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Global Positioning System

Introduction
The Global Positioning System (GPS) is a burgeoning technology, which provides unequalled accuracy and flexibility of positioning for navigation, surveying and GIS data capture. The GPS NAVSTAR (Navigation Satellite timing and Ranging Global Positioning System) is a satellite-based navigation, timing and positioning system. The GPS provides continuous three-dimensional positioning 24 hrs a day throughout the world. The technology seems to be beneficiary to the GPS user community in terms of obtaining accurate data upto about100 meters for navigation, metre-level for mapping, and down to millimetre level for geodetic positioning. The GPS technology has tremendous amount of applications in GIS data collection, surveying, and mapping.

Geopositioning -- Basic Concepts
By positioning we understand the determination of stationary or moving objects. These can be determined as follows:
  1. In relation to a well-defined coordinate system, usually by three coordinate values and
  2. In relation to other point, taking one point as the origin of a local coordinate system.
The first mode of positioning is known as point positioning, the second as relative positioning. If the object to be positioned is stationary, we term it as static positioning. When the object is moving, we call it kinematic positioning. Usually, the static positioning is used in surveying and the kinematic position in navigation.

Some Interesting Links :
  1. GPS Basics
    An Introduction to GPS by GPS Scales Waypoint Enterprises
  2. Introduction to GPS
    An article submitted by Mark Bohrer
  3. How GPS Works
    An Introduction by Craig Haggart
  4. Global Positioning Systems
    An Overview by The Geographer's Craft


GPS - Components and Basic Facts
The GPS uses satellites and computers to compute positions anywhere on earth. The GPS is based on satellite ranging. That means the position on the earth is determined by measuring the distance from a group of satellites in space. The basic principle behind GPS are really simple, even though the system employs some of the most high-tech equipment ever developed. In order to understand GPS basics, the system can be categorised into

FIVE logical Steps
  1. Triangulation from the satellite is the basis of the system.
  2. To triangulate, the GPS measures the distance using the travel time of the radio message.
  3. To measure travel time, the GPS need a very accurate clock.
  4. Once the distance to a satellite is known, then we need to know where the satellite is in space.
  5. As the GPS signal travels through the ionosphere and the earth's atmosphere, the signal is delayed.
To compute a positions in three dimensions. We need to have four satellite measurements. The GPS uses a trigonometric approach to calculate the positions, The GPS satellites are so high up that their orbits are very predictable and each of the satellites is equipped with a very accurate atomic clock.

Components of a GPS
The GPS is divided into three major components
  • The Control Segment
  • The Space Segments
  • The User Segment
The Control Segment
The Control Segment consists of five monitoring stations (Colorado Springs, Ascesion Island, Diego Garcia, Hawaii, and Kwajalein Island). Three of the stations (Ascension, Diego Garcia, and Kwajalein) serve as uplink installations, capable of transmitting data to the satellites, including new ephemerides (satellite positions as a function of time), clock corrections, and other broadcast message data, while Colorado Springs serves as the master control station. The Control Segment is the sole responsibility of the DoD who undertakes construction, launching, maintenance, and virtually constant performance monitoring of all GPS satellites.

The DOD monitoring stations track all GPS signals for use in controlling the satellites and predicting their orbits. Meteorological data also are collected at the monitoring stations, permitting the most accurate evaluation of tropospheric delays of GPS signals. Satellite tracking data from the monitoring stations are transmitted to the master control station for processing. This processing involves the computation of satellite ephemerides and satellite clock corrections. The master station controls orbital corrections, when any satellite strays too far from its assigned position, and necessary repositioning to compensate for unhealthy (not fully functioning) satellites.

The Space Segment
The Space Segment consists of the Constellation of NAVASTAR earth orbiting satellites. The current Defence Department plan calls for a full constellation of 24 Block II satellites (21 operational and 3 in-orbit spares). The satellites are arrayed in 6 orbital planes, inclined 55 degrees to the equator. They orbit at altitudes of about 12000, miles each, with orbital periods of 12 sidereal hours (i.e., determined by or from the stars), or approximately one half of the earth's periods, approximately 12 hours of 3-D position fixes. The next block of satellites is called Block IIR, and they will provide improved reliability and have a capacity of ranging between satellites, which will increase the orbital accuracy. Each satellite contains four precise atomic clocks (Rubidium and Cesium standards) and has a microprocessor on board for limited self-monitoring and data processing. The satellites are equipped with thrusters which can be used to maintain or modify their orbits.

