GRT INSTITUTE OF ENGINEERING AND TECHNOLOGY, TIRUTTANIDEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

ADVANCED SATELLITE BASED SYSTEMSUNIT I NAVIGATION, TRACKING AND SAFETY SYSTEMSSYLLABUS :

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.

INTRODUCTIONWhat 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.GPSwas 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 USAs NAVSTAR Global Positioning System (GPS)and Russias 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 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. Accuracyrefers to the difference between the measured and the real position, speed or time of the receiver.

2. Integrityrefers to a systems capacity to provide confidence thresholds as well as alarms in the event that anomalies occur in the positioning data.

3. Continuityrefers to a navigation systems ability to function without interruption.

4. Availabilityrefers 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 aglobal navigation satellite systemorGNSS.HISTORY AND THEORY

Early predecessors were the ground basedDECCA,LORAN,GEEandOmegaradio navigationsystems, which used terrestriallongwaveradiotransmittersinstead of satellites. Thesepositioning systemsbroadcast 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 afix.

The first satellite navigation system wasTransit, a system deployed by the US military in the 1960s. Transit's operation was based on theDoppler effect: the satellites traveled on well-known 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, theUS 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 anatomic clockto maintain synchronization of all the satellites in the constellation. The receiver compares the time of broadcast encoded in the transmission with the time of reception measured by an internal clock, thereby measuring the time-of-flight to the satellite. 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 oftrilateration: seeGNSS positioning calculationfor details.

Each distance measurement, regardless of the system being used, places the receiver on a spherical shell at the measured distance from the broadcaster. By taking several such measurements and then looking for a point where they meet, a fix is generated. However, in the case of fast-moving receivers, the position of the signal moves as signals are received from several satellites. In addition, the radio signals slow slightly as they pass through the ionosphere, and this slowing varies with the receiver's angle to the satellite, because that changes the distance through the ionosphere. The basic computation thus attempts to find the shortest directed line tangent to four oblate spherical shells centered on four satellites. Satellite navigation receivers reduce errors by using combinations of signals from multiple satellites and multiple correlators, and then using techniques such as Kalman filteringto combine the noisy, partial, and constantly changing data into a single estimate for position, time, and velocity.

CLASSIFICATION

Satellite navigation systems that provide enhanced accuracy and integrity monitoring usable for civil navigation are classified as follows:[3] GNSS-1is the first generation system and is the combination of existing satellite navigation systems (GPS and GLONASS), withSatellite Based Augmentation Systems(SBAS) orGround Based Augmentation Systems(GBAS). In the United States, the satellite based component is theWide Area Augmentation System(WAAS), in Europe it is theEuropean Geostationary Navigation Overlay Service(EGNOS), and in Japan it is theMulti-Functional Satellite Augmentation System(MSAS). Ground based augmentation is provided by systems like theLocal Area Augmentation System(LAAS). GNSS-2 is the second generation of systems that independently provides a full civilian satellite navigation system, exemplified by the European Galileo positioning system. These systems will provide the accuracy and integrity monitoring necessary for civil navigation; including aircraft. This system consists of L1 and L2 frequencies for civil use and L5 for system integrity. Development is also in progress to provide GPS with civil use L2 and L5 frequencies, making it a GNSS-2 system.

Core Satellite navigation systems, currently GPS (United States), GLONASS (Russian Federation), Galileo (European Union) and Compass (China).

Global Satellite Based Augmentation Systems (SBAS) such as Omnistar andStarFire.

Regional SBAS including WAAS (US), EGNOS (EU), MSAS (Japan) andGAGAN(India).

Regional Satellite Navigation Systems such as China'sBeidou, India's yet-to-be-operationalIRNSS, and Japan's proposedQZSS.

Continental scale Ground Based Augmentation Systems (GBAS) for example the Australian GRAS and the US Department of Transportation NationalDifferential GPS(DGPS) service.

Regional scale GBAS such as CORS networks.

