Thursday, 24 December 2015

Global Positioning System (GPS) " Research "

1- Introduction:

With development of science and technology, GPS measuring technique also makes some progress nowadays and plays an increasingly important role in practice of engineering surveying.
Although GPS measuring technique has many advantages, such as high technical content, high measurement accuracy and short measuring time, it still has some problems in practical application, which we need pay attention to. This thesis simply analyzes composition, technical advantages and application of GPS measuring technique, focuses on studying problems of GPS in engineering practice and proposes solutions via exploration.

GPS just refers to global positioning system, which belongs to radio navigation positioning system in essence. It is jointly composed of global communication satellite and signal reception devices and can provide accurate navigation, time information and three-dimensional coordinate for users.
As the latest satellite positioning navigation system, GPS can not only realize all-weather, global and uninterrupted three-dimensional navigation positioning function but also have strong confidentiality and anti-interference performance. As global digitization process accelerates, GPS technique develops rapidly, is widely applied to engineering surveying and plays an important role.

2- Composition of GPS system:

Global positioning system (GPS) mainly contains two sub-systems, i.e., ground monitoring system and space satellite group system. According to hardware facilities, it also contains satellite receiving equipment. The space satellite group includes 24 satellites in all and all satellites are averagely distributed to 6 orbits surrounding the earth, whose distance to the ground is 0.2 million kilometers. On such orbits, operational cycle of satellites is 11 hours and 58 minutes. In this way, any site on the earth can receive GPS signals sent by 4 satellites at least and 11 satellites      at most
simultaneously anytime. Devices contained by GPS user terminal part involve receiver, data processing equipment, corresponding user equipment, meteorological      instrument and computer etc.

3- Advantage of GPS surveys :

• Three Dimensional .
• Site visibility 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 .
•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.

4- Methods of GPS Surveys :

-Static GPS Surveys :
Static GPS survey procedures allow various systematic errors to be resolved when high-accuracy positioning is required. Static procedures are used to produce baselines between stationary GPS units by recording data over an extended period of time during which the satellite geometry changes.

-Fast-static GPS Surveys :
Fast-static GPS surveys are similar to static GPS surveys, but with shorter observation periods (approximately 5 to 10 minutes). Fast-static GPS survey procedures require more advanced equipment and data reduction techniques than static GPS methods. Typically, the fast-static GPS method should not be used for corridor control or other surveys requiring horizontal accuracy greater than first order.

-Kinematic GPS Surveys :
Kinematic GPS surveys make use of two or more GPS units. At least one GPS unit is set up over a known (reference) station and remains stationary, while other (rover) GPS units are moved from station to station. All baselines are produced from the GPS unit occupying a reference station to the rover units. Kinematic GPS surveys can be either continuous or “stop and go”. Stop and go station observation periods are of short duration, typically under two minutes. Kinematic GPS surveys are employed where third-order or lower accuracy standards are applicable.

-OPUS GPS Surveys :
The NGS On-line Positioning User Service (OPUS) allows users to submit individual GPS unit data files directly to NGS for automatic processing. Each data file that is submitted is processed with respect to 3 CORS sites. OPUS solutions shall not be used for producing final coordinates or elevations on any Caltrans survey; however OPUS solutions may be used as a verification of other procedures.

5 - Equipment :

Post processed GPS surveying equipment generally consists of two major components: the receiver and the antenna.

-Receiver Requirements:
First-order, second-order, and third-order post processed GPS surveys require GPS receivers that are capable of recording data. When performing specific types of GPS surveys (i.e. static, fast-static, and kinematic), receivers and software shall be suitable for the specific survey as specified by the manufacturer. Dual frequency receivers shall be used for observing baselines over 9 miles in length. During periods of intense solar activity, dual frequency receivers shall be used for observing baselines over 6 miles in length.

-Antennas :
Whenever feasible, all antennas used for a project should be identical. For vertical control surveys, identical antennas shall be used unless software is available to accommodate the use of different antennas. For first-order and second-order horizontal surveys, antennas with a ground plane attached shall be used, and the antennas shall be mounted on a tripod or a stable supporting tower. When tripods or towers are used, optical plummets or collimators are required to ensure accurate centering over marks. Fixed height tripods are required for third-order or better vertical surveys. The use of range poles and/or stake-out poles to support GPS antennas should only be employed for third-order horizontal and general-order surveys.

-Miscellaneous Equipment Requirements

All equipment must be properly maintained and regularly checked for accuracy. Errors due to poorly maintained equipment must be eliminated to ensure valid survey results. Level vials, optical plummets, and collimators shall be calibrated at the beginning and end of each GPS survey. If the duration of the survey exceeds a week, these calibrations shall be repeated weekly for the duration of the survey. For details regarding equipment repair, adjustment, and maintenance.

