Global Positioning Systems (GPS) where originally developed by the US military starting on 1973 with the first satellites being launched in 1978. Originally GPS was limited to military use only but is now accessible to anyone with a GPS receiver. The accuracies that may be obtained from the system depend on the degree of the user’s available access and the complexity of GPS receiver being used. Using survey grade equipment in a static position with proper technique millimetre level accuracy can be achieved anywhere on or above earth as well as at sea. GPS technology is readily available and used as part of most people’s daily lives. GPS has become a part of daily life to many sectors of business including construction machine control, heavy haul trucking fleet management, cargo ships as well as most new vehicles and smart phones for personal navigation. Naturally GPS has been well adapted for Land Surveying and Construction Surveying. Land Surveyors GPS instruments and equipment are significantly more accurate than that of a consumer grade handheld unit. GPS equipment is an indispensable tool for any Land Surveyors and Construction Surveying.
How GPS works in a nutshell.
The GPS Navigation System relies on satellites that continually broadcast their location in space and maybe thought of as a control station in space. GPS receivers use a clock exactly synchronized to the GPS clock system and precisely measure the time delay of a satellite GPS time of transmission to the time the users GPS receiver detects the signal. Time delay multiplied by the mean speed of light along the path of transmission from the satellite to the users GPS receiver will give a range from the satellites known position. The users GPS receiver will have an error or offset so the user is not able to directly measure the range to any satellite, but the ‘Pseudo Range’ which is the actual range with an unknown but instantaneously fixed offset. Pseudo Range is the clock error times the speed of light. If three ranges are simultaneously observed the users 3-dimensional position can be calculated through triangulation.
There are 4 unknown parameters that need to be solved for a navigation solution. The three coordinates and the receivers clock offset. At least 4 satellites must always be visible with the GPS received wherever the user is on or above the earth. These satellites must be in good geometric configuration relative to the position of the receiver. Below is a basic overview of some of the pros and cons of how GPS can be used in Construction and Land Surveying.
GPS offers the following advantages over conventional surveying:
- The results from measuring a single line, also referred to as a baseline will yield the distance between the start and end stations as well as well as their component parts such as Northing/Easting and Height or Latitude/Longitude and Height.
- No line of sight is required between stations unlike conventional surveying where all stations must have a clear line of sight between a foresight and back sight station. Although a clear sky view is required for the GPS receivers to have a clear path to the satellites.
- GPS surveying equipment is extremely weatherproof so observations may occur in any weather, day or night. Even thick fog will not affect GPS measurements.
- GPS receivers and current survey data collection software allow for one-person operation. This saves significant time and money.
- GPS surveyors do not require a significant amount of technical knowledge for basic operation.
- Accurate GPS measurements may take place anywhere on land, sea or air.
- Baselines of hundreds of kilometers maybe measured eliminating the need for very extensive geodetic survey networks of conventional survey observations.
- Continuous measurement observations may be carried out for real time deformation monitoring on structures such as buildings, dams, bridges, overpasses and landslides.
GPS is not a solution for every survey task and maybe subject to the following difficulties:
- A clear sky view is required so that satellites can be observed and tracked.
- GPS surveying cannot take place indoors, underwater or underground.
- Survey grade receivers are very expensive sometimes as much as 2-3 Total Stations depending on the manufacturer.
- The value of GPS heights is not immediately understood and must be understood by the survey technician. The coordinate system of GPS is Earth mass centered, so any position on the earths surface will be relative to some arbitrary defined datum, such as an ellipsoid surface. If the height above the Geoid or Mean Sea Level is required then the separation between then the separation between the chosen ellipsoid and geoid model will have to be calculated. Some data collection and survey software may have a model to help solve this problem.
GPS Systems can broadly be divided into three segments,
- The Space Segment-The Space Segment consists of satellites om an almost constant orbit at a height of approximately 20,200 kilometres above the earth. This is about three times the radius of the earth and orbit the earth in just under 12 hours. The six orbital planes are equally spaced at 55 degrees to the equator. A satellite may appear for up to 5 hours above the horizon. The GPS constellation is designed so a minimum of 4 satellites are always in view at least 25 degrees above the horizon. Each satellite has an antenna that transmits the satellites position to the user. Each satellite carries two rubidium and two caesium atomic clocks to ensure precise timing. GPS receivers broadcast on two L Band carrier frequencies, L1 and L2. The carriers are phase modulated to carry 2 codes known as the P code, Precise code or Precise Positioning Service as well as the C/A or Course/Acquisition code or SPS (Standard Positioning Service). It is relatively simple for the GPS equipment to obtain lock onto the C/A code as it short, simple and repeats 1000 times per second. The P code is long and complex and this is the key to selective access. Only users approved by the US Department of Defence can utilize the P code.
- Control Segment-The control segment is a network of ground stations that continually monitor the velocity and shape of satellites orbit. Monitoring Stations are remote stations each with a very precise GPS receiver, clock, meteorological sensors, data processing and communication. Their function is to is to observe and broadcast satellite navigation messages and satellite clock errors and drifts.
- User segment-The user segment consists of a GPS receiver with a processor. The processor is basically a microcomputer containing software for processing field data.
A typical GPS equipment setup for Land Surveying and Construction Surveying typically includes 2-GPS receivers (a base and rover), a tripod to setup the base GPS receiver, a survey range pole to mount the rover GPS receiver and a data collector, similar to a handheld computer with software. This is a standard GPS instrument setup for land surveys in Kelowna.
GPS surveying is a great option for many land surveying tasks in the Kelowna area due to our undulating topography and densely forested areas. For more information on how GPS can be used for Land Surveying in the Kelowna area please consult with a registered British Columbia or Canadian Land Surveyor.