GLOBAL POSITIONING SYSTEM
7.1 Definition: The Global Positioning System (GPS) is a satellite based navigation system developed by the US Department of Defense in the early 1970s. GPS provides continuous positioning and timing information, anywhere in the world under any weather conditions. 7.2 Basic Idea of GPS: If the distances from a point on the earth ( a GPS receiver) to three GPS satellites are known along with the satellite locations, then the location of the point can be determined by simply applying the concept of resection Our distance from the satellite is measured by calculating the time it takes for a radio wave to travel from the satellite to our GPS receiver We multiply this time by the speed of light to get our distance.
7.3 Working of GPS: Suppose we measure our distance from the satellite and find it to be 20,000 kilometers. Knowing that were 20,000 kilometers from a particular satellite narrows down all the possible locations we could be in the whole universe to the surface of a sphere that is centered on this satellite and has a radius of 20,000 kilometers.
20,000 kms
Next, say we measure our distance to a second satellite and find out that its 22,000 kilometers away. That tells us that were not only on the first sphere but were also on the sphere thats 22,000 kilometers from the second satellite. Or in other words, were somewhere on the circle where these two spheres intersect.
Two measurements puts us somewhere on this circle
If we then make a measurement from a third satellite and find that were 21,000 kilometers from that one, that narrows our position down even further, to the two points where the 21,000 kilometers sphere cuts through the circle thats the intersecton of the first two spheres. So by ranging from these satellites we can narrow our position to just two points in space. To decide which one is our true location we could make a fourth measurement. But usually one of the two points is a ridiculous answer (either too far from the earth or moving at an impossible velocity) and can be rejected without measurement.
One of these two points is the accurate position
To be able to fix our position with only three satellites requires that there be accurate clocks not only in the satellites but also in the receiver units. Because these clocks are so expensive, it is impossible to put them in receivers. Instead, receivers use the measurement from a fourth satellite to remove clock errors. 7.4 GPS Segments 7.4.1 Space Segment surface The space segment consists of the 24 satellite constellation. The satellites are at an altitude of 20000 kilometers above the earths Each satellite orbits the earth in about 12 hours There are four satellites in each of six distinct orbital planes
7.4.2 Control Segment The control segment consists of a master control station (MCS), a world wide network of monitor stations and ground control stations The MCS, located near Colorado Springs, Colorado, is the central processing facility of the control segment There are five monitor stations, located in Colorado Springs (with the MCS), Hawaii, Kwajalein, Diego Garcia and Ascension Island. Monitor stations are for the purpose of continuous tracking of all the GPS satellites The GPS observations collected at the monitor stations are transmitted to the MCS for the processing 7.4.3User Segment The user segment includes all military and civilian users. With a GPS receiver connected to a GPS antenna, a user can receive the GPS signals, which can be used to determine the his position anywhere in the world
7.5 GPS Signals: 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 19 cm wavelength 1575.42 M Hz C/A Code Navigation
L2 Career 24 cm wavelength 1227.6 M Hz P Code 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 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. 7.6 Sources of Errors in GPS GPS receivers have potential position errors due to the result of the accumulated errors due to primarily of the following sources o Ionosphere and troposphere delays 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. o Signal multi-path Occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors
Receiver clock errors Since it is not practical to have an atomic clock in the GPS receiver, the built-in-clock can have timing errors Orbital errors Also known as ephemeris errors, these are inaccuracies of the satellites reported location Number of satellites visible 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. Satellite geometry This refers to the relative position of the satellites at any given time. Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. The satellite geometry effect can be measured by a dimensionless number called the Geometric Dilution of Precision (GDOP). The lower the value of the GDOP number, the better the geometric strength. The best results are obtained when PDOP is less than about 7.
POOR GDOP
GOOD GDOP
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). 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.
