GPS Surevy and Applications | Geology Optional for UPSC PDF Download

Introduction

• In the previous units, you learned about the basic functions, features, accuracy, and types of GPS technology. You also studied how GPS works and its different segments. Now, we will focus on planning a GPS survey, the techniques used for measuring with GPS, and the steps involved in carrying out a GPS survey, along with some of its applications.
• GPS technology has made a significant impact on our lives by providing accurate and efficient solutions for various surveying and navigation tasks. It allows us to determine the precise location of points on Earth, measure distances and angles between them, and navigate with ease. However, it is important to remember that GPS is not suitable for every survey task, and proper planning and understanding of its capabilities and limitations are crucial for a successful survey.

Objectives

After studying this unit, you should be capable of:

• Distinguishing between GPS Navigation and GPS Survey.
• Planning a GPS survey task and understanding GPS field survey procedures.
• Describing GPS measuring techniques.
• Listing the general steps for conducting a GPS survey.
• Discussing the applications of GPS in various fields.

Advantages and Disadvantages of GPS Over Conventional Surveying Methods

The GPS-based method, like any other approach, comes with its own set of advantages and disadvantages. It is merely one of the many tools that should be part of a surveyor's toolkit. Let's explore some of the advantages and disadvantages of GPS compared to conventional surveying methods.

Advantages of GPS Satellite Surveying over Conventional Surveying

• No Need for Intervisibility: In GPS satellite surveying, it is not necessary for the survey stations to be visible to each other. This is a significant advantage over conventional surveying methods, which require a clear line of sight between stations.
• Weather Independence: GPS surveying is not affected by weather conditions because it uses radio frequencies to transmit signals. This makes it a reliable option regardless of weather circumstances.
• Homogeneous Accuracy: GPS surveying offers consistent accuracy, allowing survey points to be placed wherever needed. Unlike conventional methods, there is no requirement for survey points to be evenly distributed to ensure intervisibility or network geometry.
• Round-the-Clock Operation: GPS can be used at any time of day or night for data logging purposes, providing flexibility in survey operations.
• Three-Dimensional Information: GPS provides detailed three-dimensional information for user point coordinates, enhancing the quality of the data collected.
• High Accuracy with Less Effort: GPS survey results are generally more accurate and can be achieved with relatively less effort compared to conventional methods.
• Efficiency and Flexibility: GPS surveying is more efficient, flexible, and less time-consuming. The absence of the need for intervisibility between stations and the network-independent site selection procedure contribute to these advantages.

Disadvantages of GPS Surveying

• Signal Obstruction: GPS surveying is not effective in areas where the GPS signals are blocked, such as under overhanging structures. While raising the antenna above the obstruction can help, GPS cannot be used underground and has limited applications in densely populated urban areas or thick forests.
• Efficiency and Cost: Although GPS is efficient, it requires minimizing travel time between stations to justify the savings in on-site time. This can increase costs, especially in rugged and inhospitable terrain where transporting and supporting multiple field teams is challenging. Helicopters may be needed for such surveys, further driving up expenses.
• Specificity: GPS surveys are typically designed to meet specific survey needs. They may require establishing two intervisible stations to provide azimuth data for conventional survey methods. Additionally, the horizontal and vertical coordinates obtained from GPS may need to be transformed for use in traditional surveying applications.
• Planning and Data Analysis: GPS surveying requires careful planning of field operations for data collection and the development of different data analysis methods.

GPS Navigation vs. GPS Surveying

GPS (Global Positioning System) technology is used in both navigation and surveying, but they serve different purposes and have distinct characteristics. Here are the key differences between GPS navigation and GPS surveying:

• Purpose: GPS surveying is primarily used for traditional tasks such as establishing geodetic control, supporting engineering construction, conducting cadastral surveys, and creating maps. In contrast, GPS navigation is focused on ensuring the safe passage of vessels or aircraft from their point of departure, while en route, and to their destination.
• Operational Aspects: GPS navigation relies on real-time absolute positioning, which is crucial for guiding vehicles accurately. On the other hand, GPS surveying involves post-processed relative positioning, where data is analyzed after collection.
• Cost of Equipment: GPS surveying receivers are more expensive due to their advanced capabilities and the type of measurements they make. In contrast, GPS navigation receivers are relatively low-cost.
• Measurement Techniques: In GPS navigation, the primary measurement is the pseudorange, which is handled in a casual and relaxed manner. However, GPS surveying requires a more careful treatment of biases during data processing, making the measurements more precise and accurate.