Local GBAS typified by a single GPS reference station operatingReal Time Kinematic(RTK) corrections.

BlockLaunchPeriodSatellite launchesCurrently in orbitand healthy

Suc-cessFail-ureIn prep-arationPlan-ned

I19781985101000

II1989199090000

IIA19901997190006

IIR199720041210012

IIR-M2005200980007

IIFFrom 201060606

IIIAFrom 2014000120

IIIB00080

IIIC000160

Total64263631

BASIC CONCEPTS OF GPS

A GPS receiver calculates its position by precisely timing the signals sent by GPS satellites high above the Earth. 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. Each of these distances and satellites' locations define a sphere. The receiver is on the surface of each of these spheres when the distances and the satellites' locations are correct. These distances and satellites' locations are used to compute the location of the receiver using the navigation equations. This location is then displayed, perhaps with a moving map display or latitude and longitude; elevation or altitude information may be included. Many GPS units show derived information such as direction and speed, calculated from position changes.

In typical GPS operation, four or more satellites must be visible to obtain an accurate result. Four sphere surfaces typically do not intersect. [a] Because of this, it can be said with confidence that when the navigation equations are solved to find an intersection, 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, thereby eliminating the need for a very large, expensive, and power hungry clock. The very accurately computed time is used only for display or not at all in many GPS applications, which use only the location. A number of applications for GPS do make use of this cheap and highly accurate timing. These include time transfer, traffic signal timing, and synchronization of cell phone base stations.

Although four satellites are required for normal operation, fewer apply in special cases. If one variable is already known, a receiver can determine its position using only three satellites. For example, a ship or aircraft may have known elevation. Some GPS receivers may use additional clues or assumptions such as reusing the last known altitude, dead reckoning, inertial navigation, or including information from the vehicle computer, to give a (possibly degraded) position when fewer than four satellites are visible.

STRUCTURE

The current GPS consists of three major segments. These are the space segment (SS), a control segment (CS), and a user segment (US).The U.S. Air Force develops, maintains, and operates the space and control segments. GPS satellitesbroadcast signalsfrom space, and each GPS receiver uses these signals to calculate its three-dimensional location (latitude, longitude, and altitude) and the current time.

The space segment is composed of 24 to 32 satellites inmedium Earth orbitand also includes the payload adapters to the boosters required to launch them into orbit. The control segment is composed of a master control station, an alternate master control station, and a host of dedicated and sharedground antennasand monitor stations. The user segment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial, and scientific users of the Standard Positioning Service (seeGPS navigation devices).

SPACE SEGMENT

The space segment (SS) is composed of the orbiting GPS satellites, or Space Vehicles (SV) in GPS parlance. The GPS design originally called for 24SVs, eight each in three approximately circularorbits,but this was modified to six orbital planes with four satellites each.The six orbit planes have approximately 55inclination(tilt relative to Earth'sequator) and are separated by 60right ascensionof theascending node(angle along the equator from a reference point to the orbit's intersection).The orbital period is one-half asidereal day, i.e., 11 hours and 58 minutes so that the satellites pass over the same locationor almost the same locationsevery day. The orbits are arranged so that at least six satellites are always withinline of sightfrom almost everywhere on Earth's surface.The result of this objective is that the four satellites are not evenly spaced (90 degrees) apart within each orbit. In general terms, the angular difference between satellites in each orbit is 30, 105, 120, and 105 degrees apart which sum to 360 degrees. Orbiting at an altitude of approximately 20,200km (12,600mi); orbital radius of approximately 26,600km (16,500mi),each SV makes two complete orbits eachsidereal day, repeating the same ground track each day.This was very helpful during development because even with only four satellites, correct alignment means all four are visible from one spot for a few hours each day. For military operations, the ground track repeat can be used to ensure good coverage in combat zones.

As of December 2012,there are 32 satellites in the GPSconstellation. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements. With the increased number of satellites, the constellation was changed to a non uniform arrangement. Such an arrangement was shown to improve reliability and availability of the system, relative to a uniform system, when multiple satellites fail.About nine satellites are visible from any point on the ground at any one time (see animation at right), ensuring considerable redundancy over the minimum four satellites needed for a position.

