in humans and animals. As regards the industrial scale of food and beverages production, the maximum extension of the shelf life is an essential task. Peracetic acid (CH<sub>3</sub>COOOH) is frequently applied in sanitation procedures of production piping and product tanks, but if it is not rinsed well, could drastically affect organoleptic characteristics of the products. Silver water (containing Ag<sup>+</sup>) could replace the use of peracetic acid. A device for efficient production of silver water is constructed. The silver content of the water produced is determined by atomic absorption spectroscopy, UV-Vis and IR spectral analysis. It is found that minimal inhibitory concentration (MIC) of 1 μg silver content inhibited the yeast (<em>Saccharomyces cerevisiae</em> ATCC 200060) growth (24 ± 1 mm mean inhibition zone) on a similar rate as 30 μg kanamycin (27 ± 1 mm). Bacterial (<em>Escherichia coli</em> ATCC 47093) growth inhibited by 1.5 μg silver content (18 ± 1 mm) is comparable to 10 μg ampicillin (17 ± 1 mm) but not to 30 μg kanamycin (35 ± 1 mm).</p> Irina Alexandar Sophia Titorenkova Vladimir Titorenkov Copyright (c) 2022 Proceedings of the Bulgarian Academy of Sciences 2022-06-30 2022-06-30 75 6 821 826 10.7546/CRABS.2022.06.05 The Ratio of RAGE Isoforms Is Affected by HMGB1 and Its Truncated Form Only in A549 but not in H1299 Lung Cancer Cell Lines <p>The Receptor of Advanced Glycated End products (RAGE) could exist in two forms, a membrane bound and a soluble one. RAGE is highly expressed during embryonic development and decreases in all tissues except the lung in adult organisms. Lung RAGE mediates lung cancer, pulmonary fibrosis, chronic obstructive pulmonary disease (COPD), cystic fibrosis, etc. The lung cancers, among the most invasive of tumours, are reported to express low levels of RAGE. A specific RAGE ligand is the nuclear protein HMGB1. We examined the effect of HMGB1 and its truncated form lacking the C-tail (HMGB1ΔC) on RAGE expression in NSCLC cancer cell lines with different invasive capacity: A549 with better prognosis and H1299 with negative outcome. In A549 upon addition of ligands two important results are observed: (i) total RAGE expression is augmented 1.5 times and 1.7 times for HMGB1 and HMGB1ΔC, respectively, and (ii), a full-length membrane variant (flRAGE) is observed and in the presence of HMGB1ΔC its expression is comparable with the soluble one. In control H1299 lung cancer cells the dominant form of RAGE was the flRAGE while sRAGE represents one third of the whole amount of the receptor. HMGB1 and HMGB1ΔC do not affect the total RAGE amount and the ratio between flRAGE and sRAGE. In the case of lung cancer, the higher amount of RAGE is related to better prognoses and less metastatic ability which means that the stimulation of RAGE expression by HMGB1 and HMGB1ΔC in A549 is considered a positive tendency. In the case of the more aggressive lung cancer H1299 both ligands do not change the RAGE behaviour and in this way do not affect the invasive potential of the cancer cells. </p> Jordana Todorova Maria Petrova Evdokia Pasheva Iva Ugrinova Copyright (c) 2022 Proceedings of the Bulgarian Academy of Sciences 2022-06-30 2022-06-30 75 6 827 834 10.7546/CRABS.2022.06.06 Chitinase Profile of Arabidopsis, Subjected to the Combined Stress of Soil Salinity, Cuscuta campestris Parasitism and Herbivores <p>Plants are continuously subjected to changing environmental factors, often growing in suboptimal conditions. Due to the negative impact on agricultural yield numerous researches are published every year, studying the stress response and adaptation in plants to stress factors. Most of the studies, however, are dedicated to a single stress factor. In the present study, we examined the effect of a different combination of stresses on the chitinase profile of Arabidopsis plants. First, soil salinity (abiotic stress) was applied by irrigation with 150 mM NaCl. The plants were either infected or not with the parasitic flowering plant <em>Cuscuta campestris</em> Yunck. (dodder) and further challenged with the herbivorous attack – the larvae of either the sciarid fly <em>Lycoriella ingenua</em> (Dufour, 1839) or the citrus flatid planthopper <em>Metcalfa pruinosa</em> (Say, 1830). It was found that salinity and <em>Cuscuta</em> parasitism suppressed the response of <em>Arabidopsis</em> to sciarid flies, simultaneously with induction of chitinases in terms of the