NOPR Collection: Special Issue: Atmospheric constraints and its mitigation for present and future navigation systems (Guest Editor: Prof A D Sarma)

Troposphere induced GPS navigation error, its effect on GPS-INS integrated system performance and mitigation strategies

2013-07-05T16:36:30Z

Title: Troposphere induced GPS navigation error, its effect on GPS-INS integrated system performance and mitigation strategies Authors: Harikumar, G; Karthikeyan, K; Syamala, S; Pillai, C Radhakrishna; Hemachandran, S; Shukkoor, A Abdul; Mohanlal, P P Abstract: Conventional Global Positioning System (GPS) receivers track GPS satellites above 5 degree horizon to avoid multipath effects. In order to improve vertical dilution of precision (VDOP), it is an attractive prospect to utilize GPS satellites below the horizon in addition to those above, for navigation computations in GPS receivers used by vehicles operating at high altitude trajectories. This paper, however, exposes a possible risk of navigation output degradation in adopting this scheme due to unintentional occultation of GPS signals from satellites below the earth tangent horizon. The error on GPS range and range rate measurements induced by troposphere are simulated using a model based approach. Results indicate position errors up to 150 m and velocity errors up to 14 ms^-1 over a period of 15 s under practical conditions. Such slowly varying errors being uncharacteristic of GPS receivers are not rejected by the usual protection features in GPS-Inertial Navigation System (INS) integration schemes. Thereby, the integrated system performance is degraded. In the uncoupled GPS-INS scheme studied, GPS error of the above order resulted in integrated system output error of 100 m in position and 5 ms^-1 in velocity, respectively. An altitude based elevation cut off scheme that can be incorporated in the GPS receiver for retaining the advantages of using negative elevation satellites, while avoiding occulted measurements, is presented in this paper. A method for strengthening integrated scheme protection, using innovation residuals, for rejecting such slow GPS errors is also presented. Page(s): 150-158

Pratham satellite: Faraday rotation based TEC measurement

2013-07-05T16:37:42Z

Title: Pratham satellite: Faraday rotation based TEC measurement Authors: Prashanth, Giri; Kumar, Avnish; Mulay, Sanyam Abstract: Pratham, the student satellite of Indian Institute of Technology Bombay is a landmark project undertaken by the students of the institute in collaboration with ISRO. The project is in its environment testing phase now. Total electron content (TEC) measurement was chosen as the payload for the satellite keeping in mind the current scientific atmosphere in India with the Indian Regional Navigational Satellite System (IRNSS) and GPS-Aided Geo-Aumented Navigation (GAGAN) system in the process of development. This paper describes the use of Faraday rotation method for TEC measurements. The key features of the method, n-pi ambiguity resolution, possible sources of error and simulations carried out to get the initial estimates of the TEC values are also discussed. The on-board and on-ground hardware implementation are briefly explained. Page(s): 197-203

GPS signal Rician fading model for precise navigation in urban environment

2013-07-05T16:37:40Z

Title: GPS signal Rician fading model for precise navigation in urban environment Authors: Rao, G Sasi Bhushana; Kumar, G Sateesh; Kumar, M N V S S Abstract: Global Positioning System (GPS) usage is not limited to the aircraft en-route navigation and missile guidance where the user receives the satellite signals from the open sky. Currently, GPS has become an essential utility in the car navigation, mobile phones, surveying and aircraft landing applications. The received signal strength of a GPS satellite at a given location on or near the earth surface can be predicted by analyzing the propagation characteristics of the channel with an appropriate propagation model. The signal propagation characteristics particularly the short term variations severely affect the quality, availability and continuity of the system. The short term propagation characteristics of GPS signal is modeled and analyzed in this paper. Short term variations are mainly due to multipath reflections and Doppler shift which degrades the quality of received signal particularly in urban environments. The variation of signal quality with respect to user velocity is observed using Rician fading model. Page(s): 192-196

An optimal method for parameter retrieval from Radio Occultation Missions

2013-07-09T16:36:59Z

Title: An optimal method for parameter retrieval from Radio Occultation Missions Authors: Rajarajan, D; Babu, R; Saha, Shibu; Rathnakara, S C Abstract: Radio occultation (RO) is the Global Navigation Satellite System (GNSS) based remote sensing of Earth’s atmosphere for atmospheric parameter retrieval and total electron content (TEC) computation. Several RO missions have been launched by ISRO, viz. Meghatropiques and Oceansat-2, for atmospheric studies. Current implemented methods are able to provide the parameters only in post processed mode and hence, are available after some duration. This is due to the limitation of availability of precise satellite orbit for GNSS and LEO satellites which serves as one of the fundamental input to the process. However, a faster turnaround time is possible if the parameters are computed in near real time from the on-board solutions. This paper highlights the work that has resulted in optimizing the atmospheric parameter retrieval and TEC computation using the radio occultation technique. Page(s): 187-191