IntroductiontoGlobal Positioning System

INTRODUCTIONThe Global Positioning System (GPS) is a technology, which provides unequalled accuracy and flexibility of positioning for navigation, surveying and GIS data capture. The GPS NAVSTAR (Navigation Satellite timing and Ranging Global Positioning System) is a satellite-based navigation, timing and positioning system. GPS provides continuous 3D positioning 24-hrs a day throughout the world.

INTRODUCTIONThe technology is beneficiary to the GPS user community in terms of obtaining accurate data up to about 100 meters for navigation, metre-level for mapping, and down to milli-metre level for geodetic positioning. The GPS technology has tremendous amount of applications in GIS data collection, surveying, and mapping.

Geopositioning - Basic Concepts By positioning we understand as the determination of position of stationary or moving objects.

These can be determined as follows: i) In relation to a well-defined coordinate system, usually by three coordinate values and ii) In relation to other point, taking one point as the origin of a local coordinate system.

Geo-positioning - Basic ConceptsThe first mode of positioning is known as point positioning, the second as relative positioning.

If the object to be positioned is stationary, it is known as STATIC POSITIONING. When the object is moving, then it is known as KINEMATIC POSITIONING.

Usually, the static positioning is used in surveying and the kinematic position in navigation.

GPS - Components and Basic Facts

The GPS uses satellites and computers to compute positions anywhere on earth and is based on satellite ranging.

That means the position on the earth is determined by measuring the distance from a group of satellites in space.

Measurements of distanceDistance measurementstart: 0.00 s

GPS - Components and Basic Facts

The basic principle behind GPS is really simple, even though the system employs some of the most high-technology based equipment ever developed.

In order to understand GPS basics, the system can be categorised into FIVE logical Steps

STEP ITo compute a position in three dimensions, four satellites have to be observed.

STEP IITo triangulate, the GPS measures the distance using the travel time of the radio message. To measure travel time, the GPS needs a very accurate clock.

STEP IIIOnce the distance to a satellite is known, then its location in space is required.

STEP IVAs the GPS signal travels through the ionosphere and the earth's atmosphere, the signal is delayed.

Components of a GPS

The GPS is divided into three major components

The Space Segment The Control Segment The User Segment

SPACE SEGMENT

CONTROL SEGMENT The Control Segment consists of five monitoring stations Colorado Springs, Ascension Island, Diego Garcia, Hawaii, and Kwajalein Island.

CONTROL SEGMENT

CONTROL SEGMENTThree of the stations (Ascension, Diego Garcia, and Kwajalein) serve as uplink installations, capable of transmitting data to the satellites, including new ephemerides i.e. satellite positions as a function of time, clock corrections, and other broadcast message data, Colorado Springs serves as the Master Control station.

CONTROL SEGMENTThe Control Segment is the sole responsibility of the Department of Defence (DoD) who undertakes construction, launching, maintenance, and virtually constant performance monitoring of all GPS satellites.The DoD monitoring stations track all GPS signals for use in controlling the satellites and predicting their orbits.

Control SegmentMeteorological data also are collected at the monitoring stations, permitting the most accurate evaluation of tropospheric delays of GPS signals.

Satellite tracking data from the monitoring stations are transmitted to the master control station for processing. This processing involves the computation of satellite ephemerides and satellite clock corrections.

SPACE SEGMENT

The Space Segment consists of the Constellation of NAVSTAR earth orbiting satellites.

The current Department of Defence plan calls for a full constellation of 24 Block II satellites (21 operational and 3 in-orbit spares).

The satellites are arrayed in 6 orbital planes, inclined 55 degrees to the Equator.

They orbit at altitudes of about 12000, miles each, with orbital periods of 12 sidereal hours (i.e., determined by or from the stars), or approximately one half of the earth's periods, approximately 12 hours of 3-D position fixes.

SPACE SEGMENT

The next block of satellites is called Block IIR, and they will provide improved reliability and have a capacity of ranging between satellites, which will increase the orbital accuracy. Each satellite contains four precise atomic clocks (Rubidium and Cesium standards) and has a microprocessor on board for limited self-monitoring and data processing. The satellites are equipped with thrusters which can be used to maintain or modify their orbits.

USER SEGMENT

The user segment is a total user and supplier community, both civilian and military. It consists of all earth-based GPS receivers which can vary greatly in size and complexity, though the basic design is rather simple.

USER SEGMENT

The typical receiver is composed of an antenna and preamplifier, radio signal microprocessor control and display device, data recording unit, and power supply. The GPS receiver decodes the timing signals from the 'visible' satellites (four or more) and, having calculated their distances, computes its own latitude, longitude, elevation, and time.

GPS Hand-held Type

USER SEGMENT

This is a continuous process and generally the position is updated on a second-by-second basis, output to the receiver display device and, if the receiver provides data capture capabilities, stored by the receiver-logging unit.

SATELLITE RANGING

GPS positions are based on the measurement of the distance from the satellite to the GPS receiver on earth. The GPS receiver can determine the distance to each satellite. The basic idea of determination of position is that of resection or trilateration, which many surveyors use in their daily work.

SATELLITE RANGING If the distance of three points relative to unknown position is known, then the position of unknown point relative to these three points can be determined.Similarly, if the distance of one satellite is known, then the position of the receiver must be at some point on the surface of an imaginary sphere of radius equal to that distance with origin at the satellite. By intersecting three imaginary spheres the receiver position can be determined accurately.