number of isoforms and relative activity in the parasite itself. Meanwhile, in the presence of <em>Cuscuta, M. pruinosa</em> tend to feed predominantly on the parasitic plant, completely ignoring the host. This was accompanied by the appearance of new chitinases, also suppressed by salinity. These results suggest complicated, antagonistic interactions between abiotic and different biotic stresses. It was also shown that the parasite is sensing the herbivore attack indirectly (in the case of <em>L. ingenua</em>) and transferring the stress signal to the host (in the case of <em>M. pruinosa</em>). </p> Lyuben Zagorchev Ivanela Albanova Yana Lozanova Mariela Odjakova Denitsa Teofanova Copyright (c) 2022 Proceedings of the Bulgarian Academy of Sciences 2022-06-30 2022-06-30 75 6 835 844 10.7546/CRABS.2022.06.07 Visual and Brainstem Auditory-evoked Potentials Correlate with Specific Motor and Non-motor Symptoms in Parkinson's Disease <p>Parkinson’s disease (PD) is a common and complex neurodegenerative disease characterized by progressive motor and non-motor symptoms. The current study investigated the dysfunction of brain hemispheres and brainstem of PD patients and its relation to motor and non-motor symptoms using brainstem auditory evoked potentials (BAEP) and pattern reversal visual evoked potentials (p-VEP). We hypothesized that some motor and non-motor symptoms of PD due to specific dysfunctions of brainstem and brain hemispheres could be related to changes in p-VEP and BAEP. Accordingly, the aim of this study was to explore the abnormalities of BAEPs and p-VEPs in patients with PD and its correlation to the motor and non-motor symptoms of PD. Seventy-six nondemented PD patients took part in the p-VEP evaluations and 66 nondemented PD patients were included in BAEP evaluation. In order to compare the p-VEP performance of PD group, we included a control group comprising 32 age-matched participants. A BAEP substudy involved 29 age-matched healthy controls. All patients underwent a comprehensive motor and non-motor neuropsychological assessment. We found significant changes in all tested parameters of p-VEPs in PD patients compared to control subjects. In addition, these lower scores in p-VEP correlate with the results of some tests for cognition related to frontal-striatal dysfunction and contralateral rigidity, but not with the results of overall motor evaluation or other non-motor symptoms studied. In BAEP, PD patients have a significant prolongation of III and V wave latencies compared to controls. In addition, when examining the possible associations of BAEP with other non-motor manifestations of the disease, such association was observed only by the apathy scale. In conclusion, the current study confirms the visual and auditory abnormalities among PD patients that<br />reflect brain hemispheres and brainstem pathological changes. It also correlates these abnormalities to some motor and non-motor features of the disease, providing a localizing tool for associated brain hemispheres and brainstem dysfunctions. Furthermore, abnormal visual evoked potentials are associated with motor and non-motor symptoms due to the basal ganglia dysfunction. In contrast, abnormal auditory evoked potentials are connected with apathy, which is considered to be related with brainstem raphe nuclei dysfunction. </p> Mariya Petrova Georgi Angov Latchezar Traykov Copyright (c) 2022 Proceedings of the Bulgarian Academy of Sciences 2022-06-30 2022-06-30 75 6 882 891 10.7546/CRABS.2022.06.12 Effects of Diabetic Neuropathy on the Postural Stability of Adults with Type 2 Diabetes Mellitus <div> <div>Diabetic peripheral neuropathy (DPN) is one of the most common complications, which occurs in 50–70% of people with Type 2 diabetes mellitus (T2DM). The disease usually affects the peripheral nerves and can cause loss of sensation in the feet, leading to impaired body equilibrium during quiet stance and gait, and elevated risk of falls. A total of 38 non-insulin-dependent T2DM adults (mean age 53.21 ± 7.74 years) – 18 with DPN and 20 without DPN, and 20 age- and sex-matched controls took part in the study. The DPN group was selected with standard clinical criteria and a Bulgarian version of the Michigan neuropathy screening instrument. Postural stability was evaluated with static posturography under two visual conditions (eyes open and eyes closed) on firm and soft support. Both diabetic groups exhibited significantly higher postural instability compared with the healthy controls, as well as stronger visual dependence. While standing on firm support, subjects with DPN exhibited significantly higher postural instability than those without DPN in the closed eyes condition only, whereas when standing on soft support, their postural stability was significantly more impaired compared with the diabetic subjects without DPN in both experimental conditions (open and closed eyes). A better understanding of the changes in postural stability in persons with T2DM would contribute to early diagnostics, rehabilitation and prevention of DPN.