GRT INSTITUTE OF ENGINEERING AND TECHNOLOGY, TIRUTTANIDEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING ADVANCED SATELLITE BASED SYSTEMS UNIT I NAVIGATION, TRACKING AND SAFETY SYSTEMS SYLLABUS : Global Navigation Satellite Systems - Basic concepts of GPS. Space segment, Control segment, user segment, GPS constellation, GPS measurement characteristics, selective availability (AS), Anti spoofing (AS). Applications of Satellite and GPS for 3D position, Velocity, determination as function of time, Interdisciplinary applications. Regional Navigation Systems- Distress and Safety- Cospas – Sarsat - Inmarsat Distress System- Location-Based service. INTRODUCTION What is GPS? The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense. GPS was originally intended for military applications, but in the 1980s, the government made the system available for civilian use. What is Satellite? It is an artificial body placed in orbit round the earth or another planet in order to collect information or for communication. What is Global Navigation Satellite Systems (GNSS)? The term ‘global navigation satellite system’ (GNSS) refers to a constellation of satellites providing signals from space transmitting positioning and timing data. By definition, a GNSS provides global coverage. GNSS receivers determine location by using the timing and positioning data encoded in the signals from space. The USA’s NAVSTAR Global Positioning System (GPS) and Russia’s Global'naya Navigatsionnaya Sputnikovaya Sistema (GLONASS) are examples of GNSS. Europe is in the process of launching its own independent GNSS, Galileo. When it becomes operational in 2014, Galileo will provide positioning and timing services through a ADVANCED SATTELITE BASED SYSTEMS S V Dharani Kumar, Asst Prof. 1 GRT INSTITUTE OF ENGINEERING AND TECHNOLOGY, TIRUTTANI DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING network of 30 satellites and an associated ground infrastructure. Galileo will be interoperable with GPS and GLONASS. This interoperability will allow manufacturers to develop terminals that work with Galileo, GPS and GLONASS. The performance of a satellite navigation system is assessed according to four criteria: 1. Accuracy refers to the difference between the measured and the real position, speed or time of the receiver. 2. Integrity refers to a system’s capacity to provide confidence thresholds as well as alarms in the event that anomalies occur in the positioning data. 3. Continuity refers to a navigation system’s ability to function without interruption. 4. Availability refers to the percentage of time during which the signal fulfils the accuracy, integrity and continuity criteria. A satellite navigation system with global coverage may be termed a global navigation satellite system or GNSS. HISTORY AND THEORY Early predecessors were the ground based DECCA, LORAN, GEE and Omega radio navigation systems, which used terrestrial longwave radio transmitters instead of satellites. These positioning systems broadcast a radio pulse from a known "master" location, followed by repeated pulses from a number of "slave" stations. The delay between the reception and sending of the signal at the slaves was carefully controlled, allowing the receivers to compare the delay between reception and the delay between sending. From this the distance to each of the slaves could be determined, providing a fix. The first satellite navigation system was Transit, a system deployed by the US military in the 1960s. Transit's operation was based on the Doppler effect: the satellites traveled on wellknown paths and broadcast their signals on a well knownfrequency. The received frequency will differ slightly from the broadcast frequency because of the movement of the satellite with respect to the receiver. By monitoring this frequency shift over a short time interval, the receiver can determine its location to one side or the other of the satellite, and several such measurements combined with a precise knowledge of the satellite's orbit can fix a particular position. Part of an orbiting satellite's broadcast included its precise orbital data. In order to ensure accuracy, the US Naval Observatory (USNO) continuously observed the precise orbits of these satellites. As a satellite's orbit deviated, the USNO would send the updated information to the satellite. Subsequent broadcasts from an updated satellite would contain the most recent accurate information about its orbit. Modern systems are more direct. The satellite broadcasts a signal that contains orbital data (from which the position of the satellite can be calculated) and the precise time the signal was transmitted. The orbital data is transmitted in a data message that is superimposed on a code that serves as a timing reference. The satellite uses an atomic clock to maintain synchronization of all the satellites in the constellation. The receiver compares the time of broadcast encoded in ADVANCED SATTELITE BASED SYSTEMS S V Dharani Kumar, Asst Prof. 2 Satellite navigation receivers reduce errors by using combinations of signals from multiple satellites and multiple correlators. in the case of fast-moving receivers. a fix is generated. Asst Prof. 3 . ADVANCED SATTELITE BASED SYSTEMS S V Dharani Kumar. However. By taking several such measurements and then looking for a point where they meet. In addition. and this slowing varies with the receiver's angle to the satellite. thereby measuring the time-of-flight to the satellite. CLASSIFICATION Satellite navigation systems that provide enhanced accuracy and integrity monitoring usable for civil navigation are classified as follows:[3] GNSS-1 is the first generation system and is the combination of existing satellite navigation systems (GPS and GLONASS). and in Japan it is the Multi-Functional Satellite Augmentation System (MSAS). with Satellite Based Augmentation Systems (SBAS) or Ground Based Augmentation Systems (GBAS). and velocity. In the United States.GRT INSTITUTE OF ENGINEERING AND TECHNOLOGY. partial. and then using techniques such as Kalman filtering to combine the noisy. the radio signals slow slightly as they pass through the ionosphere. because that changes the distance through the ionosphere. and constantly changing data into a single estimate for position. The basic computation thus attempts to