cET 318 The Seventh Lecture 1. Introduction 7. Field Surveying with GPS Book:p.133-179 Dr Guoqing Zhou 1.1 Terminology Definitions 3. Point Positioning ys. Relative positioning The coordinates of a single point are determined when 1. Code Range vs, Carrier Phase Code measurements(meter level) Carrier phase measurements(millimeter range). Single point positioning, 2. Real-time Processing vs Postprocessing Relative positioning(P. 134) 4. Static ys, Kinematic 于m( Kinematic and dy namIc ? temporary loss of signal lock Comparison between Point Static Point Positioning(P 135): Positioning and relative positioning Kinematic Point Positioning(P. 135) Static Relative Positioning(P 135): Point Positioning Relative Positioning Kinematic Relative Positioning(P 135) With SA, hSA,·lppm 300m tho SA, 10mSA
1 Dr. Guoqing Zhou 7. Field Surveying with GPS CET 318 Book: p. 133-179 1. Introduction 1.1 Terminology Definitions 1. Code Range vs. Carrier Phase GPS observables are pseudoranges derived from 1. Code measurements (meter level), 2. Carrier phase measurements (millimeter range). 2. Real-time Processing vs. Postprocessing 1. Real-time, 2. quasi (or near) real-time, 3. Instantaneous navigation of moving vehicles 3. Point Positioning vs. Relative Positioning The coordinates of a single point are determined when using a single receiver. • Point positioning, • Single point positioning, • Absolute point positioning ("relative“) Relative positioning (P. 134) 4. Static vs. Kinematic Static: stationary observation location, Kinematic: motion. A temporary loss of signal lock in static mode is not as critical as in kinematic mode. Kinematic and dynamic ? • Static Point Positioning (P.135): • Kinematic Point Positioning (P.135): • Static Relative Positioning (P.135): • Kinematic Relative Positioning (P.135): • 1ppm to • cm level 0.1ppm • With SA, 100m • Without SA, ? • With SA, 300m • Without SA, 10m Static Kinematic Static Kinematic Point Positioning Relative Positioning Comparison between Point Positioning and Relative Positioning
1. 2 Observation Technique 2. Differential GPS (GPS) The selection of the observation technique ular requirements of the projec The desired accuracy especially I. Two(or more)receivers, where one(stationary)reference (base) receiver is located at a known point and the 1. Point positioning position of moving remote receiver is to be determined. For the SPs only the C/A-code is available and SA is incorporated into the service. This limits the (real-time accuracy The known position is used to calculate corrections to the 1. Horizontal accuracy: 100 m(95%), 300 m(99.99%) GPs derived position or to the observed pseudoranges 2. Vertical accuracy: 156 m(95%), and 500 m(99.99%) 4. These corrections are then transmitted via telemetry (i.e. controlled radio link)to the roving receiver The pps has access to both codes The higher accuracy is based on the fact that GPS error sources 1. Horizontal accuracy: 16 m(95%) re very similar over a distance of about 500 km and are 2. Vertical accuracy: 23 m(95%) therefore, virtually eliminated by the differential technique. 2. Two Correction Methods 3. Accuracy of DGPS I Position Correction. 1. Point positioning can not reach meter level because of The difference (differential") of the known and the SA but can be achieved by DGP calculated position yields position corrections. These 2. Using C/A-code ranges, accuracies at the 3-5 m level values are then applied to the roving receiver to obtain an can be routinely achieved improved position (conceptually simple, more comple 3. Phase smoothed code ranges or high performance C/A at. selection) code receivers can obtain the submeter level 2. Pseudorange Corrections 4. Carrier phases can obtain sub-decimeter level for up The difference between calculated ranges and observe to 20 km, to achieve this accuracy, the ambiguit (code or phase) pseudoranges at the re must be resolved on-the-fly and therefore(generally) site may be lual frequency receivers are required corrected by applying pseudorange corrections of the 1. The accuracy requirements of GPS users vary from reference station(more flexible, higher accuracy several hundred meters and centimeter level thera 2. Interested a real-time accuracy at the meter level. 