CONTROL SEGMENT

The control segment is composed of:

1. a master control station (MCS),

2. an alternate master control station,

3. four dedicated ground antennas, and

4. six dedicated monitor stations.MASTER CONTROL STATION :

The master control station in Colorado is where 2SOPS performs the primary control segment functions, providing command and control of the GPS constellation. The MCS generates and uploads navigation messages and ensures the health and accuracy of the satellite constellation. It receives navigation information from the monitor stations, utilizes this information to compute the precise locations of the GPS satellites in space, and then uploads this data to the satellites.

The MCS monitors navigation messages and system integrity, enabling 2SOPS to determine and evaluate the health status of the GPS constellation. 2SOPS uses the MCS to perform satellite maintenance and anomaly resolution. In the event of a satellite failure, the MCS can reposition satellites to maintain an optimal GPS constellation.MONITOR STATIONS :

Monitor stations track the GPS satellites as they pass overhead and channel their observations back to the master control station. Monitor stations collect atmospheric data, range/carrier measurements, and navigation signals. The sites utilize sophisticated GPS receivers and are operated by the MCS.

There are 16 monitoring stations located throughout the world, including six from the Air Force and 10 from the National Geospatial-Intelligence Agency (NGA).GROUND ANTENNAS :

Ground antennas are used to communicate with the GPS satellites for command and control purposes. These antennas support S-band communications links that send/transmit navigation data uploads and processor program loads, and collect telemetry. The ground antennas are also responsible for normal command transmissions to the satellites. S-band ranging allows 2SOPS to provide anomaly resolution and early orbit support.

There are four dedicated GPS ground antenna sites co-located with the monitor stations at Kwajalein Atoll, Ascension Island, Diego Garcia, and Cape Canaveral. In addition, the control segment is connected to the eight Air Force Satellite Control Network (AFSCN) remote tracking stations worldwide, increasing visibility, flexibility, and robustness for telemetry, tracking, and command.CONTROL SEGMENT MODERNIZATION :As part of the GPS modernization program, 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. To view the schedule for control segment modernization, visit the GPS Modernization page.LEGACY ACCURACY IMPROVEMENT INITATIVE (L - AII)

The Legacy Accuracy Improvement Initiative, completed in 2008, expanded the number of monitoring sites in the operational control segment from six to 16. This tripled the amount of data collected on GPS satellite orbits, enabling a 10% to 15% improvement in the accuracy of the information broadcast from the GPS constellation.

The L-AII effort added 10 operational GPS monitoring sites owned and operated by the National Geospatial-Intelligence Agency (NGA). NGA originally fielded these sites to help it define the Earth reference frame used by GPS.ARCHITECTURE EVOLUTION PLAN :

In 2007, the Air Force implemented the Architecture Evolution Plan, replacing the original, mainframe-based master control station with an entirely new one built on modern IT technologies. 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.

Utilizing commercial off-the-shelf products, AEP also improved GPS monitor stations and ground antennas, substantially enhancing sustainability and accuracy. AEP is capable of managing all satellites in the constellation, including the new Block IIF satellites. AEP features an alternate master control station, a fully operational backup for the MCS.

The AEP system received several upgrades, with the final version declared fully operational in April 2011.Launch And Early Orbit, Anomaly Resolution, And Disposal Operations (LADO) :

The GPS master control station can command and control a constellation of up to 32 satellites. In 2007, 2SOPS fielded the LADO system to handle GPS satellites outside the operational constellation. These include newly launched satellites undergoing checkout, satellites taken out of service for anomaly resolution, residual satellites stored in orbit, and satellites requiring end-of-life disposal.

The LADO system serves three primary functions. The first is telemetry, tracking, and control. The second is the planning and execution of satellite movements during LADO. The third function is LADO simulation of different telemetry tasks for GPS payloads and subsystems. The LADO system uses the AFSCN remote tracking stations only, not the dedicated GPS ground antennas.