find the shortest directed line tangent to four oblate spherical shells centered on four satellites. in Europe it is the European Geostationary Navigation Overlay Service (EGNOS). Ground based augmentation is provided by systems like the Local Area Augmentation System (LAAS). the satellite based component is the Wide Area Augmentation System (WAAS). time. regardless of the system being used. the position of the signal moves as signals are received from several satellites. TIRUTTANI DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING the transmission with the time of reception measured by an internal clock. places the receiver on a spherical shell at the measured distance from the broadcaster. Several such measurements can be made at the same time to different satellites. allowing a continual fix to be generated in real time using an adapted version of trilateration: see GNSS positioning calculation for details. Each distance measurement. These systems will provide the accuracy and integrity monitoring necessary for civil navigation. India's yet-to-beoperational IRNSS.GRT INSTITUTE OF ENGINEERING AND TECHNOLOGY. This system consists of L1 and L2 frequencies for civil use and L5 for system integrity. Regional SBAS including WAAS (US). Regional Satellite Navigation Systems such as China's Beidou. 4 . Regional scale GBAS such as CORS networks. Core Satellite navigation systems. Galileo (European Union) and Compass (China). Global Satellite Based Augmentation Systems (SBAS) such as Omnistar and StarFire. making it a GNSS-2 system.Fail. TIRUTTANI DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING GNSS-2 is the second generation of systems that independently provides a full civilian satellite navigation system. EGNOS (EU). MSAS (Japan) and GAGAN (India). Local GBAS typified by a single GPS reference station operating Real Time Kinematic (RTK) corrections.Plan- Currently in orbit and healthy cess ure aration ned I 1978–1985 10 1 0 0 0 II 1989–1990 9 0 0 0 0 IIA 1990–1997 19 0 0 0 6 IIR 1997–2004 12 1 0 0 12 0 0 0 7 From 2010 6 0 6 0 6 IIIA From 2014 0 0 0 12 0 IIIB — 0 0 0 8 0 IIIC — 0 0 0 16 0 64 2 6 36 31 IIR-M 2005–2009 8 IIF Total ADVANCED SATTELITE BASED SYSTEMS S V Dharani Kumar. and Japan's proposed QZSS. GLONASS (Russian Federation). currently GPS (United States). Asst Prof. including aircraft. exemplified by the European Galileo positioning system. Continental scale Ground Based Augmentation Systems (GBAS) for example the Australian GRAS and the US Department of Transportation National Differential GPS (DGPS) service.In prep. Block Launch Period Satellite launches Suc. Development is also in progress to provide GPS with civil use L2 and L5 frequencies. The U. This location is then displayed. and each GPS receiver uses these signals to calculate its three-dimensional location (latitude. expensive. A number of applications for GPS do make use of this cheap and highly accurate timing. The control segment is composed of a master control station. These are the space segment (SS). traffic signal timing.S. and operates the space and control segments. For example. Many GPS units show derived information such as direction and speed. 5 . and a user segment (US). and synchronization of cell phone base stations.GRT INSTITUTE OF ENGINEERING AND TECHNOLOGY. TIRUTTANI DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING BASIC CONCEPTS OF GPS A GPS receiver calculates its position by precisely timing the signals sent by GPS satellites high above the Earth. and power hungry clock. The space segment is composed of 24 to 32 satellites in medium Earth orbit and also includes the payload adapters to the boosters required to launch them into orbit. maintains. Although four satellites are required for normal operation. GPS satellites broadcast signals from space. longitude. to give a (possibly degraded) position when fewer than four satellites are visible. a control segment (CS). [a] Because of this. a receiver can determine its position using only three satellites. In typical GPS operation. The receiver is on the surface of each of these spheres when the distances and the satellites' locations are correct. or including information from the vehicle computer. calculated from position changes. inertial navigation. perhaps with a moving map display or latitude and longitude. and a host of dedicated and shared ground antennas and monitor stations. Asst Prof. These include time transfer. Each satellite continually transmits messages that include: The time the message was transmitted The satellite position at time of message transmission The receiver uses the messages it receives to determine the transit time of each message and computes the distance to each satellite using the speed of light. elevation or altitude information may be included. a ship or aircraft may have known elevation. this solution gives the position of the receiver along with the difference between the time kept by the receiver's on-board clock and the true time-of-day. Air Force develops. dead reckoning. thereby eliminating the need for a very large. STRUCTURE The current GPS consists of three major segments. fewer apply in special cases. Four sphere surfaces typically do not intersect. four or more satellites must be visible to obtain an accurate result. If one variable is already known. which use only the location. it can be said with confidence that when the navigation equations are solved to find an intersection. The very accurately computed time is used only for display or not at all in many GPS applications. an alternate master control station. These distances and satellites' locations are used to compute the location of the receiver using the navigation equations. The user segment is composed of hundreds of ADVANCED SATTELITE BASED SYSTEMS S V Dharani Kumar. Each of these distances and satellites' locations define a sphere. and altitude) and the current time. Some GPS receivers may use additional clues or assumptions such as reusing the last known altitude. The orbits are arranged so that at least six satellites are always within line of sight from almost everywhere on Earth's surface.GRT INSTITUTE OF ENGINEERING AND TECHNOLOGY. SPACE SEGMENT The space segment (SS) is composed of the orbiting GPS satellites. the angular difference between satellites in each orbit is 30.600 km (16. each SV makes two complete orbits each sidereal day. