Rao Et Al 4_navcom12 India_dec12

V. Rao & G. Lachapelle, NAVCOM 2012 Pearl Jubilee International Conference on Navigation and Communication, Hyderabad, India, 20-21 December 2012

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Proposed Third Frequency of Operation for IRNSS and its Advantages

Vyasaraj Guru Rao PhD Candidate, University of Calgary & Sr. Manager

GNSS Technology Accord Software & Systems Pvt. Ltd. Bangalore, India [email protected]

Dr. Gérard Lachepelle Professor PLAN Group

University of Calgary, Canada [email protected]

Abstract: The Indian Regional Navigation Satellite System (IRNSS) is in the intial stages of signal design. Based on the open source information, it will be a Dual use Dual frequency system. As a first objective of this research, a third frequency of operation is proposed for IRNSS over the Indian subcontinent.

Satellite Based Augmentation System (SBAS) is a defacto standard used in aviation grade receivers [1, 1996]. With the achieved accuracies, the consumer grade GPS receivers have integrated SBAS as additional channels as part of their firmware. With the proposed third frequency of opeartion, the research proposes methods to use this for the SBAS specifically IRNSS. This minimizes the operational costs drastically from a satellite payload point of view with an advantage of position accuracy improvement.

I. INTRODUCTION The proposed signals to be transmitted by the IRNSS satellites are in the L5 and S1 band of operation. The civilian signal adopts Binary Phase Shift Keying (BPSK) modulation, while the Restricted Service (RS) will have Binary of Carrier (BOC) modulation. The center frequency will be 1176.45 and 2492.028 MHz [2, 2012]. One of the main signal design issues during the initial phases of frequency selection will be interoperability with other existing systems, where the signal transmitted by a new system should co-exist with existing operational systems. Second, from a GNSS serviceability point of view, the signal should typically be in the L band, which is relatively optimal from Ionosphere and Troposphere related effects [3, 2006]. Third, at the time of frequency filing, the signal should be available and should not have been filed by another country [2]. Given the above trade-offs, the IRNSS signal has evolved with the above mentioned frequency constraints. If a new frequency needs to be added as a part of the proposed IRNSS system, one needs to assess the tradeoffs w.r.t satellite payload and in addition to the above mentioned design constraints. Given that IRNSS is still in the initial stages of design, it is a worthwhile exercise. GPS was declared operational in 1995 and is a dual use system. Prior to 1st May 2000, the GPS satellites were transmitting dithered GNSS signals (Selective Availability (SA)) denying civilian users in achieving optimal accuracy [4, 1997]. To counter this, the user community developed

techniques such as Differential GPS (DGPS) to improve accuracy. This was restricted to a limited area [1]. The success of DGPS drove the administrators to rethink the SA, which was effectively turned off in May 2000. Post removal, the major error contributor to the user measurement is the ionosphere. Extending the benefits of DGPS, SBAS became an alternative from a global perspective. Apart from the accuracy improvement, it serves to provide integrity information as well, which is as follows: As a part of the navigation data, the satellites health is transmitted. This parameter is updated once every two hours for the GPS L1 signal. However, if there is any anomaly on board a satellite, it only gets communicated in the next broadcast. For aviation applications where the information integrity information is of paramount importance, a faster update of this parameter is required. As a consequence, in addition to other messages, SBAS provides health information within 6 s as a part of its NAV data [5, 2006]. As a part of the receiver User Equivalent Range Error (UERE), the major error component is Ionosphere for single frequency applications [4,1997]. With SBAS corrections, the error is totally minimized. Consumer GPS receivers now support SBAS, which improves position accuracy drastically. The interface control document for IRNSS is yet to be released. In addition, there is no open source literature available which addresses or comments on the SBAS for IRNSS. Literature highlighting the rational for selecting L5 and S1 bands is not available for IRNSS. Further, there is no literature that discusses proposals for a third frequency for IRNSS. Assuming L5 and S1, this paper proposes such a third frequency of operation. Based on this, SBAS incorporating IRNSS is proposed. To the best of authors’ knowledge, this paper is first to address the two subjects described above. The rest of the paper is organized as follows: Section II describes the proposed satellite constellation for IRNSS and the existing GPS Aided GEO Augmented Navigation (GAGAN). Section III proposes the new frequency onboard IRNSS. The implementation methodology is explained in Section IV. Section V describes the test methodology and the results. Finally, conclusions and future work are presented in Section VI.


