35 A Brief of WAAS, IRNSS, GAGAN, GLONASS and Galileo

Kumari Anamika Kumari Anamika

  Objectives

• Student will get to know the need of navigation system.
• Student will acquire skill how satellite constellation works to help us in navigation.
• Student will be equipped with knowledge to study further about GPS, GLONASS, NAVSTAR etc.

Outline

Introduction

GPS and WAAS systems

GPS WAAS Air Navigation User Segment

GPS WAAS Corrections and Integrity

Indian Regional Navigation Satellite System (IRNSS) GPS Aided Geo Augmented Navigation (GAGAN)

Glonass Galileo

  Wide Area Augmentation System (WAAS): The WAAS is a Satellite Based Augmentation System (SBAS) for North America that augments GPS SPS by broadcasting differential GPS (DGPS) correction messages from GEO satellites. The WAAS Service provides augmentation of GPS integrity via integrity data included in the WAAS message broadcasts. The WAAS Service is specifically designed to meet high accuracy, integrity, continuity, availability standards of aviation users, but is an open service that has the capability to support other applications as well. WAAS provides a ranging function throughout the entire satellite footprint that improves the availability of GPS positioning for SBAS users. WAAS also provides differential corrections as well as satellite status for GPS satellites. Global Positioning System (GPS) navigation signals alone are not adequate to support aviation navigation. GPS accuracy is acceptable for all but precision approach applications, but integrity, continuity & and availability are lacking. WAAS is a combination of ground based and space based systems that augment the GPS Standard Positioning Service (SPS) .WAAS provides the capability for increased availability and accuracy in position reporting, allowing more time for uniform and high quality worldwide air traffic management. WAAS provides coverage over the entire National Airspace, with a precision approach capability at over 3,000 runway.

GPS and WAAS system

GPS

•  Approved for Aviation use in 1993

•  Established as global leader and gold standard for satellite navigation

•  Extensive modernization efforts underway that will make available additional civil signals (L1C, L2C, L5) WAAS

•  Currently augments the GPS L1 signal providing improved accuracy and integrity

•  WAAS modernization efforts tied directly to GPS modernization

The primary mission of WAAS is to augment GPS SPS (Solar Powered Satellite)for air navigation. The WAAS signal provides the integrity, accuracy, availability, and continuity required for instrument approach operations in terminal, en route (ER), and final approach phases of flight in all signal coverage areas. In the primary area of coverage, the WAAS SIS also supports vertically guided instrument approach operations called Localizer Performance with Vertical (LPV) guidance and enables an aircraft to descend as low as 200 feet height above touchdown (HAT). WAAS provides an en route navigation and approach landing capability (lateral and vertical guidance) that; (a) improves safety by reducing controlled flight terrain on approach, (b) increases the number of candidate runway ends that can have vertically guided approach procedures, and (c) supports area navigation (RNAV) route for aircraft that are not equipped with inertial navigation and flight management systems. WAAS provides the additional accuracy, availability, continuity and integrity necessary to enable users to rely on GPS for all phases of flight, from route through approaches with vertical guidance, at all qualified airports within the WAAS LPV coverage area. This WAAS service enables development of more standardized approach procedures, missed approach procedures, and departure guidance for numerous runway ends and heliport/helipads in the National Airspace System (NAS). GPS-only operations depend on the receiver-based integrity monitoring technique called receiver autonomous integrity monitoring (RAIM). GPS WAAS operations add an additional WAAS SIS broadcast from a geostationary earth orbit (GEO) satellite to improve the accuracy of the GPS SPS SIS and to provide integrity by alerting when not to use satellites as part of the WAAS solution. The GEO broadcast may also be used as an additional ranging signal to improve the availability of the augmented GPS service.