The User Segment
The user segment is a total user and supplier community, both civilian and military. The User Segment consists of all earth-based GPS receivers. Receivers vary greatly in size and complexity, though the basic design is rather simple. The typical receiver is composed of an antenna and preamplifier, radio signal microprocessor, control and display device, data recording unit, and power supply. The GPS receiver decodes the timing signals from the 'visible' satellites (four or more) and, having calculated their distances, computes its own latitude, longitude, elevation, and time. This is a continuous process and generally the position is updated on a second-by-second basis, output to the receiver display device and, if the receiver display device and, if the receiver provides data capture capabilities, stored by the receiver-logging unit.

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GPS Positioning Types

Absolute Positioning
The mode of positioning relies upon a single receiver station. It is also referred to as 'stand-alone' GPS, because, unlike differential positioning, ranging is carried out strictly between the satellite and the receiver station, not on a ground-based reference station that assists with the computation of error corrections. As a result, the positions derived in absolute mode are subject to the unmitigated errors inherent in satellite positioning. Overall accuracy of absolute positioning is considered to be no greater than 50 meters at best by Ackroyd and Lorimer and to be + 100 meter accuracy by the U.S. Army Corps of Engineers.

Differential Positioning
Relative or Differential GPS carries the triangulation principles one step further, with a second receiver at a known reference point. To further facilitate determination of a point's position, relative to the known earth surface point, this configuration demands collection of an error-correcting message from the reference receiver.

Differential-mode positioning relies upon an established control point. The reference station is placed on the control point, a triangulated position, the control point coordinate. This allows for a correction factor to be calculated and applied to other roving GPS units used in the same area and in the same time series. Inaccuracies in the control point's coordinate are directly additive to errors inherent in the satellite positioning process. Error corrections derived by the reference station vary rapidly, as the factors propagating position errors are not static over time. This error correction allows for a considerable amount of error of error to be negated, potentially as much as 90 percent


GPS setup


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Accuracy of GPS?
There are four basic levels of accuracy - or types of solutions - you can obtain with your real-time GPS mining system:

Autonomous Accuracy 15 - 100 meters 
Differential GPS (DGPS)  Accuracy 0.5 - 5 meters
Real-Time Kinematic Float (RTK Float)  Accuracy  20cm - 1 meter
Real-Time Kinematic Fixed (RTK Fixed) Accuracy 1cm - 5 cm


GPS satellites broadcast on three different frequencies, and each frequency (or career wave) has some information or codes on it. You can think of it as three different radio stations broadcasting several different programs. The table below lists the signals and the contents:

L1 Career L2 Career L3 Career
19 cm wavelength 24 cm wavelength Data not available
1575.42 M Hz 1227.6 M Hz
C/A Code P Code
Navigation Navigation Message
  • P Code : Reserved for direct use only by the military
  • C/A Code : Used for rougher positioning
  • For Single frequency use only L1 career is used
  • For Double frequency, L1/L2/L3 career is used
  • The navigation message (usually referred to as the ephemeris) tells us where the satellites are located, in a special coordinate system called WGS-84. If you know where the satellites are at any given time, then you can compute your location here on earth.
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Different types of answers given by a GPS.

Autonomous Positions
Uses……….. C/A code only
Requires….. Only one receiver
Data from at least four satellites
Provides…… An accuracy range of about 15 - 100 meters

This solution is designed for people who just need an approximate location on the earth, such as a boat at sea or a hiker in the mountains.

Real-Time Differential GPS (DGPS) Positions
Uses……….. C/A code only
Requires….. Two receivers
A radio link between the two receivers
  • Reference receiver at a known location broadcasts RTCM (Radio Technical Commission for Maritime Services) corrections.
  • Rover receiver applies corrections for improved GPS positions
Data from at least four satellites - the same four at both the references and rover (common satellites) Provides…… An accuracy range of about 0.5 - 5 meters depending upon the quality of receiver and antennae used.

This solution gives much better results because here we have a known position at a reference receiver. However it must have a radio link between the reference receiver and the roving (moving) receiver.