7.7 GPS Positioning Modes: Positioning with GPS can be performed by either of two ways: absolute positioning or relative positioning Absolute positioning uses a single GPS receiver and does not require known survey control. Absolute positions can provide instantaneous (real-time) or postprocessed positions. Topographic surveyors can find out stations location by using absolute positioning. In it we can get location of a point with accuracy of sub-meter in post processing mode and accuracy of 25 m in the worst condition. Accuracy will increase if we will collect the data for long duration. Differential positioning uses two or more GPS receivers. One GPS receiver (reference or base receiver) is resident over a known control point. The remaining receivers (rovers) are used to position points of interest. Differential positioning can be performed in real time or through postprocessing. If real-time positioning results are required, a communications link that is capable of transmitting digital data must be established at the reference- and remote-receiver locations. This method supports distances up to 30 kilometers between the reference and remote stations.
7.8 Differential Surveying Techniques: 7.8.1 Reference Station: For differentia; survey 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. Once it is set, 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. 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 Accessibility of the station
7.8.2 Measurement Techniques: There are several measurement techniques that can be used by most of GPS survey receivers. The basic techniques are: o Static Surveying o Rapid Static Surveying o Kinematic Surveying o Real-time Kinematic (RTK) Surveying
7.8.2.1 Static Surveying This is the most widely used differential technique for control and geodetic surveying. It involves long observation times (1-2 hours) Two GPS receivers are used to measure a GPS baseline distance. The line between a pair of GPS receivers from which simultaneous GPS data have been collected and processed is a vector referred to as a baseline. GPS receiver pairs are set up over stations of either known or unknown locations. Typically, one of the receivers is positioned over a point whose coordinates are known (reference receiver) and the second is positioned on the other end of the baseline point whose coordinates are unknown (rover receiver). During the observation, the following points should be noted o All receivers must track during the same time period o All receivers must track the same constellation of satellites o All receivers must record data for the same epochs The occupation time for baselines is influenced by
o The length of the baseline o The number of satellites observed and the satellite geometry o The precision of the result required After the observation session has been completed, the received GPS signals from both receivers are processed in a computer to calculate the three dimensional baseline vector components between the two observed points. These vectors may be computed and adjusted. This process is called post processing. Applications of static method of survey o Geodetic control over large areas o National and continental networks o Monitoring tectonic movements o High accuracy survey networks 7.8.2.2 Rapid Static Surveying: Rapid Static survey is similar to the static method, but consists of a shortened site occupation time at the rover. In rapid static surveys, a reference point is chosen and one or more rovers operate with respect to it.
Reference receiver is usually setup at a known point. If no known point is available, it can be setup everywhere within the network by continuous observation for long duration. Rover receivers are then moved to each of the required points. The length of time that the rovers must observe at each point is related to the baseline length and the geometry of the satellites. Rapid static technique is used mainly for detailing the existing network. 7.8.2.3 Kinematic Surveying The reference receiver remains fixed on known control point while the rover receiver collects data on a constantly moving platform (vehicle, aircraft, etc.) rate. While the rover moves, the user can record its positions at a predefined recording
The observation data are later post-processed on a computer to calculate relative vector to the roving receiver. 7.8.2.3 Realtime Kinematic Surveying (RTK) RTK is a method that can offer positional accuracy in a real time very near to static positioning. The observations are corrected in a real time. This method is suitable when
o The survey involves a large number of unknown points located in the vicinity o The coordinates of the unknown points are required in realtime o The line of sight is relatively unobstructed
The base receiver remains stationary over the known point and is attached to a radio transmitter. The rover receiver is attached to a radio receiver.
The base receiver measurements and coordinates are transmitted to the rover receiver through the communication radio link. The built-in software in the base receiver uses the precisely known base coordinates and the computed ranges to obtain pseudorange errors. These corrections are transmitted to the rover through a communication link. The rover then applies the corrections to correct the measured pseudoranges at the rover. Finally the corrected pseudoranges are used to compute the rover coordinates. 7.9 Accuracy of GPS There are four basic levels of accuracy - or types of solutions - you can obtain with your realtime GPS mining system: Type of solution Autonomous Differential GPS (DGPS) Real-Time Kinematic Float (RTK Float) Real-Time Kinematic Fixed (RTK Fixed) Accuracy 15 - 100 meters 0.5 - 5 meters 20cm - 1 meter 1cm - 5 cm