GPS WAAS Air Navigation User Segment: GPS WAAS is a combination of ground-based and space-based equipment that augments the GPS SPS. A network of ground monitoring stations with precisely surveyed GPS antennas is strategically positioned to collect GPS satellite data across the NAS including sites in Alaska, Hawaii, and Puerto Rico. Additional stations are located in Canada and Mexico to further improve availability in the NAS and extend coverage to most of Canada and Mexico. Using site measurements of the GPS SIS, GPS WAAS calculates errors in the GPS signal and then provides users with correction data to compensate for these GPS SIS errors as well as integrity information associated with those corrections. WAAS GEO satellites, used to relay the correction and integrity information to users, also provide an additional GPS-like ranging capability that further improves the availability of GPS positioning. Hence, WAAS provides for increased availability and accuracy in position reporting, enabling more uniform and high-quality NAS-wide air traffic management. The end result is a GPS WAAS service that supports navigation from the en route phase of flight to approach with ceiling and visibility minimums equivalent to Category (CAT) I. GPS WAAS augments the GPS SPS SIS with integrity and corrections to make the GPS SPS a trusted air navigational aid. Figure 1 depicts aircraft with a GPS-only operation with an FAA Technical Standard Order (TSO) C129 compliant receiver and an aircraft with TSO-C145/146 compliant WAAS receiver.

The GPS-only receiver compliant with TSO-C129 uses RAIM to assure the integrity of its solution. A GPS receiver solves for position and time using signals from a minimum of four GPS satellites. 1st Edition Page 6 31 October 2008 GPS WAAS PS RAIM requires a fifth satellite, or barometric aiding, to perform a consistency check to detect a fault on a single satellite. The GPS WAAS receiver compliant with TSO-C145/146 also uses RAIM for instances when the augmentation signal becomes unavailable. The WAAS receiver adds a fault detection & exclusion (FDE) feature requiring a minimum of 6 satellites to detect and exclude a faulted satellite. Instead of declaring GPS SPS service unusable with a RAIM alert, RAIM/FDE excludes the bad satellite and continues to provide an integrity-assured solution provided the geometry of the remaining satellites in view is sufficient. The WAAS GEO broadcast also provides an additional ranging source for improved availability of navigation services. When a WAAS receiver is using the corrections and integrity messages broadcast by the GEO, only four GPS or GEO satellites are needed, which increases the availability of service versus RAIM or RAIM/FDE.

Fig:1. WAAS system

Source:http://substance.etsmtl.ca/precision-and-performance-analyses-of-a-gps-augmented-solution-with-waas-l1l5-real-time-corrections-research-paper-introduction-rpi/

GPS WAAS Corrections and Integrity: The WAAS receiver uses the WAAS broadcast-corrections in conjunction with GPS SPS signals to calculate an accurate position with high-confidence error bounds on this position. Correction data is used to differentially correct errors in the broadcast GPS range measurements. Integrity data is used to calculate integrity bounds called protection levels. Depending on the flight operation, the user equipment may either simply compute a Horizontal Protection Level (HPL) or both a HPL and a Vertical Protection Level (VPL). The user receiver compares the computed protection levels with the alert limit thresholds established for the selected phase of flight. If one of the protection levels exceeds the corresponding alert limit, the receiver provides an annunciations to the pilot. The overall system design ensures that the user receiver does not exceed a time-to-alert of 6.2 seconds. WAAS estimates GPS satellite clock, ephemeris errors, ionosphere delays for satellites and ionospheric grid points that are adequately viewed by WAAS wide-area reference stations, calculates corrections for those errors, and broadcasts the corrections through the GEO satellites. Three types of corrections are broadcast:

1) Fast Corrections (FCs) for each GPS satellite clock’s rapid, short-term errors; calculated and broadcast every 6 seconds for each satellite,

2) Long Term Corrections (LTCs) for each GPS satellite clock’s slow drift errors and slow ephemeris errors; calculated every 256 seconds and broadcast at least every 120 seconds, and

