SURVEYING WITH GPS. GPS has become a standard surveying technique in most surveying practices

Transcription

1 SURVEYING WITH GPS Key Words: Static, Fast-static, Kinematic, Pseudo- Kinematic, Real-time kinematic, Receiver Initialization, On The Fly (OTF), Baselines, Redundant baselines, Base Receiver, Rover GPS has become a standard surveying technique in most surveying practices This could be attributed to both the ease of use as well as reduction in hardware costs Advantage of GPS surveys Three Dimensional Site Intervisibility Not Needed Weather Independent Day or Night Operation Common Reference System Rapid Data Processing with Quality Control High Precision Less Labor Intensive/Cost Effective Very Few Skilled Personnel Needed GPS eliminates the need for establishing control before a survey

2 GPS can establish control as and when needed and establish points at strategic locations to start and close conventional traverses All or any of the following values could be available directly in the field or after post-processing the data Latitude, longitude, geodetic height and X, Y, Z Cartesian coordinates State Plane or Project coordinates Forward and back geodetic azimuth of the baseline Geodetic distance or Monument to Monument slope distance of baselines Vertical angle from point to point GPS determines the geodetic azimuth between two points directly thereby eliminating the need for converting an astronomic azimuth to geodetic azimuth by applying Laplace correction State plane coordinates can be directly computed from the latitudes and longitudes obtained from GPS The slope distances can be reduced to the ellipsoid very accurately as ellipsoidal height is known Note, however that, even though the baseline components such as distances and azimuths are accurate, the accuracy of coordinates of new points are dependent on the quality of known points included in the survey

3 GPS OBSERVATION TECHNIQUES In surveying applications, accepted method is relative positioning although Differential GPS with code receivers could be used in projects that require a very low order of accuracy, e.g. locating cellular phone transmission towers In relative positioning, two or more receivers make simultaneous phase measurements on the carrier frequencies from four or more satellites Only the data collected from the same satellites by the receivers occupying the ends of a baseline can be used in the computation of this baseline As stated earlier, accurate baseline components can be computed from simultaneous carrier frequency measurements from four or more satellites by two receivers If accurate coordinates of a point are needed, one of the receivers must occupy a point whose coordinates are known accurately For this reason, at least one receiver must occupy a known point in the WGS84 datum as GPS satellite positions are defined in WGS84 datum coordinate system In practice, however, points in the National Spatial Reference System (NSRS) which are in NAD83 datum are used

4 For projects requiring geodetic accuracy, National Geodetic Survey (NGS) must be contacted or NSRS database must be accessed to determine points available in the project area before the project begins If there are points previously established which are in the National Reference System and meet the accuracy requirements, these points can be used instead of NGS points Coordinates of points determined in the WGS84 (NAD83) system could be transformed to any other coordinates such as State Plane coordinates or project coordinates either in the field, if real time coordinates are needed, or after post processing State Plane coordinates can be directly computed from NAD83 coordinates by using software available from NGS or other parties If accurate project coordinates are needed, an accurate transformation needs to be done by including a sufficient number of points whose coordinates are known in both systems In less precise work such as in construction projects, project coordinates can be determined by a field calibration of the site independently using an approximate transformation

5 There are several different field techniques for GPS surveying using carrier frequency These techniques could be used singly or in combination in some surveying projects The mode used depends on Accuracy needed Type of equipment and post processing software available Sky visibility in the project area Other considerations such as mode of transportation, urgency of the project, etc. The primary objective of each of the field techniques is to resolve the integer ambiguity resulting from carrier wave measurements Unlike conventional surveys, planning is an important part of any GPS survey, regardless of the technique used Following are the techniques that are commonly used now Static

6 Fast Static (Rapid Static) Kinematic Pseudo-kinematic(Pseudo-static) Real Time Kinematic STATIC MODE OF GPS SURVEYING This method, sometimes called static surveying, is used surveying projects that require high accuracy In this method, each receiver at each point logs data continuously for a pre-planned length of time The duration of data collection depends on required precision number of visible satellites satellite geometry(dop) whether the receivers are single frequency or dual frequency distance between receivers The duration of data collection, however, should be long enough for the post processing software to resolve the integer ambiguity Most new generation receivers and processing software are capable of resolving the integer ambiguity with small amount of data