</div> </div> Mariya Irikeva Katerina Kirilova Dorina Petrova Georgi Angov Katerina Stambolieva Copyright (c) 2022 Proceedings of the Bulgarian Academy of Sciences 2022-06-30 2022-06-30 75 6 892 900 10.7546/CRABS.2022.06.13 Combined Artificial Collapse and Assisted Hatching Increase the Success Rate in Frozen Embryo Transfer <p>Human embryo cryopreservation is a contemporary procedure, widely used in assisted reproductive technology. It is of exceptional importance to assess all existing methods for improving embryo survival rate and increasing pregnancy and live birth rates. In the present study we examine the effects of artificial collapse on the survival rate of vitrified blastocysts and the impact of assisted hatching on the clinical pregnancy rate when frozen embryo transfer is performed. In addition we provide statistically significant data demonstrating the beneficial effect of combination of these procedures on the success rate for pregnancy after frozen embryo transfer. Embryologists might benefit from the approaches presented in this study in order to improve assisted reproductive technology outcomes. </p> Lyuboslava Valkova Tanya Milachich Tanya Timeva Atanas Shterev Copyright (c) 2022 Proceedings of the Bulgarian Academy of Sciences 2022-06-30 2022-06-30 75 6 901 907 10.7546/CRABS.2022.06.14 GPs' View on Barriers to Patients with Erectile Dysfunction: A Bulgarian Cross-sectional Study <p>Epidemiological studies show that erectile dysfunction (ED) is widespread worldwide and its prevalence tends to increase. ED influences quality of life, preceding or accompanying different diseases and it is a proven risk factor for cardiovascular disease. Different factors were found impeding patients to discuss the ED problem during the consultations.</p> <p>The aim of the study is to identify GPs’ view on barriers to patients with ED.</p> <p>A cross-sectional study, using questionnaire, was conducted among randomly selected GPs. For the statistical analysis descriptive statistics, chisquared test, Fisher’s exact test and SPSS 17.0 were used. The sample is nationally representative. Only 3% of all participants believe that patients share ED-related problems easily. The following most common barriers for the patients were identified by GPs: I<em>nappropriate conditions – </em><em>lack of privacy and lack of time in the doctor’s office</em> (62.8%); <em>embarrassment</em> (60.2%), and <em>the patient expects the erectile problem to be resolved spontaneously</em> (55.0%). Physicians’ personal characteristics age, gender and education affect their perception for the role of the different factors.</p> <p>Embarrassment remains among the leading barriers for the consultation about ED. At the same time there is a national specific barrier, connected with the organization of primary health care. Another important direction is education. Knowledge of the barriers is a prerequisite for their overcoming. Further researches are needed.</p> Georgi Tsigarovski Zdravko Kamenov Gergana Foreva Radost Assenova Nonka Mateva Arman Postadzhiyan Copyright (c) 2022 Proceedings of the Bulgarian Academy of Sciences 2022-06-30 2022-06-30 75 6 908 915 10.7546/CRABS.2022.06.15 Genetic Polymorphisms in Bulgarian Children with Non-alcoholic Fatty Liver Disease <p>Non-alcoholic fatty liver disease (NAFLD) is a multifactorial, highly prevalent liver disease in children. There is growing evidence that some genetic variants are associated with its development and rapid progression. The aim of this study was to assess the prevalence of specific single-nucleotide polymorphisms – <em>PNPLA3</em> I148M, <em>GCKR</em> P446L, <em>TM6SF2</em> E167K in Bulgarian children with NAFLD.