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A typical VSAT network depicting two- way communications from remote terminals through a VSAT satellite to a central hub. VSAT satellite space segment providers offer three types of satellite beam: spot, hemisphere, and global. Spot beams are available in both Ku-band (12-16 GHz) and C-band (4-6 GHz) and are focused on population centers. Spot beams are generally high power, thus allowing smaller antenna dishes to be used at remote sites (for example 1.8 m for Ku and 3 m for C band). Hemisphere and Global beams have a much larger footprint and weaker signal strength. The majority of hemisphere and all global beams are C -band. C-band’s lower frequencies are less sensitive to rain-induced signal degradation (attenuation of 1 to 2 dB ) compared to the Ku-band signal (attenuation of 5 to 10 dB) but suffers from greater interference relative to Ku-band (Maral, 1995). Because the antenna gain depends on the frequency, the C-band signal has a smaller gain per unit area and thus requires a larger antenna diameter relative to Ku-band systems. For Ku-band transmissions, rain-induced s ignal degradation or rain fade, can cause significant loss
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The University NAVSTAR Consortium (UNAVCO) Boulder Facility is assessing Very Small Aperture Terminal (VSAT) technology for near real-time transmission of GPS data from a remote receiver to a central processing facility. The study is motivated by the need for a robust, cost-effective data communications solutions to transfer GPS data from remote si...
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... Small Aperture Terminal (VSAT) networks use geostationary satellites orbiting in the equatorial plane of the earth at an altitude of 35786 km to transmit data from a network of remote sites to a data hub (Figure 1) (Maral, 1995). VSAT satellite space segment providers offer three types of satellite beam: spot, hemisphere, and global. ...
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... Figure 10 we look at the number of retransmission requests from the hub to each remote station. The hub sends out a retransmission request when it determines that a data packet has been missed based on a discontinuity in the packet sequence numbers. ...
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... occur due to problems at the remote, a power loss for example, attenuation due to rain-fade, local LAN problems, or bandwidth limitations. Most of the retransmission requests seen in Figure 10 are for data housekeeping at the hub. Figure 11 shows the number of data packets lost which were not attributed to remote power issues. Figure 11 illustrates that of the majority of retransmission requests indicated in Figure 10, the actual number of lost data packets is small and can be traced to specific causes. ...
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... of the retransmission requests seen in Figure 10 are for data housekeeping at the hub. Figure 11 shows the number of data packets lost which were not attributed to remote power issues. Figure 11 illustrates that of the majority of retransmission requests indicated in Figure 10, the actual number of lost data packets is small and can be traced to specific causes. ...
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... of the retransmission requests seen in Figure 10 are for data housekeeping at the hub. Figure 11 shows the number of data packets lost which were not attributed to remote power issues. Figure 11 illustrates that of the majority of retransmission requests indicated in Figure 10, the actual number of lost data packets is small and can be traced to specific causes. Note that a packet of data can contain between 1 and 232 bits. ...
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... of the retransmission requests seen in Figure 10 are for data housekeeping at the hub. Figure 11 shows the number of data packets lost which were not attributed to remote power issues. Figure 11 illustrates that of the majority of retransmission requests indicated in Figure 10, the actual number of lost data packets is small and can be traced to specific causes. Note that a packet of data can contain between 1 and 232 bits. ...
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... for scale, the outage at the MARS on day 271 was 20721 packets or the equivalent of 24 Mb of data or about ¼ of the days data. The results from Figure 11 indicate that over a 72-day period the Libra network is 99.8% reliable. The windstorm on days 322 and 323 that misaligned the hub resulted in data loss from all remote sites. ...
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... es of satellite beam: spot, hemisphere, and global. Spot beams are focused on population centers and are generally high power, thus allowing smaller antenna dishes to be used at remote sites. Hemisphere and global beams have a much larger footprint and weaker signal strength and thus requires a larger antenna diameter relative to spot beam systems (Jackson et. al, 2001). VSAT may provide a cost effective solution when shared with other users, but the large power consumption and large dish size makes it less desirable for standalone GNSS applications. ...