4. Wide Area DGPS- WADGPS 5. Data Link WADGPS uSes a network of gPs reference stations with UHF (ultra high frequency)radio links for terrestrial coverage of a larger territo data links 2. RDS (radio data system) which is a standardized Main Advantages of wadgPs ethod for distributing digital data along with the 1. A more consistent accuracy throughout the regio onventional upported by the network(regular DGPS, the accuracy 3. LEO worldwide telecommunication satellite allows decreases at a rate of approximately 1 cm per 1 km) high frequencies in the GHz range and enables data 2. Inaccessible regions can be covered, e.g., large bodies rates up to 1200 bits per second over long distances. Correction update rates of 10 seconds or better are 3. The network will still maintain a relatively high level of adequate to remove SA effects at the 2 m accuracy level integrity and reliability compared to a collection of because SA is characterized by variations of the individual DGPS reference stations pseudorange error with an 100 m each about 10 minutes
2 1.2 Observation Technique The selection of the observation technique • Particular requirements of the project • The desired accuracy especially 1. Point Positioning For the SPS only the C/A-code is available and SA is incorporated into the service. This limits the (real-time) accuracy 1. Horizontal accuracy: 100 m (95%), 300 m (99.99%) 2. Vertical accuracy: 156 m (95%), and 500 m (99.99%). The PPS has access to both codes. 1. Horizontal accuracy: 16 m (95%) 2. Vertical accuracy: 23 m (95%) 2. Differential GPS (DGPS) 1. Two (or more) receivers, where one (stationary) reference (base) receiver is located at a known point and the position of moving remote receiver is to be determined. 2. At least four common satellites must be tracked simultaneously. 3. The known position is used to calculate corrections to the GPS derived position or to the observed pseudoranges. 4. These corrections are then transmitted via telemetry (i.e., controlled radio link) to the roving receiver. 1. Basic Principle The higher accuracy is based on the fact that GPS error sources are very similar over a distance of about 500 km and are, therefore, virtually eliminated by the differential technique. 1. Position Correction: The difference ("differential") of the known and the calculated position yields position corrections. These values are then applied to the roving receiver to obtain an improved position (conceptually simple, more complex sat. selection). 2. Two Correction Methods 2. Pseudorange Corrections • The difference between calculated ranges and observed (code or phase) pseudoranges at the reference site. • The observed pseudoranges at the roving site may be corrected by applying pseudorange corrections of the reference station (more flexible, higher accuracy, general use). 1. Point positioning can not reach meter level because of SA but can be achieved by DGPS. 2. Using C/A-code ranges, accuracies at the 3-5 m level can be routinely achieved. 3. Phase smoothed code ranges or high performance C/Acode receivers can obtain the submeter level. 4. Carrier phases can obtain sub-decimeter level for up to 20 km, to achieve this accuracy, the ambiguities must be resolved on-the-fly and therefore (generally) dual frequency receivers are required. 3. Accuracy of DGPS 1. The accuracy requirements of GPS users vary from several hundred meters and centimeter level. 2. Interested a real-time accuracy at the meter level. 4. Wide Area DGPS-WADGPS WADGPS uses a network of GPS reference stations with coverage of a larger territory. 1. A more consistent accuracy throughout the region supported by the network (regular DGPS, the accuracy decreases at a rate of approximately 1 cm per 1 km). 2. Inaccessible regions can be covered, e.g., large bodies of water, 3. The network will still maintain a relatively high level of integrity and reliability compared to a collection of individual DGPS reference stations. Main Advantages of WADGPS 5. Data Link 1. UHF (ultra high frequency) radio links for terrestrial data links. 2. RDS (radio data system) which is a standardized method for distributing digital data along with the conventional program. 3. LEO worldwide telecommunication satellite allows high frequencies in the GHz range and enables data rates up to 1200 bits per second over long distances. Correction update rates of 10 seconds or better are adequate to remove SA effects at the 2 m accuracy level because SA is characterized by variations of the pseudorange error with an 100 m each about 10 minutes
