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.
-Networks:
-Redundancy:
9- Office Procedures:
1-Civilian Applications of GPS :
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 :
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.
two sessions.
-Networks:
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.
-Redundancy:
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:
-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 :
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).