Satellite locationGiven 1 satellite

Satellite locationWe can locate our position on the surface of a sphere

Satellite locationGiven 2 satellites

Satellite locationGiven 2 satellites

Satellite locationWe can locate our position on the intersection of 2 spheres (a circle)

Satellite locationGiven 3 satellites

Satellite locationWe can locate our position on the intersection of 3 spheres (2 points)

Satellite locationGiven 4 satellites we can locate our position on the intersection of 4 spheres (1 point)

Satellite locationThe point should be located on the earths surface

SATELLITE RANGING

The GPS receiver also calculates the distance from the receiver to the satellite using the equation, Distance = Velocity x Time wherevelocity = the velocity of the radio signal, i.e. 290,000 km per second (speed of light) and time = the time taken by the radio signal to travel from the satellite to the receiver.

SATELLITE COMMUNICATION

GPS satellites communicate all the information to receivers by using codes.It broadcasts two carrier waves which are modulated by the coded information signal. The two GPS carrier waves are radio waves called L1 and L2, in the L-Band (390 MHz to 1550 MHz).These are derived from the fundamental frequency of 10.23 MHz, generated by a very precise atomic clock.

SATELLITE COMMUNICATION

They travel to the earth at the speed of light.These high frequency transmissions from the satellite travel in straight lines and have very low power. The power of transmission from the satellite is about 50 watts. Hence it is essential that the antenna of the GPS receiver have a direct view of the satellite.

SATELLITE COMMUNICATION

L1 carrier is broadcasted at 1575.42 MHz (10.23 x 154). L2 carrier is broadcasted at 1227.60 MHz (10.23x 120). The L1 carrier has two codes modulated upon it:The Coarse/Acquisition code (C/A code) modulated at 1.023 MHz The Precision code ( P-code 10.23 MHz). The L2 carrier has only one code modulated upon it, the L2 P-code, modulated at 10.23 MHz.

SATELLITE COMMUNICATION

The Navigation Message (the information that the satellites transmit to a receiver) contains:- - the satellite orbital and clock information, - general system status messages and - an ionospheric delay model. The navigation code has a low frequency of 50 Hz and is modulated both on the L1 and L2 carriers. It communicates the data in a message called GPS message or navigation message.

GPS CODES

GPS receivers use different codes to distinguish between satellites. These codes can also be used as a basis for making pseudo-range measurements which enable the calculation of position.GPS codes are binary in nature. The three basic codes in GPS are - the Precise code or the P-code, - the Coarse/ Acquisition code or C/ A code, - the Navigation code.

GPS CODES

The modulated C/ A code and P-code are referred to as Pseudo-Random Noise (PRN) code. This PRN code is actually a sequence of very precise time which permits the ground receivers to compare and compute the time of transmission between the satellite and ground station. From this transmission time, the range to the satellite can be derived and is the basis behind GPS range measurements. The C/A code pulse intervals are approximately 300 m in range and the more accurate P-code intervals have a range of 30 m.

IMPORTANCE OF PRN CODE

The PRN code is a complex pattern, thus ensuring that the receiver does not accidentally synchronize with some other signal. The patterns are so complex that it is highly unlikely that a stray signal will have exactly the same shape.Each satellite has its own unique PRN code which ensures that the receiver will not accidentally pick up a signal from another satellite. Hence all the satellites can use the same frequency without signal jamming.

IMPORTANCE OF PRN CODE

This makes it difficult for any hostile force to jam the system. In fact, the PRN code gives the DoD a complete control the access to the system.Also, the codes make it possible to use information theory to amplify the GPS signal. Further, the complexity of the PRN code makes GPS economical. That is why GPS receivers do not need big satellite dishes to receive the GPS signals.

PSEUDO-RANGE

It is a measure of the apparent signal propagation time from GPS satellite to the GPS receiver antenna, scaled into distance by speed of light. The apparent propagation time is the difference between the time of signal reception and the time of emission. Hence pseudo-range is the time delay between the satellite clock and the receiver clock, as determined from C/A code or P-code pulses.

PSEUDO-RANGE

If a satellite is right over the head of an observer, the travel time of signal would be about 0.06 seconds.

This time difference gives range measurements but is called a pseudo-range, since at the time of the measurement the receiver clock is not synchronized to the satellite clock.

PSEUDO-RANGE

In most cases, an absolute 3D real time navigation position can be obtained by observing at least four simultaneous pseudo-ranges. Pseudo-range differs from the actual range due to - the influence of satellite orbital errors, - user clock error, and - ionospheric delays.

STANDARD POSITIONING SERVICE

The Standard Positioning Service uses the less precise C/A code pseudo ranges for position calculation and for real-time GPS navigation.

PRECISE POSITIONING SERVICE (PPS)

In Precise Positioning Service, pseudo-ranges are obtained using the higher pulse rate P-code on both frequencies (L1 and L2), thereby giving higher accuracy. Real time 3D accuracies at sub-meter level (below 10 m horizontal) can only be achieved with PPS. The P-code is encrypted to prevent unauthorized civil or foreign use and requires a special key to obtain the accuracy offered by PPS.

CARRIER PHASE MEASUREMENTS

Carrier frequency tracking measures the phase differences between the Doppler shifted satellite and receiver frequencies. The phase differences are continuously changing due to the changing satellite-earth geometry and can be resolved in the receiver and subsequent post-processing of data. When carrier phase measurements are observed and compared between two stations, baseline vector accuracy between the stations, below the centimeter level, is attainable in 3D.

GPS BROADCAST

Each NAVSTAR GPS satellite periodically broadcasts data concerning clock corrections, system and satellite status, and most critically, its position or ephemeris data.

There are two basic types of ephemeris data- the broadcast and - the precise.