Planning a GPS Survey

Planning a GPS survey requires careful consideration of different planning, execution, and processing techniques, as it differs from conventional surveying. Systematic planning maximizes the chances of achieving the desired accuracy within a reasonable time and budget. Before starting the planning process, it is essential to determine the ultimate purpose of the survey and the desired level of accuracy.

Elements of GPS Survey Planning Process

Unlike traditional surveying methods, there is often not enough time for surveyors to rely on conventional knowledge to carry out GPS surveys effectively. Therefore, meticulous planning is crucial and involves the following elements:

• Project Design
• Observation Scheduling
• Instrumentation and Personnel Consideration
• Logistical Considerations
• Reconnaissance

Let's discuss each of these elements in detail.

3.4.1 Project Design

Project Design is a crucial aspect of planning for GPS surveys. When designing a project, surveyors need to consider several important factors to ensure the success and accuracy of the survey.

• Definition of the Network: This involves determining the size and shape of the entire network, the number of stations required, their spacing, intervisibility requirements, and the inclusion of both new and existing (known) stations.
• Spacing of the Existing Stations: This includes assessing the need to densify existing control stations and determining transformation parameters.
• Accuracy and Standards: Surveyors must consider the accuracy requirements specified by the client and the standards set by geodetic control authorities. Both vertical and horizontal surveys should be taken into account during the project design.

After deciding on the number and approximate locations of GPS stations, surveyors should also think about other factors that could impact the performance of GPS-based calculations, such as potential locations for additional stations or refinements to the network design.

Observation Scheduling

To prepare an observation schedule for a GPS survey, consider the following:

• Observation Parameters: Determine how many observations to make and for how long.
• Satellite Geometry: Consider the distribution of satellites in the sky and their geometry.
• Logistical Design: Plan the number of observation sessions per day and multiple site occupancies.

Satellite Considerations:

• Define the satellite constellation to be tracked, including:
• Rise and Set Times: Satellites are typically tracked at elevations above 15° to 20° to avoid atmospheric refraction.
• Satellite Health Status: Avoid satellites with known health issues indicated in their navigation message.
• Minimum Observation Period: Determine the minimum length of observation per station based on the satellite constellation and baseline length.

Estimating Observation Length:

• The appropriate length of an observing session is challenging to estimate as it depends on:
• Baseline length
• Environmental factors
• Satellite constellation

Ionospheric Considerations:

• GPS signals pass through the ionosphere, a zone of charged particles that affects the speed of the signal.

Satellite Receiver Geometry:

• The accuracy of GPS-derived coordinates depends on:
• Measurement precision
• Systematic errors
• Processing strategy
• Receiver satellite geometry

Position Dilution of Precision (PDOP):

• PDOP is influenced by:
• Geographic location
• Time of day
• Obstructions blocking satellite views
• Low PDOP: Good accuracy when satellites are spread out.
• High PDOP: Weaker signals when satellites are close together.

Instrumentation and Personnel Considerations for Project Planning

When planning a project, it is important to consider the following factors related to instrumentation and personnel:

• Number of Available GPS Receivers: To ensure a better network and faster progress in surveying, it is crucial to have a large number of receivers and directly connected stations. The optimum number of receivers is typically around four to six.
• Receiver Type: While all geodetic GPS receivers produce similar datasets and accuracies, mixing different types of receivers for the same network calculations can lead to problems.
• Single or Dual Frequency GPS Receivers: For high accuracy applications, dual frequency instruments are preferred as they allow for compensation of ionospheric delays based on GPS signals.

