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GPS machine control systems all have many things in common. Scrapers, excavators, blades, and even tractors all must use the same basic philosophy when utilizing GPS. All GPS systems receive signals from satellites in the sky broadcasting positions; a base station on your job site picks up these signals. The base station “learns” where it is and “locks” in it’s position. Once the base knows where it is, it can read the GPS signals and send the working machines the positional error corrections. The computers onboard the moving machines receive these messages via radio or cell phone signals over 20 times a second, as their location changes, and generate coordinates for the computer to compare to grade.
While learning about GPS Machine Control, think of the machines as rovers, with blades on them. Basically the machine is trying to attain grade, while the rover is trying to determine deviance from that grade. Imagine doing both at the same time and then having computerized movement of the blade fixes the difference. Thus the reason GPS machine control has been pursued, all off the work is done from the same reference point in space, in accordance to the plans of the job, making work faster and more accurate.
Why is this GLONASS / GPS integration such a huge advantage?
In the mid 1990’s the first dual constellation receiver boars appeared on the market that integrated single-frequency GPS plus GLONASS signals. Now we explained that in order to generate coordinates one must be viewing 4 satellites, but to truly generate “Good Coordinates” you would like to see 6 satellites. Finally to have the “best” accuracy you need to receive signals from a minimum of eight satellites. Well, there are times during the day when it is impossible to get eight GPS satellites in view, even on a flat job site! Now, add in some trees, a few buildings—you could be lucky to receive 4 or 5 GPS satellites. What does that do to your accuracy, if you’re able to keep working at all? Ultimately, when you invest in a positioning solution, whether for a surveying crew or for stake less 3-D grading, you want “uptime†not “downtime,†and that uptime must be accurate.
Therefore, the one overwhelming advantage to GPS and GLONASS integral systems is this — the integration provides access to 33 percent more satellites than ordinary GPS. That’s because GPS with GLONASS has the ability to track not only both frequencies of all 24 GPS satellites, it can also receive the signals from the 9 GLONASS positioning satellites (with another 6 GLONASS satellites scheduled for launch this year). The accessibility to more satellites directly relates to the precision obtained. A satellite availability comparison shows that at all times the Dilution of Precisions are lower numbers when GLONASS satellite information is included with GPS satellite positioning information, in comparison to GPS satellite positioning information alone.
For surveyors working in high latitudes, adding GLONASS to GPS can also add strength to positioning. With the inclination of GPS orbits being 55°, all satellites are south of the observer’s zenith in the second and third quadrants. In Fairbanks, Alaska, for example, the only satellites to the north are on the other side of the North Pole and just barely above the horizon. With the inclination of GLONASS orbits almost 10° higher, observers in Fairbanks will have observable GLONASS satellites in all four quadrants.
That is what global positioning is all about–accuracy, all the time. So if the highest degree of accuracy and productivity is important to you, you need a system that receives both GPS plus GLONASS positioning systems information.
If and when Galileo Systems come online from the European Union even more satellite signals will be available. If and when this happens there is a completely new set of signals that will be provided by those satellites. Many of the manufacturers of GPS systems on the market have already developed, tested, and are delivering chips that have these capabilities on board. Ask your dealer; you might already have the Galileo chipset in your machine.
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GPS Vehicle Tracking in realtime! This is ABC news coverage of the only true realtime tracking system on the market that does 5-second and 10-second updates! Used with permission.
GALILEO is a global navigation satellite system (GNSS) which is being developed by Europe. The European Union GNSS initiatives started in 1994. The system will provide a highly accurate, guaranteed global positioning service under civilian control, as opposed to military control as all other systems. The first satellite was launched in late 2005. The full system, not yet operational, will have 30 satellites positioned in three circular Medium Earth Orbit (MEO) planes at 23616 km altitudes, and inclinations of 56 degrees with reference to the equatorial plane. Having completed the system, the Galileo navigation signals will provide good satellite coverage and decrease the dilution of precision or DOP, associated with northern latitudes up to 75 degrees. The large number of satellites, together with the optimization of the constellation, and the availability of the three active spare satellites, will ensure that the loss of one satellite has no discernible effect on end users.
GALILEO systems will offer the world a number of benefits over GPS systems alone.
* GALILEO has been designed and developed as a non-military satellite platform.