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements. In general terms. and 105 degrees apart which sum to 360 degrees.S. or Space Vehicles (SV) in GPS parlance. The orbital period is one-half a sidereal day. repeating the same ground track each day.e. orbital radius of approximately 26. With the increased number of satellites. the constellation was changed to a non uniform arrangement. and scientific users of the Standard Positioning Service (see GPS navigation devices). This was very helpful during development because even with only four satellites. ADVANCED SATTELITE BASED SYSTEMS S V Dharani Kumar. For military operations. 11 hours and 58 minutes so that the satellites pass over the same location or almost the same locations every day.200 km (12. Such an arrangement was shown to improve reliability and availability of the system.600 mi). eight each in three approximately circular orbits. but this was modified to six orbital planes with four satellites each. relative to a uniform system. The GPS design originally called for 24 SVs. TIRUTTANI DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING thousands of U. Orbiting at an altitude of approximately 20. commercial. the ground track repeat can be used to ensure good coverage in combat zones. About nine satellites are visible from any point on the ground at any one time (see animation at right). there are 32 satellites in the GPS constellation. correct alignment means all four are visible from one spot for a few hours each day. ensuring considerable redundancy over the minimum four satellites needed for a position. and tens of millions of civil. 6 . i. 120. Asst Prof. when multiple satellites fail.. 105. and allied military users of the secure GPS Precise Positioning Service. The six orbit planes have approximately 55° inclination (tilt relative to Earth's equator) and are separated by 60° right ascension of the ascending node (angle along the equator from a reference point to the orbit's intersection). The result of this objective is that the four satellites are not evenly spaced (90 degrees) apart within each orbit.500 mi). As of December 2012. MASTER CONTROL STATION : The master control station in Colorado is where 2SOPS performs the primary control segment functions. There are 16 monitoring stations located throughout the world. and then uploads this data to the satellites. Diego Garcia. an alternate master control station. 7 . enabling 2SOPS to determine and evaluate the health status of the GPS constellation. including six from the Air Force and 10 from the National Geospatial-Intelligence Agency (NGA). TIRUTTANI DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING CONTROL SEGMENT The control segment is composed of: 1. The MCS monitors navigation messages and system integrity. The MCS generates and uploads navigation messages and ensures the health and accuracy of the satellite constellation. and robustness for telemetry. GROUND ANTENNAS : Ground antennas are used to communicate with the GPS satellites for command and control purposes. 2SOPS uses the MCS to perform satellite maintenance and anomaly resolution. and 4. Asst Prof. the MCS can reposition satellites to maintain an optimal GPS constellation. and navigation signals. and collect telemetry. In the event of a satellite failure. S-band ranging allows 2SOPS to provide anomaly resolution and early orbit support. The sites utilize sophisticated GPS receivers and are operated by the MCS. 2. flexibility. tracking. These antennas support S-band communications links that send/transmit navigation data uploads and processor program loads. 3. a master control station (MCS). Monitor stations collect atmospheric data. and Cape Canaveral. six dedicated monitor stations. ADVANCED SATTELITE BASED SYSTEMS S V Dharani Kumar. MONITOR STATIONS : Monitor stations track the GPS satellites as they pass overhead and channel their observations back to the master control station. It receives navigation information from the monitor stations. range/carrier measurements. utilizes this information to compute the precise locations of the GPS satellites in space.GRT INSTITUTE OF ENGINEERING AND TECHNOLOGY. the control segment is connected to the eight Air Force Satellite Control Network (AFSCN) remote tracking stations worldwide. increasing visibility. four dedicated ground antennas. There are four dedicated GPS ground antenna sites co-located with the monitor stations at Kwajalein Atoll. providing command and control of the GPS constellation. and command. The ground antennas are also responsible for normal command transmissions to the satellites. In addition. Ascension Island. The second is the planning and execution of satellite movements during LADO. Launch And Early Orbit.GRT INSTITUTE OF ENGINEERING AND TECHNOLOGY. LEGACY ACCURACY IMPROVEMENT INITATIVE (L . expanded the number of monitoring sites in the operational control segment from six to 16. The L-AII effort added 10 operational GPS monitoring sites owned and operated by the National Geospatial-Intelligence Agency (NGA). This tripled the amount of data collected on GPS satellite orbits. Utilizing commercial off-the-shelf products. with the final version declared fully operational in April 2011. The AEP system received several upgrades. The LADO system uses the AFSCN remote tracking stations only. 8 . completed in 2008. TIRUTTANI DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING CONTROL SEGMENT MODERNIZATION : As part of the GPS modernization program. satellites taken out of service for anomaly resolution. the Air Force implemented the Architecture Evolution Plan. And Disposal Operations (LADO) : The GPS master control station can command and control a constellation of up to 32 satellites. residual satellites stored in orbit. The third function is LADO simulation of different telemetry tasks for GPS payloads and subsystems.AII) The Legacy Accuracy Improvement Initiative. including the new Block IIF satellites. The first is telemetry. and satellites requiring end-of-life disposal. NGA originally fielded these sites to help it define the Earth reference frame used by GPS. substantially enhancing sustainability and accuracy. mainframe-based master control station with an entirely new one built on modern IT technologies. To view the schedule for control segment modernization. not the dedicated GPS ground antennas. In 2007. AEP features an alternate master control station. tracking. 