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WAAS cannot always be intercepted; even when one is in the open and at the time of writing EGNOS is not yet operational. Both systems are expected to increase coverage as more stations are installed.
Additionally, a European satellite version of GPS called Galileo is also expected to be operational by 2009. This all adds up to better accuracy, availability and reliability of global navigation systems. A new term in fact is emerging to refer to these multiple systems, global navigation satellite systems or GNSS.
8.2.3 Accuracy of GPS: Some Technical Issues
Several different techniques are used to try to enhance the accuracy of GPS devices. Three such techniques are discussed here.
Differential GPS
By placing a stationary GPS receiver at a known location, it can be used to determine exactly which errors the satellite data contains. This receiver acts like a static reference point and it is called a “Pseudolite”. The stationary receiver can transmit an error-correcting message to any other GPS receivers that are in the local area. These additional receivers can use the error message to correct their positional solutions (Figure 8.7). This concept works because the satellites are so far above the earth that errors measured by one receiver will be almost exactly the same for any other receiver in a given area. This will reduce ionospheric, timing, and atmospheric errors, as well as any other errors introduced intentionally as was the case when selective availability (SA) was on.
Figure 8.7. Differential correction: the Pseudolite principle
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Before S/A was turned off this form of correction was becoming very popular and a number of companies began putting up the infrastructure needed to place these Pseudolites all over the country. But once S/A was turned off, subscribers of this service did not feel the improved accuracy offered was worth the cost of subscription, and as a result these companies either went out of business or specialized in specific applications, such as setting up a Pseudolite receiver to correct GPS errors in farming or surveying applications.
How trilateration can lead to an exact position
In GPS, when the distance to an object is known, that object’s location can be represented by the equation of a circle. Without any other point of reference the location is merely somewhere on that circumference of the circle, at a radial distance away from its centre. The mathematical equation of a circle is
r2 = (x − xc)2 + (y − yc)2
where r is the radius of the circle, distances x and y are the coordinates along the circumference of the circle, and xc and yc are the coordinates of the centre of the circle.
Now for a 2D solution, the distance to three other objects are required where the exact locations must be known. This will give three equations with two unknowns, and these can be solved for x and y to obtain the exact location of the object’s position The three quadratic equations would given as
(x − longitude1)2 + (y − latitude1)2 = (distance1)2
(x − longitude2)2 + (y − latitude2)2 = (distance2)2 (x − longitude3)2 + (y − latitude3)2 = (distance3)2
Now for a 3D solution where altitude can also be determined, the location of a fourth object, specified through its radial distance, is required and then four equations can be solved for the three unknowns, latitude, longitude, and altitude.
The Kalman Filter
The Kalman filter is a multi-input/output recursive linear filter. It takes inputs from multiple sources and can estimate the state of the system based on its noisy outputs. The use of a Kalman filter system can reduce the error of the noisy outputs by real-time analysis of the mean-square of the estimated error.
The Kalman filter can also be used for combining multiple sources of locationbased information and reducing its error by weighting each source accordingly as to reduce the error. The more inputs there are to the system the better the accuracy that can be obtained. The Kalman filter could be used to combine the inputs from GPS, DGPS, Inertial guidance, and WAAS to provide the best accuracy of a desired location.
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8.2.4 Frequency Spectrum of GPS, Present and Future
The GPS system operates in the UHF (ultra high frequency) spectrum, which is between 300 MHz and 3 GHz, and within the L-Band frequency range. The L-Band consists of frequencies between (390 MHz–1.55 GHz) and is used for GPS satellites, satellite phones, SETI outer space exploration, and miscellaneous communication satellites. The primary frequency for civilian use is L1 = 1575.42 MHz. L1 carries a Pseudo Random Noise Course-Acquisition (PRN C/A) code, as well as an encrypted precision P(Y) code. L2 = 1227.6 MHz carries only the P(Y) code (Figure 8.8). The P(Y) code is used in military applications and the encryption keys needed to use the P(Y) code are rotated on a daily basis by the US government.
Since civilian applications only have access to the C/A code signal on the single frequency at L1, the accuracy is limited to approximately 10 m (without selective availability). Consequently, these applications cannot use dual-frequency corrections to remove the delay caused by the signals travelling through the ionosphere.
In December 2005, the first of the next generation GPS satellites was launched. The (Block IIR-M) satellite provides three new signals, two military signals (L1M, L2M), and a second civilian signal, L2C. Since the second signal for civilian use is in L2, civilian receivers can finally remove the ionosphere delay by using dualfrequency correction. In an open sky environment, this would probably improve the accuracy to less than 5 m.
Planned for launch in 2007, will be the second generation GPS satellite (Block IIF), which will add an additional frequency L5 = 1176.45 MHz.
Figure 8.8. GPS signals: present and future
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Figure 8.9. GPS signals at L1
Planned for launch in 2013 will be the third generation GPS satellite (Block III) that will add another civilian signal L1C. See Figure 8.9 for the proposed GPS signals at L1.
8.2.5 Other GPS Systems
8.2.5.1 Galileo System
Like GPS this is a global navigation satellite system constructed by the European Union. Galileo’s first satellite GIOVE-A, was launched in December, 2005. Galileo is now scheduled to have a test constellation of four satellites by 2008, but no date for initial operating capability has been set, although it is assumed the system will be in operation by 2010.
Galileo is interoperable with the U.S. GPS system; it consists of 30 satellites in three Medium Earth Orbit planes at an altitude of 23,616 km. Each plane contains nine satellites plus one active spare. The higher orbital plane of Galileo results in better coverage over the northern parts of Europe where GPS coverage is weak due to its lower orbital plane. One revolution around the earth takes a Galileo satellite 14 h 4 min.
Galileo uses the same L1 frequency as GPS and has two signals L1F and L1P (Figure 8.10).
8.2.5.2 GLONASS – Russia
GLONASS (global navigation satellite system) consists of 21 satellites with three active spares in three circular orbital planes at an altitude of 19,100 km. Each satellite completes an orbit in approximately 11 h 15 min.
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Figure 8.10. GPS and Galileo at L1
GLONASS was in full operation by 1995, but with the fall of the Soviet Union only eight satellites were operational by 2002. Three new improved GLONASS-M satellites, with a life span of seven years, were launched in 2004 with three more launched in 2005. The next generation of GLONASS-K satellites, which are lighter in weight, with improved functionality and a longer 10-year life span are to begin being launched starting 2006. Russia has said it plans to have the GLONASS operational by 2008 with 18 satellites covering all of Russia. A complete constellation of 24 satellites with full global coverage is planned by 2010.
Unique to GLONASS is that each satellite transmits its data at a different frequency determined by the frequency channel number of the satellite, thus allowing the user’s receiver to identify the satellite. Two different carrier frequencies are used to transmit a precision (SP) signal and high precision (HP) signal, along with a standard C/A positioning code:
L1 : f1(k) = 1602 MHz + k × 9/16 MHz
L2 : f2(k) = 1246 MHz + k × 7/16 MHz
(where k is the channel number).
8.2.5.3 QZSS – Japan
Quasi Zenith Satellite System (QZSS) or Jun-Ten-Cho in Japanese is a modified geosynchronous orbit covering Japan and Southern Asia (Figure 8.11). The QZSS constellation will consist of three satellites moving in periodical highly elliptical orbits (HEOs) over the Asia region and should be functional by 2008. This system uses WAAS-like data on L1, L2, and L5. The satellites will have L, S, and Kuband capabilities (L-band for positioning, S-band for broadcasts and low-speed communications, Ku-band for high-speed communications).