GPS postioning modes and GPS processing.pptx

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1. GPS POSITIONING MODES

2. GPS Positioning GPS work is sometimes divided into three categories; positioning, navigation, and timing (PNT). Most often, GPS surveying is concerned with the first of these, positioning. positioning with GPS can be performed by either of two ways: point positioning or relative positioning.  Point positioning: employs one GPS receiver that measures the code pseudoranges to determine the user’s position instantaneously, as long as four or more satellites are visible at the receiver. Relative positioning: employs two GPS receivers simultaneously tracking the same satellites.

3. GPS point positioning employs one GPS receiver that measures the code pseudoranges to determine the user’s position instantaneously, as long as four or more satellites are visible at the receiver. The expected horizontal positioning accuracy from the civilian C/A-code receivers has gone down from about 100m (2 drms ) when selective availability was on, to about 22m (2 drms ) in the absence of selective availability GPS point positioning is used mainly when a relatively low accuracy is required. This includes recreation applications and low-accuracy navigation .

4. GPS point positioning , also known as standalone or autonomous positioning, involves only one GPS receiver. That is, one GPS receiver simultaneously tracks four or more GPS satellites to determine its own coordinates with respect to the centre of the Earth Almost all of the GPS receivers currently available on the market are capable of displaying their point-positioning coordinates.

5. Point positioning The receiver gets the coordinates of the satellites from the navigation message while the pseudorange are determined from C/A code or P-code

6.  determine the receiver’s point position  at any time, the satellite coordinates as well as a minimum of four ranges to four satellites are required. The receiver gets the satellite coordinates through the navigation message, while the ranges are obtained from either the C/A-code or the P(Y)-code, depending on the receiver type (civilian or military). the measured pseudoranges are contaminated by both the satellite and receiver clock synchronization errors. Correcting the satellite clock errors may be done by applying the satellite clock correction in the navigation message; the receiver clock error is treated as an additional unknown parameter in the estimation process

7. This brings the total number of unknown parameters to four: three for the receiver coordinates and one for the receiver clock error. This is the reason why at least four satellites are needed. It should be pointed out that if more than four satellites are tracked, the so-called least-squares estimation or Kalman filtering technique is applied . As the satellite coordinates are given in the WGS 84 system, the obtained receiver coordinates will be in the WGS 84 system as well. However, most GPS receivers provide the transformation parameters between WGS 84 and many local datums used around the world.

8. Relative Positioning GPS relative positioning , also called differential positioning, employs two GPS receivers simultaneously tracking the same satellites to determine their relative coordinates  Of the two receivers, one is selected as a reference, or base, which remains stationary at a site with precisely known coordinates. The other receiver, known as the rover or remote receiver, has its coordinates unknown. The rover receiver may or may not be stationary, depending on the type of the GPS operation.

9. A minimum of four common satellites  is required for relative positioning. However, tracking more than four common satellites simultaneously would improve the precision of the GPS position solution. Carrier-phase and/or pseudorange measurements can be used in relative positioning. A variety of positioning techniques are used to provide a postprocessing (post mission) or real-time solution.

10.   GPS relative positioning provides a higher accuracy than that of autonomous positioning. Depending on whether the carrier-phase or the pseu-dorange measurements are used in relative positioning, an accuracy level of a sub centimeter to a few meters can be obtained. This is mainly because the measurements of two (or more) receivers simultaneously tracking a particular satellite contain more or less the same errors and biases . The shorter the distance between the two receivers, the more similar the errors. Therefore, if we take the difference between the measurements of the two receivers (hence the name "differential positioning"), the similar errors will be removed or reduced.

11. The key point of relative positioning is to keep the coordinates of the reference station fixed. Carrier phase or pseudorange observables provide a post processing (post mission) or real time solutions GPS relative positioning provides a higher accuracy (cm level) Depending on whether carrier phase or pseudorange measurements Two satellite contain more or less similar errors or biases. The shorter the distance between the two receivers, the more similar the errors are. The similar errors will be removed or reduced.

12. Static GPS Static GPS surveying is a relative  positioning technique that depends on the carrier-phase measurements . It is Based on the carrier phase measurements. It employs two (or more) stationary receivers simultaneously tracking the same satellites One receiver, the base receiver, is set up over a point with precisely known coordinates such as a survey monument (sometimes referred to as the known point). The other receiver, the remote receiver, is set up over a point whose coordinates are sought (sometimes referred to as the unknown point). The base receiver can support any number of remote receivers, as long as a minimum of four common satellites is visible at both the base and the remote sites.