2SOPS fielded the LADO system to handle GPS satellites outside the operational constellation. The LADO system serves three primary functions. Asst Prof. and control. AEP also improved GPS monitor stations and ground antennas. visit the GPS Modernization page. ARCHITECTURE EVOLUTION PLAN : In 2007. The AEP system improves the flexibility and responsiveness of GPS operations and paves the way forward for the next generation of GPS space and control capabilities. a fully operational backup for the MCS. enabling a 10% to 15% improvement in the accuracy of the information broadcast from the GPS constellation. These include newly launched satellites undergoing checkout. Anomaly Resolution. ADVANCED SATTELITE BASED SYSTEMS S V Dharani Kumar. AEP is capable of managing all satellites in the constellation. replacing the original. the Air Force has continuously upgraded the GPS control segment over the past few years and will keep doing so in the years to come. including the ability to fully control the modernized civil signals (L2C. tuned to the frequencies transmitted by the satellites. In general. and tens of millions of civil.GRT INSTITUTE OF ENGINEERING AND TECHNOLOGY. a component of the LCC. The LCC component of OCX will be delivered prior to OCX Block 1 in order to support the launch and checkout of the first GPS III satellite. scheduled for 2015. Any increments beyond OCX Block 2 will be phased to support future satellite generations. Originally limited to four or five. and control additional navigation signals. receiver-processors. receivers typically have between 12 and 20 channels. The Air Force awarded the contract for the provision of the LCC to Lockheed Martin in January 2012. and L1C). the Air Force awarded a contract to Raytheon for development of the Next Generation Operational Control System. L5. At the same time. the Air Force operationally accepted a new version adding GPS Block IIF capability. Asst Prof. and allied military users of the secure GPS Precise Positioning Service. ADVANCED SATTELITE BASED SYSTEMS S V Dharani Kumar.S. USER SEGMENT The user segment is composed of hundreds of thousands of U. as of 2007. and a highly stable clock (often a crystal oscillator). OCX Block 2 will support. this has progressively increased over the years so that. A receiver is often described by its number of channels: this signifies how many satellites it can monitor simultaneously. the Air Force awarded Raytheon a contract for the development of the Launch and Checkout System (LCS). The LCC will ensure a timely launch so constellation availability remains optimal and not impacted by the late discovery of problems. monitor. GPS receivers are composed of an antenna. commercial and scientific users of the Standard Positioning Service. following testing during the launch of the first GPS IIF satellite. OCX Block 1 is scheduled to enter service in 2017. Unlike today's LADO system. 9 . In October 2010. TIRUTTANI DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING The LADO system has been upgraded several times since 2007. This version will introduce the full capabilities of the L2C navigation signal. the LCC will be fully integrated with OCX. including L1C and L5. OCX will be delivered in increments. NEXT GENERATION OPERATIONAL CONTROL SYSTEM (OCX) In 2008. OCX Block 0 will launch and checkout the GPS III satellites. which operates separately from the master control station. They may also include a display for providing location and speed information to the user. LAUNCH CHECK OUT CAPABILITY (LCC) The Launch Checkout Capability is a command and control center that will checkout all GPS III satellites. This approach will allow the operation of a single OCX-centric system that can sustain the GPS constellation from launch to disposal. OCX will add many new capabilities to the GPS control segment. both are used for GPS navigation (position. the acquisition and maintenance of signal tracking). GPS MEASUREMENT : There are two range-type measurements that can be made on the GPS signals: Pseudo-ranges. the carrier-tracking loop must. Many GPS receivers can relay position data to a PC or other device using the NMEA 0183 protocol. even low-cost units commonly includeWide Area Augmentation System (WAAS) receivers. hence correlation techniques are used to obtain the satellite signals. the receiver will carry out a "sky search". such as the SiRF and MTK protocols. The two tracking loops have to work together in an iterative manner. or only a very poor estimate of PVT is available. and on the stored satellite almanac information residing within the receiver. Before studying these measurements it is useful to consider the overall GPS hardware tracking operation (in a much abbreviated form!). and both have a role in the specialised data processing that characterises GPS surveying. adjust the frequency of the receiver-generated carrier until it matches the incoming carrier frequency. The receiver will then decode the Navigation Message and read the almanac information about all the other satellites in the constellation. allowing open source tools like gpsd to read the protocol without violating intellectual property laws. Satellite visibility is estimated from predictions of present PVT.GRT INSTITUTE OF ENGINEERING AND TECHNOLOGY.determination). Receivers can interface with other devices using methods including a serial connection. The amount of this offset is the "beat" frequency which ADVANCED SATTELITE BASED SYSTEMS S V Dharani Kumar. using the RTCM SC-104 format. or Bluetooth. The receiver's carrier-tracking loop will locally generate an L1 carrier frequency (or L2 if the receiver is capable of tracking this frequency) which differs from the received carrier signal due to a Doppler offset of the carrier frequency. TIRUTTANI DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING GPS receivers may include an input for differential corrections. Both are a product of the operation of the GPS receiver (that is. USB. Asst Prof.[clarification needed] Other proprietary protocols exist as well. Although this protocol is officially defined by the National Marine Electronics Association (NMEA). In order to maintain lock on the carrier. in effect. [citation needed] As of 2006.