Broadcast EphemerisThe broadcast ephemeris is actually predicted satellite positions broadcasted within the navigation message transmitted from the satellites in real time. A receiver capable of acquiring either the C/A or P-code can acquire the ephemeris in real time. The broadcast ephemeris is computed using past tracking data of the satellites.

BROADCAST EPHEMERIS

The satellites are tracked continuously by the monitor stations to obtain more recent data to be used for the orbit predictions. The data is analyzed by the Master Control Stations and new parameters for the satellite orbit are transmitted back to the satellites. This upload is performed daily with new predicted orbital elements transmitted every hour by the navigation message.

PRECISE EPHEMERIS

The precise ephemeris is based on actual tracking data that is post-processed to obtain more accurate satellite positions. This ephemeris is available at later date and is more accurate than the broadcast ephemeris and are based on the actual tracking data and not predicted data. For most survey applications, the broadcast ephemeris is adequate to needed accuracies.

ALMANAC DATA

GPS receiver stores the data about the position of the satellites at any given time in its memory. This data is called the almanac data received from the satellites. When the GPS receiver is not turned long time, the almanac gets outdated as the latest corrected data is not by the receiver for a long time. This condition is called as a cold receiver.

ALMANAC DATA

When the GPS receiver is cold, it would take longer time to acquire satellite. A receiver is considered warm, when the data has been collected satellites within the last four to six hours.While purchasing a new GPS receiver, the cold and warm acquisition specifications must be noted, as the time taken by the GPS unit to lock on to the satellite signals and calculate a position is important. Once the locked onto enough satellites to calculate a position, it is ready navigation or for surveying.

CALCULATING LOCATIONS

The signal emitted from the satellite, contains three components in the symbolic form (L1, C/A, D), (L1, P, D), and (L2, P, D). The aim of the signal processing by the GPS receiver is the recovery of the signal components, including the reconstruction of the carrier wave and the extraction of the codes for the satellite clock readings and the navigation message as,

Calculating Locations:

CALCULATING LOCATIONS

A GPS receiver determines its position by using the signals that it observes from different satellites. Since the navigation message supplies the satellite positions and the code measurements provide pseudo range (PR) between the receiver and the satellite, the receiver computes its position using resection techniques. Since the receiver must solve for its position (X,Y,Z) and the clock error (x), four SVs are required to solve receiver's position using the following four equations: R1= SQRT{(X-x1)2 +(Y-y1)2 +(Z-z1)2 +x2} R2= SQRT{(X-x2)2 +(Y-y2)2 +(Z-z2)2 +x2} R3= SQRT{(X-x3)2 +(Y-y3)2 +(Z-z3)2 +x2} R4= SQRT{(X-x4)2 +(Y-y4)2 +(Z-z4)2 +x2}

Calculating Locations

CALCULATING LOCATIONS

where (x1,y1) (x2,y2) (x3,y3) and (x4,y4) stand for the location of satellites and R1, R2, R3, R4 are the distances of satellites from the receiver position. Hence solving the four equations for four unknowns X,Y, Z and x, the position or location of the station is calculated. However, the accuracy of position determination depends upon the code used in calculation.

CALCULATING LOCATIONS

Post processed static carrier-phase data can provide 1-5 cm relative positioning within 30km of the reference receiver with measurement time of 15 minutes for short baselines (10km) and one hour for long baselines (30km). Rapid static or fast static surveying can provide sub-m level accuracies with 20km baselines and 10-20 minutes of recording time. The Real-time-Kinematic (RTK) surveying technique provides centimetre measurements in real time over 10km baselines tracking five or more satellites and real-time radio links between the reference and remote receivers.

DIFFERENT TYPES OF GPS POSITIONING

POSITIONING MODES FOR GPS

Absolute or Point positioning where coordinates are in relation to a well-defined global reference system.

Differential or relative positioning where coordinates are in relation to some other fixed point. In GPS surveying this is referred to as baseline determination.

POSITIONING MODES FOR GPS

Static positioning where coordinates of stationary points is either absolute or relative mode. This is generally synonymous with the surveying mode of positioning, based on the analysis of carrier phase observations.

Kinematic positioning where coordinates of moving points is either in absolute or relative mode. This is generally the navigation mode of positioning, based on pseudo-range observations (absolute positioning) and surveying mode in relative or differential positioning.

NAVIGATION MODE

As all GPS observations are overwhelmed with biases, hence for both navigation and surveying applications, an appropriate combination of measurement and processing strategies must be used to minimise their effect on the positioning results. There are some distinctions to be made in data processing to minimise the effect of biases in the measurements.

NAVIGATION MODE

In the point positioning mode, satellite clock error is ignored, as it is assumed to be smaller than the measurement noise. Receiver clock error is estimated in real time through redundant measurements, because all data is contaminated by the same biases.In the relative positioning mode, all satellite and propagation biases are significantly reduced.

NAVIGATION MODE

In navigation mode of positioning, results are obtained in real time, when four or more pseudo-ranges are processed simultaneously.

Relative navigation is of higher accuracy as the primary biases due to orbit error, atmospheric refraction and selective availability (if switched on) are minimized.

SURVEYING MODE

Integrated carrier beat phase data is very precise, hence any contamination by systematic errors is of greater concern than in the case of pseudo-range measurements. Appropriate processing techniques must therefore be used. However, the primary drawback of this data type is its range ambiguity. In GPS surveying the major biases are accounted for in the following ways:

SURVEYING MODE

Differencing the data collected simultaneously from two or more GPS receivers for several GPS satellites, between satellites and between receivers. This eliminates, or significantly reduces, most of the biases. All position results are therefore expressed relative to (fixed) datum stations.

