The new GPS (Global Positioning System) devices launched have multiple and diverse functions. They have made the previous techniques employed not just obsolete but also inefficient economically, as they consumed much more time and effort. This research project intends to review Leica and Tremble satellite navigation systems and assess their relevance and level of assistance provided in various civil engineering jobs. The modes employed in this project will be two. First mode will be based upon research work and listing of the functions of these devices. The software of these devices will be analyzed. In the second mode the intention is to study more practical uses of these devices and collect stats of readings and data outputs from them. Furthermore categorize their outputs and rank their user interface based upon the ease of use.
The scope of this project will further cover understanding the need and demands of civil engineering jobs namely design surveys, boundary surveys, and construction staking. By working through these activities there will be gathering and enlisting of all the data and values required, for carrying out these jobs. Beyond this methodologies will be devised with which the GPS devices can be used to do these same jobs. Finally with these information step by step workflows to carry out these jobs using Leica and Tremble Satellite navigation systems will be formulated and their limitations will be stated.
To make a brief comparison on the use Satellite navigation system and the conventional system employed in civil engineering.
Introduction to Satellite Navigation System
Global positioning system GPS is the true application of the Satellite navigation system. It is a marvel of modern age that makes possible the determination of position of any point on or near the surface of the earth, to a level of accuracy unimagined before.
GPS comprises of three groups of satellites. These groups work in isolation to each other and are sometimes referred to as satellite constellations. These are the NAVSTAR global positioning system, GLONASS global positioning system and the Galileo global positioning system.1
NAVSTAR (NAVigation Satellite Timing And Ranging) is at present the single fully operational positioning system, and is owned and maintained by the United States. 2
GLONASS is the GPS system developed by Russia. Like the US NAVSTAR it comprises of a satellite constellation which continuously transmits coded signals all over the world to users who can receive them on their GPS devices to obtain their position and velocity. GLONASS, which stands for The Global Navigation Satellite System was completed in 1995 but soon after completion, the system quickly fell into a bad condition with the collapse of the Russian financial system. Starting from 2001, Russia took it upon itself to make the GPS system functional again. Moreover, in the more recent years it has brought in the Indian government as a partner, and sped up the program with an objective of restoring worldwide coverage as early as possible.3
GALILEO is the European GPS system, being developed by the EU, but has faced many problem since its inception. Galileo is intended to provide more precise measurements than available through GPS or GLONASS (Galileo will be accurate down to the meter range) including the height (altitude) above sea level, and better positioning services at high latitudes.4
1 http://en.wikipedia.org/wiki/Introduction_to_the_Global_Positioning_System
2 "Federal Special-Purpose Program" Global Navigation system. Russian Federal Govt. 2001-08-20
3 ICAO Completes Fact-Finding Investigation. International Civil Aviation Organization: 2008-09-15.
4 http://news.bbc.co.uk/2/hi/science/nature/7120041.stm
As the only fully operational GPS system is the US NAVSTAR, the discussion will be more or less limited to it.
Use of Satellite Navigation in Civil Engineering
The use of satellite navigation in civil engineering has brought revolution in the field of civil engineering. It has made the tasks of surveying and setting out much less time consuming and economical.
In the field of Civil engineering, Satellite navigation makes its use in the following areas;
Setting up roads and railways alignments
Oil and gas pipelines alignment and passages
Alignment of transmission lines
Establishing high accuracy controls
Densifying existing control networks
Setting out structures (Construction Staking)
Boundary surveys
Design Surveys
It must be noted that not all civil engineering related surveying jobs are simplified with the use of satellites. The reason is the initial high cost. Most of small construction jobs, spanning over a limited area or in an urban setting might not need satellite navigation at all, because bench marks are available nearby. But tasks such as oil or gas pipelines, high rise construction and the likes are highly benefitted from the modern techniques.5
5 http://www.gmat.unsw.edu.au/snap/gps/gps_survey/principles_gps.htm
Conventional Systems in Civil Engineering
Before the advent of the modern electronic surveying instruments, coupled with GPS, the Boundary and design surveys and construction staking were carried out using the conventional techniques and approaches.