[69] references to this protocol have been compiled from public records. which limits the accuracy of the signal sent using RTCM. aiding each other in order to acquire and track the satellite signals. velocity and time -PVT -. and Carrier phase observations. (If no stored almanac information exists. attempting to randomly locate and lock onto a signal.) A carriertracking loop is used to track the carrier frequency while a code-tracking loop is used to track the C/A and/or P code signals. 10 .[citation needed] Receivers with internal DGPS receivers can outperform those using external RTCM data.800 bit/s speed. Data is actually sent at a much lower rate. This is typically in the form of an RS-232 port at 4. This Doppler offset is proportional to the relative velocity along the line-of-sight to the satellite. A typical satellite tracking sequence begins with the receiver determining which satellites are visible above the horizon. The received satellite signal level is actually less than the background noise level. GRT INSTITUTE OF ENGINEERING AND TECHNOLOGY. What role does the code-tracking loop play in this process? In order for the carriertracking loop to acquire the incoming satellite signal in the first place the carrier signal must be made visible above the background noise. One-way ranging using PRN codes. A by-product of code-tracking are the pseudo-range measurements. PSEUDO – RANGES MEASUREMENT : Ranging with the PRN Codes Consider for a moment a perfect system where all satellite clocks are synchronized to the same time system: GPS Time. which is used to determine the receiver's velocity. This is generally done by the code-tracking loop using the code-correlating technique to "reconstruct" the carrier wave (see discussion below under "Carrier Phase Measurements"). Asst Prof. Multiplying the time offset required to align the two code sequences within the code-tracking loop (one from the received satellite signal and the other an internally generated code) by the speed of electromagnetic radiation yields the satellite-receiver range. TIRUTTANI DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING can be processed to give a periodic carrier phase measurement. Furthermore. Measuring ranges simultaneously in this fashion to three satellites would fix the receiver's position at the intersection of three spheres of known radii (the satellite ranges). Under these circumstances. Now suppose the satellite starts transmitting its L1 carrier (modulated with the combined C/A code and navigation data). the satellite and receiver generated C/A codes would be output in unison. when the satellite signal is received it will be lagging the receiver generated code due to the signal transit time. and at the same instant the receiver begins generating the C/A code corresponding to that particular satellite (see Figure below). Figure 1. as illustrated in Figure 2. However. and none of the clocks drift with respect to the GPST scale. the ground receiver's clock also maintains the same synchronization. ADVANCED SATTELITE BASED SYSTEMS S V Dharani Kumar. 11 . centred at each satellite whose coordinates can be calculated from the Navigation Message. The derivative of this phase measurement is the "Doppler" measurement. to determine position using pseudo-range data. The geometric problem of 3-D positioning from ranges. Consequently each range is contaminated by the receiver clock error.section 3. and implementation of the policy of Selective Availability.GRT INSTITUTE OF ENGINEERING AND TECHNOLOGY. Hence. Asst Prof. 12 . as one of the accuracy limitations of the GPS system. This is the basis of GPS (real-time) navigation as described insection1. Ranging (and hence receiver position determination) can be carried out using the C/A code or the P code.2). and its allies). results in the provision of two GPS positioning services: the Precise Positioning Service based on P code (dual-frequency) ranging. This distinction between the ranging codes.2.7). The absence of a C/A code on L2 is intentional. a minimum of four satellites must be tracked and the position determination problem is therefore one requiring the solution of four equations (one per observation). and hence the C/A derived ranges are subject to greater measurement "noise". and the associated policies for their use (in peacetime and in times of global emergencies). or a linear combination of the L1 and L2 pseudo-ranges that largely eliminates the bias due to ionospheric refraction (section 6. Furthermore. Others are the ability under the policy of Anti-Spoofing to restrict access to the secret Y code to only "authorized" users (such as the military and those working in the "national interest" of the U. the C/A code resolution is "coarser".4. and the Standard Positioning Service based on single frequency C/A code ranging. each containing four unknowns: the three-dimensional position components and the receiver-clock offset (from GPST).2.S. In reality the situation is more complex: Receivers are generally equipped with quartz crystal clocks that do not necessarily keep the same time as the more stable satellite clocks (these clocks can be approximately synchronized to GPST using the clock correction model transmitted in the Navigation Message -. TIRUTTANI DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING Figure 2. This is the reason this range measurement is referred to as a “pseudorange". P code ranging can be performed on either of the two frequencies.3. ADVANCED SATTELITE BASED SYSTEMS S V Dharani Kumar. 1993). or the "delay-lock loop" electronics. Figure 3. This process is summarised in Figure 3 below. As soon as the incoming signal and the receiver C/A code sequences are aligned within the receiver (by sliding the received code sequence against that internally generated sequence). contained within the Navigation Message. then use a timing mark known as the "Handover Word". 