The ambiguity bias is often estimated, though a weaker solution can be obtained from the appropriate triple-difference observable.

SURVEYING MODE

This is the surveying mode.

The fact that the receivers are stationary, and that data is collected over some observation period, it permits the ambiguities to be reliably estimated and a strong solution obtained.

There are alternative means of estimating ambiguities that permit real time kinematic baseline determination to be carried out as well.

ABSOLUTE POSITIONING

This mode of positioning relies upon a single receiver station. It is also referred to as 'stand-alone' GPS, because ranging is carried out strictly between the satellite and the receiver station. As a result, the positions derived in absolute mode are subject to the unmitigated errors inherent in satellite positioning. It is, however, the most widely used military and commercial GPS positioning method for real time navigation and location determination.

ABSOLUTE POSITIONING

The accuracies obtained by GPS absolute positioning are dependent on the user's authorization. A Standard Positioning Service (SPS) user can obtain real-time point positional accuracies of 25 m without selective availability (S/A). The Precise Positioning Service (PPS) user (with a receiver capable of tracking P-code) can use a decryption device to achieve a point positional (3D) accuracy in the range of 10-12 m with a single-frequency receiver.

ABSOLUTE POSITIONING

It can be further divided into two categories

Absolute positioning using carrier phase, andAbsolute positioning using C/A-code (pseudo-ranging).

ABSOLUTE POINT POSITIONING WITH THE CARRIER PHASE

Here positional information gathered using a GPS receiver, which is capable of tracking both the C/A-code and carrier phase. By using broadcast ephemeris, the user is able to use pseudo-range values in real time to determine absolute point positions with an accuracy of 3 m in the best of conditions and 25 m in the worst. By using post-processed ephemeris, the user can expect absolute point positions with sub-meter accuracy in the best of conditions and 15 m in the worst.

ABSOLUTE POINT POSITIONING WITH PSEUDO-RANGING

By pseudo-ranging, the GPS user measures an approximate distance between the antenna and the satellite by correlation of a satellite-transmitted code and a reference code created by the receiver, without any corrections for errors in synchronization between the clock of the transmitter and that of the receiver. The distance the signal has traveled is equal to the velocity of the transmission of its satellite multiplied by the elapsed time of transmission, with satellite signal velocity changes due to tropospheric and ionospheric conditions being considered.

APP WITH PSEUDO-RANGING

Four pseudo-range observations are needed to resolve a 3D GPS position. Three pseudo-range observations are needed for a 2D location. In practice often more than four observations are taken. More pseudo-ranges are required to resolve the clock biases contained in both satellite and ground-based receiver. Thus in solving for the X-Y-Z coordinates of a point, a fourth unknown (i.e. the clock bias) must also be included in the solution.

DIFFERENTIAL POSITIONING

Relative or Differential GPS carries the triangulation principles one step further, with a second receiver at a known reference point. To further facilitate determination of a point's position, relative to the known earth surface point, this configuration demands collection of an error-correcting message from the reference receiver.Differential-mode positioning relies upon an established control point.The reference station is placed on the control point, a triangulated position, the control point coordinate.

A base station receiver is set up on a location with coordinates known.Signal time at reference location is compared to time at remote location.Time difference represents error in satellites signalReal-time corrections transmitted to remote receiverSingle frequency (1-5 m)Dual frequency (sub-meter)DIFFERENTIAL GPS

DIFFERENTIAL GPS

This allows for a correction factor to be calculated and applied to other roving GPS units used in the same area and in the same time series. Inaccuracies in the control point's coordinate are directly additive to errors inherent in the satellite positioning process. Error corrections derived by the reference station vary rapidly, as the factors propagating position errors are not static over time. This error correction allows for a considerable amount of error to be negated, potentially as much as 90 percent.

DIFFERENT METHODS OF DGPS

There are eight basic DGPS surveying techniquesStatic surveying Rapid Static surveying Stop-and-Go Kinematics surveying True Kinematic surveyingPseudo-kinematic surveyingKinematic on-the-fly (OTF) surveyingReal-time Kinematic (RTK) surveyingReal-time DGPS (code/carrier) surveying

STATIC SURVEYING

It is the primary and most widely used differential technique for control and geodetic surveying. It involves long observation time (1-2 hours depending on number of visible satellites) in order to resolve the integer ambiguities between the satellite and the receive. Relative static positioning involves several stationary receivers collecting simultaneously from at least four satellites during an observation session which usually lasts 30 minutes to 2 hours.

STATIC SURVEYING

Post processing software analyzes all data from the receivers simultaneously and obtains the differential position between the two receivers. This method is used for long lines, geodetic networks, tectonic plate studies etc. This method offers high accuracy of 1cm to 0.1 cm over long distances like 10 kilometers.

RAPID STATIC SURVEYING

This method is the latest one added to GPS positioning procedures. The concept behind rapid static surveying is to measure baselines and determine positions up to centimeter level with short observation time of about 5-20 minutes. The observation time is dependent on the length of the baseline and number of visible satellites. In rapid static surveys, a reference is chosen and one or more rovers operate with respect to it.

RAPID STATIC SURVEYINGRapid static technique is used for detailing the existing network, establishing control points etc. It is similar to the static method, but consists of a shortened site occupation time.

RAPID STATIC SURVEYING

Rapid static technique can provide the user with nearly the same accuracy available from a 1-2 hour session of static positioning with observations of 5-20 minutes. This is because it uses a technique called wide laning which is based on the linear combination of the measured phases from both GPS frequencies, L1 and L2. Carrier phase measurements can be made on L1 and L2 separately, but when they are combined, two distinct signals result.