6- General Post Processed GPS Survey Specifications:

-Network Design
Baselines (Vectors)
Baselines are developed by processing data collected simultaneously by GPS units at each end of a line. For each observation session, there is one less independent (non-trivial) baseline than the number of receivers collecting data simultaneously during the session. Notice in Figure 6A-1 that three receivers placed on stations 1, 2, and 3 for Session “A” Baselines are developed by processing data collected simultaneously by GPS units at each end of a line. For each observation session, there is one less independent (non-trivial) baseline than the number of receivers collecting data simultaneously during the session. Notice in Figure 6A-1 that three receivers placed on stations 1, 2, and 3 for Session “A” yield two independent baselines and one dependent (trivial) baseline. Magnitude (distance) and direction for dependent baselines are obtained by separate processing, but use the same data used to compute the independent baselines. Therefore, the errors are correlated. Dependent baselines shall not be used to compute or adjust the position of stations.

-Loops :
A loop is defined as a series of at least three independent, connecting baselines, which start and end at the same station. Each loop shall have at least one baseline in common with another loop. Each loop shall contain baselines collected from a minimum of 
two  sessions.

Networks shall only contain closed loops. Each station in a network shall be connected with at least two different independent baselines. Avoid connecting stations to a network by multiple baselines to only one other network station. First-order and second-order GPS control networks shall consist of a series of interconnecting closed-loop, geometric figures.

First-order, second-order, and third-order GPS control networks shall be designed with sufficient redundancy to detect and isolate blunders and/or systematic errors. Redundancy of network design is achieved by:
• Connecting each network station with at least two independent baselines
• Series of interconnecting, closed loops
• Repeat baseline measurements

7- Satellite Geometry:

Satellite geometry factors to consider when planning a GPS survey are:
• Number of satellites available
• Minimum elevation angle for satellites (elevation mask)
• Obstructions limiting satellite visibility
• Positional Dilution of Precision (PDOP)
• Vertical Dilution of Precision (VDOP) when performing vertical GPS surveys

 8- Field Procedures:

-Reconnaissance :
Proper field reconnaissance is essential to the execution of efficient, effective GPS surveys. Reconnaissance should include:
• Station setting or recovery
• Checks for obstructions and multipath potential
• Preparation of station descriptions (monument description, to-reach descriptions, etc.)
• Development of a realistic observation schedule

-Station Site Selection :
The most important factor for determining GPS station location is the project’s requirements (needs). After project requirements, consideration must be given to the following limitations of GPS:
• Stations should be situated in locations, which are relatively free from horizon obstructions. In general, a clear view of the sky is required. Satellite signals do not penetrate metal, buildings, or trees and are susceptible to signal delay errors when passing through leaves, glass, plastic and other materials.
• Locations near strong radio transmissions should be avoided because radio frequency transmitters, including cellular phone equipment, may disturb satellite signal reception.
Avoid locating stations near large flat surfaces such as buildings, large signs, fences, etc., as satellite signals may be reflected off these surfaces causing multipath errors.
With proper planning, some obstructions near a GPS station may be acceptable. For example, station occupation times may be extended to compensate for obstructions.

-Weather Conditions:
Generally, weather conditions do not affect GPS survey procedures with the following exceptions:
• GPS observations should never be conducted during electrical storms.
• Significant changes in weather or unusual weather conditions should be noted in the observation log (field notes). Horizontal GPS surveys should generally be avoided during periods of significant weather changes. Vertical GPS surveys should not be attempted during these periods.
With proper planning, some obstructions near a GPS station may be acceptable. For example, station occupation times may be extended to compensate for obstructions.

-Antenna Height Measurements :
Blunders in antenna height measurements are a common source of error in GPS surveys because all GPS surveys are three-dimensional whether the vertical component will be used or not. Antenna height measurements determine the height from the survey monument mark to the phase center of the GPS antenna. With the exception of fixed-height tripods and permanently mounted GPS antennas, independent antenna heights shall be measured in both feet and meters (use conversion between feet and meters as a check) at the beginning and end of each observation session. A height hook or slant rod shall be used to make these measurements. All antenna height measurements shall be recorded on the observation log sheet and entered in the receiver data file. Antenna height measurements in both feet and meters shall check to within ± 0.01 feet. When a station is occupied during two or more observation sessions back to back, the antenna/tripod shall be broken down, reset, and re-plumbed between sessions. When adjustable antenna staffs are used (e.g., kinematic surveys), they should be adjusted so that the body of the person holding the staff does not act as an obstruction. The antenna height for staffs in extended positions shall be checked continually throughout each day.
When fixed-height tripods are used, verify the height of the tripod and components (antenna) at the beginning of the project.