The LADO system has been upgraded several times since 2007. In October 2010, the Air Force operationally accepted a new version adding GPS Block IIF capability, following testing during the launch of the first GPS IIF satellite.

NEXT GENERATION OPERATIONAL CONTROL SYSTEM (OCX)In 2008, the Air Force awarded a contract to Raytheon for development of the Next Generation Operational Control System.

OCX will add many new capabilities to the GPS control segment, including the ability to fully control the modernized civil signals (L2C, L5, and L1C).

OCX will be delivered in increments. OCX Block 0 will launch and checkout the GPS III satellites. This version will introduce the full capabilities of the L2C navigation signal. OCX Block 1 is scheduled to enter service in 2017.

OCX Block 2 will support, monitor, and control additional navigation signals, including L1C and L5. Any increments beyond OCX Block 2 will be phased to support future satellite generations.

LAUNCH CHECK OUT CAPABILITY (LCC)

The Launch Checkout Capability is a command and control center that will checkout all GPS III satellites. Unlike today's LADO system, which operates separately from the master control station, the LCC will be fully integrated with OCX. This approach will allow the operation of a single OCX-centric system that can sustain the GPS constellation from launch to disposal.

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. The LCC will ensure a timely launch so constellation availability remains optimal and not impacted by the late discovery of problems.

The Air Force awarded the contract for the provision of the LCC to Lockheed Martin in January 2012. At the same time, the Air Force awarded Raytheon a contract for the development of the Launch and Checkout System (LCS), a component of the LCC.

USER SEGMENT

The user segment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial and scientific users of the Standard Positioning Service. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly stable clock (often acrystal oscillator). They may also include a display for providing location and speed information to the user. A receiver is often described by its number of channels: this signifies how many satellites it can monitor simultaneously. Originally limited to four or five, this has progressively increased over the years so that, as of 2007, receivers typically have between 12 and 20channels.GPS receivers may include an input for differential corrections, using theRTCMSC-104 format. This is typically in the form of anRS-232port at 4,800bit/s speed. Data is actually sent at a much lower rate, which limits the accuracy of the signal sent using RTCM.[citation needed]Receivers with internal DGPS receivers can outperform those using external RTCM data.[citation needed]As of 2006, even low-cost units commonly includeWide Area Augmentation System(WAAS) receivers.

Many GPS receivers can relay position data to a PC or other device using theNMEA 0183protocol. Although this protocol is officially defined by the National Marine Electronics Association (NMEA),[69]references to this protocol have been compiled from public records, allowing open source tools likegpsdto read the protocol without violatingintellectual propertylaws.[clarification needed]Other proprietary protocols exist as well, such as theSiRFandMTKprotocols. Receivers can interface with other devices using methods including a serial connection,USB, orBluetooth.GPS MEASUREMENT :

There are two range-type measurements that can be made on the GPS signals:

Pseudo-ranges, and

Carrier phaseobservations.

Both are a product of theoperationof the GPS receiver (that is, the acquisition and maintenance of signal tracking), both are used for GPSnavigation(position, velocity and time -- PVT -- determination), and both have a role in the specialised data processing that characterises GPSsurveying. Before studying these measurements it is useful to consider the overall GPS hardware tracking operation (in a much abbreviated form!).

The received satellite signal level is actually less than the background noise level, hence correlation techniques are used to obtain the satellite signals.A typical satellite tracking sequence begins with the receiver determining which satellites are visible above the horizon. Satellite visibility is estimated from predictions of present PVT, and on the stored satellite almanac information residing within the receiver. (If no stored almanac information exists, or only a very poor estimate of PVT is available, the receiver will carry out a "sky search", attempting to randomly locate and lock onto a signal. The receiver will then decode the Navigation Message and read the almanac information about all the other satellites in the constellation.) A carrier-tracking 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. The two tracking loops have to work together in an iterative manner, aiding each other in order to acquire and track the satellite signals.

