Survey Manual Chap 4 GPS Surveys
Land surveying is an ancient but still much-used activity. Now you can use Global Positioning System (GPS) technology to accurately survey and mark land. Land surveyors measure horizontal positions in geographic or plane .. Diagram showing signal time difference between GPS satellite and GPS receiver. Surveying and mapping was one of the first commercial adaptations of GPS. But what is GPS and also the 3 methods of measurement that surveyors use.
GPS is a current state-of-the-art surveying technology with its prominence in contemporary and future surveys. GPS has several major advantages such as both day and night operation, intervisibility between stations is not required, mostly weather independent, geodetic accuracy, and is very productive with a possible one man operation. GPS is becoming the first choice surveying technique for all types of surveys. In this chapter, GPS will be discussed with the same approach that other surveying techniques were discussed in Chapter 3.
The discussion will include a short introduction to GPS, error issues, and field procedures. GPS surveying is an evolving technology. As GPS hardware and processing software are improved, and state-of-the-art new techniques are developed, new guidelines and specifications will be considered.
The specifications contained herein are referred to as of the date of this issue. The specifications included in this chapter are general recommendations. Though initially designed as a military system, it became freely available for international civil use with certain restrictions to civilians for positioning. The system has at least a complete set of at least 24 satellites orbiting the earth in a carefully designed pattern.
Space, Control, and User. The Space Segment consists of the orbiting satellites making up the constellation. The initial design was for a 24 satellite constellation, each orbiting at an altitude of approximately 20, km above the earth, in one of six orbital planes. Each satellite broadcasts a unique "bar code", known as Pseudo Random Noise PRN code, which enables GPS receivers to identify the satellites from where the signals came, and makes positioning possible. The Control Segment, under DoD's direction, oversees the building, launching, orbital positioning, monitoring, and providing GPS positioning services.
A master control station updates the information message component of the GPS signal with satellite ephemeris data and other announcements to the users. This information is then decoded by the receiver and used in the positioning process. The User Segment is the most important segment of the system and is comprised of all users making observations with GPS receivers.
The civilian GPS user community has increased dramatically in recent years, due to the emergence of low cost, portable GPS receivers and the ever-expanding areas of applications in which GPS has been found to be very useful.
Some of these applications are: First, there must be a relatively clear "line of sight" between the receiver's antenna and several orbiting satellites. Buildings, trees, overpasses, and other obstructions that block the line of sight between the satellite and the observer GPS antenna make it impossible to work with GPS.
Anything shielding the antenna from a satellite can potentially weaken the satellite's signal to such a degree that it becomes too difficult to make reliable positioning.
Chapter 5: Land Surveying and GPS
Any obstruction that can block or interfere with a radio signal can effectively block or interfere with GPS signals. Bouncing of the signal off nearby objects may present another problem, that of distinguishing between the signals coming directly from the satellite and the "echo" signal that reaches the receiver indirectly.Mod-01 Lec-3 GPS Positioning Methods
This phenomenon is referred to as multipath. Multipath refers to the existence of signals reflected from objects in the vicinity of a receiver's antenna that corrupt the direct line-of-sight signals from the GPS satellites, thus degrading the accuracy of both code-based and carrier phase—based measurements.
Particularly difficult is close-in multipath in which the reflected secondary signals arrive only slightly later within about nanoseconds than does the direct-path signal, having been reflected from objects only a short distance from the receiver antenna.
In areas that possess these types of characteristics, longer observational times or traditional surveying techniques must be used instead of GPS positioning or to complement the GPS positioning.
The receiver must receive signals from at least four satellites to be able to make reliable position measurements. In addition, these satellites must be in a favorable geometrical arrangement. The four satellites used by the receiver for positioning must be fairly spread apart. In areas with a relatively open view of the sky, this will almost always be the case because of the way these satellites were placed in orbit. Position Determination by Measuring Distances to Satellites 4.
The GPS receiver "knows" where each of the satellites is at the instant in which the distance was measured. These distances will intersect only at one point, the position of the GPS receiver antenna. The receiver "knows" the position of the satellites, because this information comes from the broadcast ephemeris that is downloaded when the GPS receiver is turned on. The way in which a GPS receiver determines distances called pseudo ranges to the satellites depends on the type of GPS receiver.