For the most part, the benchmarks established by the federal agencies were used is the main source of reference and then distances and level were found out using conventional methods. Although there is no particular classification as to what is a Conventional or Modern system, Conventional systems might include
Tapes and right triangles
Theodolites and Stadia Measurement
Electronic Distance Measurement, EDM
Tapes and Right Angles
The survey using tapes and employing right triangles using tapes are the simplest and cheapest sort of surveys. Well-liked in the years around 1950s, tape surveys while being accurate for distance, were considerably deficient in their accuracy of reading angles and bearings. This problem was resolved by using bearing compasses and other similar instruments.
Theodolites and Stadia Measurement
After the tapes, the Engineer's Transit and subsequently, Theodolites were born. These Theodolites were able to measure the angles between two points quite accurately, but were not able to measure the distances. Therefore, the technique of Stadia Measurement was developed and the suppliers started producing Theodolites which had stadia hair above and below the cross-hair and also gave the multiplying and additive constants required to measure the distance using the stadia hair and the leveling staff, right from a single theodolite.
But there were great problems associated with the accuracies. For instance, with an instrument having a multiplicative constant of 100, a difference of 1cm meant an error of 1m in the distance reading.
Electronic Distance Measurement
Then came the era of EDMs, popularly known as distomats, these were add-in devices which were mountable on top of a theodolite, and used infrared or other waves to measure the distances. The prism of the EDM instrument had to be placed, and depending upon the instrument, the mean of several readings will give the distance. But as with all systems, there were problems associated with it. The problem of carrying and maintain, two different instruments hindered with the progress of the work. Also, there was no combined interface of the two and both had to be operated simultaneously to get the angles and the distances. These problems were removed in the instrument we now call the Total Station.
As clear from the above discussion, the methodologies suited well to the small scale surveys, but when the job of the survey became more demanding, these methods were simply not enough. The biggest problem was of relating the work with a suitable permanent benchmark. Many a times, the benchmarks of suitable accuracy were at far off places, and building and other features made it almost impossible to have a direct sight through the benchmark. These problems gave birth to the use of GPS in the field of surveying.6
6 Surveying techniques: http://en.wikipedia.org/wiki/Surveying
Satellite Based Navigation
Now as discussed earlier the currently operational GPS is NAVSTAR. NAVSTAR GPS is an acronym for NAVigation System with Time And Ranging Global Positioning System.
GPS was originally designed by the USA for military use so that position could be calculated from anywhere and at any time from the surface of the earth. This allowed the US army to navigate in various areas with ease. In 1983, After Korean Air Lines Flight 007 was shot down by USSR, after straying into their prohibited airspace, President Ronald Reagan of the US issued orders that after sufficient development of GPS has taken place it should be available civilians. The first satellite of US NAVSTAR was launched in 1989, and the 24th and last satellite was launched in 1994. Hence the GPS system came into civilian use. The applications which were to emerge in the civilian use of GPS technology were marine navigation and surveying. 7
The whole GPS arrangement includes three discrete segments:
The Space Segment - It consists of the GPS satellites transmitting the signals to the earth
The Control Segment - It consists of Stations located near the Earth's equator to manage the satellites
The User Segment - Users of GPS Devices who receive the GPS signal. 8
7 United States Updates Global Positioning System Technology". America.gov. February 3, 2006.
8 John Pike. "GPS III Operational Control Segment (OCX)". Globalsecurity.org. Retrieved 2009-12-08.
Basic Working of a GPS
The GPS device basically locates its position by receiving radio waves from the GPS satellites. These GPS satellites continuously send out messages which carry the following information
The Precise time at which the message is sent.
Information on the exact location of the satellite in the orbital
The common system health and rough orbits of all of the GPS satellites.
The GPS device which receives this message calculates the transit time using which the device calculates the distance between it and the satellite. 9
Calculating Distance:
To calculate the distance of the point from the satellite the relationship used is
Distance = Velocity x Time
Where, Distance is between the satellite and GPS device
Velocity is that of a radio signal, which is equal to the speed of light.
Time is the transit time for the satellite signal to reach the receiver.
This method is very simple and it is the same as calculating distance of an object travelling at a constant velocity. It is derived from the relation s = ut + ½at2
As the acceleration is zero the distance is simply computed by the product of speed and time. 10
Calculating position:
To calculate the position the first step is to calculate the distance of the GPS device from the satellite. This is done by the method mentioned above. The number of satellites required to compute the position are at the minimum three. The basic ideas which are used are resection and trilateration. This is a method which allows you to determine your position if your distance from three objects and their respective location are known. If the distance from one satellite is known then the location of the GPS device must lie on an imaginary sphere with radius equal to the computed distance and centre being at the location of the satellite. The location of receiver can be computed by finding the intersection of three spheres formed around the three satellites from which the receiver is obtaining signals.11
9 "NAVSTAR GPS User Equipment Introduction" (PDF). US Government.