7 However, a higher accuracy for the baseline components can be achieved by collecting data for a longer period of time Collection of data using two or more receivers for a period of time is called a session The slope line between any two antennas is called a baseline vector or simply baseline If more than two receivers are used multiple baseline vectors can be determined simultaneously Most GPS survey projects consist of multiple baselines or networks, and the baselines can be measured individually using only two receivers or several at a time using multiple receivers For economic reasons it is preferable to use more than two receivers for multiple baselines When the baseline between a known point and a new point is measured the new point can be used as a known point for other baselines Unlike in conventional surveys, the accuracy obtainable from networks is independent of the network geometry

8 Accuracy can be increased by increasing the number of redundant measurements Redundant measurements are those that are over and above the ones required to determine the coordinates of unknown points A redundant measurement should also be independent i.e. a measurement that is not related to or could not be generated from other measurements In a single session using more than two receivers, there are both independent (non-trivial) and dependent (trivial) baselines A baselines measured in separate sessions are always independent Redundant measurements can also be used to check for blunders In a network of GPS baselines, blunders can be detected by checking the closures of loops formed by connecting, independent baselines If the network is such that the loops are elongated in a east-west direction a higher accuracy in the positions can be obtained(gps measurements are stronger in north-south direction) and this should be considered at the planning stage of the network

9 Networks should also have several control points, located at strategic locations, in order to strengthen the network These control points should be preferably above or at least equal to the order of accuracy expected of new points The number and locations of control points depend on the size and shape of network (See Geometric Geodetic Accuracy Standards and Specifications for Using GPS Relative Positioning Techniques, Federal Geodetic Control Sub-committee, 1988) FAST STATIC MODE OF GPS SURVEYS Fast Static or Rapid Static was a method developed for dual frequency receivers A new algorithm was developed to reduce the amount of data needed to resolve integer ambiguity Lately, because of modifications in processing algorithms and because a larger number of satellites are available, the amount of data needed can be reduced even with single frequency receivers Sometimes, the manufacturers call this also fast static Field requirements and procedure for fast static are same as those for static except for the short session lengths

10 However, fast static is only suitable for low order control surveys, e.g. ground control for photogrammetric mapping KINEMATIC MODE OF GPS SURVEYING This is the mode of positioning from a moving platform. i.e. when the antenna is in motion This is the mode used in navigation where usually only a single receiver is used But, unlike in navigation, the kinematic method used in surveying is a relative positioning method where one antenna+receiver is stationary and one antenna+receiver is moving When the moving receiver is in constant motion as in navigation it is called continuous kinematic In most surveying applications, a method called stop-and-go kinematic is used The stationary receiver, called the base receiver, is placed at a known point while a second receiver called "rover' will visit all unknown points Rover will occupy each unknown point for a very short time (less than two minutes); Hence the term "Stop-and-Go" surveying

11 It is possible to combine both continuous and stop and go methods in the same survey It also is possible to operate more than one rover with the same base station The accuracy obtained is not as good as that obtained from static surveying but is better than that obtained in most surveys The single most advantage of stop and go surveying is its speed This method also has certain limitations An initialization process to determine the integer biases of at least 4 satellites is needed at the beginning The lock on the same four or more satellites must be maintained during the entire survey For this reason, kinematic GPS surveying is suitable for an area where there are no large over-hanging trees, over-passes or such structures in rover s route If for any reason a cycle slip occurs, the rover must return to any previous point which had been determined without cycle slip

12 The Initial integer bias term can be determined in one of 3 ways Using a known baseline less than 20 km in length and having an accuracy of less than 5 cm. Antenna swap Perform a static mode survey first for one of the base lines When using a known baseline, it is necessary to use one end of the baseline as the base station The rover will occupy the other end to collect 3 or 4 epochs of data (less than 2 minutes) Antenna swap is done by first occupying the known point with the base receiver and another point feet away with the rover After collecting data for 3-4 epochs two receivers + antennas are swapped while maintaining lock Collect data for another 3-4 epochs, return the base receiver + antenna to the base and continue the survey with the rover as usual In the third method, a baseline is measured by static method with the base receiver at the known base This now becomes a known baseline and the rest is similar to the first method For highest accuracy more than 6 satellites, well distributed over the sky is preferable