Real Time Kinematic (RTK) Float Positions
Uses……….. C/A code and career waves.
Requires….. Two receivers

  • Reference receiver at a known location tracks satellites and then broadcasts this satellite data over a radio link in a format called CMR. (CMR is a Trimbleβ - defined format)
  • Rover receiver receives data from both the satellites and the reference station.
A radio link between the two receivers.
Data from atleast four common satellites.

Provides…… An accuracy range of about 20 cm to 1 meters.
This solution uses more of the satellite signal than the autonomous or DGPS solution. The CMR data is carrier phase data. The float solution is actually an intermediate step towards the most precise answer, which we'll discuss next.

Real Time Kinematic (RTK) Fixed Solutions
Uses……….. C/A code and career waves.

Requires….. Two receivers
  • Reference receiver at a known location tracks satellites and then broadcasts CMR data over a radio link.
  • Rover receiver receives data from both the satellites and the reference station.
A radio link between the two receivers.

Initialization, which is achieved most easily with dual-frequency receivers. Data from at least five common satellites to initialize on-the-fly (in motion) Tracking at least four common satellites after initializing.

Provides…… An accuracy range of about 1 - 5 cm.

We noticed that with each increasing level of precision, there are more requirements. The most important unique requirement for the RTK fixed solution is something called an initialization. Here it is not feasible to explain what's happening in an initialization, but it is relevant to mention that initialization is necessary to work at centi-meter level accuracy. Dual frequency receivers can perform this process automatically.

If the receiver looses the initialization - which can happen if it fails to track enough satellites - then your working accuracy will drop to the float solutions status temporarily. Remember, however both of these solutions require a radio link to your reference receiver. If, for any reason, you loose your radio link, you will drop back to the autonomous level - the least precise - until the radio link is regained.

Factors that affect GPS
There are a number of potential error sources that affect either the GPS signal directly or your ability to produce optimal results:
  • Number of satellites - minimum number required:
    You must track atleast four common satellites - the same four satellites - at both the reference receiver and rover for either DGPS or RTK solutions. Also to achieve centimeter -level accuracy, remember you must have a fifth satellite for on-the fly RTK initialization. This extra satellite adds a check on the internal calculation. Any additional satellites beyond five provide even more checks, which is always useful.

  • Multipath - reflection of GPS signals near the antennae:
    Multipath is simply reflection of signals similar to the phenomenon of ghosting on our television screen. GPS signals may be reflected by surfaces near the antennae, causing error in the travel time and therefore error in the GPS positions.

  • Ionosphere - change in the travel time of the signal:
    Before GPS signals reach your antenna on the earth, they pass through a zone of charged particles called the ionosphere, which changes the speed of the signal. If your reference and rover receivers are relatively close together, the effect of ionosphere tends to be minimal. And if you are working with the lower range of GPS precisions, the ionosphere is not a major consideration. However if your rover is working too far from the reference station, you may experience problems, particularly with initializing your RTK fixed solution.

  • Troposphere - change in the travel time of the signal:
    Troposphere is essentially the weather zone of our atmosphere, and droplets of water vapour in it can effect the speed of the signals. The vertical component of your GPS answer (your elevation) is particularly sensitive to the troposphere.

  • Satellite Geometry - general distribution of the satellites:
    Satellite Geometry - or the distribution of satellites in the sky - effects the computation of your position. This is often referred to as Position Dilution of Precision (PDOP).

    PDOP is expressed as a number, where lower numbers are preferable to higher numbers. The best results are obtained when PDOP is less than about 7.

    PDOP is determined by your geographic location, the time of day you are working, and any site obstruction, which might block satellites. You can use planning software to help you determine when you'll have the most satellites in a particular area.

    When satellites are spread out, PDOP is Low (good).

    When satellites are closer together, PDOP is High (weak).

  • Satellite Health - Availability of Signal:
    While the satellite system is robust and dependable, it is possible for the satellites to occassionally be unhealthy. A satellite broadcasts its health status, based on information from the U.S. Department of Defense. Your receivers have safeguards to protect against using data from unhealthy satellites.

  • Signal Strength - Quality of Signal :
    The strength of the satellite signal depends on obstructions and the elevation of the satellites above the horizon. To the extent it is possible, obstructions between your GPS antennae and the sky should be avoided. Also watch out for satellites which are close to the horizon, because the signals are weaker.