3) Ionospheric Grid Point (IGP) Corrections (ICs) for the estimated ionosphere signal propagation delays; calculated for every 5 degrees latitude and longitude grid point over the LPV coverage area and broadcast every 5 minutes. 1st Edition Page 7 31 October 2008 GPS WAAS PS WAAS calculates integrity data associated with its generated corrections at the required level of integrity for the intended flight operation. Integrity data is provided in the form of error bounds which are used to compute the protection levels taking all relevant error sources into account. The integrity data consists of the User Differential Range Error (UDRE) and the Grid Ionospheric Vertical Error (GIVE). UDRE characterizes the residual error in the FC and LTC. The UDRE is transmitted with the FC message. GIVE characterizes the residual error in the IC for the estimated ionosphere signal delays calculated for IGPs, defined in the IGP mask and broadcast every 5 minutes. The GIVE is transmitted with the IC message. If using WAAS vertical guidance, the user receiver utilizes this integrity data to calculate a “protection cylinder” as defined in the RTCA Inc. (formerly Radio Technical Commission for Aeronautics) Minimum Operational Performance Standard (MOPS) (RTCA/DO-229), incorporated by reference, in TSO-C145/146. A simplified depiction is shown in Figure The user receiver applies the various

WAAS corrections described above and calculates a user position using the WAAS corrected range and ephemeris data. Then, the user receiver applies the UDRE, GIVE (and other error characteristics for residual troposphere delay and receiver errors) values to calculate VPL and HPL. These protection limits can be envisioned as defining a protection cylinder, depicted as the dark cylinder in WAAS Architecture and Operational Environment. The GPS WAAS architecture and operational environment are shown in Figure 2. The space segment consists of the GPS and GEO satellites. The GPS constellation nominally consists of 24 satellites located in orbital slots distributed in 6 different orbital planes (an average of approximately 8-10 satellites are in view to a user at any time). The GPS SPS PS contains a complete description of the GPS constellation. Each GPS satellite transmits signals on two frequencies that are used by WAAS. The Link 1 (GPS L1) 1575.42 MHz Coarse Acquisition (C/A) coded signal contains the orbital and timing parameters that a receiver requires to calculate the user’s position. The L2 signal is the 1227.60 MHz P(Y) coded signal. Currently, the operations & maintenance (O&M) workstations located at the National Operations and Control Center (NOCC) and Pacific Operations Control Center (POCC). Figure 2 shows the WAAS data processing path. WAAS receives the GPS satellite signals and processes the GPS satellite data to determine the corrections and integrity data for each satellite. The resultant correction and integrity data for each GPS satellite is referred to as the WAAS message. The WAAS message is uplinked to the GEO satellites for broadcast on the GPS L1 frequency. The user receives the signal with a unique GEO satellite pseudo-random noise (PRN) number coding on the SIS. The user receiver extracts the various corrections and integrity data for the GPS and GEO satellites to calculate an accurate position and associated protection level(s). WAAS also controls the timing of the GEO satellite signal and generates the GEO satellite navigation message data (GEO satellite position and clock) that enables use of the GEO satellites as an additional GPS-like ranging source. WAAS user receiver receives only the L1 signal. The WAAS is used to produce a Satellite Based Augmentation System (SBAS) signal-in-space (SIS) that contains GPS corrections and integrity data messages. The WAAS includes Wide-area Reference Stations (WRSs), Terrestrial Communication Network (TCN), Wide-area Master Stations (WMSs), Ground Earth Stations (GESs), and GEO satellites. WAAS is controlled from two 1st Edition Page 9 31 October 2008 GPS WAAS PS operations & maintenance (O&M) workstations located at the National Operations and Control Center (NOCC) and Pacific Operations Control Center (POCC).

Fig:2. GPS WAAS Corrections and Integrity

Source:http://slideplayer.com/slide/9384990/

Indian Regional Navigation Satellite System (IRNSS): Indian Regional Navigation Satellite System (IRNSS) is an independent, indigenously developed satellite navigation system fully planned, established and controlled by the Indian Space Research Organization (ISRO).

IRNSS Architecture mainly consists of:

Space Segment

Ground Segment

User Segment

Fig:3. IRNSS architecture

Source: http://www.navipedia.net/index.php/NAVIC

IRNSS SPACE SEGMENT Based on various considerations the IRNSS constellation is worked out to be a combination of GSO and IGSO satellites.