13. In principle,  this method is based on collecting simultaneous measurements at both the base and remote receivers for a certain period of time, which, after processing, yield the coordinates of the unknown point. The observation, or occupation, time varies from about 20 minutes to a few hours, depending on the distance between the base and the remote receivers (i.e., the baseline length), the number of visible satellites, and the satellite geometry. The measurements are usually taken at a recording interval of 15 or 20 seconds, or one sample measurement every 15 or 20 seconds. Occupation time: <10 km – 30 m, > 10 km - 1 hr , >15 km - 2 hr , >100 km - 4 hr Resolving the ambiguity parameters would be a key issue to ensure a high-precision positioning. Recording interval of 15 or 20 seconds Accuracy -- 5mm+1ppm

14. After completing the field measurements , the collected data is downloaded from the receivers into the PC for processing. Different processing options may be selected depending on the user requirements, the baseline length, and other factors. For example, if the baseline is relatively short, say, 15 or 20 km, resolving the ambiguity parameters would be a key issue to ensure high-precision positioning

15. Rapid Static GPS One receiver always remains on a control station while the others are moved progressively from one unknown point to the next during the entire observation session. The observation time or the occupation time for rapid static surveying is relatively small. Collects data for about 2 to 10min depending on the distance to the base and the satellite geometry. After collecting and downloading the GPS data is processed.

16. Stop and go GPS The data is usually collected at 1-to-2 seconds recording rate for a period of 30 seconds per each stop. The survey starts by first determining the initial integer ambiguity parameters, a process known as receiver initialization. Once the initialization is performed successfully, centimeter-level positioning accuracy can be obtained instantaneously. A special case of Stop and Go GPS surveying is known as kinematic GPS surveying. Both methods are the same in principle. However; the latter one requires no stop at unknown stations.

17. antenna swapping Antenna swapping in GPS refers to the technique of alternating or switching antennas between two GPS receivers to mitigate certain types of systematic errors, such as multipath or phase center variations, and to improve positioning accuracy, especially in high-precision applications like geodetic surveys or monitoring networks .

18. antenna swapping Antenna swapping methods including repeatedly swapping between antennas, and related wireless electronic devices. of a start baseline with a static survey prior to kinematic operations, short observation on a known baseline, and antenna swapping -- static initialization

19. RTK GPS a carrier phase-based relative positioning technique. This methods are required that are independent of static initialization techniques, and that include the capacity to recover cycle slips and/or to resolve ambiguities during motion. These techniques are referred to as ambiguity solution on the way, or on the fly Only with such methods at hand can kinematic surveying be purely or truly kinematic.

20. " Solution on the Way " and " On the Fly " are terms commonly used in Real-Time Kinematic (RTK) GPS to describe two different approaches for resolving carrier-phase ambiguities, which are crucial for achieving high-precision positioning.

21. Solution on the Way (Also called Post-Processing): Definition : This method refers to resolving ambiguities and correcting errors after data collection, rather than in real-time. Process : GPS data is recorded by both the base and the rover receivers. After the data is collected, it is processed later (post-processed) to resolve ambiguities and compute precise positions. Accuracy : Provides centimeter-level accuracy by resolving ambiguities after gathering enough data over a certain period. Use Cases : Used in situations where real-time positioning is not necessary, such as some land surveying applications, geodetic measurements, and scientific research.

22. Cont’d Advantages: Often yields highly accurate results because it uses more complete data, correction models, and can handle more complex environmental factors. Disadvantages: The main drawback is that the results are not available in real-time, as the corrections and ambiguity resolution are applied after data collection.

23. On the Fly (OTF) "On the fly" refers to the real-time resolution of carrier-phase ambiguities while the rover is in motion. As the rover receives GPS signals, it continuously processes data from the base station, resolving the integer ambiguities in real time without needing to stop. Provides centimeter-level accuracy in real time, which is critical for dynamic applications like machine control, precision agriculture, and real-time navigation. Used when immediate positioning is needed, such as in real-time surveys, construction, and vehicle navigation.

