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A study on the utility of combined use of GPS and GLONASS constellations in India  P. Banerjee National Physical Laboratory,New Delhi banerjee@csnpl.ren.nic.in Anindya Bose Physics Department Burdwan University Burdwan A. Dasgupta Institute of Radio Physics & Electronics Calcutta University Calcutta
Abstract
GPS has been operational with its full constellation since early 90’s. The GLONASS constellation is also now operational. The effort of integrating two constellations resulted in the production of receiver, which may use both constellations in combination. Utilising this type of receiver GPS and GLONASS signals were monitored for one calendar year in different places in India simultaneously. The aim of this monitoring was to study aspects of visibility of satellites and the limits of accuracy. It has been observed that on the average 95% of time, 8 or more GPS satellites are available anywhere in India. Number of satellites in GLONASS constellation has been found to be gradually depleting form 16 to 7 during the course of this study. So 3D solution thus was hardly possible by the use of only GLONASS satellites. GLONASS alone, however, may be used for timing application as a single satellite can also give timing solution at a predetermined location. However, appreciable improvement in PDOP was observed in the combined mode. Before the withdrawal of GPS SA significant improvement of position accuracy could be observed in the combined mode. After the removal of GPS-SA, the accuracy of the combined mode and that of only GPS mode have been found to be of the same order. It apparently does not reflect any advantage. But it indirectly confirms that both, GPS and GLONASS systems have similar accuracy limits of accuracy and also confirms that interoperability of two systems has optimally been achieved. These studies reveal that the use of GLONASS along with GPS will always be beneficial to the varying degree depending on different application and circumstances. This paper elaborates these observations. Introduction Both GPS and GLONASS will offer, independently, precise location and time transfer continuously anywhere in the world and, indeed, in space itself. The reliability and availability of GPS in Indian subcontinent have already been exhaustively studied and presented in the report of SRAGI project. It has been felt that GPS alone fall short of meeting the requirements of a global sole means, or stand-alone, navigation system. GLONASS alone also cannot serve the same purpose. The performance capabilities of GLONASS are substantially similar to those of GPS as claimed by the respective authorities. But GPS has achieved its operation status in full, while the prospects of GLONASS are less clear. The two systems taken together, however, should offer amply redundant measurements to all users, and seem capable of meeting these requirements. To study these aspects, GPS and GLONASS signals were monitored for one year in different places in India. The number of satellites with good signal strength, the geometry of the avalable satellites and the corresponding achievable accuracy were recorded during this study. This paper attempts to examine the performance capabilities of GLONASS and compare them with those of GPS based on monitored data Constellation Status of GLONASS and GPS It is necessary to have a prior knowledge of constellation status of both GPS and GLONASS in order to have the comprehensive conclusion on the availability of the satellites. The GLONASS satellite was launched in 1982. 24-satellite was available for the first time on 18 January 1996. This status, however, was relatively short-lived and the constellation has gradually fallen into decline, as failed satellites have not been replaced. Since December 1995 there was no launch till December1998 when three new satellites were launched. For replenishment after a gap of two years three satellites were launched in December 2000 but only one of them has been successfully operational. The status of the constellation during the period of Dec. 1998 to Feb 2001 has been tabulated in Table. 1. GPS constellation has been full with 24 satellites or more during the entire period of this study. It may be noted that there were 29 satellites operational. Out of these, 6 satellites were launched in the year 1900 or before. Availability of GLONASS and GPS Satellites in India The monitoring of GPS and GLONASS satellites has been carried out at three sites (New Delhi, Calcutta and Bangalore) independently and the recorded data had been analysed. The number of satellites in GLONASS constellation varied widely during the period of study. So the availability of GLONASS should be referenced to its constellation status. The type of receivers used in this campaign may be operated in any one of three modes namely in GPS only mode, GLONASS only mode and GPS and GLONASS combined mode. Constellation of GPS was full with 24 or more satellites during the entire period of observation. So GPS constellation status may be assumed to be uniform during the period of this study. When the receiver operates in GPS+GLONASS mode, each sample of observation displays the elevation and azimuth of all Table 1: Status of the constellation.
GPS and GLONASS satellites being used for finding position solution. These satellites are counted. These numbers have been analysed in a very comprehensive manner. As the availability of GPS and GLONASS signal has been extensively studied at three sites well scattered over India, the observation may be assumed to general observation for Indian subcontinent. The observation does not project any diurnal or seasonal nature variability. Also it has been seen that the availability of signal from both the constellation is almost similar in nature with insignificant variability over whole of India.  Fig.1: Probability of a certain number or more GPS satellites on different days. Some of observations on GPS availability is shown in Fig.1. On the average 95% of time, 8 or more GPS satellites are available any where in India. Even maximum number of 13 satellites was available in Calcutta in Bangalore but New Delhi never received more than 12 satellites.
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