Basically, there are two broad classes: These receivers measure distances or pseudo ranges to visible satellites by determining the number N of whole wavelengths l and measuring the partial phase signal wavelength F between the satellites and the receiver's antenna. It is then a straight forward, albeit complex task to compute a baseline distance and azimuth between any pair of receivers operating simultaneously.
With one receiver placed on a point with precisely known 3-D position and with the calculated baseline distance between 2 pointsthe coordinate for the unknown point may be determined.
Carrier Phase Based Ranging Carrier phase receivers come in two models: Dual-frequency receivers are required for all NJDOT projects where observations are greater than 10kms.
Dual-frequency receivers are the preferred type of receiver for ALL observations, regardless of length. The time interval Dt is determined by comparing the time in which a specific part of the "bar code" left the satellite with the time it arrived at the antenna. Pseudo ranges from at least four satellites are needed in order for a receiver to perform essentially a triangulation and produce a position fix. Position fixes are made by the receiver roughly every second, and the more advanced receivers enable the user to store the position fixes in a file that can be downloaded to a computer for post processing.
Code Based Pseudo Ranging 4.
What is GPS & How Do Surveyors Use It? | Jurovich Perth
Traditional terrestrial survey activities that would have normally taken months to complete, now take only a few days utilizing GPS. Real-time applications of GPS enable surveyors to perform quick data collection and construction stakeout activities. In fact, a complete construction design plan can be loaded into a receiver and staked out by following the guidance of the receiver.
GPS is used exclusively to densify control networks. GPS is a positioning system that can also be used as a real world digitizer for mapping point and line features, such as roads or wetland boundaries. However, for large volume data collection which includes measuring many points, mapping, contouring, etc. GPS, as a surveying tool, has many standard traditional surveying errors as well.
For example, an instrument setup error applies across the board to all surveying measurements regardless of the instrument or technique used. It does not make a difference what instrument is used, if a GPS receiver, a total station or a range pole is on the wrong point, or not properly set up on the proper point, the survey is erroneous. There are two major categories of error sources in GPS surveys.
The first is System Errors and the other is Operational Errors. Most of these errors can be eliminated if GPS positioning is performed in a relative mode and with dual frequency receivers. GPS surveys are always made relative to a known control point, thus, many system errors cancel out. Ephemerides Errors — To compute a position with GPS, it is necessary to know the exact position of each observed satellite. The positions of the satellites derived from the broadcast navigation message broadcast orbitsare predictions of where the satellites are expected to be.
These predictions could have an error of a few meters. For most practical purposes these errors are insignificant in a relative differential positioning mode. Precise orbits that have typically sub-decimeter orbital accuracy may be required for specific projects.
The double-differencing data processing technique processing observation data from 2 satellites and 2 receivers simultaneously can eliminate the impact of this error. In GPS surveying, the standard positioning computation is double-differencing. Tropospheric Delay — The troposphere is the lower part of the atmosphere extending from the Earth's surface to a height of approximately 15 km. This is an electrically neutral and non-dispersive medium for frequencies as high as about 30 GHz.
Within this medium, group and phase velocities of the GPS signal on both L1 and L2 frequencies are equal. The GPS satellites transmit on two L-band frequencies: An electromagnetic signal propagating through the neutral atmosphere is affected by the constituent gases. The effect of the troposphere on GPS signals appears as an extra delay in the measurement of travel time from the transmitter to the receiver.
This delay is caused at the lower part of the atmosphere and is a function of the atmospheric conditions such as barometric pressure, temperature, and humidity. GPS signals are refracted due to moisture in the lower atmosphere.
Reasonably established models for the index of refraction and other atmospheric models can correct this error. Ionospheric Delay — An error introduced by the outer region of the atmosphere, which causes the GPS signal to disperse and change its traveling speed. The magnitude of the impact of the Ionosphere on the GPS signal depends on the intensity of the sun spot activity or the solar radiation. Solar activity has an year epicycle. At that time, accurate positioning with GPS will become more difficult.
