Ionospheric Delay Estimation during Ionospheric Depletion Events for Single Frequency Users of IRNSS

The IRNSS (Indian Regional Navigation System) navigation users estimate their position by using a receiver which receives the navigation signal from the IRNSS satellites which will be operating on L5 (1176.45MHz) and S (2492.028MHz) frequencies. There are 3 types of IRNSS users: 1) Dual frequency (L5 and S), 2) Single frequency (L5) and 3) Single frequency (S). The signal from the satellites before reaching the user receiver passes through the ionospheric layer of the atmosphere and suffers a delay. The delay in the signal introduces error in the position computed by the user. The dual frequency users of IRNSS correct the ionospheric error by taking advantage of the dispersive nature of ionosphere. On the other hand, single frequency user requisite an algorithm for computing the ionospheric delay along his line of sight. In IRNSS, the ionospheric error corrections for single frequency (L5 or S) users will be provided by two ways: 1) Grid based and 2) Coefficient based. These corrections may not be valid when an abnormal behavior of ionosphere occurs due to geomagnetic storm, solar coronal mass ejections or any other disturbances in the earth’s magnetic field. The abnormal behavior may result in increase or decrease of the TEC (Total Electron Content) in the ionosphere. Ionospheric depletion event is one such, where there is a sudden drop in TEC forming plasma bubbles travelling through the ionosphere. A user, whose line-of-sight when crosses such a TEC depleted area of ionosphere suffers from an extra error due to depletion. The amount of error is proportional to the depth of depletion. This error in the range ultimately results in the user position accuracy degradation. In this paper a novel algorithm has been designed and developed which will estimate the ionospheric delay, thereby providing ionospheric corrections even at times of depletions. The developed technique in turn provides achievable position accuracy during times of ionospheric depletions. The developed technique has been tested with GAGAN (GPS Aided GEO Augmented Navigation) INRES (Indian Reference Stations) data and IRNSS IRIMS (IRNSS Range and Integrity Monitoring Stations) data having deep ionospheric depletions. The fully tested and validated ionospheric delay estimation algorithm is proposed to be implemented in IRNSS single frequency (L5/S) receivers. Keywords: IRNSS Single Frequency User, Ionospheric Error, Ionospheric Depletion, Ionospheric Delay Estimation, Kalman Filte

Modeling of IRNSS System Time-Offset with Respect to other GNSS

The IRNSS System Time started at 00:00 UT on Sunday August 22nd 1999 (midnight between August 21st and 22nd). At the start epoch, IRNSS system time was ahead of UTC by 13 leap seconds. (i.e. IRNSS time, August 22nd 1999, 00:00:00 corresponds to UTC time August 21st 1999,23:59:47). IRNSS time is a continuous time without leap second corrections determined by the IRNSS System Precise Timing Facility (IRNPT) with an ensemble of Caesium and Hydrogen maser standard atomic clocks.Combining of multi GNSS satellites provides very significant advantages a) paves the way for computing the user position with increased number of satellites. b) Reduced horizontal and vertical Dilution of Precision (DOP) factors. And c) Decreased occupation time which means faster positioning results.This paper presents the 1.IRNSS time offset generation with respect to other GNSS timescales such as GPS, GLONASS system and traceability to UTC,UTC(NPLI)/UTC(K) 2.Validation of predicted time offsets with actual offsets.3.The IRNSS time offsets are derived from GNSS navigation message using UTC offsets to validate the predicted IRNSS time offsets. IRNSS times offset from GNSS are broadcasted in the form of coefficients in one of the IRNSS navigation messages. This broadcast message also allows the user to recover UTC and UTC (NPLI)/UTC(K) time for precise timings. Keywords: IRNSS, IRNSS Time offsets, IRNWT, UTC, UTC (NPLI) and GNS