24. Advantages : Provides immediate positioning solutions without the need for post-processing, making it ideal for time-sensitive applications. Disadvantages : It may be less reliable in areas with poor satellite visibility (e.g., near tall buildings or dense vegetation) and can lose accuracy when subjected to signal interruptions. On the Fly" is more commonly used in real-time operations, while "Solution on the Way" is preferred when post-processing is possible

25. Suitable methods for ambiguity resolution while the receiver is moving are code/carrier combination using the extra wide lining technique, and ambiguity search functions for six or more satellites. Methods for cycle slip recovery in true kinematic mode are use of redundant satellites (≥ 4 four satellites), use of dual frequency data, and use of code/carrier combination

26. On the fly procedure The base and rover measurements are combined in the double differenced mode and an initial adjustment by, for example, the least squares, is then performed. The outcome of this initial adjustment is an initial rover position along with estimates (real values) for the ambiguity parameters and their uncertainty values, or the covariance matrix. The covariance matrix can be represented geometrically to form a region, known as the confidence region, around the estimated real-value ambiguity parameters 27

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28. Based on statistical evaluation, only one point is selected as the most likely candidate for the integer ambiguity parameters. Once the ambiguities are correctly resolved, a final adjustment is performed to obtain the rover coordinates at centimeter-level accuracy. It should be pointed out that the OTF technique, although designed mainly for resolving the ambiguity parameters in real time, could also be used in the non- realtime mode. 29

29. Real Time DGPS Real time DGPS occurs when the base station calculates and broadcasts corrections for each satellite as it receives data. As a result ,the position displayed and logged to the data file of the roving GPS receiver as a differentially corrected position. The correction is received by the roving receiver by a radio signal if the source is land based by a satellite signal if it is satellite based and applied to the position it is calculating code based relative positioning technique that employs two or more receivers simultaneously tracking same satellites.

30. GPS Data Processing

31. GPS DATA PROCESSING

32. Solving Initial Integer Ambiguity In carrier phase measurements, the signal is pure sinusoidal wave, which means that all cycles look the same. Therefore the receiver has no means to identify one cycle from another When the receiver is switched on, it cannot count the total number of cycles between the satellite and the receiver. It can only measure the fractional part of the cycle and the number of complete cycles will remain ambiguous or unknown and which is known as integer ambiguity.

33. The integer ambiguity will be the same throughout the survey and can be represented by a single bias term ( N ). The integer ambiguity will change only if the receiver loses lock on the satellite.

34. Preprocessing Transfer of data to storage medium. RINEX (Receiver independent exchange format) format can be used. Satellite positions determination from orbital data at selected epoch. Approximate point coordinate determination by single point positioning from the code data. Phase difference observables formation from the raw phase data. Cycle slip detection and repair. Determination of base line vectors from the double difference observables.

35. Main Processing Assumed that all the data is clean, and free of cycle slips. Phase data processing in single baseline mode. The main steps of a baseline solution using phase data are: Preparation: a priori coordinates, ephemeris file to be used, baseline to be processed (if more than one observed in a session), data file names, antenna height and offsets, etc.

36. Con’d Selection of parameters:  this is dependent on the baseline to be processed, the ambiguity model used in the software, differencing scheme adopted, etc. Selection of options:  a priori standard deviation of parameters and observations, criteria for data rejection, data reduction methods, whether correlations are to be considered, elevation cutoff, satellites to be excluded from solution, differencing strategy to be used, etc.

37. Con’d Processing the data and get solution. In such case there are three types of phase solutions Triple difference Solution. Double difference Solution (Ambiguity-free) Double Difference Solution (Ambiguity-fixed).In the process converting ambiguous phase data to range data. There are two types of double-difference phase data solutions: Ambiguity-free solution (A mbiguity- float solution ) Ambiguity-fixed solution - in which some or all the ambiguity parameters have been, resolved to their integer values. Such a solution is very strong as it only contains the station coordinate parameters

38. Ambiguity-Free Solution refers to a technique in GPS data processing that aims to achieve high-precision positioning without the need to resolve the integer ambiguities associated with carrier-phase measurements. This method is particularly valuable in applications where immediate positioning results are required, and it is especially relevant in Real-Time Kinematic (RTK) positioning systems. An ambiguity-fixed solution in GPS positioning refers to a scenario where the integer ambiguities in carrier-phase measurements have not only been resolved but have been fixed to specific integer values. This solution leads to highly accurate positioning results, typically within the centimeter range. Ambiguity fixing is essential for applications that require precision, such as surveying, navigation, and geodesy.

39. Feature Ambiguity-Free Solution Ambiguity-Fixed Solution Definition A solution where ambiguities are resolved but not yet assigned specific integer values (often referred to as "float" ambiguities). A solution where ambiguities are resolved and fixed to specific integer values. Integer Resolution Ambiguities are estimated as real numbers (floats), leading to potential inaccuracies. Ambiguities are resolved to precise integers, providing a higher degree of confidence in the position. Accuracy Typically offers lower accuracy compared to fixed solutions, often in the range of centimeters to decimeters . Provides high accuracy, usually within centimeters , due to the fixing of ambiguities. Reliability May have more variability and uncertainty in positioning results. More stable and reliable results, with reduced likelihood of drift or errors. Usage Context Often used in scenarios where immediate precision is not critical, or data is collected over long periods. Commonly used in high-precision applications like surveying, geodesy, and real-time navigation where immediate accuracy is crucial. Processing Complexity Simpler and faster to compute as it does not require fixing to integers. More complex processing involved to accurately resolve and fix ambiguities.