RAPID STATIC SURVEYING

One is called a narrow lane, which has a short wavelength of 10.7 cm and the other is known as a wide lane having 86.2 cm wavelength. The frequency of the wide lane is 347.82 MHz, which is three times lower than the original carriers. Further it is 86.2 cm wavelength is about four times longer than the wavelength of L1 (19 cm) and L2 (24.4 cm). These changes greatly increase the spacing of the phase ambiguity, thereby making its resolution much easier. For rapid static surveys, the receivers used must be capable of dual-frequency tracking.

STOP-AND-GO TECHNIQUE IN KINEMATIC METHOD

The term Kinematic is applied to GPS surveying methods where the rover receivers are in continuous motion. However, for relative positioning the more typical arrangement is a STOP-AND-GO technique, a method developed by Dr. Benjamin Remondi.This method is sometimes referred to as semi-kinematic survey.

STOP-AND-GO METHOD1234567Ref. Stn

STOP-AND-GO METHODIn this method a reference station is established. At least four satellites have to be tracked without signal loss for this method. Good geometry of satellite resulting in good GDOP and strong satellite constellation is needed with favorable ionospheric conditions. The roving receiver starts from an initial point for initial rapid static fix or starts from a known position coordinate.

STOP-AND-GO METHODThen it moves to other points maintaining lock on the satellites. The rover remains only for a small time for two epochs on each detail point 1, 2, 3,4, 5 in serial order. Using a post processing software these points can be plotted.This technique is similar to rapid static method in which all the receivers observe the same satellites simultaneously, and the reference receivers occupy the same control point throughout the survey.

STOP-AND-GO METHODApplications(i)Detailed and engineering surveys in open areas.(ii)When points are too close together. Advantages(i)It is a fast and economical method.(ii) One of the fastest way to survey detail points. Disadvantages(i)New static or rapid static fix is needed if complete loss of satellite lock occurs.(ii)Must maintain phase lock to at least four satellites for a successful survey.

KINEMATIC SURVEYING METHOD

Kinematic surveying is often referred to as dynamic surveying. It is faster than static methods. It uses two single frequency L1 receivers for recording observations simultaneously. One receiver is set over a known point (reference station) and the other is used as rover (i.e. moved from point to point or along a path).

KINEMATIC SURVEYING METHODBefore the rover receiver can rove, a period of static initialization or antenna swap must be performed. The reference and rover are switched on and remain absolutely stationary for 5-20 minutes, collecting data. The actual time depends upon the baseline length from the reference and the number of satellites observed. This period of static initialization is dependent on the number of satellites visible.

After this period the rover starts to move freely, the user can record its positions at a predefined recording rate (say at 1 or 2 or 5 seconds interval). This part of the measurements is commonly called kinematic chain.

ADVANTAGESIn very short sessions or real time, it can produce the large number of positions within a short period of time.Only slight degradation in the accuracy of the work. In this method, the receiver resolves the phase ambiguity, once and only once, at the beginning of the project. Then by keeping a continuous lock on the satellites signals, it maintains that solution throughout the work. The kinematic technique needs initialization. The receivers can occupy each end of a baseline between two control points and since the distance between the points is known, the phase ambiguity is resolved in a few minutes.

Applications of Kinematic Method(i) Measuring trajectory of moving objects.(ii) In hydrographic surveys. (iii) In surveying centre of a road. (iv) Photogrammetry with ground control. (v) Collection of data for the preparation of highly accurate topographic maps.

Advantages (i) Fast and economical(ii) Continuous measurementsDisadvantages (i) New static or rapid static fix needed in case of complete loss of satellite lock.(ii) Occupied stations should be free of overhead obstructions. (iii) The route between stations must be clear.

PSEUDO-KINEMATIC GPS SURVEY

Pseudo-kinematic GPS surveying is similar to stop-and-go techniques except that loss of satellite lock is tolerated when the receiver is transported between occupation sites. This feature provides the surveyor with a more favourable positioning technique since obstructions such as bridge overpasses, tall buildings, and overhanging vegetation are common. Loss of lock that may result due to these obstructions is more tolerable when pseudo-kinematic techniques are employed.

Pseudo-kinematic techniques require that one receiver be placed over a known control station. A rover receiver occupies each unknown station for 5 minutes. After 1 hour of the initial station occupation, the same rover receiver must re-occupy each unknown station. The pseudo-kinematic technique requires that at least four common satellites are observed between initial station occupation and the requisite re-occupation.

Suppose a rover receiver occupies Point A for 5 minutes and tracks satellites 6, 9, 11, 12, 13.After 1 hour later, during the second occupation of Point A, it now tracks satellites 2, 6, 8, 9, 19. So only satellites 6 and 9 are common to the two sets, hence the data cannot be processed as four common satellites have not tracked during both occupation.Thus, prior mission planning is essential in conducting a successful pseudo-kinematic survey. It critical to determine whether or not common satellite coverage will be present for the desired period of survey.

COMPARISON OF METHODS

Pseudo-kinematic and Stop-and-Go techniques are considered as the ideal GPS measurement techniques for large scale surveying purposes. Pseudo-kinematic technique can be used advantageously in areas where there is a fear of signal shading due to vegetation and built up areas, as there is no requirement for the rover receiver to maintain its lock to the satellite during movement. For open areas, Stop-and-Go technique proves more useful.