13 . Typically a P code receiver must acquire lock on the C/A code first. the determination of the amount by which the receiver generated PRN code must be shifted to align it with the incoming signal. and the travel time of the signal from the satellite to the receiving antenna). Within the electronics of a receiver tracking "channel" the L1 carrier modulated by the C/A code is mixed with a locally generated replica C/A code. The local C/A code is generated on a different time scale to that of the incoming C/A code (due to nonsynchronization of the receiver clock to GPST. the extraction of the pseudo-range. how the incoming satellite signal is processed within the GPS receiver. for example. TIRUTTANI DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING Recovery of PRN Ranging Codes from the Incoming Signals The PRN codes are accurate time marks that permit the receiver's navigation computer to determine the time-of-transmission of any portion of the satellite signal. a sliding correlation technique as described above for the C/A code cannot be used in practice without a very good estimate of GPST and receiver position. or more precisely. Extraction of the Pseudo-Ranges As mentioned already. LANGLEY. leaving the incoming carrier signal modulated only by the binary Navigation Message. the "0"s and "1"s of the two codes cancel.GRT INSTITUTE OF ENGINEERING AND TECHNOLOGY. to enable the correct portion of the P code to be generated within the receiver and thus initialise the P code delay-lock loop. Alignment of the incoming signal with the receiver generated C/A code is carried out by the code-tracking loop. Because of the complexity of the P code sequence (its length and higher chipping rate). Before examining this in detail it is necessary to consider. Asst Prof. 1987. is carried out with the aid of a PRN code correlator in some delaylock loop scheme (see.023Mbps. Recovery of ranging code. How accurate is this carried out? The C/A code has a chip rate of 1. in general terms. TALBOT. corresponding to a wavelength of about ADVANCED SATTELITE BASED SYSTEMS S V Dharani Kumar. and for P code ranging it is of the order of 0. This makes static GPS positioning more accurate and reliable than in the case of positioning a moving GPS receiver (using either pseudo-range or carrier phase data). As a "rule-of-thumb": the alignment of the incoming and receiver generated codes is generally possible to within about 1-2% of the chipping rate. on the other hand. Both the P and C/A code ranges are susceptible to multipath (though the susceptibility is inversely proportional to the signal frequency). buildings. hence the measurement precision of C/A code ranging is of the order of 3-5m.compared to the C/A and P code chip lengths. the multipath effect changes. chimneys. and hence higher measurement precision. As the receiver-satellite geometry changes (and hence the angle of incidence and reflection of the signal with respect to the reflective surface changes). and the causes of multipath tend to be permanent features (metallic fences. The P code is modulated on both the L1 and L2 carriers. time-oftransmission information for the L-band signal cannot be imprinted onto the carrier wave as is ADVANCED SATTELITE BASED SYSTEMS S V Dharani Kumar. The P (or Y) code.approximately 19cm for L1 and 24cm for L2 -. and hence a wavelength of about 30m. Assuming a measurement resolution of 1-2% of the wavelength. (Modern "narrow correlator" technology has demonstrated 10 times better correlation performance for the C/A code than that above. 14 . and generally "averages out" over a period from several minutes to a quarter of an hour.5m.). For pseudo-ranges this could mean tens or hundreds of meters.) The main advantages gained by using the P code therefore are: Because of the higher chipping rate. has a chip rate of 10. or more.3-0. Unfortunately.23Mbps. Multipath is receiver-satellite geometry dependent. a phase measurement is "ambiguous" as it cannot discriminate one (either L1 or L2) wavelength from another. hence the ionospheric signal delay can be overcome. Multipath is caused by extraneous reflections from nearby metallic objects or water surfaces reaching the antenna and causing the signal measurement process to become noisy than normal. Asst Prof.GRT INSTITUTE OF ENGINEERING AND TECHNOLOGY. superstructure. P code receivers are better suited to high dynamic environments and resist signal jamming better than C/A code receivers. etc. In other words. TIRUTTANI DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING 300m (speed of light divided by the frequency). P code ranging translates into a more accurate position fix. Some characteristics of multipath are Multipath can cause "jumps" in the signal measurement of the order of its (effective) wavelength. this means that carrier phase can be measured to millimeter precision compared with a few meters for C/A code measurements (and several decimeters for P code measurements). but of the order of only centimeters for carrier phase measurements. water surfaces. CARRIEER PHASE MEASUREMENTS : The wavelengths of the carrier waves are very short -. hence the multipath effect will generally repeat on a daily basis at the same receiver site. (This is analogous to making terrestrial distance measurements using only the "reader" portion of a steel band. Integrated Carrier Beat Phase Raw carrier phase measurements are generally the by-product of all GPS receivers. then the integrated carrier phase observation could be generated: ADVANCED SATTELITE BASED SYSTEMS S V Dharani Kumar. These phase measurements cannot be used as "range" observations because they are ambiguous.GRT INSTITUTE OF ENGINEERING AND TECHNOLOGY. 15 . a phase-lock loop. rather than 154 or 120 times lower in the case of the P code. Squaring. for example. and high precision kinematic positioning. Figure 1. and time. and furthermore. Asst Prof. or otherwise processing the received signal without using a knowledge of the ranging codes. More complex signal processing is required to make carrier phase measurements on the L2 signal under conditions of Anti-Spoofing. There are essentially two means by which the carrier wave can be recovered from the incoming modulated signal: Reconstruct the carrier wave by removing the ranging code and broadcast message modulations. In the first technique the ranging codes (C/A and/or P code) must be known. It is nevertheless the basis for GPS surveying. TIRUTTANI DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING done using PRN codes (this would be possible only if the PRN code frequency was the same as the carrier wave. The extraction of the Navigation Message can then be easily performed by reversing the process by which the bi-phase shift key modulation was carried out in the satellite. and 1540 or 1200 times lower for the C/A code).) It is very difficult to resolve the continuously changing unknown ambiguity in a navigation solution (as can be done in the case of the receiver clock bias). The basic phase measurement is therefore in the range 0° to 360° (see Figure 1 below). the ambiguity changes continuously. The ambiguity is therefore a function of both the receiver channel tracking the satellite. But all is not lost! If it were possible to keep track of the number of whole wavelengths of the carrier wave as it is sampled within. Carrier phase measurements. In the latter method no knowledge of the ranging codes is required. 