-Documentation :
The final GPS Survey project file should include the following information:
• Project report
• Project sketch or map showing independent baselines used to create the network
• Station descriptions
• Station obstruction diagrams
• Observation logs
• Raw GPS observation (tracking) data files
• Baseline processing results
Loop closures
• Repeat baseline analysis
• Least squares unconstrained adjustment results
• Least squares constrained adjustment results
• Final coordinate list

9- Office Procedures: 

-General :
For first-order, second-order, and some third-order Post-Processed GPS surveys, raw GPS observation (tracking) data shall be collected and post processed for results and analysis. Post processing and analysis are required for first-order and second-order GPS surveys. The primary post-processed results that are analyzed are:
• Baseline processing results
• Loop closures
• Repeat baseline differences
• Results from least-squares network adjustments

Post-processing software shall be capable of producing relative-position coordinates and corresponding statistics which can be used in a three-dimensional least squares network adjustment. This software shall also allow analysis of loop closures and repeat baseline observations.

-Loop Closure and Repeat Baseline Analysis:
Loop closures and differences in repeat baselines are computed to check for blunders and to obtain initial estimates of the internal consistency of the GPS network. Tabulate and include loop closures and differences in repeat baselines in the project documentation. Failure of a baseline in a loop closure does not automatically mean that the baseline in question should be rejected but is an indication that a portion of the network requires additional analysis.

-Least Squares Network Adjustment :
An unconstrained (free) adjustment is performed, after blunders are removed from the network, to verify the baselines of the network. After a satisfactory standard deviation of unit weight (network reference factor) is achieved using realistic a priori error estimates, a constrained adjustment is performed. The constrained network adjustment fixes the coordinates of the known reference stations, thereby adjusting the network to the datum and epoch of the reference stations. A consistent control reference network (datum) and epoch shall be used for the constrained adjustment. The NGS Horizontal Time Dependent Positioning (HTDP) program may be used to translate geodetic positions from one epoch to another.

10-Applications of GPS :

1-Civilian Applications of GPS :

1. Road Transport

2. Aviation
3. Shipping & Rail Transport
4. Science
5. Security
6. Heavy Vehicle Guidance
7. Surveying, Mapping and Geophysics
8. Telecommunications
9. Financial Services
10. Social Activities

2- Military Applications :

Aircraft navigation systems usually display a "moving map" and are often connected to the autopilot for en-route navigation. Cockpit-mounted GPS receivers and glass cockpits are appearing in general aviation aircraft of all sizes, using technology such as WAAS or LAAS to increase accuracy. Many of these systems may be certified for instrument flight rules navigation and some can also be used for final approach and landing operations. Glider pilots use GNSS Flight Recorders to log GPS data verifying their arrival at turn points in gliding competitions. Flight computers installed in many gliders also use GPS to compute wind speed aloft, glide paths to way points such as alternate airports or mountain passes, and to aid en route decision making for cross-country-soaring.

Boats and ships can use GPS to navigate all of the world's lakes, seas and oceans. Maritime GPS units include functions useful on water, such as "man overboard" (MOB) functions that allow instantly marking the location where a person has fallen overboard, which simplifies rescue efforts. GPS may be connected to the ships self-steering gear and Chart plotters using the NMEA 0183 interface. GPS can also improve the security of shipping traffic by enabling AIS.

Heavy Equipment can use GPS in construction, mining, and precision agriculture. The blades and buckets of construction equipment are controlled automatically in GPS-based machine guidance systems. Agricultural equipment may use GPS to steer automatically or as a visual aid displayed on a screen for the driver. This is very useful for controlled traffic, row crop operations, and when spraying. Harvesters with yield monitors can also use GPS to create a yield map of the paddock being harvested.

Bicyclists often use GPS in racing and touring. GPS navigation allows cyclists to plot their course in advance and follow this course, which may include quieter, narrower streets, without having to stop frequently to refer to separate maps. Some GPS receivers are specifically adapted for cycling with special mounts and housings.

Hikers, climbers, and even ordinary pedestrians in urban or rural environments can use GPS to determine their position with or without reference to separate maps. In isolated areas, the ability of GPS to provide a precise position can greatly enhance the chances of rescue when climbers or hikers are disabled or lost (if they have a means of communication with rescue workers) .

11- References :

1) California Department of Transportation  CALTRANS SURVEYS MANUAL

2) Wu Yingjie. Experience about GPS in engineering surveying [J]. Municipal Engineering of China, 2005(2).

3) Li Jun. GPS measuring technique and its application to engineering surveying [J]. Journal of Qiqihaer Vocational College, 2012(5).

4) Ma Jie. Discussion on improvement in accuracy and reliability of GPS RTK landmarks [J]. Zhongzhou Coal, 2006(2).

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