  • Distance from the Reference Receiver :
    The effective range of a rover from a reference station depends primarily on the type of accuracy you aere trying to achieve. For the highest real time accuracy (RTK fixed), roveres should be within about 10-15 Km (about 6-9 miles) of the reference station. As the range exceeds this recommended limit, you may failto initialize and be restricted to RTK float solutions (decimeter accuracy).

  • Radio Frequency (RF) Interference:
    RF interference may sometimes be a problem both for your GPS reception and your radio system. Some sources of RF interference include:
    • Radio towers
    • Transmitters
    • Satellite dishes
    • Generators
    One should be particularly careful of sources which transmit either near the GPS frequencies (1227 and 1575 MHz) or near harmonics (multiples) of these frequencies. One should also be aware of the RF generated by his own machines.

  • Loss of Radio Transmission from Base:
    If, for any reason, there is an interruption in the radio link between a reference receiver and a rover, then your rover is left with an autonomous position. It is very important to set up a network of radios and repeaters, which can provide the uninterrupted radio link needed for the best GPS results.

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Reference Station
Why?

We have already discussed that there are different levels of accuracy in GPS positions. For any level except autonomous (which can have a large amount of error in it), you must have a reference receiver, which is stationary, and a rover, which can be mobile or stationary.

The GPS reference station normally operates continuously, 24 hours a day. The coordinates of this station must be known before you can begin using GPS on any of your machines. First a proper site for the reference station is to be selected, then a GPS survey is performed to obtain the known coordinates. This is usually done as part of the installation, either by the installation team or other qualified personnel.

Once it is installed, the GPS reference station can perform two functions simultaneously:
  • Receive data from the satellites
  • Broadcast GPS data to the rovers in the mine
One reference station can support unlimited rovers. The primary constraint may be distance, because your accuracy may suffer if you're working too far from the reference station. This maximum distance will vary with your accuracy requirements and environment. Selecting the Reference Station

Some of the features of a good reference site are:
  • Clear View to the Sky
  • Proximity to your Working Areas This is both a GPS issue and a radio issue. Remember, RTK is generally limited to about 10-15 Km (6-9 miles) for reliable initializations, due primarily ot potential errors from the ionosphere. Therefore, you should select a reference site that is within about 10-15 Km of where your rovers expect to work.
  • Absence of RF Interference Try to place the reference station away from sources of radio interference, which arise from radio towers, transmitters, television or other satellite dishes, high-voltage power lines,and any other obvious source of interference.
  • Minimal Sources of Multipath Multipath at your reference site can cause inaccurate answers or interfere with your rover's ability to initialize.
  • Continuous AC / DC Power Source
  • Stable Monumentation One should have a stable survey monument or other similarly well-defined physical point at the reference station. Without this, we will have to survey and compute new coordinates to the reference station anytime you move the GPS antennae.
  • Stable Antennae Mount Not only the monument should be stable, but also the GPS antennae itself should be secure and stable to minimize the movement.
  • Accessibility of the station
Reference Station Equipment:
  • GPS receiver
  • GPS antenna
  • Radio and antenna, Power supply, & Cables
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Radios
We have seen that each GPS rover must receive information from the reference station to achieve accurate positions. To maintain constant communication between your reference station and rover, you need these items at the reference station and at each rover:
  • Radio
  • Radio Antenna
  • Cables
The radios are cabled directly into the GPS receiver. Power may be provided to the radio through the GPS receiver. At the reference site, GPS data is broadcast through the radio. At the rover site, the reference GPS data is received by the radio and routed into the rover receiver, where it is processed together with rover's GPS data/ The rover radio can also draw power from the GPS receiver.

Repeater Radios: If, for any reason, the reference station transmission cannot reach your rovers, then you must use one or more repeaters. A repeater relays the data from your reference or another repeater. The maximum number of repeaters you can use depends on your type of radio. Repeaters differ from your reference and rover radios in two important ways: they must have their own source of power, and they can be moved as the needs change. The radios draw very low power, but they require uninterrupted power. Because repeaters may need to be moved to accommodate your needs, batteries or compact solar power units are normally used.