Modernized IRNSS Broadcast Ephemeris Parameters

India has successfully stepped into satellite Navigation system with the launch of its first three IRNSS satellites IRNSS 1A, 1B and 1C.  IRNSS provides two types of services, Standard Posting Service (SPS), which is open for civilian use and the Restricted Service (RS), for authorized users.  The system is set to change the facet of navigation, surveying, transportation, precision agriculture, disaster management and telecommunication in India. In any navigation system, broadcast navigation parameters are of paramount importance in arriving user position solution at user receiver end. IRNSS Navigation data is classified as primary and secondary Navigation parameters. Primary navigation data of a satellite principally represents its own orbit and onboard clock offset in the form of quasi-keplerian elements and clock coefficients (Bias, Drift and Drifts rate) respectively. Whereas secondary navigation parameters includes satellite almanac, ionosphere delay correction messages, differential corrections, Earth orientation parameters and  IRNSS Time offset with respect to other GNSS. In existing IRNSS system satellite ephemeris of primary navigation parameters are broadcast in the form of 15 quasi-keplerian elements valid for a period of 2 hours or more. Spacecraft ephemeris which represents orbit in the form of 9 parameters, i.e., position, velocity and acceleration component of spacecraft in Cartesian coordinate system are chosen from Russian Global Navigation satellite system (Glonass) to improve Time to First Fix (TTFF) of IRNSS system with similar existing orbit accuracy. In addition, two models of user receiver orbit propagation algorithms with proposed ephemeris are briefed and their results are compared with standalone Glonass model. Generation of IRNSS ephemeris in Cartesian coordinate system and description of user receiver orbit propagation algorithms using new type of ephemeris to get user position solution is the scope of this paper.. Keywords: IRNSS, TTFF (Time to First Fix), Broadcast ephemeri

Simulation of IRNSS Navigation Payload Operations for End to End Payload Testing

Fault free operations of space vehicles have always been a challenging task. Every space mission requires stringent qualification process on ground for qualification of the space vehicle for mission operations. This paper deals with the simulation of IRNSS navigation payload operations on ground for end to end payload testing and qualification of the payload for broadcast of IRNSS navigation parameters. IRNSS is an emerging Indian regional navigation satellite system for providing the satellite based navigation service over India and neighboring region. The system is optimally designed for its space and ground segment to provide the best in class navigation service. The space segment comprises of 7 satellites with 4 satellites in geo-synchronous orbit and 3 in geo-stationary orbit. The navigation payload on-board every IRNSS spacecraft comprises of navigation signal generation unit, atomic clocks and ranging subsystems. For every IRNSS spacecraft, a series of tests are carried out during different phases of spacecraft integration and testing. The core elements of IRNSS navigation operations such as IRNSS navigation software, payload test receiver, atomic clocks and telecommand and telemetry subsystem all participate in simulation and end to end testing of navigation payload. This paper describes in detail the simulation of various mission scenarios with respect to navigation payload operations considering different phases of satellite operations, subsystems involved and environment. The simulation has been key to successful operations of IRNSS 1A and IRNSS 1B which are operational in IRNSS space segment. Keywords: IRNSS, Navigation, payload, simulatio

An optimal method for parameter retrieval from Radio Occultation Missions

187-191Radio 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

Single frequency ionospheric error correction using coefficients generated from regional ionospheric data for IRNSS

125-130Indian Regional Navigation Satellite System (IRNSS) is the satellite based navigation system being developed by India for locating user’s position in 3-dimensional space and time over IRNSS service area. The users estimate their position by using a receiver which receives the navigation signal from the IRNSS satellites which is operating on L5 (1176.45 MHz) and S (2492.028 MHz) frequencies. There are three types of IRNSS users: dual frequency (L5 and S); single frequency (L5); and single frequency (S). The signal from the satellites before reaching the user receiver passes through the ionospheric layer of the atmosphere and suffers a delay. The delay in the signal introduces error in the position computed by the user. The dual frequency users of IRNSS correct the ionospheric error by taking advantage of the dispersive nature of ionosphere. On the other hand, single frequency user requisite an algorithm for computing the ionospheric delay along his line-of-sight. In IRNSS, the ionospheric error correction for single frequency (L5 or S) user is provided through a set of eight ionospheric coefficients. These coefficients are computed on ground by using previous 24-hour data and broadcasted as secondary navigation parameters (messages) to the user. These corrections are available for any user in the primary service area (extending up to 1500 km from Indian boundary) of IRNSS with a validity period of one day. The algorithm designed and developed for the estimation of these coefficients is explained in this paper which uses the global ionospheric model based on cosine curve approximation of diurnal variation of ionospheric delay. The estimation technique has been tested initially with simulated data and then with measurement driven model data for days of different ionospheric activity. The results are promising when compared with GPS transmitted Klobuchar coefficients over IRNSS primary service area. This algorithm is a part of the IRNSS navigation software which is operational at IRNSS Navigation Center (INC), Bangalore