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. This Doppler offset is proportional to the relative velocity along the line-of-sight to the satellite. In order tomaintain lock on the carrier, the carrier-tracking loop must, in effect, adjust the frequency of the receiver-generated carrier until it matches the incoming carrier frequency. The amount of this offset is the "beat" frequency which can be processed to give a periodic carrier phase measurement. The derivative of this phase measurement is the "Doppler" measurement, which is used to determine the receiver's velocity.

What role does the code-tracking loop play in this process?In order for the carrier-tracking loop toacquire the incoming satellite signalin the first place the carrier signal must be made visible above the background noise. This is generally done by the code-tracking loop using thecode-correlating techniqueto "reconstruct" the carrier wave (see discussion below under "Carrier Phase Measurements"). A by-product of code-tracking are the pseudo-range measurements.PSEUDO RANGES MEASUREMENT :

Ranging with the PRN CodesConsider for a moment a perfect system where all satellite clocks are synchronized to the same time system:GPS Time. Furthermore, the ground receiver's clock also maintains the same synchronization, and none of the clocks drift with respect to the GPST scale. Now suppose the satellite starts transmitting its L1 carrier (modulated with the combined C/A code and navigation data), and at the same instant the receiver begins generating the C/A code corresponding to that particular satellite (see Figure below). Under these circumstances, the satellite and receiver generated C/A codes would be output in unison. However, when the satellite signal is received it will be lagging the receiver generated code due to thesignal transit time. 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.

Figure 1. One-way ranging using PRN codes.

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), centred at each satellite whose coordinates can be calculated from the Navigation Message, as illustrated in Figure 2.

Figure 2. The geometric problem of 3-D positioning from ranges.

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 theNavigation Message -- section 3.3.2). Consequentlyeach range is contaminated by the receiver clock error. This is the reason this range measurement is referred to as a pseudo-range". Hence, to determine position using pseudo-range data, 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), each containing four unknowns: the three-dimensional position components and the receiver-clock offset (from GPST). This is the basis of GPS (real-time) navigation as described insection1.4.2.

Ranging (and hence receiver position determination) can be carried out using the C/A code or the P code. P code ranging can be performed on either of the two frequencies, or a linear combination of the L1 and L2 pseudo-ranges that largely eliminates the bias due to ionospheric refraction(section 6.2.7). Furthermore, the C/A code resolution is "coarser", and hence the C/A derived ranges are subject to greater measurement "noise". The absence of a C/A code on L2 is intentional, as one of the accuracy limitations of the GPS system. Others are the ability under the policy ofAnti-Spoofingto 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.S. and its allies), and implementation of the policy ofSelective Availability.

This distinction between the ranging codes, and the associated policies for their use (in peacetime and in times of global emergencies), results in the provision of two GPS positioning services: thePrecise Positioning Servicebased on P code (dual-frequency) ranging, and theStandard Positioning Servicebased on single frequency C/A code ranging.

Recovery of PRN Ranging Codes from the Incoming SignalsThe 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.Before examining this in detail it is necessary to consider, in general terms, how the incoming satellite signal is processed within the GPS receiver. 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 non-synchronization of the receiver clock to GPST, and the travel time of the signal from the satellite to the receiving antenna). Alignment of the incoming signal with the receiver generated C/A code is carried out by the code-tracking loop, or the"delay-lock loop" electronics.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), the "0"s and "1"s of the two codes cancel, leaving the incoming carrier signal modulated only by the binary Navigation Message. This process is summarised in Figure 3 below.

Figure 3. Recovery of ranging code.Because of the complexity of the P code sequence (its length and higher chipping rate), asliding correlationtechnique as described above for the C/A code cannot be used in practice without a very good estimate of GPST and receiver position. Typically a P code receiver must acquire lock on the C/A code first, then use a timing mark known as the "Handover Word", contained within the Navigation Message, to enable the correct portion of the P code to be generated within the receiver and thus initialise the P code delay-lock loop.