IRNSS GROUND SEGMENT Ground Segment is responsible for the maintenance and operation of the IRNSS constellation. The Ground segment comprises of:

ISRO Navigation Centre, IRNSS Spacecraft Control Facility, IRNSS Range and Integrity Monitoring Stations, IRNSS Network Timing Centre, IRNSS CDMA Ranging Stations, Laser Ranging Stations, Data Communication Network.

USER SEGMENT The User segment mainly consists of Single frequency IRNSS receiver capable of receiving SPS signal at L5 or S band frequency. A dual frequency IRNSS receiver capable of receiving both L5 and S band frequencies. A receiver compatible to IRNSS and other

GNSS signals. Figure 2 specifies the radio frequency interface between space and user segments.

Each IRNSS satellite provides SPS signals in L5 and S bands.

Fig:4.IRNSS Interface with User Segment

Source: https://www.quora.com/Can-civilians-use-IRNSS-NAVIC-Indias-equivalent-of-GPS-on-normal-phones-like-Samsung-and-Iphone

IRNSS Services: Standard Position Services (SPS), an open service without encryption and Restricted Service (RS), an authorized with encryption are the basic services.

IRNSS Signal Characteristic

IRNSS Frequency Band:

The IRNSS SPS service is transmitted on L5 (1164.45 – 1188.45 MHz) and S (2483.5-2500 MHz) bands. The frequency in L5 band has been selected in the allocated spectrum of Radio Navigation Satellite Services as indicated in Figure 3 and S band as indicated in Figure 4. offered by IRNSS.

Fig:5.IRNSS CARRIER FREQUENCIES the IRNSS carrier frequencies and the bandwidths of transmission for the SPS service is shown in

Source: https://www.slideshare.net/AkshayKumar671/irnss-62098816

Table1

    Applications of IRNSS

Terrestrial, Aerial and Marine Navigation Disaster Management

Vehicle tracking and fleet management Integration with mobile phones

Visual and voice navigation for drivers Precise Timing

Mapping and Geodetic data capture

   GPS Aided Geo Augmented Navigation (GAGAN): The GPS aided geo augmented navigation or GPS and geo-augmented navigation system (GAGAN) is a planned implementation of a regional satellite-based augmentation system (SBAS) by the Indian government. It is a system to improve the accuracy of  a GNSS receiver by providing  reference

The project is being implemented in three phases through 2008 by the Airport Authority of India with the help of the Indian Space Research Organization’s (ISRO) technology and space support. The goal is to provide navigation system for all phases of flight over the Indian airspace and in the adjoining area. It is applicable to safety-to-life operations, and meets the performance requirements of international civil aviation regulatory bodies.

To begin implementing an SBAS over the Indian airspace, Wide Area Augmentation System (WAAS) codes for L1 frequency and L5 frequency were obtained from the United States Air Force and U.S Department of Defense on November 2001 and March 2005. The system will use eight reference stations located in Delhi, Guwahati, Kolkata, Ahmedabad, Thiruvananthapuram, Bangalore, Port Blair, and a master control center at Bangalore.GAGAN, after its final operational phase completion, will be compatible with other SBAS systems such as the Wide Area Augmentation System (WAAS), the European Geostationary Navigation Overlay Service (EGNOS) and the Multi-functional Satellite Augmentation System (MSAS) and will provide seamless air navigation service across regional boundaries. While the ground segment consists of eight reference stations and a master control centre, which will have sub systems such as data communication network, SBAS correction and verification system, operations and maintenance system, performance monitoring display and payload simulator, Indian land up linking stations will have dish antenna assembly. The space segment will consist of one geo-navigation transponder

With Trimble DGPS Systems has the capability to capture the GAGAN satellites and give the real time correction on the Ground. With this system, the real time accuracy of the system Juno Series and Geo Series such as Geo-XT 3000 and Geo-XH 6000, the accuracy and productivity of the system increases.