Logistical Considerations

Logistical considerations in a GPS project significantly increase with the rise in various factors:

• Stations to be Surveyed: More stations require more planning and resources.
• Receivers Deployed: An increase in receivers adds to the logistical complexity.
• Common Stations Occupied Between Sessions: More common stations mean more coordination.
• Fixed Stations to be Occupied: Additional fixed stations increase the workload.
• Sessions Per Day: More sessions per day require tighter scheduling and resources.

These considerations are further complicated by factors such as:

• Terrain and Transport: The type of terrain and the mode of transport (e.g., four-wheeler or helicopter) impact logistics.
• Instrumental Factors: Special equipment may be needed for receivers to download data.
• Contingency Plans: Having plans for unforeseen events is crucial.
• Quality Control and Data Processing: Daily transfer of data to the field office for quality control and partial processing is necessary.
• Accommodation: Availability of nearby accommodation reduces travel time and enhances efficiency.

Reconnaissance

After plotting the GPS points on the map and gathering directions to the existing control points, it is time to conduct a field reconnaissance. It is important to ensure easy access to the site to save time between sessions. Sites with numerous obstructions require additional considerations. Therefore, proper field reconnaissance is essential for any control survey.

This reconnaissance should include:

• Review of Existing Control Networks: Assessing the current control networks in place.
• Mark Recovery and Maintenance: Evaluating the recovery and maintenance of existing marks.
• Station Selection: Choosing appropriate stations for the survey.
• Setting of New Monuments: Establishing new monuments as necessary.

GPS Measuring Techniques

There are several measuring techniques that can be used by most GPS Survey Receivers are listed below.

Static

The Static technique is employed for measuring long baselines, typically 20 km or more. This method offers high accuracy over extended distances but is relatively slow. In a static survey, one receiver is positioned on a point with known coordinates in WGS84, referred to as the Reference Receiver. The other receiver is placed at the opposite end of the baseline and is called the Rover Receiver. Data is recorded simultaneously at both stations at the same rate, which may be set to 15, 30, or 60 seconds. The duration for data collection is influenced by factors such as the number of satellites observed, satellite geometry (dilution of precision or DOP), and the length of the line. As a general guideline, a minimum observation time of 1 hour is recommended for a 20 km line with 5 satellites, with longer lines requiring extended observation times. Once sufficient data has been collected, the receivers can be turned off, and the Rover receiver can be moved to the next baseline.

Step 1

• UVWXY is the network to be measured using three receivers placed on points U, V, and Y. The coordinates of point U are known in WGS84. GPS data is recorded for the required duration.
• After the necessary time, the receiver at point Y moves to point X, and the receiver at point V moves to point W. The triangle UWX is then measured.

Step 2

• The receiver at U moves to Y, and the receiver at W moves to V. The triangle VXY is measured.

Step 3

• The receiver at V moves back to W, and the line YW is measured.

Step 4

• The entire network UVWXY is measured.

Rapid Static Survey

Rapid Static Survey is a method used for densifying existing networks and setting up local control networks. It provides high accuracy for baselines of up to 20 km and is significantly faster than the traditional static technique.

In a Rapid Static Survey, a Reference Point is selected, and one or more Rovers operate in relation to this point. When starting in an area where no GPS surveying has been done before, the initial task is to observe several points with accurately known coordinates in the local system. At least four known points on the perimeter of the area of interest should be observed.

The Reference Receiver is typically set up at a known point. If a known point is not available, it can be set up anywhere within the network. The Rover Receivers then visit each of the known points. The observation time required for each point depends on the baseline length from the Reference and the GDOP (Geometric Dilution of Precision).

To illustrate this, consider an example where the network UVWXY needs to be measured from Reference station R using three GPS receivers.

• Step 1: Setup the reference station and have one Rover occupy point U while the other occupies point W.
• Step 2: After the required observation time, one Rover moves to point V while the other moves to point X.
• Step 3: One Rover can return while the other measures point Y.
• Step 4: The final results are obtained.