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II. SIGNAL CONSTELLATION OF EXISTING GAGAN AND PROPOSED IRNSS

The Indian Space Research Organization (ISRO) has taken the initiative of deploying SBAS to cover the Indian subcontinent. The program started in 2001 and currently has one satellite transmitting corrections over India. Figure 1a) depicts the space and the ground segment for the GAGAN constellation. When fully deployed, GAGAN will have three satellites. At this time, one GAGAN satellite is operational [6, 2012]. The top-level principle of operation of SBAS is as follows: The GPS satellites visible over the Indian subcontinent are continuously tracked at several stations. These stations are equipped with state of the art reference station grade receivers, which provide precise estimates of the pseudorange and carrier phase measurements, typically dual frequency systems. The stations will also have the antenna located at surveyed locations. With these inputs and traits, measurements are formulated. In addition, these receivers also provide estimates on anomalies if any. The integrity stations are spread across the Indian land mass [6, 2012] and all these stations will relay the data to the master control station in Bangalore. Based on the data collected from various stations, the messages as described in [7, 2007] are generated. This data is uplinked in C-band to the GAGAN satellite as shown in Figure 1 [6, 2012]. The GAGAN satellites will transmit signals similar to WAAS over the Indian subcontinent. The GPS receivers equipped to track SBAS signals will be able to acquire and track this signal. From a user perspective, the service being available freely eliminates the need of a modem to achieve DGPS connectivity for most applications. In addition, permanent DGPS stations are no longer required, thereby reducing operational overheads to maintain the system [1, 1996]. Points that are noteworthy from a new signal proposal point of view are:

• Signals transmitted in L1 band at 1575.42 MHz • Signals transmitted at -160 dBW, similar to GPS L1

C/A • The codes used are from the GPS L1 C/A family • The basic data rate of the signal is 250 Hz • Rate ½ 7 bit Viterbi encoding employed on data bits • Accuracy achievable with a GPS L1 C/A receiver

over the Indian land mass will be similar to that achieved over US with or without GAGAN.

Given the above points, it is clear that the signal is transmitted 5 times faster than the GPS L1 C/A but the signal power has remained same. This could be due to the following reasons: First, to achieve performance similar to

that of GPS L1 C/A code, 15 dB additional power is required. This will act as an in-band jammer and lift the noise floor of the L1 channels. Second, to accommodate the former requirement, the absolute power required would be -145 dBW, which violates the guidelines for space based signal transmission [8, 2008]. With -160 dBm, the measurement performed on SBAS would be relatively inferior to GPS L1 [4]. However, the primary objective of SBAS is to provide messages and not ranges. In addition, it uses integrity checks that are more robust than GPS L1 [1, 1997]. As a second initiative in the space based navigation, ISRO is planning to have an independent regional navigation system to cover its territorial footprint and slightly beyond. The system is expected to be operational by the end 2016. The system of space and control segment for IRNSS is as shown in Figure 1b. The top level shows that the Space and Control segments are as follows: The space segment will have a total 7 satellites, namely 4 in Geo-stationary and 3 in GEO synchronous orbits. Detailed explanations on the achievable accuracies are given in [9]. With respect to the control segment, the IRNSS Regional Integrity Monitoring Stations (IRIMS) will be deployed at several places in the Indian subcontinent. These stations will be equipped with high end receivers which will provide all relevant information about the satellite. With the signal transmission from the first satellite, these receivers will perform measurements and collect the NAV data. The data will be relayed to the master control station located in Bangalore [9]. In turn the master control station will generate Keplerian parameters of all the satellites, the clock correction terms and the secondary NAV data information. Unlike GAGAN, this is an involved activity that determines the overall system accuracy. However, the following points are of importance from a new signal design proposal:

• The IRNSS L5 and S1 will transmit signals for civilian use and will adopt the BPSK modulation scheme [9, 2011]

• The ICD is yet to be released. The codes used onboard the satellites and signal power is unknown at this time.

Based on the above details, the following section proposes the third signal as part of IRNSS.

III. PROPOSED NEW SIGNAL DESIGN FOR IRNSS The space initiatives of GAGAN and IRNSS have been undertaken by ISRO and are fully under the control of the latter. The coverage of IRNSS and GAGAN is mainly intended for operation over the Indian subcontinent. As can be seen in Figure 1, the GEO satellites are present as part of both IRNSS and GAGAN systems. With GAGAN codes,


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Figure 1: GAGAN-IRNSS Space and Ground Segments.

the IRNSS along with its existing L5 and S1 frequency bands can transmit on the L1 frequency as well. As stated earlier the power levels on the L5 and S1 civilian codes are unknown. However this is an engineering proposition, which can be easily answered. This third frequency can be used for SBAS related operations, which drastically reduce the payload requirements exclusively onboard GAGAN satellites. In addition, the operational overheads to maintain separate tracking and control stations can be avoided. The following paragraph addresses interoperability issues: The GAGAN and IRNSS effectively cover the Indian subcontinent [6]. In this region, if the GAGAN or IRNSS were to beam the SBAS signals, the L1 GPS C/A user receiver’s performance will effectively remain the same. As such, without violation or frequency filing issues, SBAS can

be augmented on the GEO satellites of IRNSS. Proposed implementation is discussed next.