6. RTCM Format 3. Relative Positioning Radio Technical Commission for Maritime Services Static Relative Positioning Observation periods depending on C/A-code differential corrections baseline length number of visible satellites rrelated with the I Delta differential corrections geometric configuration, andt baseline length and · he method used. (or even better). 10 P-code differential corrections Rapid Static Technique pecial message Use code and carrier phase combinations on both Dual frequency receivers and optimum satellite geometry are required. e corrections Restricting 20 km baselines, with sub cm level Semi-Kinematic Relative Positioning(Stop-&-Go) The semikinematic(stop-and-go)is characterized by stopping In practice. it is best to use a mixture of the three methods and moving one receiver. The most important feature is several measurement epochs at when using single frequency receivers. the stop locations are accumulated and averaged This technique is often referred to as simply kinematic survey. Static and pseudokinematic methods can be used to Relative positional accuracies at the centimeter level can establish a broad framework of control and to set for baselines up to some 20 km points on either side of obstructions such as bridges Phase ambiguities of Kinematic positioning: Kinematic surveys can then be employed to The initialization by static or kinematic technique etermine the coordinates of the major portion of are(for dual points, using the static points as control and check requires 1-2 minutes(baselines up to 20 km) kinematically Lock must be maintained over 4 satellites all entire survey A thorough reconnaissance is required for these Best suited for wide open areas. mixed surveys. 1.3 Impact of SA on Positioning SA Impact Reduction 1. Authorized P-code by decrypting Sa to get SA has two components correction d-process dithers the satellite clock frequency(some 2. Differentia 3. Relative te 4. Modeling the behavior of sa and to remove its effect E-process truncates the ephemerides data in the by appropriate filters. navigation message(some hours) Both processes induce variations in the code and phase 1.Impact on Point Positioning: pseudoranges which in turn translate into like position errors 2. with SA on, the poston cror:1 m level
3 6. RTCM Format Radio Technical Commission for Maritime Services 21 Code range corrections 20 Carrier phase corrections 19 Raw code range measurements 18 Raw carrier phase measurements 16 Special message 10 P-code differential corrections 6 Null frame 3 Reference station parameters 2 Delta differential corrections 1 C/ A-code differential corrections Type Meaning RTCM Message Types 64 Message Types 3. Relative Positioning Static Relative Positioning • Observation periods depending on • baseline length, • number of visible satellites, • geometric configuration, and t • he method used. The accuracy is correlated with the baseline length and amounts to 1-0.1ppm (or even better). Rapid Static Technique: • Based on fast ambiguity resolution techniques • Use code and carrier phase combinations on both frequencies. • Dual frequency receivers and optimum satellite geometry are required. • Restricting 20 km baselines, with sub cm level Semi-Kinematic Relative Positioning (Stop-&-Go): • The semikinematic (stop-and-go) is characterized by stopping and moving one receiver. • The most important feature is several measurement epochs at the stop locations are accumulated and averaged. • This technique is often referred to as simply kinematic survey. • Relative positional accuracies at the centimeter level can be achieved for baselines up to some 20 km. Phase Ambiguities of Kinematic Positioning: • The initialization by static or kinematic techniques. • Commercial software (for dual frequency receivers) only requires 1-2 minutes (baselines up to 20 km) kinematically. • Lock must be maintained over 4 satellites all entire survey. • Best suited for wide open areas. In practice, it is best to use a mixture of the three methods when using single frequency receivers. For example: • Static and pseudokinematic methods can be used to establish a broad framework of control and to set points on either side of obstructions such as bridges. • Kinematic surveys can then be employed to determine the coordinates of the major portion of points, using the static points as control and check points. • A thorough reconnaissance is required for these mixed surveys. 