Asian Science Citation Index is committed to provide an authoritative, trusted and significant information by the coverage of the most important and influential journals to meet the needs of the global scientific community.  
ASCI Database
308-Lasani Town,
Sargodha Road,
Faisalabad, Pakistan
Fax: +92-41-8815544
Contact Via Web
Suggest a Journal
 
Articles by M. Abdullah
Total Records ( 6 ) for M. Abdullah
  M. Abdullah , H.J. Strangeways and S.S.N. Zulkifli
  Since the United States government discontinued Selective Availability (SA) on 1 May 2000, ionospheric effects have been responsible for the largest errors in GPS systems. The standard Differential GPS (DGPS) method is incapable of completely eliminating the ionospheric error. This paper describes a new approach to determine the differential ionospheric error between geographically distributed receiver stations. The ray paths of GPS signals were simulated using a modified Jones 3D ray tracing programme that includes the effect of the geomagnetic field. A Nelder–Mead optimisation algorithm was embedded in the program to precisely determine the satellite-to-station path. A realistic ionospheric model is essential for accurate ray tracing results and for estimates of differential error that are accurate on sub-centimetre scales. Here, the ionospheric model used in the ray tracing programme was developed by fitting realistic ionosphere profiles with a number of exponential functions. Results were compared to the theoretical approach. Results show that the differential delay is about 1–5 cm at low elevation angles for a short baseline of 10 km, as reported in other literature. This delay is often neglected in DGPS application. The differential delay also shows a pattern similar to that predicted by the Klobuchar model. The method proposed here can be used to improve future GPS applications.
  M. Abdullah , A.F.M. Zain , M.H. Jusoh , N. Misran and W.A. Mubarak
  Problem statement: Ionospheric precursors of earthquake have been studied by scientists and seismologists. This study aims at examining the relationship between the ionosphere and earthquake precursors. The effects of the anomalous electric field that penetrates the ionosphere on the electron concentration can be measured experimentally. This study reports on the variability of the Total Electron Content (TEC) during the December 26, 2004 earthquake in North Sumatra (epicentre: 3.295°N, 95.982°E) which measured 9.3 on the Richter scale. Approach: The ionospheric TEC near North Sumatra between December 19 to 31 was calculated between 22:00 and 24:00 and between 02:00 and 06:00 local time (LT) using a dual frequency Global Positioning System (GPS) receiver. It was recorded for a period of 13 days, which is seven days before and five days after the earthquake. The GPS data was taken from the Department of Survey and Mapping Malaysia in the north of Malaysia (4.1°N, 99.8°E), which is near North Sumatra. Four sets of data were selected from different GPS satellites that passed near the epicentre. Results: Results show good agreement with the existence of earthquake precursors. TEC variability was detected at night and in the early morning of 21 December 2004 (five days before the earthquake) and 25 December 2004 (a day before). Findings show an increase in the electron concentration level at the station closest to the epicentre and the TEC varies from 2-10 TEC unit. Conclusion/Recommendations: The findings correspond with previous research and literature in this field. Further studies on the parameters that cause the change in the ionospheric TEC due to earthquakes are needed if this is to be used as part of an earthquake early warning prediction system.
  C.H. Chia , S. Zakaria , S. Ahamd , M. Abdullah and S. Mohd. Jani
  Magnetic papers have been successfully prepared from unbleached kenaf (hibiscus cannabinus) kraft pulps via the lumen loading and in situ synthesis process. The lumen loading method allows the magnetic fillers to be introduced into the lumen of fibres and at the same time leaving the external surfaces free from filler, resulting the better inter-fibre bonding and subsequently its papermaking properties are not affected. Fillers, which deposited in the lumen of fibres will be protected from further mechanical influences. For the lumen loading process, commercial magnetite (Fe3O4) powders have been introduced into the lumen by mixing the pulp and magnetic powder. For the in situ synthesis process, nanosized magnetite particles have been precipitated in the presence of pulp fibres and deposited inside the lumen of fibres via chemical coprecipitation process. The samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) and their magnetization properties were examined by a vibrating sample magnetometer (VSM). The loading degree of the papers was estimated by using the Thermogravimetric analyzer (TGA). The VSM results show that the magnetic properties of the lumen loaded papers are higher compared to the in situ synthesis papers in which can be explained that the nanosized magnetite particles possess superparamagnetic properties. However, the SEM micrograph shows that the size and the distribution of the magnetic particles inside the lumen for the in situ synthesis papers are much better than that of the lumen loading papers.
  Norsuzila Y. , M. Abdullah and M. Ismail
  The signals from the satellites of the Navstar Global Positioning System (GPS) travel through the earth's ionosphere on their way to GPS receivers. However, ionospheric delay is one of the main sources of error in GPS. The magnitude of the ionospheric delay is influenced by the Total Electron Content (TEC) along the radio wave path from a GPS satellite to the ground receiver. This study investigates the TEC using GPS data collected from Wisma Tanah, Kuala Lumpur, KTPK (3° 10’ 15.44"N, 101° 43’ 03.35"E) station and processed and analyzed under quiet geomagnetic day at the equatorial region on 8 November 2005. This research assesses the errors translated from the code-delay to the carrier-phase ionospheric observable by the so-called leveling process, which was applied to reduce multipath from the data. It was found that the leveled carrier-phase ionosphere observable was affected by a systematic error, produced by code-delay multipath through the leveling procedure. The effects, however, do not cancel after averaging all the data. Dual frequency carrier-phase and code-delay GPS observations are combined to obtain ionospheric observable related to the slant TEC (TECs) along the satellite-receiver line of sight (los). This results in the absolute differential delay and the remaining noise was discarded. These are the first results obtained using TEC-GPS technique for TEC measurement in Malaysia.
  M. Abdullah , A.F.M. Zain , Y.H. Ho and S. Abdullah
  Problem Statement: Ionospheric scintillations, which cause significant effects on satellite signals for communication and navigation, often takes place in equatorial region such as Malaysia. However, this disturbance is not fully understood due to few studies performed. This research reports the study and monitoring activity on Total Electron Content (TEC) and ionospheric scintillation in Malaysia using GPS measurements. Approach: One dual-frequency GPS receiver was positioned at the main station in Parit Raja, West Malaysia (1.86° N, 103.8° E) and Sipitang, East Malaysia (5.10° N, 115.56° E) respectively. Dual-frequency GPS data collected during the one-month ionospheric experimental campaign was used for TEC and scintillation computation and analysis. The TEC with 15 sec interval were computed from combined L1 and L2 code-pseudorange and carrier phase measurements. Whereas, the scintillation parameter S4 index was computed as a standard deviation of the received signal power normalized to average signal power every 60 sec on L1. A corrected S4 (without noise effects) was also computed and used in the analysis. Results: It was found that the daily maxima vTEC for Parit Raja (PR) ranged from 38-100 TECU, which is generally higher than those of Sipitang, which ranged from 30-42 TECU. However, a general consistency for both stations can be seen during the 1 month campaign period. Conclusions/Recommendations: In conclusion, these results show good agreement in the existence of the equatorial anomaly observed during moderate solar flux conditions and undisturbed geomagnetic condition. This will contributes to the knowledge of equatorial ionosphere and help in space weather condition. However, to better understand and characterize the ionosphere over Malaysia, more campaigns should be encouraged.
  K. Singh , M. Ismail , K. Jumari , M. Abdullah and K. Mat
 