IV. IMPLEMENTATION METHODOLOGY Accord Software & Systems has developed a test rig to validate algorithms developed internally. The simulator is capable of supporting a maximum of 36 channels similar to GPS. Of these, 7 were configured to IRNSS L5 and 7 to IRNSS S1. To demonstrate the inclusion of SBAS, 3 GEO satellites were assigned PRN id 33, 34 and 35 of the GPS L1 C/A code. The test apparatus is shown in Figure 2. The outputs were taken independently to the receiver hardware. To demonstrate the performance of SBAS, only the Type-26 (Ionosphere correction message) was implemented. To test the above scheme, the receiver as used in [2] was taken as a reference platform. The only inclusion was the addition of 3 new channels with C/A codes 33, 34 & 35 of GPS to receive the SBAS signals.

Figure 2: Test Set-up.


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V. TEST METHODOLOGY AND RESULTS To demonstrate the effective performance of SBAS along with IRNSS, the simulation was performed with the Ionosphere errors being enabled on the L5 and S1 channels. The simulator was initially configured to transmit L5 and S1 IRNSS signals with ionosphere errors enabled. In addition, to demonstrate the benefits of the SBAS corrections, the dual frequency ionosphere estimation or Ionosphere correction based on the broadcast parameters were disabled in the receiver software.

Figure 3: IRNSS with SBAS on L1 performance.

With the above test conditions, the receiver was powered on and the position errors were profiled. With time and measurements from 4 satellites, IRNSS positions were computed. Following this, the SBAS correction signals were enabled on the simulator. Subsequently, the receiver was able to acquire and track SBAS signals. With the corrections from SBAS, the receiver was able to correct the ionosphere errors and the position estimation was relatively accurate compared to the no-correction state, as shown in the Figure 3.

VI. CONCLUSIONS This research proposed two engineering solutions: First, an optimization w.r.t to the satellite signal payloads distributed across GAGAN and IRNSS was deduced. Based on this, the third frequency of operation for IRNSS was designed.

Finally, GAGAN signals as part of IRNSS were proposed. The proposal was experimentally analysed and verified. This research has presented a scheme to optimize the satellite payloads and thereby achieve a drastic reduction in the satellite hardware and associated ground station networks.

VII. ACKNOWLEDGEMENTS The first author would like to thank Accord Software & Systems Pvt. Ltd for the partial financial assistance towards his doctoral studies. The contents expressed herein are the views of the authors and are not binding on any organization

VIII. REFERENCES [1] Spilker, J.J, and B W Parkinson, Global Positioning System:

Theory and Applications Volume II, AIAA Volume 164 [2] V G Rao & G. Lachapelle (2012), Proposed Single

Frequency Signal Design for optimal TTFF in IRNSS. Accepted for Publication in the proceedings of NERTU/NAVCOM -2012, 6 pages

[3] KAPLAN, E. D. and C.J Hegarty (2006), Understanding GPS: Principles and Applications, 2nd edition Artech House

[4] ARINC, Navstar GPS Space Segment / Navigation User Interfaces Interface Control Document (ICD), Revision 200C, prepared and published for GPS JPO, revised September 1997, ION, Salt Lake City, USA

[5] Mishra, P. and P. Enge (2006), Global Positioning System: Signals, Measurements, and Performance, Second Edition, Ganga – Jamuna Press

[6] Ganeshan A S, GAGAN: Status and Update, Coordinates VOL VIII, Issue 9 September 2012

[7] WAAS ICD (2011), Wide Area Augmentation System Interface Control Document FAA Washington [8] Singh A K, A K Sisodia, V. Garg, Candidate Modulation Designs For Indian Regional Navigation Satellite Systems (2008), International Aeronautical Federation [9] Vyasaraj, G, G. Lachapelle and S. B. Vijaykumar (2011), “Analysis of IRNSS over Indian Subcontinent,” in Proceedings of ITM11, The Institute of Navigation, San Diego, 24-26 Jan, Session E5, 13 pages.

Rao Et Al 4_navcom12 India_dec12

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