GAGAN Stability tests were successfully completed in June 2013. The overall performance of the systems was reviewed by the review committee. As part of certification activity, DGCA personnel visited GAGAN complex, Kundalahalli in Bengaluru and carried out final inspection activities on Indian Land Uplink Station (INLUS), Indian Master Control Centre (INMCC), Indian Reference Earth Station (INRES) and other facilities. The implementation of GAGAN has numerous benefits to the aviation sector in terms of fuel saving saving in equipment cost flight safety increased air space capacity efficiency enhancement of reliability reduction in work load for operators coverage of oceanic area for air traffic control high position accuracy

The quantum of benefits in the aviation sector would depend on the level of utilisation of such benefits. Some of the benefits GAGAN is expected to bring for Civil Aviation sector are:

Safety benefits – Vertical guidance improves safety, especially in adverse weather conditions Reduction of circling approaches.

Environmental benefits–Approach with Vertical Guidance procedures will help facilitate better energy and descent profile management during the final approach

Global seamless navigation for all phases of flight including arrival, departure, oceanic and enroute

Allow direct routings, multiple approaches resulting in considerable fuel savings to airlines and provide for capacity enhancement of airports and airspace

In addition to aviation sector, GAGAN is expected to bring benefits to other sectors like: Navigation and Safety Enhancement in Railways, Roadways, Ships, Spacecraft

Geographic Data Collection Scientific Research for Atmospheric Studies Geodynamics Natural Resource and Land Management Location based services, Mobile, Tourism, etc Indian Regional Navigation Satellite System (IRNSS)

The development plan consists of three different phase

• Technology Demonstration System (TDS)
•  Initial Experimental Phase (IEP)
• Final Operational phase (FOP)

     The TDS phase was completed in August 2007 using the navigation payload of the INMARSAT 4F1 satellite. The Initial Experimental Phase (IEP), initially planned to be finished by 2009, is currently being implemented concurrently with TDS Phase.

On 15 April 2010, it was attempted to launch the first GAGAN navigation payload. The equipment was installed in the satellite, GSAT-4, but, unfortunately, the launch failed and the GEO satellites never reached its nominal orbit. In May 2011, the GSAT-8 satellite carrying a GAGAN SBAS payload was successfully launched by the Ariane-V launch vehicle of Arianespace from Kourou, French Guiana. Later on 28 September 2012, another Ariane 5 rocket successfully launched the India’s GSAT-10 satellite which carries the second GAGAN payload

GAGAN Stability tests were successfully completed in June 2013. The overall performance of the systems was reviewed by the review committee of the DGCA (Director General of Civil Aviation)

The main components of the GAGAN Architecture are:

Space segment: three operational GEO satellite. The GSAT-8 and GSAT-10 satellites were successfully launched in March 2011 and April 2012, respectively. The remaining satellite is schedule to be launched during 2014 aboard an Arianne 5 launch vehicle.

Ground segment: On the ground, the GPS data is received and processed in the 15 Indian Reference Stations (INRES), located at Ahmedabad, Bengaluru, Bhubaneswar, Kolkata,

Delhi, Dibrugarh, Gaya, Goa, Guwahati, Jaisalmer, Jammu, Nagpur, Porbandar, Portblair, Trivandrum. The Indian Master Control Center (INMCC) composed by two sites and located in Bangalore, processes the data from the INRESs to compute the differential corrections and the estimate of its level of integrity. The SBAS message generated by the two INMCC is uplinked to the GEO satellites through its corresponding Indian Land Uplink Station (INLUS)[11].

User segment: GAGAN-enabled GPS receivers, with the same technology as WAAS Receivers, capable to use the GAGAN Signal-in-Space (SIS). User equipment for civil aviation shall be certified against several standards (see article SBAS Standards).

The company Raytheon was awarded in 2009 with the contract to modernize the Indian air navigation system. Raytheaon was responsible for building the Ground Stations being supplied by some ground equipments by ISRO.