Kinematic

The Kinematic Survey method is commonly employed for detailed surveying and tracking trajectories. It is an effective way to measure numerous points that are in close proximity to each other. This technique involves a moving Rover whose position is calculated relative to a Reference point.

Step 1: Initialisation

• Initially, the Rover must perform an initialisation, similar to measuring a Rapid Static point.
• The Reference and Rover units are activated and need to remain completely stationary for a duration of 5 to 20 minutes.

Step 2: Rover Movement

• After the initialisation period, the Rover is free to move.
• The user can record positions at a predefined interval, at specific points, or a combination of both.
• This process is referred to as a kinematic chain.

Important Note

• If the Rover receiver is tracking fewer than four satellites at any point, the process must be stopped.
• The Rover should be moved to a location where four or more satellites can be tracked, and initialisation should be performed again before continuing.

Real Time Kinematic (RTK)

Real Time Kinematic (RTK) is a surveying technique that uses a radio data link to transmit satellite information from a Reference station to a Rover in real time. This process allows for the calculation and display of coordinates as the survey is conducted.

In an RTK survey:

• The Rover receives a signal broadcast from the Reference station via a radio link.
• The Rover also receives satellite data directly from the satellites.

By processing these two sets of data together, the Rover can resolve ambiguities and obtain a highly accurate position relative to the Reference receiver.

The initialisation process begins when the Rover is tracking satellites and receiving data from the Reference. Once initialisation is complete, the Rover can start recording points with coordinate data. At this stage, baseline accuracies typically range from 1 to 5 centimeters.

It is crucial to maintain contact with the Reference Receiver throughout the survey to ensure accuracy.

Stop-and-Go

Stop-and-go surveying, also referred to as semi-kinematic GPS surveying, shares the same principle as kinematic surveying. However, the key difference lies in the fact that kinematic surveying does not require stops at unknown points. Stop-and-go surveying is expected to offer higher positional accuracy because errors are averaged out when the receiver pauses at the unknown points.

The survey begins with the determination of the initial integer ambiguity parameters, a process known as receiver initialization. Once this initialization is successfully completed, centimetre-level positioning accuracy can be achieved instantaneously. After initialization, the rover proceeds to the first unknown point, where data is collected for approximately 30 seconds. Without switching off, the rover then moves to the second point, and the procedure is repeated.

It is crucial to track at least four satellites during the movement; otherwise, the initialization process must be repeated by reoccupying the previous point.

GPS Field Survey Procedures

1) Receiver Setup:

• Set up the GPS receiver according to the manufacturer's specifications before starting any survey task.
• Mount base station antennas on a tripod and kinematic rover receivers and antennas on fixed-height range poles.
• If conducting real-time kinematic observations, establish radio or satellite communication links.

2) Antenna Setup:

• Calibrate and adjust all tribrachs used in the project before starting the survey.
• It is strongly recommended to use both optical plummets and standard plumb bobs to minimize centering errors, which are a significant source of error in all survey work, including GPS surveying.

3) Height of Instrument Measurements:

• Height of instrument (HI) refers to the accurate measurement of the distance between the GPS antenna and the reference point it is mounted on.
• Measure HI both before and after each observation session.

4) Observation Recording Procedures:

• Complete field recording books, log sheets, or log forms for each station and/or session.
• The level of detail in record keeping depends on the project. Typical data to include are:
• Project name or order number.
• Station designation and file number.
• Date, weather conditions, and start/stop times (local and UTC).
• Receiver, antenna, data recording unit, and tribrach make, model, and serial numbers.
• Antenna height measurements (vertical or diagonal) in inches (or feet) and meters (or centimeters).
• Space vehicle (SV) designations of observed satellites.
• Sketch of station location and approximate geodetic location and elevation.
• Any problems encountered.

5) Calibration and Initialization:

• For kinematic surveys, calibrate the base station to a known local coordinate point and reference datum.
• Some types of kinematic surveys require an initialization process, which should follow the manufacturer's recommendations.
• Clearly note these calibrations in the survey log records.

6) Processing and Verification:

• It is advisable to process and verify GPS data in the field when possible.
• This helps identify and rectify any issues encountered during the survey before leaving the field.