Dual frequency receivers are more useful in handling this error because they can compute it and apply the necessary corrections. Single frequency receivers must rely on an external correction model to overcome this error and may produce poor results at the peak of the solar activity. High quality clocks are very expensive and even they are subject to errors. Receiver clock errors can be eliminated by utilizing the double differencing computation method.
Chapter 5: Land Surveying and GPS
Receiver Noise — GPS receivers are not perfect devices. Some level of noise always contaminates the observations and produces positioning errors. The position that is determined with GPS is the position of the antenna phase center. Every antenna is calibrated by the vendor to determine the offset between the center of the physical center of the antenna used to place the antenna directly above a point and the phase center.
Each antenna has a setup orientation mechanism to enable the user to orient all antennas used in a given session to the same usually north direction. If this is done and the same type of antenna is used in the session, the antenna phase error can be eliminated. This is one of the reasons why it is not recommended to mix antennas from different manufacturers in a given session, unless this error is known and corrected for.
Bulls-Eye Level Bubble Collimation Error — The integrity of the bulls-eye level bubble on the 2-meter fixed height pole and the rover bi-pod pole must be checked before, after and during the project if suspect. The findings must be documented in the final survey report. An out of adjustment bubble can cause an antenna centering error of several centimeters. Tribrach Misalignment — This is the same error that can be committed when setting up a total station or a theodolite over a point.
The GPS antenna is mounted on a tribrach and, in turn, the tribrach is used to place the antenna directly above a point on the ground. If the tribrach is misaligned, the position determined by GPS is not exactly the one of the intended point, but has a small offset which depends on the misalignment error. This error is usually larger than in the case of total station misalignment because GPS antennas are mounted higher than total stations to avoid low obstructions.
The verification of both the bulls-eye level bubble and reticular sighting components of the tribrach must be documented. The rules of point selection in traditional surveying, mainly maintaining line of sight, do not apply in GPS surveys. Since the direct line of sight has to be with the satellites, points have to be selected in such a way that the clearest signal is received at that point.
The following are errors that can impact the results of GPS surveys: Multipath — Multipath is receiver-satellite geometry dependent, hence the effect of the multipath error on positioning will generally repeat on a daily basis for the same baseline.
A signal can arrive at a receiver directly from the satellite, but also from a nearby reflective surface. The reflected signal travels a longer path than the direct one, which results in an observation error. The point to be GPS occupied must be selected in such a way that it is not adjacent to a reflective surface.
If possible, avoid locations of stations near large flat surfaces such as buildings and large signs.
For this reason, the vehicle used during the survey should not be parked near the GPS antenna, or the antenna should be mounted higher than the vehicle. The first is a solid obstruction that completely blocks the antenna from the incoming signal. This will cause fewer actual observations to fewer satellites than planned and a weaker positioning solution.
Every point to be GPS surveyed must be inspected for such obstructions and the obstructions must be properly mapped. The observation planning software should be updated with these obstructions to provide better session planning and, eventually, better results.
Constant signaling can then update speed and direction information for moving receivers. GPS was originally developed for military use but since the s has been open for civilian use and is now used in such common applications as mobile phones, car navigation systems, and of course surveying and mapping.
Surveying and mapping was one of the first commercial adaptations of GPS, as it provides a latitude and longitude position directly without the need to measure angles and distances between points. In practice, GPS technology is often incorporated into a Total Station to produce complete survey data. GPS receivers used for base line measurements are generally more complex and expensive than those in common use, requiring a high quality antenna. There are three methods of GPS measurement that are utilised by surveyors.
Static GPS is used for determining accurate coordinates for survey points by simultaneously recording GPS observations over a known and unknown survey point for at least 20 minutes. The data is then processed in the office to provide coordinates with an accuracy of better than 5mm depending on the duration of the observations and satellite availability at the time of the measurements.
This is where one receiver remains in one position over a known point — the Base Station — and another receiver moves between positions — the Rover Station.
The position of the Rover can be computed and stored within a few seconds, using a radio link to provide a coordinate correction.