KINEMATIC ON THE FLY (OTF) OTF surveying is similar to kinematic differential GPS surveying as it requires two receivers recording observations simultaneously and allows the rover receiver to be moving. Unlike the kinematic surveying, OTF surveying technique uses dual frequency Ll/L2 GPS observations and can handle loss of satellite lock. Since this method uses the L2 frequency, the GPS receiver must be capable of tracking the L2 frequency during anti-spoofing.

In OTF method, successful ambiguity resolutions are required for baseline formulations. The OTF technology allows the rover receiver to initialize and resolve the ambiguity integers without a period of static initialization. With OTF, if loss of satellite lock occurs, initialization can be done while on motion. The integers can be resolved at the rover within 10-30 seconds, depending upon distance from the reference station. OTF uses the L2 frequency transmitted by the GPS satellites for the ambiguity resolution.

In this method, one of the GPS receivers is set over a known point, and the other is moving or kept on a mobile platform. If the survey is performed in real time, a data link and processor is required and the method is known as Real time Kinematic Surveying (RTK Method).

REAL TIME KINEMATIC SURVEYING (RTK)RTK is a method that can offer positional accuracy in real time very near to static carrier-phase positioning. RTK is capable of delivering 5 cm accuracy. Unlike DGPS, RTK is a differential GPS method that uses carrier phase observations corrected in real-time and therefore, depends upon the fixing of the integer cycle ambiguity.

RTK systems resolve the integer ambiguity i.e. resolve the carrier phase ambiguity. The method requires dual frequency GPS receivers capable of making both carrier phase and precise pseudo-range measurements. Observations on L1 and L2 are combined into a wide lane (ambiguity = 86 cm), and the integer ambiguity is solved in the first pass. This information is used to determine the kinematic solution on L1.

Therefore, RTK suitable where there is good correlation of atmospheric biases at both ends of the baseline and hence distance between the base and rover should be less than 20 km. RTK requires a radio link between the receivers at the base station and the rover, and both must be tuned to the same frequency. Usually RTK GPS surveying equipment operate between 450-470 MHz.The configuration operates at 4800 or 9600 baud rate.

REAL-TIME DGPS SURVEYING

The code phase differential GPS system is commonly used for positioning hydrographic survey vessels and dredges. It also used for topographic, small-scale mapping surveys and input to GIS database. Real-time DGPS is a method that improves GPS pseudo-range accuracy. This is also known as real time (code) DGPS surveying.

Differential GPS involves the usage of two receivers; one stationary and other roving around making position measurements. The stationary receiver is known as reference station, base station or reference receiver. The second receiver that is roving is known as rover receiver, mobile receiver or navigator. The reference receiver antenna is mounted on a previously measured with known coordinates and placed on a known survey station in an area having an unobstructed view of sky.

A RTK DGPS consists of a GPS receiver, GPS antenna, processor having a communication link (radio-link). The reference receiver is switched on and it begins to track satellites. The reference station measures the timing and ranging information broadcasted by the satellites and computes and format range corrections for broadcast to the user equipment.

It calculates its own position from the received signals from the satellites. The actual co-ordinates of the known station of the reference receiver antenna is fed manually. The reference receiver works out the difference between the computed and measured value of the ranges to the satellites. These differences known as pseudo-range corrections.

Since the roving receiver may use any satellite to calculate its position, the reference receiver quickly runs through all the visible satellites and computes errors of all the visible satellites. It then transmits all the corrections to the rover receiver through the radio link. The rover, in turn, calculates ranges to the satellites and then applies the transmitted corrections to the corresponding satellite ranges. This enables the rover receiver to calculate its position more accurately.

Further, multiple rover receivers can receiver corrections from one single reference. Also, the base station takes a little time to calculate these errors and transmit them through a radio link. The rover receivers this transmitted data from the reference station, decodes the data and applies it through its software.

The time is called as latency of the communication between the reference and the rover. It may be as quarter of a second or a couple of seconds. Since the base stations corrections are only accurate for the instance they are generated for, the base station must also send a range rate correction along with them. Using this correction, the rover is able to give correction corresponding to the instant it makes an observation.

Accuracy of GPSThere are four basic levels of accuracy - or types of solutions - you can obtain with your real-time GPS mining system:

Autonomous15 100 metersDifferential GPS (DGPS)0.5 5 metersReal-Time Kinematic Float (RTK Float) 20 cm 1 meterReal-Time Kinematic Fixed (RTK Fixed) 1 cm 5 cm

SOLUTIONS GIVEN BY A GPS

Absolute Positions Uses.. C/A code onlyRequires.. Only one receiverData from at least four satellitesProvides An accuracy range of about 15 - 100 metersThis solution is designed for people who just need an approximate location on the earth, such as a boat at sea or a hiker in the mountains.

Real-Time Differential GPS (DGPS) PositionsUses.. C/A code onlyRequires.. Two receiversA radio link between the two receivers. Reference receiver at a known location broadcasts RTCM (Radio Technical Commission for Maritime Services) corrections. Rover receiver applies corrections for improved GPS positions

Data from at least four satellites - the same four at both the references and rover (common satellites) Provides An accuracy range of about 0.5 - 5 meters depending upon the quality of receiver and antennae used.This solution gives much better results because here we have a known position at a reference receiver. However it must have a radio link between the reference receiver and the roving (moving) receiver.

Real Time Kinematic (RTK) Float PositionsUses.. C/A code and career waves.Requires.. Two receivers Reference receiver at a known location tracks satellites and then broadcasts this satellite data over a radio link in a format called CMR. (CMR is a Trimble - defined format) Rover receiver receives data from both the satellites and the reference station.

A radio link between the two receivers.Data from at least four common satellites.Provides An accuracy range of about 20 cm to 1 meters. This solution uses more of the satellite signal than the autonomous or DGPS solution. The CMR data is carrier phase data.

