Browsing by Author "Yasser Asrul Ahmad, Ph.D"

Star sensors play a crucial role in space navigation, providing reliable attitude determination for spacecraft, which is especially important for future lunar missions such as the International Lunar Research Station (ILRS). Operating on the lunar surface, however, introduces unique challenges. Strong solar reflections from the lunar regolith, together with cosmic and solar radiation generate significant noise that can degrade image quality and disrupt the star detection as well as reducing the centroiding accuracy. This research aims to develop an enhanced centroiding algorithm to overcome the noises in the lunar environment as well as to evaluate the performance of the proposed algorithm. The methodology incorporates several preprocessing stages, including Point Spread Function (PSF) modelling to simulate optical blurring and median filtering for noise suppression. Star regions are then segmented using the global thresholding method, where the threshold value is defined as a function of the maximum image intensity. The centroid of each detected star is computed using an intensity-weighted Centre of Mass (COM) approach. The accuracy of the proposed method is validated by transforming pixel-based centroid coordinates into celestial coordinates through an affine transformation, followed by star identification using the Hipparcos star catalogue. Experimental evaluation was conducted to imitate the lunar surface environment under simulated noise including Poisson-Gaussian noise, salt-and-pepper noise, speckle noise, and sunlight glare. The results show that the proposed algorithm outperforms conventional methods such as COM, Gaussian Fitting, and Sieve Search Algorithm (SSA), achieving the lowest average RMSE of 1.218 pixels and Euclidean distance of 1.143 pixels. It also maintains a low False Detection Rate (FDR) of 6.716% and angular distance errors below 0.05°. These results demonstrate that the algorithm is robust under low SNR conditions, making it suitable for reliable star sensing in long-term lunar missions. Keywords: Star sensor, lunar surface navigation, centroiding algorithm, image preprocessing, attitude determination.

Latency is a key performance factor for satellite internet, especially for real-time applications like web browsing, video calls, Virtual Private Network (VPN), and remote access used in rural, maritime, emergency, and military environments. While Geostationary (GEO) High Throughput System (HTS) systems still experience high latency due to their orbital altitude, Starlink’s Low Earth Orbit (LEO) constellation now offers globally available low-latency, high-bandwidth satellite internet. Despite their high-speed benefits, both HTS systems in GEO and LEO are using Ku-band which is highly susceptible to rain-induced signal attenuation, particularly in regions with heavy rainfall such as the tropics and equatorial countries. The rain attenuation at Ku-band not only weakens the radio frequency signals but also increases real time latency in the satellite communication link. This research aims to determine the latency of HTS satellites in LEO and GEO induced by rain and evaluate its performance under heavy rainfall conditions. This study uses real satellite link paid services from Société Européenne des Satellites (SES) operator from Luxembourg, SES-12 a geostationary HTS located at 95° east and Starlink LEO constellation operated by Starlink Services a subsidiary of American space company, SpaceX for rain-induced latency performance assessment. A 1.2m VSAT terminal for SES-12 and a Starlink terminal were installed to measure and record latency continuously for six months at the same location with a collocated rain gauge to measure rainfall rate. Site-specific latency datasets were collected to evaluate the Round Trip Time (RTT) using Ku-band link under various rainfall intensities for both satellite services. The empirical measurements were further developed into rain-induced latency correlations, using power law and logarithmic based equations that directly relate to rainfall rate for both satellite systems. These new relationships capture the nominal and tail latency behaviors, representing the normal operating conditions and the degraded latency performance during severe rain-fade conditions, respectively. The results show that the SES-12 GEO terminal latency ranges from 600 ms to 3,000 ms, while the LEO Starlink terminal maintains a much lower latency between 20 ms and 100 ms. Starlink achieved higher link availability (99.6 %) compared to 94 % for SES-12 in the same tropical environment. The findings indicate that extreme rainfall significantly impacts the GEO link, causing prolonged signal degradation and delay expansion, whereas the LEO link remains less affected due to its low-orbit architecture and dynamic satellite switching capability. Keywords: Low Earth Orbit (LEO), Geostationary Earth Orbit (GEO), Rain-Induced Latency, Ku-Band Satellite Communications, Tropical Regions, Empirical Measurements, Link Availability.