7) Session Designations:

• A survey session in GPS terminology refers to a single period of observations.
• Designate and input sessions and stations into the data collector using alphanumeric characters.
• Ensure station and session designations correlate with log form entries.
• Record the date of each survey session.
• Consider factors such as satellite visibility and site reconnaissance data when determining station/session designations.

8) Station Log Forms:

• Use standard bound field survey books or separate log/work sheets for recording station information.
• Ensure log forms capture all necessary details, including project name, agency, locality, observer, equipment details, and session data.

Procedure for Carrying Out a GPS Survey

GPS (Global Positioning System) survey is a method used to determine precise locations on the Earth's surface using signals from satellites. While there are various types of GPS receivers available, this guide focuses on the basic steps for using a handheld GPS device.

Familiarity with GPS Receiver Keys

• Power/Backlight Key: Used to turn the GPS unit on and off, as well as to adjust the backlighting.
• Up/Down/Right/Left Keys: Allow you to select different items on the screen by navigating through the menu.
• ENTER/MARK Key: Used to select or change highlighted items on the screen and to mark or record your current location.
• PAGE/COMPASS Key: Helps you move to different screens, such as the map or main menu, and close the on-screen keyboard.
• QUIT Key: Takes you to the previous screen or exits a page.

Steps for Using a Handheld GPS Receiver

Finding Your Location

• Turn on the GPS by pressing the POWER key. The main menu screen will appear.
• The GPS will scan for satellites and display a screen showing available satellites and battery power.
• Hold the GPS at eye level with an unobstructed view of the sky. Once the GPS fixes your location, the coordinates will be displayed.
• If the GPS cannot fix your location, check the datum and units settings, and move to a more open area.

Saving Your Location

• When the GPS displays your coordinates, you can save the location by pressing the MARK key.
• A new screen will appear showing a WAYPOINT number, which represents your saved location.
• To access saved waypoints later, turn on the GPS, press the PAGE key, select WAYPOINT LIST, and press ENTER.

Getting from One Point to Another

• To navigate to a saved waypoint, press the GOTO button, select the desired waypoint from the list, and press ENTER.
• A new screen will display the direction and distance to the selected waypoint.

Changing Units and Datum

• To change units or datum settings, go to the MAIN MENU, select unit SETUP, and adjust the units as needed.
• To change the datum, navigate to the datum settings and select the appropriate datum (e.g., WGS84 or India/Bangladesh).
• Press QUIT to return to the previous screen after making changes.

Check Your Progress II

What are the basic steps to find user location details using a GPS device?  

Applications of GPS

(a) Introduction

• GPS, or Global Positioning System, is known for its unique feature of providing positioning signals to users anywhere in the world, at any time.
• Initially developed for military purposes, GPS has found extensive applications in various fields such as navigation, surveying, and scientific research.

(b) Uses of GPS

• GPS is widely used in defence for navigation and positioning.
• It is also used in geo-science for measuring accurate time and frequency, which is essential for studies related to the ionosphere, atmosphere, global climate change, polar motion, and Earth rotation rate.
• GPS helps in mapping the gravity field, detecting seismo-ionospheric effects, and is increasingly becoming a part of various technologies and utilities.
• In India, GPS is being used by various organisations for a wide range of applications, and its use is expected to grow further.

Military Applications of GPS

GPS technology was initially created for real-time military positioning and navigation. It is widely used in military applications across airborne, marine, and land domains.

Uses in Military Aircraft

• Navigation: GPS helps military aircraft navigate accurately during missions.
• Target Designation: It assists in identifying and marking targets for precision strikes.
• Close Air Support: GPS guides aircraft providing support to ground troops by ensuring they strike the right targets.
• Weapon Technology: GPS is integral to the development and use of precision-guided munitions.
• Rendezvous: It helps in coordinating and timing rendezvous points for aircraft.