Extraction of the Pseudo-RangesAs mentioned already, the extraction of the pseudo-range, or more precisely, the determination of the amount by which the receiver generated PRN code must be shifted to align it with the incoming signal, is carried out with the aid of a PRN code correlator in some delay-lock loop scheme (see, for example,TALBOT, 1987;LANGLEY, 1993).How accurate is this carried out?The C/A code has a chip rate of 1.023Mbps, corresponding to a wavelength of about 300m (speed of light divided by the frequency). The P (or Y) code, on the other hand, has a chip rate of 10.23Mbps, and hence a wavelength of about 30m.

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, hence the measurement precision of C/A code ranging is of the order of 3-5m, and for P code ranging it is of the order of 0.3-0.5m. (Modern "narrow correlator" technology has demonstrated 10 times better correlation performance for the C/A code than that above.)

The main advantages gained by using the P code therefore are:

Because of the higher chipping rate, and hence higher measurement precision, P code ranging translates into a more accurate position fix.

The P code is modulated on both the L1 and L2 carriers, hence the ionospheric signal delay can be overcome.

P code receivers are better suited to high dynamic environments and resist signal jamming better than C/A code receivers.

Both the P and C/A code ranges are susceptible tomultipath(though the susceptibility is inversely proportional to the signal frequency). 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. Some characteristics of multipath are

Multipath can cause "jumps" in the signal measurement of the order of its (effective) wavelength.For pseudo-ranges this could mean tens or hundreds of meters, but of the order of only centimeters for carrier phase measurements.

Multipath is receiver-satellite geometry dependent, and the causes of multipath tend to be permanent features (metallic fences, buildings, chimneys, superstructure, water surfaces, etc.), hencethe multipath effect will generally repeat on a daily basis at the same receiver site.

As the receiver-satellite geometry changes (and hence the angle of incidence and reflection of the signal with respect to the reflective surface changes),the multipath effect changes, and generally "averages out" over a period from several minutes to a quarter of an hour, or more.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).CARRIEER PHASE MEASUREMENTS :

The wavelengths of the carrier waves are very short -- approximately 19cm for L1 and 24cm for L2 -- compared to the C/A and P code chip lengths. Assuming a measurement resolution of 1-2% of the wavelength, this means thatcarrier phase can be measured to millimeter precision compared with a few meters for C/A code measurements (and several decimeters for P code measurements).Unfortunately, a phase measurement is "ambiguous" as it cannot discriminate one (either L1 or L2) wavelength from another. In other words, time-of-transmission information for the L-band signal cannot be imprinted onto the carrier wave as is done using PRN codes (this would be possible only if the PRN code frequency was the same as the carrier wave, rather than 154 or 120 times lower in the case of the P code, and 1540 or 1200 times lower for the C/A code). The basic phase measurement is therefore in the range 0 to 360 (see Figure 1 below). It is nevertheless the basis for GPS surveying, and high precision kinematic positioning.

Figure 1. Carrier phase measurements.

There are essentially two means by which the carrier wave can be recovered from the incoming modulated signal:

Reconstruct the carrier waveby removing the ranging code and broadcast message modulations. Squaring, or otherwise processing the received signal without using a knowledge of the ranging codes.

In the first technique the ranging codes (C/A and/or P code) must be known. 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. In the latter method no knowledge of the ranging codes is required. More complex signal processing is required to make carrier phase measurements on the L2 signal under conditions of Anti-Spoofing.

Integrated Carrier Beat PhaseRaw carrier phase measurements are generally the by-product of all GPS receivers. These phase measurements cannot be used as "range" observations because they are ambiguous, and furthermore, the ambiguity changes continuously. The ambiguity is therefore a function of both the receiver channel tracking the satellite, and time. (This is analogous to making terrestrial distance measurements using only the "reader" portion of a steel band.) 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).