1.3 Impact of SA on Positioning SA has two components. • δ-process dithers the satellite clock frequency (some minutes) • ε-process truncates the ephemerides data in the navigation message (some hours). Both processes induce variations in the code and phase pseudoranges which in turn translate into like position errors. 1. Authorized P-code user by decrypting SA to get correction data from the navigation message 2. Differential techniques 3. Relative techniques 4. Modeling the behavior of SA and to remove its effect by appropriate filters. SA Impact Reduction: 1. Impact on Point Positioning: 1. With SA off, the position errors: 15 m level 2. With SA on, the position errors: 100 m level
2. Impact on DGPS The errors induced by Sa are similar for the reference and remote receiver over distances of 500 km 2. In practice, time delay cannot be completely avoided Differential corrections calculating at reference station Signal propagation delay in transmitting these 2. US GPS Network Order corrections to the remote receiver 3. But this delay plays a much less stringent role in the absence of sa because the pseudoranges change smoothly and do not show the short variations caused by the dithering process. The differential pseudorange corrections accumulate ar error of about I m after 10 s 2.1 US GPS Survey Order 2.2 Observation for GPS Survey Observation Order Distance Relative Minimum time(minutes) 240120120 Pose Accuracy Accuracy Minimum satellites A0.5cm 0. 1ppm US Geodetic Reference Network. Elevation angle() Earth Surface Deformation Site number B 0.8 cm I ppm Local Earth Surface Deformation, imum baseline c 1.0 cm 10 ppm Engineering Surveying, Urban Max sides of non-synchronic loop Control Max length of non-synchronic loop 500300200 Number of atm eric measureme 2.3 Synchronic Loop Construction Definition: Several GPS receiver simultaneously survey 6 S=n(n-1)2 Radial Surveying
4 2. Impact on DGPS: 1. The errors induced by SA are similar for the reference and remote receiver over distances of 500 km. 2. In practice, time delay cannot be completely avoided because of • Differential corrections calculating at reference station • Signal propagation delay in transmitting these corrections to the remote receiver. 3. But this delay plays a much less stringent role in the absence of SA because the pseudoranges change smoothly and do not show the short variations caused by the dithering process. The differential pseudorange corrections accumulate an error of about 1 m after 10 s. 2. US GPS Network Order 2.1 US GPS Survey Order Engineering Surveying, Urban Control Surveying C 1.0 cm 10 ppm Local Earth Surface Deformation, high-accuracy Engineering Surveying B 0.8 cm 1 ppm US Geodetic Reference Network, Earth Surface Deformation A 0.5 cm 0.1ppm Relative Purpose Accuracy Distance Accuracy Order 2.2 Observation for GPS Survey Number of atmospheric measurement 2 2 1 Number of antenna measurement 2 2 1 Max length of non-synchronic loop 500 300 200 Max sides of non-synchronic loop 8 10 10 Minimum baseline 15 5 5 10 30 20 50 40 80 Site number • Over 3 • Over 2 Elevation angle (°) 15 20 20 Minimum satellites 4 3~4 3 Minimum time (minutes) 240 120 120 Observation A B C 2.3 Synchronic Loop Construction Definition: Several GPS receiver simultaneously survey S=n(n-1)/2 Radial Surveying
2.3 Non-synchronic Loop Constructio 3. Pla f GPS Surve 3.1 General Remarks L. Designing a GPS network will have to consider 3. The optimum planning of a GPS survey has to 1. Project objectives 3. Observation technique, and Site or satellite configurations 2. GPS surveying differs essentially from classical one 3. Type of receivers 4. Economic aspects 5. Data processing considerations(software allows for 2. No need for intervisibility between the sites single baseline vectors or for multipoint solutions) 3. Different planning 4. Execution 5. Processing techniques 3.2 Pre-survey Planning 4. Contrary to the design of triangulation o L Point Selection: trilateration networks. the followings are not so critic for GPS networks Small-scale maps(1: 25000 to 1: 100000)for point selection. All desired survey points are plotted on the map along Line length. with the known control points Three basic considerations in choosing a point: 1. No obstructions above 20 elevation to avoid satellite signal blockage For large projects with many sites and many 2. No reflecting surfaces(e.g, metal structures, fences receivers, planning a GPS survey could be aided by the water surfaces) in the vicinity of the antenna to avoid use of computer programs to save time and resources. 3. No nearby electrical installations(e. g, transmitters)to avoid signal disturbances