10 3.1.1 Introduction to GPS (Global Positioning System) version 1. Leica.
11 3.1.2 Introduction to GPS (Global Positioning System) version 1. Leica.
But GPS devices do not restrict themselves to use only three satellite signals for computation. Although three satellites can have intersecting signals at one point on the surface of the earth but we also have to consider the accuracy of the transit time. The satellite signals are radio waves which travel at the speed of light. When computing the distance the product of time and speed of light is taken. For a very small difference in the value of time the difference in distance is much larger. It is for this reason that many GPS devices obtain signals from four or more satellites to compute the position and time.
For normal operation of GPS devices four satellites are used. This amount can be lesser if a variable is known e.g. latitude, longitude or elevation. 12
12 "GPS Support Notes" (PDF). January 19, 2007. Retrieved 2008-11-10.
Sources of Errors:
1. Ionospheric and atmospheric delays
The satellite signals travel at the speed of light and are also electromagnetic waves. During their travel through a vacuum they are unaffected in terms of direction. But when they travel from one medium into another they tend to change direction and speed. This phenomenon is known as refraction. When GPS signals pass into the ionosphere they are refracted and hence a source of error is created in the GPS reading.
2. Satellite and Receiver Clock Errors
The satellites are fitted with atomic clocks and have an extremely high accuracy. But even though the clocks in the satellite are very accurate (to about 3 nanoseconds), they do from time to time vary by a small value and create minor errors, affecting the accuracy of the position.
3. Multipath
The path from which a signal comes is very important in calculating the position by a GPS device. If there is an object close to the GPS antenna which can reflect the signals then the path followed by the signal will not be direct but will strike the object first. This phenomenon is known as Multipath and it introduces an error in the measurement of the GPS device.
4. Dilution of Precision
If the satellites are close to each other then the overlap area of their signals will be large and will give a less precise value. The further apart they are the less length will be of the overlapping signals hence more precise will be the value.
5. Selective Availability (S/A)
US Department of Defense which controls the GPS satellite for security purposes and to prevent the civilians from having high accuracy in traduce errors in the position being calculated by a GPS device. This process is termed as selective availability and is done by two methods.
The clock transmitting time from the satellite undergoes a procedure which is known as dithering. This procedure introduces a slight variation in the time being transmitted by the satellite clock.
The second source of error generated is by aberrance of data in regard to the position of the satellite. The transmitted value of the location of the satellite has a minor difference from the actual location.
6. Anti Spoofing (A-S)
Every GPS signal carries a P-code, which cannot be decoded by GPS devices available to the civilians. This feature is Anti Spoofing and is a based on the process of Selective availability, so that enemy nations cannot have accurate values of location. This P-code allows far better calculation of range and only the military, equipped with special GPS receivers can read this encrypted P-code. 13
13 3.1.3 Introduction to GPS (Global Positioning System) version 1. Leica.
Differences
Satellite based Surveying
Conventional Surveying
1
Inter-visibility between points is not required.
Inter-visibility is either necessary or special techniques have to be employed.
2
Can be used at any time of the day or night and in any weather.(see 6)
Can only be used in day light.
3
Produces results with very high geodetic accuracy.
The accuracy is lesser because of larger number of equipment and personnel are involved.
4
More work can be accomplished in less time with fewer people.
Expensive and time consuming.
5
Tree cover may impair the work.
Tree cover does not effects
6
Depends upon the availability of enough number of satellites for accurate measurement, over which no control is present
Independent of location of satellites etc,
7
Because of automatic data logging, large amount of data can be managed very easily and put to use very effectively.
Manual data recording and manipulation is highly time and effort consuming.
14 The Numerous Advantages of a GPS Tracking Device by Jeffery Dodd.
1.2. To assess some of the brands of GPS devices and compare the features provided.
List of some top brands of GPS used for surveys:
Leica
Trimble
CMT
SOKKIA
The focus of this project is Leica and Trimble so the assessment of the features will be limited to these two.
Leica SR 53015
Trimble
Receiver Type
Dual Frequency, Geodetic Real-Time RTK receiver.