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, for example, a phase-lock loop, then theintegrated carrier phaseobservation could be generated:

Figure 2. Integrated carrier phase and the ambiguity term.

Extraction of Carrier Beat Phase: Reconstructing the Carrier WaveThis is the technique used withincode-correlating receivers.When the spread spectrum signal is received at the GPS antenna, the signal power is below the background noise (Figure 3 below). After the ranging code modulations are removed by the procedure described above, the satellite signal collapses into the original very narrow carrier frequency band and signal power is again boosted well above the background noise.

Figure 3. Spreading and de-spreading the spectrum of the carrier wave. (AfterNATO, 1991)By mixing a locally generated sine wave at the same frequency as the "reconstructed" received carrier (modulated only by the Navigation Message), the broadcast message can be extracted. The incoming and receiver-generated sine waves are continuously aligned within a "phase-lock loop"(section 4.1.3). Periodic sampling of the phase of the local carrier provides the carrier beat phase observable (Figures 3 above and Figure 4 below), which although useful for some applications such as the "phase smoothing" of pseudo-ranges, is still not suitable for survey applications. A much more useful carrier phase observable can be constructed through the "integration" of carrier phase measurements (see below).Measurement of carrier beat phase on L2 by this technique requires knowledge of the P code generating algorithm. Under the policy of Anti-Spoofing, the Y code is secret and hence cannot be used in this code-correlating mode. The easiest option for GPS instrument manufacturers is to use the "squaring" technique (or some variation of it) to make L2 phase measurements. However,the primary advantage of the code-correlating approach is that it results in a far better signal-to-noise ratio, and hence better quality measurements, than any other signal processing technique.

Figure 4. Reconstructing the carrier wave and extraction of pseudo-range data.

Extraction of Carrier Beat Phase: "Squaring" the Carrier WaveIn principle, the operation of a squaring receiver is very simple. The incoming signal is first converted to an intermediate frequency (IF) signal. The carrier, or rather the beat frequency carrier wave, is obtained simply by squaring this signal. Any phase inversions in the IF signal due to the PRN codes or message are removed. (This happens because a phase inversion is a change in the IF signal amplitude from "+1" to "-1", or from "-1" to "+1", and the instantaneous amplitude is either "+1" or "-1". Squaring the signal results in a signal with constant amplitude of unity, and hence the codes and message information are lost.) However,squaring the signal also squares the noise.Aside from resulting in a noisier measurement, the squared carrier wave measurement is made on a carrier wave ofdouble frequency. That is, the effective wavelength is of the order of 9.5cm on L1 and 12cm on L2. Figure 5 below illustrates this measurement scheme.

Figure 5. Extracting carrier phase from incoming GPS signals by carrier wave squaring.ESSENTIAL CHARACTERISTICS

From a study of the design requirements of an ideal "Global Positioning System", and as a consequence of the feasible technological features of such a system, the essential characteristics of the system can now be identified.

System Configuration: A multi-satellite system at high altitude, but not in a geostationary orbit.The number of satellites to be visible to a user is dependent upon the observation type to be employed and the positioning strategy adopted. A Control Segment responsible for tracking the satellites, and computing the ephemerides.

Satellites broadcast their ephemerides to all users.

Satellite Technology: The system should concentrate as much complexity into the satellites as possible.

The system should be passive (one-way) as far as the user is concerned, with the satellites transmitting the signals necessary to support position determination at the user station.No receiving function is to be performed by the satellites.

Measurement Technology: A one-way ranging system based on microwave transmissions would satisfy the requirements for a listen-only, high precision, simple-to-use positioning system.

To make such a system work, separate clocks must be used (in the satellites and within the user equipment), and they must be synchronised in some way.

The satellites should somehow broadcast time-of-transmission information to the user.