</p> <p>We conducted a single-centre, prospective study of 32 children (22 patients with NAFLD and 10 healthy controls). Genetic testing and abdominal ultrasound were performed in all study participants. In the patients with NAFLD were additionally analyzed anthropometric parameters and standard biochemical tests, 10 of them underwent a liver biopsy for assessing disease activity and fibrosis.</p> <p>The most common polymorphism was <em>GCKR</em> P446L, detected in 75% of the studied population, followed by <em>PNPLA3</em> I148M (50.0% of the studied population) and <em>TM6SF2</em> E167K (9.4% of the studied population). The <em>GCKR</em><br>P446L gene variant was more common in patients with NAFLD than in healthy controls (86.4% vs. 50.0%, <em>p</em> = 0:03). Furthermore, significantly more patients with NAFLD were homozygous carriers of <em>GCKR</em> P446L compared to healthy controls (54.5% vs. 10.0%, <em>p</em> = 0:025). Both patients with histologically proven significant liver fibrosis were carriers of <em>GCKR</em> P446L.</p> <p>In the present study we found a high prevalence of <em>GCKR</em> P446L polymorphism among children with NAFLD, suggesting that this gene variant is associated with development of NAFLD in the Bulgarian pediatric population.&nbsp;</p> Denitza Kofinova Iglika Yordanova Petyo Hadzhiyski Rayna Shentova-Eneva Mila Baycheva Albena Todorova Daniela Avdjieva-Tzavella Copyright (c) 2022 Proceedings of the Bulgarian Academy of Sciences 2022-06-30 2022-06-30 75 6 916 922 10.7546/CRABS.2022.06.16 Electrical Conductivity Properties of Solid Polymer Electrolytes PEO-PVP-NaIO4 Filled with TiO2 Nanoparticles <div> <div>Frequency-dependent conductivity and conduction relaxation in nanocomposite solid polymer electrolyte films consisting of poly(ethylene oxide) and polyvinyl pyrrolidone blend as host matrix, NaIO4 as ions dopant, and titanium dioxide (TiO2) nanoparticles of average size 10 nm, were studied. The films were characterized using measurements of complex electrical impedance spectra in the frequency range from 1 Hz to 1 MHz, at room temperature. The concentration of the salt NaIO4 was 10 wt%, the TiO2 percentage varied from 1 wt% to 5 wt%. The frequency spectra of both alternating-current (AC) conductivity and electric modulus were analyzed in order to elucidate the effect of TiO2 nanoparticles on the electrical transport properties of the examined electrolyte thin films with a thickness of 150 μm.</div> </div> Todor Vlakhov Yordan Marinov Georgi Hadjichristov Nicola Scaramuzza Copyright (c) 2022 Proceedings of the Bulgarian Academy of Sciences 2022-06-30 2022-06-30 75 6 804 811 10.7546/CRABS.2022.06.03 Robust Adaptive Control for Robot Manipulators Trajectory Tracking Based on Iterative Learning Observer with Time-delay <p>For the trajectory tracking of the robot manipulators, the robust adaptive control based on iterative learning observer is designed. The robot manipulators model is given, and an iterable dynamic equation is obtained by linearizing the model. An iterative learning observer is designed, so that the output state of the observer can converge to the actual state of the system and the convergence analysis of the observer is also given. Based on Lyapunov stability theory, the robust adaptive controller is given to ensure that the trajectory tracking error of the robot manipulators gradually approaches zero. Finally, the double joint robot manipulators systems are simulated, and the results verify the effectiveness of the method.&nbsp;</p> Xingjian Fu Jiahui He Copyright (c) 2022 Proceedings of the Bulgarian Academy of Sciences 2022-06-30 2022-06-30 75 6 861 872 10.7546/CRABS.2022.06.10 A Two-stage Detector for First-order Sea Clutter in HF Surface Wave Radar <p>The detection capabilities in vertical polarized, High-frequency Surface<br />Wave Radar (HFSWR) are limited by the clutter spikes in first-order sea clutter. The energy, position, and Doppler frequency of first-order sea clutter depend on the sea states, which are non-homogeneous – because of that, using a fixed expression to eliminate sea clutter gives limited results. In this paper, the authors proposed a new adaptive method using a two-stage detector based on the Order Statistic Constant False Alarm Rate (OS-CFAR) principle. The approach shows the capability to eliminate heterogeneous clutter and detect targets in the vicinities of sea clutter in the Range – Doppler map. </p> Dung Ha Hieu Le My Bui Linh Cao Copyright (c) 2022 Proceedings of the Bulgarian Academy of Sciences 2022-06-30 2022-06-30 75 6 873 881 10.7546/CRABS.2022.06.11