Measuring Modes
Static, rapid static, kinematic, On the fly L1 + L2, code, phase
Real-time RTK standard
Software
Post processing software
SKI-Pro
DGPS/RTCM standard
Applications
Survey, geodetic and
Real-Time RTK Applications
Channels
12 L1 + 12 L2.
Receiver Technology
Cleartrak - patented.
Multi-bit, SAW filters
Tracking
Good even to low satellites and in adverse conditions.
Interference resistant.
Multipath
Multipath mitigation.
Time to first phase measurement after switching ON
30 seconds
Supply voltage
Nominal 12V DC
Power consumption
7W, receiver with terminal.
Dimensions
205mm x 165mm x 72mm
Weight
(receiver only)
1.25kg
Measurement precision with AS off or on
Carrier phase
on L1: 0.2mm rms
on L2: 0.2mm rms
Code (pseudo range)
on L1: 5cm rms
on L2: 5cm rms
Display type and size
LCD, 12 lines, 32 characters per line
Display, illumination/contrast
Illumination, variable contrast
Keyboard
Full alphanumeric, function keys, user-definable keys
Manual operation with TR500 terminal
Standard method. Receiver control, operation, data input, survey-data acquisition, information display via terminal
Navigation
Full navigation information in position and stakeout displays. Position, course, speed, bearing and distance to waypoint
15 Leica GPS equipment User Manual
Understanding These Features
Static Surveys: This is a time consuming method which gives a very high accuracy of position. In this type of survey there is a complete system of baselines which are composed of sets of sessions and observations along with numerous receivers and baselines. This GPS device allows this type of survey and also provides for the requirement of post-processing software for this data.16
A Real Time Kinematic GPS In this type of survey there are at least two GPS devices one out of which has a fixed location and this device is named Control Survey Marker. The other GPS device is termed as Rover and has the freedom to move around the field. While conducting this survey observations are made from the control survey marker to the rover and then the corrections are sent to the rover. 17
Channels: The larger the number of channels greater will be the satellites which will be able to pair with the device and hence increase the accuracy of the calculation of the position. 18
Cleartrak: Cleartrak is the term employed by Leica to express its advanced receiver technology, which minimizes the following errors in the measurement by the GPS:
Anti-Spoofing
Multi-path
Interfering Signals
Planned change of satellite signal.19
16 GPS Newsletter by Leica Geo-systems
17 http://www.precisionag.org/PDF/ch2.pdf
18 http://en.wikipedia.org/wiki/Introduction_to_the_Global_Positioning_System
19 http://www.haselbachinstruments.com/pdf/clrtrak.pdf
Navigation: Latitude, Longitude and elevation are provided up to an accuracy of just a few mm and other useful values like angles and direction to a waypoint are provided to assist in conducting surveys.
1.3. To review the software used and data output given by the devices.
Leica SR530
Device Technology
ClearTrak
Software Employed:
SKI-Pro
ClearTrakâ„¢
This term is a trademark of Leica Geosystems for its set of technologies it provides in its GPS devices to enhance the receiver accuracy.
As discussed earlier the error sources of A GPS device include the following four issues:
Anti-Spoofing (AS).
Multipath Signals
Interfering signals
Planned changes in the satellite signal.1
ClearTrak minimizes these error sources to provide high accuracy to the user. In this project we will analyze the solutions provided by ClearTrak one-by-one.
Anti-Spoofing Solution
GPS devices have been altered from the past in many ways. Older GPS devices used single frequency but that practice was soon changed as there were a lot of factors which caused the data to be aberrant. Refraction from ionosphere and other ambiguities were problems associated with single frequency receivers. The advent of dual frequency eliminated the problem of ionospheric refraction and minimized the ambiguities. At this time for strategic and military reasons the US government created a Y Code which encrypted the P-Code. This encryption was not made public and only the military had access to it. This meant Civilians because of the encryption could not gain access to the L2 code at all. The difference in the two frequencies L1 and L2 could not be found and hence the large amount of ambiguities present in the readings caused aberrant data. The Jet propulsion laboratory found a solution and provided a weak yet correct value of L2 frequency, hence allowing access to the civilians. But this signal was weak and many times the Code could not be received from low satellites and due to signal interference.