What remains to be established is: Thepositioning principle to be used(related to the measurement technology and the characteristics of the satellite constellation), and

Theresidual errors remaining in the system(after application of the best available technology), and the development of strategies to overcome any unacceptably high error sources.SELECTIVE AVAILABILITY ANIT SPOOFINGConceived in the 1970s, the Global Positioning System (GPS) was originally built for military use. GPS remained a military-only technology until the early 1980s, when President Reagan decided the technology could be adapted for public use, as well. By the early 1990s, civilians could buy GPS equipment that was accurate within only about 300 feet. 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.S. This was called Selective Availability (SA).

On May 1, 2000, 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. Now, GPS is accurate within 40 feet, or much better. Military GPS is even more precise and has a margin of error of only a few centimeters.

The end of Selective Availability was a major turning point that has helped GPS to become a global utility, now being used around the world in many different applications.

After the attacks of September 11th, the industry buzzed over the possibility of a return to SA. However, on Sept. 17, 2001, the Interagency GPS Executive Board (IGEB), which governed the GPS system at that time, announced the United States has no intent to ever use Selective Availability again.ASelective Availability Anti-spoofing Module(SAASM) is used by militaryGlobal Positioning Systemreceivers to allow decryption of precision GPS coordinates, while the accuracy of civilian GPS receivers may be reduced by theUnited States militarythroughSelective Availability.[1]However, the United States Presidential Directive instructing the discontinuation of Selective Availability, along with the directive that no future GPS programs will include selective availability, makes any changes to SA unlikely.

SAASM allows satellite authentication, over-the-air rekeying, and contingency recovery. Those features are not available with the similar, but older, PPS-SM system. 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, after which that paper tape must be securely destroyed). SAASM systems can be updated with an encrypted "Black Key" that may be transmitted over unclassified channels. All military receivers newly-deployed after the end of September 2006 must use SAASM.[1]SAASM does not provide any additional anti-jam capability, 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. Future GPS upgrades, such as M-Code, will provide additional improvements to anti-jam capabilities.[citation needed]SAASM hardware is covered with an anti-tampering coating, to deter analysis of their internal operation.[citation needed]Deployment of the next generation military signal for GPS, calledM-code, commenced with the launch of IIR-M and IIF satellites, beginning in 2005. A complete constellation of 18 satellites with M-code capability is planned for 2016.

REGIONAL NAVIGATION SYSTEMS

IRNSS is an independent regional navigation satellite system being developed by India. 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, which is its primary service area. The Extended Service Area lies between primary service area and area enclosed by the rectangle from Latitude 30 deg South to 50 deg North, Longitude 30 deg East to 130 deg East.IRNSS will provide two types of services, namely, Standard Positioning Service (SPS) which is provided to all the users and Restricted Service (RS), which is an encrypted service provided only to the authorised users. The IRNSS System is expected to provide a position accuracy of better than 20 m in the primary service area.

RNSS comprises of a space segment and a ground segment. The IRNSS space segment consists of seven satellites, with three satellites in geostationary orbit and four satellites in inclined geosynchronous orbit. IRNSS-1A, the first satellite of the IRNSS constellation, has already started functioning from its designated orbital slot after extensive on orbit test and evaluation to confirm its satisfactory performance.IRNSS ground segment is responsible for navigation parameter generation and transmission, satellite control, ranging and integrity monitoring and time keeping.

The constituent elements of the ground segment are: ISRO Navigation Centre (INC) at Byalalu, is the nerve center of the IRNSS Ground Segment. INC primarily generates navigation parameters.

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.

IRNSS CDMA Ranging Stations (IRCDR)carry out precise two way ranging of IRNSS satellites.

IRNSS Network Timing Centre (IRNWT)at Byalalu generates, maintains and distributes IRNSS Network Time.

Spacecraft Control Facility (SCF)controls the space segment through Telemetry Tracking & Command network. In addition to the regular TT&C operations, IRSCF also uplinks the navigation parameters generated by the INC.

IRNSS Data Communication Network (IRDCN)provides the required digital communication backbone to IRNSS network.

Laser Ranging Stations (ILRS)is planned to be used periodically to calibrate the IRNSS orbit determined by the other techniques.

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, Asst Prof.21