Uses in Military Ships and Submarines

• Navigation: GPS is used for navigating ships and submarines, ensuring they reach their destinations safely and efficiently.
• Positioning: It helps in marking positions for strategic purposes, such as setting up temporary bases or marking minefields.

Uses in Military Land Vehicles

• Navigation: Tanks, jeeps, and other land vehicles use GPS for accurate navigation, especially in unfamiliar and non-uniform terrain.
• Positioning: GPS helps in marking the positions of underground depots and other strategic locations.

Uses in Missiles

• Targeting: GPS is crucial for guiding missiles to their targets accurately, especially over complex terrain.

Marking Minefields and Depots

• Minefields: GPS is used to mark the positions of minefields, ensuring they are accurately recorded and can be avoided by friendly forces.
• Underground Depots: It helps in marking the locations of underground depots, making them easier to locate and use when needed.

Navigation

GPS technology significantly enhances navigation across various domains, including marine navigation, traffic routing, underwater surveying, and navigation in hazardous locations. Here are some key applications and benefits of GPS navigation:

• Marine Navigation: GPS aids in navigating ships and vessels, ensuring safe and efficient travel across water bodies.
• Traffic Routing: GPS is used in vehicles to determine the best routes, saving time and fuel. It provides real-time traffic updates and alternative routes.
• Underwater Surveying: GPS helps in mapping and surveying underwater features by scaling coordinates and entering waypoints.
• Hazardous Location Navigation: GPS facilitates navigation in dangerous or difficult-to-reach areas by providing accurate location data.
• Helicopter and Ship Navigation: GPS makes it easier for helicopters and ships to navigate to specific locations, reducing travel time and increasing efficiency.
• Commercial Fishing: Fishing fleets use GPS to locate optimal fishing spots and track fish migrations, improving catch efficiency.
• Automatic Vehicle Location: GPS is used in automatic vehicle location systems to track and monitor vehicles in real-time.
• In-Vehicle Navigation Systems: Many vehicles are equipped with GPS-based navigation systems that display the vehicle’s location on an electronic map, helping drivers stay on track and find new destinations.
• Route Planning for Delivery and Emergency Vehicles: GPS helps in planning and monitoring routes for delivery vans and emergency vehicles, ensuring timely and efficient service.

Surveying and Mapping with GPS Technology

GPS technology enables rapid and highly accurate surveying and mapping results while significantly reducing the equipment and labor hours required compared to conventional methods, particularly over vast areas for geo-resources. With GPS, precise three-dimensional positioning information can be obtained for both natural and artificial features, which can then be displayed on maps and models of various global elements such as mountains, rivers, forests, endangered animals, precious minerals, and other resources.

• Mapping Applications: GPS is utilized for mapping cut blocks, road alignments, and identifying environmental hazards such as landslides, forest fires, and oil spills.
• Cadastral Mapping: High-grade GPS receivers can be used for applications like cadastral mapping that require a high degree of accuracy.
• Continuous Kinematic Techniques: These techniques can be employed for topographic surveys and accurate linear mapping.

Remote Sensing and GIS

GPS position information is a crucial input for geographic information systems (GIS), which organize, store, manipulate, and display geographically referenced data. GPS has long been seen as a technology that enhances GIS operations.

• GPS positioning can also be integrated into remote sensing methods like photogrammetry, aerial scanning, and video technology for mapping geo-resources.
• In photogrammetry, GPS is used not only to determine the coordinates of ground reference points but also for aerial survey navigation and camera positioning.
• GPS has become an effective tool for GIS data capture, benefiting the GIS user community by providing accurate locational data in various applications.
• Field users can easily link GPS to a laptop or computer with appropriate software, ensuring data consistency with minimal distortion.
• The integration of GPS within a GIS environment is well-accepted for data collection, maintenance, and information management, aiding in the construction of accurate and timely GIS databases.

Aviation

Aviators worldwide utilize GPS technology to enhance flight safety and efficiency. Aircraft pilots rely on GPS for enroute navigation and airport approaches. Satellite navigation offers precise aircraft location information anywhere on or near the Earth. The continuous global coverage provided by GPS allows aircraft to fly directly from one point to another, as long as obstacle clearance and procedural requirements are met.