* GALILEO is based on the same technology as the US GPS system and provides a similar – and possibly higher – degree of precision
* GALILEO is more reliable as it includes the error correction, “integrity messages”, and more information in the L2 code informing the user immediately of any errors
* GALILEO is a real public service and, as such, guarantees continuity of satellite positioning availability no matter the geopolitical landscape
For a few years, the United States had the market on global navigation satellite systems cornered with its Global Positioning System. But, Russia quickly caught up with their own constellation. The Russian system is known as GLONASS (Global Navigation Satellite System) “a satellite-based radionavigation system, which enables an unlimited number of users to make all –weather 3D positioning, velocity measuring and timing anywhere in the world or near-Earth space.â€
What is the difference between GPS and GLONASS?
GLONASS was developed by the former Soviet Union (USSR) and is quite similar to GPS; in fact, they are more similar than different. One of the major differences between the GPS and GLONASS satellite systems was the initial design life of their satellites. The original design life of a GLONASS satellite from the early 1980’s was one to two years. In contrast, the early experimental Block I GPS satellites, launched from 1978 to 1985, had a design life of seven to 7 ½ years. Some of the current GPS satellites, launched from 1989 to 1997 have been in orbit for more than 10 years. The latest GLONASS satellites have a design life of three years, but the new GLONASS-M satellites currently being launched incorporated advanced engineering and have a design life of seven years. The new GLONASS-K satellites will have a design life up 10 to 12 years. Because of the early design life of the early satellites, there were origionally 64 GLONASS satellites launched and there were six launch failures.
GLONASS Modernization
Much has happened with GLONASS since the fall of the Soviet Union. The emerging Russian Federation government had some financial obstacles to overcome, but now fully support the program with the help of other nations like China. GLONASS has been approved and funded through 2011, and, in addition to two newly designed satellites, will feature a new launch vehicle, the Soyuz-2 rocket.
The new GLONASS-M satellites have a second civil code. The new GLONASS-K satellites will add a third civil frequency. The addition of the second civil code to the GLONASS-M satellites is a plus for the Russian system. A second civil code is planned for GPS Block IIR (replenishment) GPS satellites, which will be called Block IIR-M satellites. This additional code messages has little impact on precision applications, but is important for GIS/mapping type receivers. The additional code message is thought to assist satellite tracking of low satellites on the horizon, which could benefit precision application slightly. Now that Russia has three satellites with the second civil code, they may be able to build GLONASS-only receivers that receive these signals.
GPS plus GLONASS
In the mid 1990s, Ashtech founder Dr. Javad Ashjaee took on the task of forming a research group of Russian scientists in Moscow to build a satellite receiver with the capabilities of receiving signals from both GPS and GLONASS satellites. He was successful and Ashtech introduced a single frequency receiver in the late 1990s.
With contributions from research and engineering, several manufacturers plan to include all satellite signals into the next generation of receivers. This includes the planned new GPS L5 frequency, L2C code message, third GLONASS frequency and the future Galileo satellite signals.
GLONASS Signals
Are there instruments that receive only GLONASS signals? Yes, but at this time the number of satellites in the GLONASS constellation is not enough to make GLONASS-only receivers competitive with GPS-only receivers. Most products utilize a combination of technologies providing access to additional satellite signals and increased performance over GPS or GLONASS alone. Today, surveyors can receive signals from both GPS and GLONASS with the receivers developed making it possible to have 13 or more satellites visible at one time.
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GPS, or the American Constellation of Global Positioning Satellites owned by the U.S. military, orbits approximately 11,000 miles above the earth at a defined azimuth and elevation. Even though the primary purpose for the American global satellite systems was military guidance, the system’s benefit for everyday purposes soon became obvious, and was released to the public*. However, the United States Military has the ability to modify these flight paths for their operations and has been known to take a satellite or two to support ongoing military operations in theaters around the world.
Each GPS satellite sends out a radio signal at a known wavelength and time so precise (from an atomic clock) that the distance to each satellite can be measured to less than 1/16 of an inch! To do so, each Satellite produces a number of codes that look like this…..
Long story short, the public has access to only one of these two, the L1 code. The L2 code broadcasts error correction so that one receiver can triangulate much more accurately. We therefore receives coordinates less accurate than the military*. We receive half the information necessary to do the mathematical calculations.