Leica developed a more efficient method which was named code aiding. Leica GPS devices actually track the P Code. Using this P-Code and a large dish antenna Leica found out that the Y Code was a multiple of a very slow encryption code and P-Code. It equipped its devices with pre-processing software which processes the Y code and hence receives the L2 frequency. This massive improvement in GPS technology was patented by Leica and was first introduced in 1988. This method of code aiding was improved and it now gives full wavelength phase measurement.
This technology was made part of the ClearTrak technology suite. And it provides the best reception of L2 signals in the market.1
1. Euler, H. J., Ziegler, C., "New Real-Time Processing Strategies in Leica's System 500", Leica Geosystems Technical Paper, March, 1999.
Multipath Mitigation Technology2
Multipath has been explained previously in this project as a major source of error in the GPS reading. As the name applies the GPS receiver not only receives satellite signals from a direct path but also obtains the same signals from indirect paths such as paths created by the reflection of signal from the ground or close by buildings. The receiver has to process all of these signals and an error is introduced. This error is equal to the difference between reading from only one direct signal and the reading from all signals combined.
To differentiate between the two signals we need to realize that the multipath signals have to travel a longer distance than the actual signal and hence take a longer time than the actual signal. The methods employed for Multipath mitigation i.e. to reduce the impact of multipath signals are four:
Antenna characteristics,
Multipath estimation with multiple correlators,
Filtering by carrier aided code smoothing
Use of reduced width correlators (RWC).
1. Antenna characteristics employ three characteristics for multipath mitigation:
Antennas have a high front-to-back gain ratio
Good circular polarization
Antennas respond very weakly to low elevation angles.
To obtain multipath mitigation Leica employs two optimized antenna characteristics.
2 Hatch, R. R., "The Synergism of GPS Code and Carrier Measurements",
2. Multipath estimation with multiple correlators
Leica Geosystems does not employ this technology for two reasons:
A large number of correlators are required for every satellite so that the major multipath signals can be restricted through a process of modeling. Therefore making the system highly complex and expensive.
Leica has developed an alternative which is cheaper, less complex and provides if not better then equally good results.
3. Filtering by carrier aided code smoothing
The objective of this technique is to very accurately observe the difference in the pseudo range by carrier phase measurements. This removes any errors which otherwise result from the motion of the satellite, the user and the oscillator drift. Leica employs this technology and was the inventor of this technique but did not patent it.
4. Use of reduced width correlators (RWC).
Initially wide correlators were employed which tracked the P code and C/A code. Later it was observed that employing reduced width correlator the multipath signals were greatly minimized after the initial signal was received. Leica has developed on this technology and created MMC i.e. Multipath mitigation correlators which have a significantly higher accuracy than the normal RWCs. According to the claims of Leica the errors for MMC are "one quarter that of the 10% RWC". The use of this technology is yet to be seen in the latest Leica devices which are to be launched.
3 Amoroso, F., "Adaptive A/D Converter to Suppress CW Interference in DSPN Spread Spectrum Communications", IEEE Transactions on Communications, October 1983.
ClearTrak Interference Protection4
The distance travelled by a GPS signal is 20200 km. at this distance the signal is extremely weak. To improve this there are three methods which are adopted.
Spread Spectrum Modulation
This method improves the quality of the C/A signal by 1000 times.
Spectrum Allocation
The GPS frequencies have been classified by governments. L1 frequency is allocated to navigation and is fixed. L2 frequency on the other hand is subjected to variation. This can cause an error introduction and the govt. will provide provision of a third frequency for the civilian use to minimize the error.
GPS Device design
Leica employs two techniques:
SAW filters
SAW filters, otherwise also termed as Brick Wall, cut of signals outside the centre band. This reduces the interfering signals which are out of band.
Adaptive, Signal Sampling
This minimizes in band interfering signals and provides more accurate measurements.
4 Maenpa, J., Balodis, M., Sandholzer, J., Walter, G., "New Interference Rejection
Technology from Leica", Proceedings of the 10th International Technical Meeting of the Institute of Navigation, ION GPS-97, Kansas City, Missouri, September, 1997
ClearTrak Provides Future GPS Signal Compatibility
The US govt. has announced the GPS satellites will be modified in a few years to broadcast a C/A code in the L2 frequency. Leica devices are already made to be compatible with this change and its technology of ClearTrak will have all of its features applicable with the changes.
SKI Pro
1.4. To enlist and describe the various applications of these GPS devices in Civil Engineering.