Real Time Kinematic (RTK) Fixed SolutionsUses.. C/A code and career waves.Requires.. Two receivers Reference receiver at a known location tracks satellites and then broadcasts CMR data over a radio link. Rover receiver receives data from both the satellites and the reference station.

A radio link between the two receivers.Initialization, which is achieved most easily with dual-frequency receivers.Data from at least five common satellites to initialize on-the-fly (in motion)Tracking at least four common satellites after initializing. Provides An accuracy range of about 1 - 5 cm.

FACTORS THAT AFFECT GPS

The GPS errors associated with absolute GPS positioning mode are:

(i) Number of satellite required.(ii) Multipath(iii) Ionospheric delays(iv) Tropospheric delays(v) Satellite Health(vi) Signal Strength(vii) Distance from the Reference receiver. (viii) Radio frequency interference(ix) Loss of radio transmission from the base.

NUMBER OF SATELLITES REQUIRED

At least four common satellites be tracked and the same four satellites by both the reference and rover receivers, for either DGPS or RTK solutions. Also to achieve centimeter -level accuracy, a fifth satellite for on-the fly RTK initialization must be tracked. This extra satellite adds a check on the internal calculation. Any additional satellites beyond five provides even more checks, which is always useful.

MULTIPATH

Multipath is simply reflection of signals similar to the phenomenon of ghosting on our television screen. With multi-path reception, the receiver collects both the direct signal from the satellite and a fractionally delayed signal that has bounced off of some nearby reflective surface then reached the receiver.

Avoid Reflective SurfacesUse A Ground Plane Antenna Use Multipath Rejection ReceiverEffects of Multipath on the GPS Signal

MULTIPATH

The problem is that the path of the signal that has reflected off some surface is longer than the direct line to the satellite. This can "confuse" some lower-end receivers resulting in an incorrect range measurement and, consequently, an incorrect position.

MULTIPATH

MULTIPATH

There are several ways to deal with this problem. Most receivers have some way of "seeing" and comparing the correct and incorrect incoming signal. Since the reflected multi-path signal has traveled a longer path, it will arrive a fraction of a second later, and a fraction weaker than the direct signal. By recognizing that there are two signals, one right after another, and that second one is slightly weaker than the first, the receiver can reject the later, weaker signal, thus minimizing the problem. This ability is referred to as the receiver's multi-path rejection capability.

MULTIPATH

Mapping and survey quality receivers use semi-directional, ground-plane antennas to reduce the amount of multi-path that the receiver will have to deal with. Semi-directional antennas are designed to reject any signal below a tangent to the surface of the Earth. This is usually seen as a large (up to 20 to 30 centimeters across) flat metal plate (usually aluminum) with the actual, much smaller, receiver antenna attached on top. The metal plate interferes with any signals that may be reflected off of low reflective surfaces below them, such as bodies of water.

IONOSPHERE - CHANGE IN THE TRAVEL TIME OF THE SIGNAL:

Before GPS signals reach the antenna on the earth, they pass through a zone of charged particles called the ionosphere, which changes the speed of the signal. If the reference and rover receivers are relatively close together, the effect of ionosphere tends to be minimal and if one is working with the lower range of GPS precisions, the ionosphere is not a major consideration.However if the rover is working too far from the reference station, one may experience problems, particularly with initializing the RTK fixed solution.

TROPOSPHERE

Troposphere is essentially the weather zone of the atmosphere, and droplets of water vapour in it can effect the speed of the signals. The vertical component of the GPS answer (i.e. elevation) is particularly sensitive to the troposphere.

SATELLITE HEALTH - AVAILABILITY OF SIGNAL

While the satellite system is robust and dependable, it is possible for the satellites to occasionally be unhealthy. A satellite broadcasts its health status, based on information from the U.S. Department of Defense. Receivers have safeguards to protect against using data from unhealthy satellites.

SATELLITE GEOMETRY

Satellite Geometry - or the distribution of satellites in the sky - effects the computation of your position. This is often referred to as Position Dilution of Precision (PDOP).PDOP is expressed as a number, where lower numbers are preferable to higher numbers. The best results are obtained when PDOP is less than about 7.

PDOP is determined by the geographic location, the time of day, and any site obstruction, which might block the satellites. It is advisable that a planning software be used to determine when one will have the most satellites in a particular area. When satellites are well spread out, PDOP is Low (good). When satellites are close together, PDOP is High (weak).

Dilution Of Precision (DOP)A Measure of The Geometry Of The Visible GPS ConstellationGood DOPPoor DOP

Dilution Of Precision VDOP = Vertical Dilution Of PrecisionPDOP = Position Dilution Of Precision (Most Commonly Used)GDOP = Geometric Dilution Of PrecisionHDOP = Horizontal Dilution Of PrecisionTDOP = Time Dilution Of PrecisionMission PlanningIs Critical to ObtainGood DOP

SIGNAL STRENGTH

The strength of the satellite signal depends on obstructions and the elevation of the satellites above the horizon. To the extent it is possible, obstructions between your GPS antennae and the sky should be avoided. Also watch out for satellites which are close to the horizon, because the signals are weaker.

DISTANCE FROM THE REFERENCE RECEIVERThe effective range of a rover from a reference station depends primarily on the type of accuracy you are trying to achieve. For the highest real time accuracy (RTK fixed), rovers should be within about 10-15 Km of the reference station. As the range exceeds this recommended limit, you may fail to initialize and be restricted to RTK float solutions (decimeter accuracy).