13 Kinematic post processing software is needed to obtain the point coordinates Kinematic GPS is similar to radial surveys with Total stations Kinematic GPS can use multiple bases and/or multiple rovers in the same survey, if necessary REAL TIME KINEMATIC GPS SURVEYS Real time kinematic (RTK) refers to a stop-and-go method where the coordinates of points are available in real time In this method, a radio communication link is maintained between the base receiver and the rover, and the base receiver supplies the pseudo-range and carrier phase measurements to the rover which in turn computes its position and display the coordinates The rover keeps updating coordinates as it moves as long as the lock on satellites is maintained Kinematic GPS surveying is generally suitable for any type of surveying or mapping, but for stakeout surveys, RTK is essential

14 Some RTK receivers have the capability of resolving the integer ambiguity On The Fly (OTF), and this technique can only be used with dual frequency receivers This means that there is no need to maintain the lock on satellites while the rover is in motion New observables are generated by taking linear combinations of observations made on these codes and carriers (wide laning) The integer ambiguity can be resolved very quickly by this technique while the receiver is still in motion Wide laning techniques are used in some high-end receivers even if OTF is not being used

15 PSEUDO-KINEMATIC (OR PSEUDO-STATIC) This is a combination of both static and kinematic methods It has the speed of kinematic method but there is no need to maintain lock on 4 satellites However newer receivers and algorithms can resolve the integer ambiguity much faster and the need for pseudo-kinematic surveys is somewhat diminished There is a reference (or base) receiver and a roving receiver, and the reference receiver remains at the reference point during the entire survey while the roving receiver visits the unknown points There is no initialization as in stop and go method Each point is occupied for 5-10 minutes for baselines 10 km. or less Each point must be revisited multiple times (at least once more) and these visits must be separated by at least 1 hour and preferably not more than 4 hours Multiple observations at the same site at different times capture different epochs along the satellite's orbit, and allow the satellite configuration to change and to resolve the integer ambiguity

16 This technique is suitable for areas where there are obstructions to signal and crew movement or if the receivers are not equipped with kinematic software Pseudo-kinematic is the least precise of all methods but is more productive than static Stop-and-Go kinematic method is suitable for details surveys as topographic mapping or boundary survey work whereas pseudokinematic is suitable for lower order control such as photogrammetric control etc. The GPS survey technique used in a given project depends on Accuracy requirements Urgency of the project Local terrain conditions Available equipment, etc. A combination of these methods can be used in some projects

17 COMMON ERRORS IN GPS OBSERVATIONS Setting up over wrong point Not using well adjusted tribrachs Not observing long enough during a session Poor planning (selection of points that may cause cycle slips or multipath or poor PDOP) Interruption due to power failure (not checking batteries prior to departure) Reading and recording wrong antenna height Some recommend that instead of using a tribrach with an optical plummet a rotatable tribrach adapter with a plate vial be used Errors such as the following may be introduced at the processing stage as well Incorrect datum or coordinates for known points Incorrect linear units for the project Entering incorrect point names and/or antenna heights

18 POSSIBLE ERRORS IN KINEMATIC SURVEYING Antenna height may change between points, especially if a prism pole with a sliding mechanism is used Not properly plumbing the antenna over the point at time of measurement If no OTF is available, it is sometimes necessary to raise the antenna over some obstructions in order to maintain lock For this reason a method to raise the antenna will be useful QUESTIONS 1. What are the advantages of GPS surveying over conventional surveying methods? 2. What is the single factor that determines whether or not a GPS survey is possible in an area and/or a project? 3. What is a baseline in GPS surveying? 4. What information/quantities pertaining to points and baselines are computed by postprocessing software? 5. What is an epoch in GPS terminology? 6. Why is the static GPS survey method so named? 7. What is the reason for minimum session length in static surveying? 8. What factors determine the length of a session in static surveying? 9. What factors determine the GPS surveying method suitable for a given area/project? 10. What is the purpose of rover initialization in kinematic surveying? 11. What are the three ways of rover initialization in stop-and-go kinematic surveying? 12. What is the fastest initialization method? 13. How much data is collected at each point in stop-and-go GPS surveying? 14. What is the purpose of re-occupation of points in pseudo-kinematic method? 15. What are the time limitations on re-occupation? 16. Which GPS surveying method would you use for establishing control with geodetic accuracy? 17. Which GPS surveying method would you use if you need to complete a job urgently?

19 18. Which GPS survey method is suitable for a project that does not need very high accuracy but the project is in a downtown area where there are tall buildings and over-passes? 19. What type of receivers are needed to do true fast static surveys? 20. Everything else being equal, real time kinematic GPS or conventional method with a total station would you prefer for a stakeout survey? Indicate your reasons.