Frequency and Bandwidth: Most radios used in GPS fall within one of the following frequency ranges:
  • 150-174 MHz (VHF)
  • 406-512 MHz (UHF)
  • 902-928 MHz (spread spectrum)
The lower-frequency radios (150-174 MHZ) tend to have more power, due to design and legal issues (not Physics), However, the bandwidth, which determines the amount of data you can transmit, is narrower in these lower ranges (also due to design, not physics).

In the nominal 450 MHz and 900 MHz ranges, the bandwidth is wider. This has positive effects both on the amount of data transmitted and on the number of repeaters possible within the radio network.

Radio Range
To guarantee steady, uninterrupted transmission over the radio, one should be aware of some of the factors that affect the radio's effective range.
  • Antenna Height: raising the radio antenna is the easiest and most effective way to increase range.
  • Antenna design: radiating patterns vary, depending on the antenna design. For best performance, be sure you understand how yoour antenna transmits signals.
  • Cable length and type: radio signals suffer loss in cables, so keep the length to a minimum. If you must use long cables, use low-loss cables.
  • Output power: doubling output power does not double your effective range. Be sure one understands the relationship between power and gain before the best system is decided.
  • Obstructions: Buildings, walls and even the machines can block or interrupt radio transmission. The repeaters should be carefully used to help minimize the effect of obstructions.
Grounding
The radio antenna may be a target for lightning. To avoid damage, you may wish to ground your reference station antenna.

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  • GPS Radio The most efficient GPS Transmission


GPS Applications
One of the most significant and unique features of the Global Positioning Systems is the fact that the positioning signal is available to users in any position worldwide at any time. With a fully operational GPS system, it can be generated to a large community of likely to grow as there are multiple applications, ranging from surveying, mapping, and navigation to GIS data capture. The GPS will soon be a part of the overall utility of technology.

There are countless GPs applications, a few important ones are covered in the following passage.

Surveying and Mapping
The high precision of GPS carrier phase measurements, together with appropriate adjustment algorithms, provide an adequate tool for a variety of tasks for surveying and mapping. Using DGPs methods, accurate and timely mapping of almost anything can be carried out. The GPS is used to map cut blocks, road alignments, and environmental hazards such as landslides, forest fires, and oil spills. Applications, such as cadastral mapping, needing a high degree of accuracy also can be carried out using high grade GPS receivers. Continuous kinematic techniques can be used for topographic surveys and accurate linear mapping.

Navigation
Navigation using GPS can save countless hours in the field. Any feature, even if it is under water, can be located up to one hundred meters simply by scaling coordinates from a map, entering waypoints, and going directly to the site. Examples include road intersections, corner posts, plot canters, accident sites, geological formations, and so on. GPS navigation in helicopters, in vehicles, or in a ship can provide an easy means of navigation with substantial savings.

Remote Sensing and GIS
It is also possible to integrate GPS positioning into remote-sensing methods such as photogrammetry and aerial scanning, magnetometry, and video technology. Using DGPS or kinematic techniques, depending upon the accuracy required, real time or post-processing will provide positions for the sensor which can be projected to the ground, instead of having ground control projected to an image. GPS are becoming very effective tools for GIS data capture. The GIS user community benefits from the use of GPS for locational data capture in various GIS applications. The GPS can easily be linked to a laptop computer in the field, and, with appropriate software, users can also have all their data on a common base with every little distortion. Thus GPS can help in several aspects of construction of accurate and timely GIS databases.

Geodesy
Geodetic mapping and other control surveys can be carried out effectively using high-grade GPs equipment. Especially when helicopters were used or when the line of sight is not possible, GPS can set new standards of accuracy and productivity.

Military
The GPS was primarily developed for real time military positioning. Military applications include airborne, marine, and land navigation.

Future of GPS Technology Barring significant new complications due to S/A (Selective Availability) from DOD, the GPS industry is likely to continue to develop in the civilian community. There are currently more than 50 manufacturers of GPs receivers, with the trend continuing to be towards smaller, less expensive, and more easily operated devices. While highly accurate, portable (hand-held) receivers are already available, current speculation envisions inexpensive and equally accurate 'wristwatch locators' and navigational guidance systems for automobiles. However, there is one future trend that will be very relevant to the GIS user community, namely, community base stations and regional receive networks, as GPS management and technological innovations that will make GPS surveying easier and more accurate.

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