NISCAIR Online Periodicals Repository Collection: IJRSP Vol.36(4) [August 2007]

GAGAN - The Indian satellite based augmentation system

Sun, 29 Jul 2007 22:58:59 GMT

Title: GAGAN - The Indian satellite based augmentation system

Authors: Rao, K N Suryanarayana

Abstract: Global Positioning System (GPS) from the USA, Global Navigation Satellite System (GLONASS) from the Russian Federation and the proposed GALILEO satellite navigation system from Europe are meant for providing position and timing information for a variety of applications. However, for Safety Critical applications the basic constellations cannot meet the requirements in terms of accuracy, integrity and availability. For this purpose, the basic constellations are augmented by an overlay system. Indian Space Research Organization (ISRO), along with Airport Authority of India (AAI) is implementing the Satellite Based Augmentation System (SBAS) for the Indian region. The project called GAGAN (GPS Aided Geo Augmented Navigation) has a full complement of the SBAS inclusive of ground and onboard segment. The first phase of GAGAN is nearing completion. This paper deals with the basic SBAS concept, GAGAN configuration, implementation and the challenges involved. The roadmap towards the final operational phase is also indicated.

Page(s): 293-302

Characteristics of the ionospheric F-region plasma irregularities over Brazilian longitudinal sector

Sun, 29 Jul 2007 22:58:59 GMT

Title: Characteristics of the ionospheric F-region plasma irregularities over Brazilian longitudinal sector

Authors: Paula, E R de; Kherani, E A; Abdu, M A; Batista, I S; Sobral, J H A; Kantor, I J; Takahashi, H; Rezende, L F C de; Muella, M T A H; Rodrigues, F S; Kintner, P M; Ledvina, B M; Mitchell, C; Groves, K M

Abstract: Based on the data obtained from a network of GPS L1 band receivers deployed in Brazil, are presented here the characteristics of the 400 m ionospheric irregularities during magnetically quiet and disturbed conditions. The network is composed of 12 GPS scintillation monitors and covers the latitudinal region from the magnetic equator up to the southern crest of the Equatorial Ionization Anomaly (EIA), which is characterized by large horizontal gradients in the electron density distribution. Some results on equatorial spread-F statistics obtained from digisonde data over Cachoeira Paulista (22.41^o S, 45^oW, dip latitude 14.89^oS) and from ionosonde data over Tucumán (64.5^o W, 27^o S, dip latitude 13.71^o S) are also used in this work to complement the results from GPS network. The effects of local time, season, latitude, longitude, background ionization, solar cycle and magnetic activity on the ionospheric irregularities are presented. The ionospheric irregularity zonal velocities determined by magnetically east-west spaced GPS receivers are also presented. The influence of the ionospheric irregularities on GPS based navigational systems is discussed. These observations, complemented by computational simulations, may improve our understanding of the factors responsible for the generation, growth and dynamics of the equatorial F-region plasma irregularities.

Page(s): 268-277

An overview of GBAS integrity monitoring with a focus on ionospheric spatial anomalies

Sun, 29 Jul 2007 22:58:59 GMT

Title: An overview of GBAS integrity monitoring with a focus on ionospheric spatial anomalies

Authors: Pullen, Sam; Enge, Per

Abstract: The Local Area Augmentation System (LAAS) or, more generally, the Ground Based Augmentation System (GBAS), has been developed over the past decade to meet the accuracy, integrity, continuity and availability needs of civil aviation users. The GBAS utilizes a single reference station (with multiple GNSS receivers and antennas) within an airport and provides differential corrections via VHF data broadcast (VDB) within a 50-km region around that airport. This paper provides an overview of GBAS integrity verification, explaining how integrity risk is allocated to various potential safety threats and how monitors are used to meet these allocations. In order to illustrate GBAS integrity monitoring in detail, this paper examines the potential threat of ionospheric spatial anomalies (e.g., during ionospheric “storms”) to GBAS and how GBAS protects users against this threat. In practice, the need to mitigate potential ionospheric anomalies is what dictates CAT I GBAS availability.

Page(s): 249-260

Nickel and iron as attenuator materials for helix TWT

Sun, 29 Jul 2007 22:58:59 GMT

Title: Nickel and iron as attenuator materials for helix TWT

Authors: Kumar, Vikas; Vohra, Anil; Srivastava, Vishnu

Abstract: Coating of lossy material is done on the helix support rods to absorb reflections in a helix travelling wave tube. Carbon is a very commonly-used material used for this type of coating, but other materials may also be employed for this purpose. In the present work coatings of carbon, nickel and iron have been done and evaluated for the attenuation performance at microwave frequencies. Three alumina rods have been coated with different methods to characterize them for attenuation. One alumina rod has been coated with carbon using the pyrolytic deposition method, while two others have been coated with iron and nickel using the electro-deposition method. An experimental set-up involving a narrow-height waveguide with a hole was used to measure the attenuation of the coated rods at 6.0 GHz frequency.

Page(s): 345-347