This study presents the development of techniques to estimate mobile user`s location (position) for Universal Intelligent Positioning System (UIPS) project. UIPS uses combination of available Location Determining Technologies (LDT) and newly developed techniques for UMTS (3G) or beyond networks. The usage of each LDT (technique) will determine Location Base Services QoS (accuracy of mobile user`s location). The new techniques developed are Close Circle Correlation (CCC) and Newton Raphsons 3 Circles (NR3C). Both techniques use time measurements observed from three Node B (base stations) in known Line of Sight (LOS) environment (multipath time delays are known). For unknown LOS environment, further enhancements on CCC technique and NR3C technique are developed, such as Averaging Estimator of CCC, First Mean Averaging Estimator of NR3C and Random Search Averaging Estimator of NR3C. The Cumulative Distribution Function of simulated results (simulation of actual data collected through drive test in UMTS network with known LOS) using NR3C technique produced 67% of the estimated user`s location error at 0 m and 95% of the estimated location error at 1.7 nm. Using CCC technique, produced 67% estimated location error at 2.04 m and 95% estimated location error at 3.2 m. NR3C produces better accuracy in known conditions of multipath delays. In unknown LOS conditions, Averaging Estimator of CCC produced 67% location error at 50.67 m and 95% error at 218 m, which is better than the other two enhanced (averaging) techniques of NR3C.

 
 
 
Copyright   |   Desclaimer   |    Privacy Policy   |   Browsers   |   Accessibility