Fig:6.Gagan Configuration

Source: https://www.researchgate.net/figure/269287990_fig6_Figure-2-Configuration-of-Indian-SBAS-GAGAN-Courtesy-AAIISRO-GAGAN-consists-of-15

Gagan Implementation

GAGAN is implemented in two Phases: – Technology Demonstration Systems (GAGAN TDS) phase has been completed in 2007 – Final Operations Phase (GAGAN FOP) has commenced in 2009 and is nearing completion.

GAGAN TDS Phase: The objective To demonstrate feasibility of SBAS implementation over Indian region with minimum set of elements .To study the ionosphere over the Indian region & to collect data for system development and necessary modifications for the Indian region for the Final Operational Phase Implementation Completed with 8 Indian Reference Stations, One Master Control Station and One Land Uplink Station.

Gagan FOP Phase Objective

To provide a certified satellite based navigation system for all phases of flight by augmenting the TDS system suitably

To provide redundancies, Implement Suitable region specific IONO model, Get safety Certification for the system for the Civil Aviation use from DGCA, the regulatory authority in India.

Table 2

Fig:7. Gagan FOP configuration

Source: http://www.isac.gov.in/navigation/gagan.jsp

Fig:8.Gagan Sites

Source: https://www.slideshare.net/huangxj73/indian-satellite-based-navigation-system-description-and-implementation-status

Glonass

Glonass refers to Global Navigation System operated for Russian government. So, it is also termed as in Russia i.e. Global Navigatsion naya Sputnikovaya Sistema. It creates an alternative to GPS which is operated for U.S government. GLONASS have the benefits of global or wide coverage and same accuracy or precision. The beginning of development of GLONASS has started in the Soviet Union in 1976 by launching 43 GLONASS satellites. After that in 1982, satellites with aided functionality have been launched by number of rockets to form constellation. In 2000’s, on the basis of government priority, restoration of the system has been increased. It is most expensive program as seen in third budget of Russia in 2010. GLONASS proves very beneficial for Russia’s territory by 2010. In 2011, restoration of system is improved to enabling full global coverage. Many upgrades of GLONASS have been launched i.e. GLONASS-K. Following features are:

Developed by Soviet Union, first launch: 1982

Declined under Russia, but now revived

Have launched 81 satellites so far Constellation

24 satellites in 3 orbital planes, 64.8 inclination.

19,100 km altitude, 11 1⁄4 hour period Signals.

3 allocated bands: G1 (1602 MHz), G2: (1245 MHz), G3 (1202 MHz)

C/A-like code: 511 chips, 1 ms code period, 50 bps.

All SVs use same PRN with frequency division multiple access (FDMA) using 16 frequency channels reused for antipodal SVs. Importance of GLONASS to INDIA Network centric warfare is one component basically depend on the navigation system (GLONASS) for precision while many variations and future weapons have been already developed. As earlier plans, alternative for GLONASS is GPS operated for U.S. but GPS dependent device would not be strategically correct. For that reason, India has to depend on autonomous choice in communications. India becomes a partner for Galileo but now days; it is used for civilian purpose in Russia.

GLONASS Architecture :The GLONASS architecture has been shown in figure

Glonass Orbit

Folowing orbit constilation are used in Glonass

Total 24 Satellite divided in 3 orbital plane containing 8 each. 1200 orbit shift along equator

Different parameter of orbit

Circular

19100 km hight

11 hour 15 min revolution time

Glonass-M space craft

Main specification are given below: 580 W power consumption

Clock stability 18*10-13

0.5 deg accuracy for altitude control Mass 250 kg

Solar panel pointing accuracy 2 deg L1,1600 MHZ

Fig:9.Glonass system Design

Source: http://www.dlr.de/kn/desktopdefault.aspx/tabid-4309/3222_read-37809/admin-1/