1. 16 . which although useful for some applications such as the "phase smoothing" of pseudo-ranges. the satellite signal collapses into the original very narrow carrier frequency band and signal power is again boosted well above the background noise. the signal power is below the background noise (Figure 3 below). Spreading and de-spreading the spectrum of the carrier wave. Integrated carrier phase and the ambiguity term. Asst Prof. Extraction of Carrier Beat Phase: Reconstructing the Carrier Wave This is the technique used within code-correlating receivers.GRT INSTITUTE OF ENGINEERING AND TECHNOLOGY. The incoming and receiver-generated sine waves are continuously aligned within a "phase-lock loop" (section 4. After the ranging code modulations are removed by the procedure described above. When the spread spectrum signal is received at the GPS antenna. the broadcast message can be extracted. (After NATO. 1991) By mixing a locally generated sine wave at the same frequency as the "reconstructed" received carrier (modulated only by the Navigation Message). is still not suitable for survey applications.3). A much more useful carrier phase observable can be constructed through the "integration" of carrier phase measurements (see below). Figure 3. ADVANCED SATTELITE BASED SYSTEMS S V Dharani Kumar. Periodic sampling of the phase of the local carrier provides the carrier beat phase observable (Figures 3 above and Figure 4 below). TIRUTTANI DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING Figure 2. Figure 5 below illustrates this measurement scheme. Extraction of Carrier Beat Phase: "Squaring" the Carrier Wave In principle. the effective wavelength is of the order of 9. Figure 5. The incoming signal is first converted to an intermediate frequency (IF) signal. (This happens because a phase inversion is a change in the IF signal amplitude from "+1" to "-1". Extracting carrier phase from incoming GPS signals by carrier wave squaring. However. The easiest option for GPS instrument manufacturers is to use the "squaring" technique (or some variation of it) to make L2 phase measurements.5cm on L1 and 12cm on L2. Reconstructing the carrier wave and extraction of pseudo-range data. the Y code is secret and hence cannot be used in this code-correlating mode. than any other signal processing technique. Any phase inversions in the IF signal due to the PRN codes or message are removed.GRT INSTITUTE OF ENGINEERING AND TECHNOLOGY. Under the policy of Anti-Spoofing. and hence better quality measurements. Squaring the signal results in a signal with constant amplitude of unity.) However. or rather the beat frequency carrier wave. The carrier. TIRUTTANI DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING Measurement of carrier beat phase on L2 by this technique requires knowledge of the P code generating algorithm. the squared carrier wave measurement is made on a carrier wave of double frequency. the primary advantage of the code-correlating approach is that it results in a far better signal-to-noise ratio. 17 . ADVANCED SATTELITE BASED SYSTEMS S V Dharani Kumar. the operation of a squaring receiver is very simple. is obtained simply by squaring this signal. and the instantaneous amplitude is either "+1" or "-1". Aside from resulting in a noisier measurement. squaring the signal also squares the noise. or from "-1" to "+1". That is. Asst Prof. and hence the codes and message information are lost. Figure 4. the Global Positioning System (GPS) was originally built for military use. The system should be passive (one-way) as far as the user is concerned. GPS remained a military-only technology until the early 1980s. TIRUTTANI DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING ESSENTIAL CHARACTERISTICS From a study of the design requirements of an ideal "Global Positioning System". as well. Measurement Technology: A one-way ranging system based on microwave transmissions would satisfy the requirements for a listen-only. System Configuration: A multi-satellite system at high altitude. and they must be synchronised in some way. and the development of strategies to overcome any unacceptably high error sources. This inaccuracy was due to the deliberate distortion of the signal in order to prevent civilian gear from being used in a military attack on the U. and as a consequence of the feasible technological features of such a system. No receiving function is to be performed by the satellites. SELECTIVE AVAILABILITY – ANIT SPOOFING Conceived in the 1970s. with the satellites transmitting the signals necessary to support position determination at the user station. Satellite Technology: The system should concentrate as much complexity into the satellites as possible. and computing the ephemerides. the essential characteristics of the system can now be identified. By the early 1990s. 18 . The number of satellites to be visible to a user is dependent upon the observation type to be employed and the positioning strategy adopted. What remains to be established is: The positioning principle to be used (related to the measurement technology and the characteristics of the satellite constellation).S. This was called Selective Availability (SA). To make such a system work. separate clocks must be used (in the satellites and within the user equipment). high precision. The satellites should somehow broadcast time-of-transmission information to the user. simple-to-use positioning system. when President Reagan decided the technology could be adapted for public use. Asst Prof. civilians