The integration of a data link enables the transmission of aircraft location to other aircraft and air traffic control. This capability is crucial for maintaining safety and improving air traffic management. GPS is continually expanding the availability of new and more efficient air routes, further optimizing flight operations.

Environment

GPS technology is instrumental in surveying disaster-stricken areas and tracking the movement of environmental phenomena such as forest fires, oil spills, and hurricanes. It enables the identification of locations that have been submerged or altered due to natural disasters. In land surveying, GPS has become the primary method for pinpointing sites in basic networks.

Role of GPS in Relief Efforts

• Global Disasters: GPS has played a crucial role in relief efforts for global disasters, such as the 2004 Indian Ocean tsunami.
• Aerial Studies: GPS technology is used to conduct aerial studies of some of the world’s most inaccessible wilderness areas. This helps evaluate wildlife, terrain, and human infrastructure in these regions.

Geodesy

• The GPS-based approach has become a widely used tool in geodetic studies of the Earth. For geodetic surveying, GPS is the most preferred positioning method due to its economy and ease of operation. High-grade GPS equipment can effectively carry out geodetic mapping and other control surveys.
• When using helicopters or in situations where the line of sight is not possible, GPS can set new standards of accuracy and productivity. It is used to conduct surveys (satellite geodesy) quickly and efficiently, achieving data accuracy within a millimeter.

Time Measurement Using GPS

• GPS not only provides information on longitude, latitude, and altitude but also offers highly accurate global time measurement.
• Each GPS satellite is equipped with multiple atomic clocks, which contribute precise time data to the GPS signals.
• GPS receivers decode these signals, synchronizing each receiver to the atomic clocks and allowing users to determine the time within 100 billionths of a second.
• This technology eliminates the need for individuals or organizations to own and operate expensive atomic clocks.
• Globally precise time measurements are crucial for synchronizing control and communication facilities. For example, wireless telephone and data networks use GPS time to keep all their base stations in perfect synchronization.
• This synchronization enables mobile handsets to share limited radio spectrum more efficiently, improving overall network performance.

Land Survey Applications

• GPS technology can be used for a wide range of activities in land surveying without competing against conventional techniques. The advantages and disadvantages of GPS for land surveying can be categorized into three classes based on the range of relative accuracies.
• Scientific Surveys (better than 1ppm): These surveys are conducted for precise engineering, deformation analysis, and geodynamic applications. They require high accuracy and are at the forefront of GPS technology development.
• Geodetic Surveys (1 to 10 ppm): These surveys involve the establishment, densification, and maintenance of control networks to support mapping. They fall within the geodetic category and represent a significant portion of GPS users.
• General Surveying (lower than 10 ppm): This category includes lower accuracy surveys for urban planning, cadastral work, geophysical prospecting, GIS, and other general-purpose mapping applications. Users in the geodetic and general surveying categories make up the majority of GPS users, while scientific category users drive the development of new technology and processing methods.

Activity

Activity 3.9: Measuring a Network using GPS Technology

Instructions: Look at the figure provided and list out the steps to measure a network (12345) by setting up two reference stations and using one rover to occupy the points.

Note: Ensure that your steps are clear and sequential for accurate measurement using GPS technology.

Summary

In this unit, we learned that:

• GPS technology can achieve high accuracies with relatively less effort, making GPS survey results generally more accurate.
• The main advantage of GPS surveys over conventional surveying techniques is that a line of sight does not need to be established between two stations.
• GPS surveys require different planning, execution, and processing techniques. Planning a GPS survey involves considering various parameters such as site or satellite configurations, the number and type of receivers to be used, and economic aspects.
• The applications of GPS are vast, ranging from surveying properties, shipping, aviation, and navigation to charting ocean depth, aerial surveys, agriculture, and forestry.
• GPS can save lives by preventing transportation accidents, aiding search and rescue efforts, and speeding up the delivery of emergency services and disaster relief.
• There are limitations to the usage of GPS units, which are primarily guided by location parameters.