RADIO FREQUENCY (RF) INTERFERENCE

RF interference may sometimes be a problem both for GPS reception and radio link system. Some sources of RF interference include: Radio towers Satellite dishesTransmitters GeneratorsOne should be particularly careful of sources which transmit either near the GPS frequencies (1227 and 1575 MHz) or near harmonics (multiples) of these frequencies. One should also be aware of the RF generated by his own machines.

Loss of Radio Transmission from Base

If, for any reason, there is an interruption in the radio link between a reference receiver and a rover, then the rover is left with an autonomous position. It is very important to set up a network of radios and repeaters, which can provide the uninterrupted radio link needed for the best GPS results.

GPS APPLICATIONSOne of the most significant and unique features of the Global Positioning Systems is the fact that the positioning signal is available to users in any position worldwide at any time. With a fully operational GPS system, it can be generated to a large community of likely to grow as there are multiple applications, ranging from surveying, mapping, and navigation to GIS data capture. The GPS will soon be a part of the overall utility of technology.

SURVEYING AND MAPPING

The high precision of GPS carrier phase measurements, together with appropriate adjustment algorithms, provide an adequate tool for a variety of tasks for surveying and mapping. Using DGPS methods, accurate and timely mapping of almost anything can be carried out. The GPS is used to map cut blocks, road alignments, and environmental hazards such as landslides, forest fires, and oil spills.

SURVEYING AND MAPPING

Applications, such as cadastral mapping, needing a high degree of accuracy also can be carried out using high grade GPS receivers. Continuous kinematic techniques can be used for topographic surveys and accurate linear mapping.

NAVIGATION

Navigation using GPS can save countless hours in the field. Any feature, even if it is under water, can be located up to one hundred meters simply by scaling coordinates from a map, entering waypoints, and going directly to the site. Examples include road intersections, corner posts, accident sites, geological formations, and so on. GPS navigation in helicopters, in vehicles, or in a ship can provide an easy means of navigation with substantial savings.

REMOTE SENSING AND GIS

It is also possible to integrate GPS positioning into remote-sensing methods such as photogrammetry and aerial scanning, magnetometry, and video technology. Using DGPS or kinematic techniques, depending upon the accuracy required, real time or post-processing will provide positions for the sensor which can be projected to the ground, instead of having ground control projected to an image. GPS are becoming very effective tools for GIS data capture.

REMOTE SENSING AND GIS

The GIS user community benefits from the use of GPS for locational data capture in various GIS applications. The GPS can easily be linked to a laptop computer in the field, and, with appropriate software, users can also have all their data on a common base with every little distortion. Thus GPS can help in several aspects of construction of accurate and timely GIS databases.

GPS ApplicationsGeodesy Geodetic mapping and other control surveys can be carried out effectively using high-grade GPs equipment, especially when the line of sight is not possible, GPS can set new standards of accuracy and productivity. Military The GPS was primarily developed for real time military positioning. Military applications include airborne, marine, and land navigation.

THE END

*The second segment we'll talk about is the Operational Control Segment. This segment consists of 5 Monitor Stations on islands near the equator (Hawaii, Ascension, Diego Garcia, and Kwajelin) and one Master Control Station located at Falcon AFB, CO. All of these stations track the GPS signals, and send them back to the Master Control Station at Falcon. A backup MCS exists at Loral Federal System in Gaithersburg, MD. The four stations track and monitor the where-abouts of each GPS satellite each day. Then land-based and space-based communications are used to connect the monitoring stations with MCS.*Dilution of Precision (DOP)The cumulative UERE (User Equivalent Range Error) totals are multiplied by a factor of usually I to 6, which represents a value of the Dilution of Precision, or DOP. The DOP is, in turn, a measure of the geometry of the visible satellite constellation.The ideal orientation of four or more satellites would be to have them equally spaced all around the receiver, including one above and one below. Because we're taking our position from only one side of the Earth thats really not possible since the planet itself blocks that part of space.The upper diagram at left illustrates the next best orientation. That is, to have one satellite directly above and the other three evenly spaced around the receiver and elevated to about 25 to 30 degrees (to help minimize atmospheric refraction). This would result in a very good DOP value.The lower diagram illustrates poor satellite geometry. In this case, all of the satellites are clustered together. This would result in a poor DOP value.A low numeric Dilution of Precision value represents a good satellite configuration, whereas a higher value represents a poor satellite con- figuration. The DOP at any given moment will change with time as the satellites move along their orbits.

*Dilution of Precision (DOP)There are a number of Dilution of Precision components. The overall GDOP, or Geometric Dilution of precision includes:PDOP, or Precision Dilution of precision, probably the most commonly used, which is the dilution of precision in three dimensions. Some- times called the Spherical DOP.HDOP, or Horizontal Dilution of Precision, is the dilution of precision in two dimensions horizontally. This value is often lower (meaning "better") than the PDOP because it ignores the vertical dimension.VDOP, or Vertical Dilution of precision, is the dilution of precision in one dimension, the vertical.TDOP, or Time Dilution of Precision, is the dilution of precision with respect to time.A DOP value of less than 2 is considered excellent-about as good as it gets, but it doesn't happen often, usually requiring a clear view of the sky all the way to the horizon. DOP values of 2 to 3 are considered very good. DOP values of 4 or below are frequently specified when equipment accuracy capabilities are given.DOP values of 4 to 5 are considered fairly good and would normally be acceptable for all but the highest levels of survey precision requirements. A DOP value of 6 would be acceptable only in low precision conditions, such as in coarse positioning and navigation. Position data generally should not be recorded when the DOP value exceeds 6,