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So how accurate is a GPS system? GPS without any calculation isn’t very precise. Looking at the picture to the right, it is easy to see how the lines bend.
Refraction is the bending of radio and light waves done by the dense layers of the atmosphere which reduces the accuracy of GPS systems. Without having a system to compute this error and transmit the “Correction” GPS systems aren’t very accurate. Military receivers can “Decode” a second error correction signal beamed from the GPS satellites, but the public is not able to receive this signal for security purposes. Without error correction, a single modern GPS receiver is accurate to roughly 8-10 feet. To get to tolerances necessary for construction a second receiver (Base Station) is used to create the “Correction” formulas for the roving receivers in it’s general area. Once all system’s data is compared, the accuracy is increased to reasonable tolerances.
GPS Systems can plot latitude and longitude very precisely (X and Y axis). But elevation, or vertical accuracy (Z axis), is more of a challenge to the system. Plotting x & y axis locations, or horizontal bearings, is easy because there are most likely satellites on all sides of you (Good DOP) reducing the error in the calculations (data available from one horizon to the other). Z axis calculations are difficult due to the GPS satellites location overhead, and a long way up. Since no GPS satellite signals are available from below the receiver, it’s impossible to be between two satellite points on the z axis and receive accurate data. The satellites location and distance reduces the accuracy range of vertical points.
So what is a typical vertical accuracy for GPS Machine Control systems without lasers? Anywhere from ½ to ¾ inch (12 to 18mm). Of course, the more GPS satellites you are receiving, the better your vertical accuracy will be.
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So we know that GPS satellites are above us, and transmit signals that come down to earth telling us in precise ways how long ago the signal was sent, and we know how fast the radio signal gets to us. For now let’s assume that those signals are perfect, straight, and not affected by anything. Therefore a simple equation, distance = rate x time will tell you how far away from the satellite you are if you know the time and rate.
With known distances to 4 satellites computed, a virtual inverted pyramid can be drawn between them. By a process of triangulation we can then compute our location on the planet. To increase the accuracy of the equations and allow movement, scientists believe that 8 satellites are necessary for real time positioning data. The error caused by the distance in the equations can be further reduced by what is known as DOP, or Dilution of Precision.
DOP or Dilution of Precision
Dilution of Precision refers to the location of the satellites that are generating the base of the pyramid if you will. The larger the base of the pyramid above, the higher the level of DOP. The smaller the pyramid’s bottom, the worse the DOP, and the less accurate any GPS system can be. To see an example of good DOP, and poor DOP see below.
Figure 1. Dilution of Precision: (a) Poor DOP, (b) Good DOP
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GPS short for Global Positioning System is a group of satellites orbiting approximately 11,000 miles above the earth. Each satellite sends out a radio signal at a known wavelength and time which is so precise that the distance to each satellite can be measured to less than 1/16 of an inch!
Okay, so we know where the satellite is. What good is that when you want to know where you (or your equipment) are located on Earth? Well, if you can receive signals from at least four satellites at the same time, then software can measure the distance to each one and accurately calculate where you are. The math is based on triangulation and is very fast.
Now most folks don’t know it, but there are 2 sets of satellites which can be utilized for positioning and there may soon be 3
1. GPS is the term used to describe the American Constellation of about 24 Global Positioning Satellites owned by the U.S. military.
2. The Soviet Union, and now the Russian government maintain a similar system in place called GLONASS consisting of more than a dozen satellites. The Soviet Military maintains the fleet of satellites.
3. In the last 3 years, the European Union has made a foray into the positioning satellite business.
Together, these two systems, maybe a third, create the Global Navigation Satellite System (GNSS).
Even though the primary purposes for both the Soviet and American global satellite systems was military guidance the system’s benefit for everyday purposes soon became obvious and was therefore released to the public*.
· Companies can pinpoint the location of their delivery trucks;
· Boat owners can navigate with the comfort of knowing their exact position at any given moment; and
· A husband will never have to ask for directions and his wife will never have to read a map again with a GPS-equipped automobile.
Because of the tremendous economic and security advantages, both US and Russian governments decided to allow civilian use of the satellite signals. Today, anybody can use the information transmitted by the Global Navigation Satellite Systems.
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