... it will take a certain amount of time to complete all the required procedures . This amount of time depends on several factors such as the number of GPS stations, the data transmission and ionospheric parameter broadcasting method, the size of GPS network, complexity of the ionospheric modeling as well as the power of the computational facilities. Jackson et al. (2002) showed that using VSAT technology to transmit 1-Hz raw GPS data will have an average latency of about 1.2$1.7 second. If the amount of time used for ionospheric model construction and ionospheric parameter broadcasting is considered, the total latency will be larger. Therefore, even if it is intended to generate the ionospheric correcti ...
Single layer ionosphere models are frequently used for ionospheric modeling and estimation using GPS measurements from a network of GPS reference stations. However, the accuracies of single layer models are inherently constrained by the assumption that the ionospheric electrons are concentrated in a thin shell located at an altitude of about 350km above Earths surface. This assumption is only an approximation to the physical truth because the electrons are distributed in the entire ionosphere region approximately from 50 to 1,000km. To provide instantaneous ionospheric corrections for the real-time GPS positioning applications, the ionospheric corrections need to be predicted in advance to eliminate the latency caused by the correction computation. This paper will investigate ionospheric total electron content (TEC) predictions using a multiple-layer tomographic method for ionospheric modeling over a local area GPS reference network. The data analysis focuses on the accuracy evaluation of short-term (5min in this study) TEC predictions. The results have indicated that the obtainable TEC prediction accuracy is at a level of about 2.8 TECU in the zenith direction and 95% of the total electron content can be recovered using the proposed tomography-based ionosphere model.
This paper analyses the reliability of the Indian Tsunami Early Warning System (ITEWS), comprising a 24 × 7 manned and automated center capable of monitoring the seismic, open sea water level and coastal tide levels and disseminating tsunami bulletins with the aid of proven prerun scenario models during a tsunamigenic earthquake. Since its inception in 2007, the ITEWS has undergone technological maturity with reliability as the prime objective. The system is expected to be in operation throughout the year and alerting the entire Indian Ocean rim countries in the event of a tsunami. Based on International Electrotechnical Commission (IEC) 61508 standards and field failure data, quantitative reliability modeling is done for the subsystems, and it is found that the seismic network, tsunami buoy network, and distress information dissemination systems conform to Safety Integrity Level SIL4, while tide gauge stations conform to SIL4 with a maintenance interval of 45 days. In case of the tsunami buoy network, the failure of one tsunami buoy degrades the network to SIL3 and needs to be restored within 8 months. The study provides confidence on ITEWS’s reliable support to tsunami early warning.
The limitation of single base “real-time kinematic” (RTK) techniques is the distance between base receiver and the rover receiver
due to distance-dependent biases, namely orbit bias, ionosphere bias and troposphere bias. Techniques have been developed
to overcome this distance dependence using a network of GPS reference stations spread over a wide geographic area. Because
the measurement biases will be modelled and corrected for, the positioning accuracy will be almost independent of the inter-receiver
distance. Since the mid-1990s investigators have been investigating the optimal means of processing reference receiver data,
and then providing ‘correction’ information to users, in real-time. This technique is now generally referred to as Network-RTK.
In 1993 the International Association of Geodesy (IAG) established a Special Study Group on “Wide Area Modelling for Precise
Satellite Positioning” . This paper focusses on the progress made during the last few years in designing Network-RTK architectures
and the associated data processing algorithms and issues. Although many university investigators have been researching the
fundamental challenges in functional and stochastic modelling, currently there is only one commercially available Network-RTK
product, the Trimble VRS. However, with the use of the Internet as the primary data communication link, it is predicted that
many more implementations of Network-RTK will come ‘online’, at various sites around the world, over the next few years.