GLONASS System Design

As with GPS, the GLONASS system uses a satellite constellation to provide, ideally, a GLONASS receiver with six to twelve satellites at most times. A minimum of four satellites in view allows a GLONASS receiver to compute its position in three dimensions, as well as become synchronized to the system time. The GLONASS system design consists of three parts: • The Control segment • The Space segment • The User segment All these parts operate together to provide accurate three-dimensional positioning, timing and velocity data to users worldwide. The Control Segment The Control Segment consists of the system control center and a network of command tracking stations across Russia. The GLONASS control segment, similar to GPS, must monitor the status of satellites, determine the ephemerides and satellite clock offsets with respect to GLONASS time and UTC (Coordinated Universal Time), and twice a day upload the navigation data to the satellites. The Space Segment The Space Segment is the portion of the GLONASS system that is located in space, that is, the GLONASS satellites that provide GLONASS ranging information. When complete, this segment will consist of 24 satellites in three orbital planes, with eight satellites per plane. Figure 1 on Page 3 shows a combined GPS and GLONASS satellite system.

The User Segment

The User Segment consists of equipment (such as a NovAtel OEMV family receiver) that tracks and receives the satellite signals. This equipment must be capable of simultaneously processing the signals from a minimum of four satellites to obtain accurate position, velocity and timing measurements. Like GPS, GLONASS is a dual military/civilian-use system. The system’s potential civil applications are many and mirror those of GPS.

Fig:10 . View of GPS and GLONASS Satellite Orbit Arrangement

Source: https://www.researchgate.net/figure/236842701_fig1_Figure-1-SNS-Space-Segment-View-of-GPS-and-GLONASS-Satellite-Orbit-Arrangement

GLONASS space segment

The geometry repeats about once every 8 days. The orbit period of each satellite is approximately 8/17 of a sidereal day such that, after eight sidereal days, the GLONASS satellites have completed exactly 17 orbital revolutions. A sidereal day is the rotation period of the Earth relative to the equinox and is equal to one calendar day (the mean solar day) minus approximately four minutes.

     •  Because each orbital plane contains eight equally spaced satellites, one of the satellites will be at the same spot in the sky at the same sidereal time each day.

•  The satellites are placed into nominally circular orbits with target inclinations of 64.8 degrees and an orbital height of about 19,140 km, which is about 1,050 km lower than GPS satellites.

• Some of the GLONASS transmissions initially caused interference to radio astronomers and mobile communication service providers. The Russians consequently agreed to reduce the number of frequencies used by the satellites and to gradually change the L1 frequencies in the future to 1598.0625 – 1605.375 MHz. Eventually the system will only use 12 primary frequency channels (plus two additional channels for testing purposes).

The GLONASS satellite signal identifies the satellite and provides:

•  position, velocity and acceleration vectors at a reference epoch to compute satellite locations

•  synchronization bits, data age and satellite health

•  offset of GLONASS time from UTC (SU) (formerly Soviet Union and now Russia)

•  almanacs of all other GLONASS satellites

GPS and GLONASS Satellite Identification

The GLONASS satellites each transmit on slightly different L1 and L2 frequencies, with P-code on both L1 and L2, and with C/A code, at present, only on L1. GLONASS-M satellites reportedly3 transmit the C/A code on L2. Every GPS satellite transmits the L1 frequency centered at 1575.42 MHz. The GPS satellites are identifiable by their Pseudorandom Noise code number (PRN) with a NovAtel receiver. Unlike GPS, all GLONASS satellites transmit the same code at different frequencies. They derive signal timing and frequencies from one of three on-board cesium atomic clocks operating at 5 MHz: For example, L1 = 1602 MHz + (n x 0.5625) MHz where n = the frequency channel number (n = 0, 1, 2 and so on) It means that satellites transmit signals on their own frequency, separated by multiples of 0.5625 MHz or 562.5 kHz, from the frequency of other satellites; see Figure below

Fig:11 . GPS and GLONASS L1 Frequencies

Source: https://www.e-education.psu.edu/geog862/print/l10.html

The signals are right-hand circularly polarized, like GPS signals, and have comparable signal strength. GLONASS accomplishes system operation (24 satellites and only 12 channels) by having antipodal satellites transmit on the same frequency. Antipodal satellites are in the same orbit plane separated by 180 degrees in argument of latitude. This is possible because the paired satellites will never appear at the same time in view of an operational receiver that is on the earth’s surface, see Figure 3 below. At the time of publication, April 2007, four pairs of operational satellites share frequencies.