40. Generally Ambiguity-Free Solution: Provides a quick estimate of position with ambiguities resolved as float values, typically leading to lower accuracy and reliability. Ambiguity-Fixed Solution : Represents a more precise and confident positioning outcome, with ambiguities resolved to specific integer values, ensuring high accuracy and reliability for critical applications.

41. Con’d Output found from main processing includes the following Coordinate parameters as Cartesian (x, y, z) or geodetic (ϕ, λ, h) values in the WGS84 datum, baseline components, for ground mark and antenna centers. Estimated standard deviation and correlation matrix or variance covariance matrix of parameters

42. application of GPS GPS (Global Positioning System) has a wide range of applications across various fields due to its ability to provide accurate positioning, navigation, and timing information. Some of the key applications of GPS include: 1. Surveying and Mapping: Land Surveying : GPS is widely used for high-precision land surveying, enabling surveyors to map boundaries, property lines, and topography with centimeter-level accuracy, especially with techniques like RTK (Real-Time Kinematic) or PPP (Precise Point Positioning). Geodetic Surveys : Used in large-scale mapping projects and the creation of geodetic control networks, where precise positioning is essential. Topographic Mapping : GPS helps in creating detailed maps of the Earth’s surface for construction, urban planning, and environmental studies.

43. 2. Navigation: Automotive Navigation : GPS is embedded in car navigation systems, providing turn-by-turn directions and real-time route optimization based on traffic conditions. Aviation : Pilots use GPS for en -route navigation, landing approaches, and monitoring flight paths, reducing reliance on traditional ground-based navigation aids. Marine Navigation : Ships and boats use GPS for safe navigation, especially in open waters where landmarks are scarce, and for tasks like docking and fishing. Personal Navigation : Smartphones and portable GPS devices help individuals navigate unfamiliar areas, find businesses, and use location-based services.

44. 3. Transportation and Logistics: Fleet Management : GPS allows companies to track their vehicles in real-time, optimize routes, and improve fuel efficiency. It’s widely used in trucking, delivery services, and public transportation. Asset Tracking : Businesses use GPS to monitor the location of valuable assets like containers, machinery, and shipments, ensuring security and efficient management. Traffic Monitoring : GPS is used in urban traffic systems to provide real-time data on vehicle movement, congestion, and accident reporting, helping to reduce traffic jams and improve road safety.

45. 4. Agriculture: Precision Agriculture : GPS enables farmers to optimize crop planting, irrigation, and harvesting by providing precise location data for tractors and equipment. This increases yield while minimizing the use of water, fertilizers. Etc.

46. discussion points What are the primary GPS observables, and how do they differ in terms of accuracy and reliability? How do pseudorange and carrier-phase measurements affect the overall positioning accuracy in GPS? What are the limitations of GPS observables in urban environments with significant multipath effects? How do the different frequencies used by GPS (L1, L2, etc.) impact the quality of observables? What role do satellite ephemeris data play in determining GPS observables?

47. Cont’d What are the differences between Standard Positioning Service (SPS) and Precise Positioning Service (PPS)? How does Real-Time Kinematic (RTK) positioning improve accuracy compared to traditional GPS methods? What are the advantages and disadvantages of Differential GPS (DGPS) compared to other positioning modes? How does the use of Network RTK differ from single-base RTK, and what are its benefits? In what scenarios would you choose to use Post-Processed Kinematic (PPK) positioning instead of real-time solutions?

48. Cont’d What are the key steps involved in GPS data processing, and how do they contribute to accuracy? How do Kalman filters enhance the quality of GPS data processing? What are common sources of errors in GPS data processing, and what techniques can be used to mitigate them? How does the double-difference method improve the reliability of carrier-phase measurements? What is the significance of ambiguity resolution in GPS data processing, and what methods are used to achieve it?

49. Application of GPS What are some of the most significant applications of GPS in various industries, and how do they impact those fields? How has GPS technology transformed agricultural practices, and what are the benefits for farmers? In what ways does GPS contribute to disaster management and emergency response efforts? What are the implications of using GPS technology in autonomous vehicles, and what challenges do they face? How does GPS play a role in scientific research, particularly in geodesy and environmental monitoring?

50. End of the course Thank you for your attention Question Comment