5 2.3 Non-synchronic Loop Construction 3. Planning of GPS Survey 3.1 General Remarks 1. Designing a GPS network will have to consider 1. Project objectives 2. Equipment, 3. Observation technique, and 4. Organization. 2. GPS surveying differs essentially from classical one because 1. Weather independent 2. No need for intervisibility between the sites 3. Different planning 4. Execution 5. Processing techniques 3. The optimum planning of a GPS survey has to consider 1. Site or satellite configurations, 2. Number of receivers 3. Type of receivers 4. Economic aspects 5. Data processing considerations (software allows for single baseline vectors or for multipoint solutions) 6. Whether 4. Contrary to the design of triangulation or trilateration networks, the followings are not so critical for GPS networks • Geometric strength, • Line length. For large projects with many sites and many receivers, planning a GPS survey could be aided by the use of computer programs to save time and resources. 3.2 Pre-survey Planning 1. Point Selection: • Small-scale maps (1:25000 to 1:100000) for point selection. • All desired survey points are plotted on the map along with the known control points Three basic considerations in choosing a point: 1. No obstructions above 20° elevation to avoid satellite signal blockage. 2. No reflecting surfaces (e.g., metal structures, fences, water surfaces) in the vicinity of the antenna to avoid multipath. 3. No nearby electrical installations (e.g., transmitters) to avoid signal disturbances
2. Observation Window: I. Satellite Visibility(P. 150) 1. The optimum window of satellite availability is the period when num of satellites can be observed Observation Window: To optimum daily observati riod and to decide how it should be subdivided into 2. The difference between sidereal time and Universal time sessions UT) is 4 minutes 3. The length of the window is a function of the location 2. Geometrie D 1. The tracked satellites should be geometrically well distributed with(ideally) one in each of the four quadrants 3. ionospheric Refraction: 1. Observations during night hours may be ate because the ionospheric effect is usually quiet 2. Daylight hours are preferred for organizational 3. Sessions Session length vs. baseline length for Sessions: The specific time period chosen for an conventional static surveying and single observation frequency receivers Observation Time: The following factors to determine the length of a particular observation 1. Length of the baseline Session min 2. Number of visible satellites(affects geometry) tive geometry of the satellites and the change in 35-60 4. SNR of the received satellite signal 55-90 Summary What have we learnt? Which parts are important?
6 2. Observation Window: Observation Window: To optimum daily observation period and to decide how it should be subdivided into sessions. 1. The optimum window of satellite availability is the period when a maximum of satellites can be observed simultaneously. 2. The difference between sidereal time and Universal Time (UT) is 4 minutes. 3. The length of the window is a function of the location. 1. Satellite Visibility (P. 150): 1. The tracked satellites should be geometrically well distributed with (ideally) one in each of the four quadrants. 2. Geometric Distribution of Satellites: 1. Observations during night hours may be appropriate because the ionospheric effect is usually quieter. 2. Daylight hours are preferred for organizational reasons. 3. Ionospheric Refraction: 3. Sessions: Sessions: The specific time period chosen for an observation. Observation Time: The following factors to determine the length of a particular observation: 1. Length of the baseline 2. Number of visible satellites (affects geometry) 3. Relative geometry of the satellites and the change in geometry 4. SNR of the received satellite signal. 20 55-90 10 35-60 5 25-45 1 20-35 Baseline [km] Session [min] Session length vs. baseline length for conventional static surveying and single frequency receivers Summary What have we learnt? Which parts are important?