could buy GPS equipment that was accurate within only about 300 feet. ADVANCED SATTELITE BASED SYSTEMS S V Dharani Kumar. Satellites broadcast their ephemerides to all users.GRT INSTITUTE OF ENGINEERING AND TECHNOLOGY. and The residual errors remaining in the system (after application of the best available technology). A Control Segment responsible for tracking the satellites. but not in a geostationary orbit. announced the United States has no intent to ever use Selective Availability again. After the attacks of September 11th. however the higher data (chipping) rate of P(Y) code can provide a higher processing gain which will provide better tracking performance in a jamming environment. President Clinton signed an order ending SA as part of an on-going effort to make GPS more attractive to civil and commercial users worldwide. while the accuracy of civilian GPS receivers may be reduced by the United States military through Selective Availability. Asst Prof. Those features are not available with the similar. Military GPS is even more precise and has a margin of error of only a few centimeters. which governed the GPS system at that time. the United States Presidential Directive instructing the discontinuation of Selective Availability. TIRUTTANI DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING On May 1. It is designed to provide accurate position information service to users in India as well as the region extending up to 1500 km from its boundary.[1] However. beginning in 2005. REGIONAL NAVIGATION SYSTEMS IRNSS is an independent regional navigation satellite system being developed by India. now being used around the world in many different applications. on Sept. along with the directive that no future GPS programs will include selective availability. and contingency recovery. Now. called M-code. However.GRT INSTITUTE OF ENGINEERING AND TECHNOLOGY. 19 . PPS-SM systems require periodic updates with a classified "Red Key" that may only be transmitted by secure means (such as physically taking the receiver to a secure facility for rekeying or having a trusted courier deliver a paper tape with a new key to the receiver. will provide additional improvements to anti-jam capabilities. over-the-air rekeying. The end of Selective Availability was a major turning point that has helped GPS to become a global utility. but older. PPS-SM system. makes any changes to SA unlikely. GPS is accurate within 40 feet. All military receivers newly-deployed after the end of September 2006 must use SAASM. 2001. The Extended Service Area lies between primary service area and area enclosed by the rectangle from Latitude 30 deg South to 50 deg North. A Selective Availability Anti-spoofing Module (SAASM) is used by military Global Positioning System receivers to allow decryption of precision GPS coordinates.[citation needed] SAASM hardware is covered with an anti-tampering coating. ADVANCED SATTELITE BASED SYSTEMS S V Dharani Kumar.[citation needed] Deployment of the next generation military signal for GPS. which is its primary service area. after which that paper tape must be securely destroyed). SAASM allows satellite authentication. 17. Longitude 30 deg East to 130 deg East.[1] SAASM does not provide any additional anti-jam capability. 2000. the Interagency GPS Executive Board (IGEB). A complete constellation of 18 satellites with M-code capability is planned for 2016. such as M-Code. SAASM systems can be updated with an encrypted "Black Key" that may be transmitted over unclassified channels. to deter analysis of their internal operation. or much better. the industry buzzed over the possibility of a return to SA. commenced with the launch of IIR-M and IIF satellites. Future GPS upgrades. IRNSS CDMA Ranging Stations (IRCDR) carry out precise two way ranging of IRNSS satellites. Laser Ranging Stations (ILRS) is planned to be used periodically to calibrate the IRNSS orbit determined by the other techniques. Asst Prof. IRNSS Network Timing Centre (IRNWT) at Byalalu generates. has already started functioning from its designated orbital slot after extensive on orbit test and evaluation to confirm its satisfactory performance. ADVANCED SATTELITE BASED SYSTEMS S V Dharani Kumar. with three satellites in geostationary orbit and four satellites in inclined geosynchronous orbit. maintains and distributes IRNSS Network Time. namely. IRNSS ground segment is responsible for navigation parameter generation and transmission. The constituent elements of the ground segment are: ISRO Navigation Centre (INC) at Byalalu. Standard Positioning Service (SPS) which is provided to all the users and Restricted Service (RS). The IRNSS space segment consists of seven satellites. The IRNSS System is expected to provide a position accuracy of better than 20 m in the primary service area. INC primarily generates navigation parameters. IRNSS-1A. IRSCF also uplinks the navigation parameters generated by the INC. is the nerve center of the IRNSS Ground Segment. RNSS comprises of a space segment and a ground segment. IRNSS Data Communication Network (IRDCN) provides the required digital communication backbone to IRNSS network. satellite control.GRT INSTITUTE OF ENGINEERING AND TECHNOLOGY. which is an encrypted service provided only to the authorised users. the first satellite of the IRNSS constellation. In addition to the regular TT&C operations. Spacecraft Control Facility (SCF) controls the space segment through Telemetry Tracking & Command network. IRNSS Range and Integrity Monitoring Stations (IRIMS) perform continuous one way ranging of the IRNSS satellites and are also used for integrity determination of the IRNSS constellation. ranging and integrity monitoring and time keeping. 20 . TIRUTTANI DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING IRNSS will provide two types of services. Asst Prof. TIRUTTANI DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING Applications of IRNSS: Terrestrial. Aerial and Marine Navigation Disaster Management Vehicle tracking and fleet management Integration with mobile phones Precise Timing Mapping and Geodetic data capture Terrestrial navigation aid for hikers and travelers Visual and voice navigation for drivers DISTRESS AND SAFETY ADVANCED SATTELITE BASED SYSTEMS S V Dharani Kumar. 21 .GRT INSTITUTE OF ENGINEERING AND TECHNOLOGY.