Fig:12 . GLONASS Antipodal Satellites

Source: https://www.novatel.com/an-introduction-to-gnss/chapter-3-satellite-systems/glonass/

GLONASS Time vs. Local Receiver Time GLONASS time is based on an atomic time scale similar to GPS. This time scale is UTC as maintained by Russia (UTC (SU)). Unlike GPS, the GLONASS time scale is not continuous and must be adjusted for periodic leap seconds. Leap seconds are applied to all UTC time references as specified by the International Earth Rotation and Reference System Service (IERS). Leap seconds are used to keep UTC close to mean solar time. Mean solar time, based on the spin of the Earth on its axis, is not uniform and its rate is gradually changing due to tidal friction and other factors such as motions of the Earth’s fluid core. GLONASS time is maintained within 1 ms, and typically better than 1 microsecond (μs), of UTC (SU) by the control segment with the remaining portion of the offset broadcast in the navigation message. As well, Moscow offsets GLONASS time from UTC (SU) by plus three hours.

Datum

Datum A datum is a set of parameters (translations, rotations, and scale) used to establish the position of a reference ellipsoid with respect to points on the Earth’s crust. If not set, the receiver’s factory default value is the World Geodetic System 1984 (WGS84). GLONASS information is referenced to the Parametri Zemli 1990 (PZ-90, or in English translation, Parameters of the Earth 1990, PE-90) geodetic datum, and GLONASS coordinates are reconciled in the receiver through a position filter and output to WGS84.

Galileo

Galileo is Europe’s own global navigation satellite system, providing a highly accurate, guaranteed global positioning service under civilian control. Currently providing Initial Services, Galileo is interoperable with GPS and Glonass, the US and Russian global satellite navigation systems. By offering dual frequencies as standard, Galileo are set to deliver real-time positioning accuracy down to the meter range.

Galileo services

The fully deployed Galileo system will consist of 24 operational satellites plus six in-orbit spares, positioned in three circular Medium Earth Orbit (MEO) planes at 23 222 km altitude above the Earth, and at an inclination of the orbital planes of 56 degrees to the equator.

Initial services became available on 15 December 2016. Then as the constellation is built-up beyond that, new services will be tested and made available, with system completion scheduled for 2020.

Once this is achieved, the Galileo navigation signals will provide good coverage even at latitudes up to 75 degrees north, which corresponds to Norway’s North Cape – the most northerly tip of Europe – and beyond. The large number of satellites together with the carefully-optimised constellation design, plus the availability of the three active spare satellites per orbital plane, will ensure that the loss of one satellite should have no discernible effect on the user.

Ground infrastructure

Two Galileo Control Centers (GCCs) have been implemented on European ground to provide for the control of the satellites and to perform the navigation mission management. The data provided by a global network of Galileo Sensor Stations (GSSs) are sent to the Galileo Control Centres through a redundant communications network. The GCCs use the data from the Sensor Stations to compute the integrity information and to synchronies the time signal of all satellites with the ground station clocks. The exchange of the data between the Control Centers and the satellites is performed through up-link stations.

As a further feature, Galileo is providing a global Search and Rescue (SAR) function, based on the operational Cospas-Sarsat system. Satellites are therefore equipped with a transponder, which is able to transfer the distress signals from the user transmitters to regional rescue co-ordination centers, which will then initiate the rescue operation.

At the same time, the system will send a response signal to the user, informing him that his situation has been detected and that help is on the way. This latter feature is new and is considered a major upgrade compared to the existing system, which does not provide user feedback.

Preparation for Galileo

Experimental satellites GIOVE-A and GIOVE-B were launched in 2005 and 2008 respectively, serving to test critical Galileo technologies, while also the securing of the Galileo frequencies within the International Telecommunications Union.

Over the course of the test period, scientific instruments also measured various aspects of the space environment around the orbital plane, in particular the level of radiation, which is greater than in low Earth or geostationary orbits.