320 Radio Engineering for Wireless Communication and Sensor Applications
2000 (IMT-2000), whereas Universal Mobile Telecommunications System
(UMTS) is used in Europe [6]. Third generation systems will operate in the 2-GHz band and will provide data rates up to 2 Mbit/s. A wideband code division multiple access (WCDMA) will be one of the access methods of the third generation systems.
12.5 Radionavigation
Radionavigation means determining position, speed, or some other quantity using radio waves for navigation, such as for steering a vehicle. Radionavigation is used in aviation, in shipping, and on land [7]. Radionavigation systems can be divided into two groups:
1.Base station systems. The positions of stations on the ground or on satellites are known. The position of a vehicle can be determined by measuring distances, differences in distances (hyperbolic systems), or directions to the base stations.
2.Autonomous systems. Doppler navigation is an example of autonomous navigation: Antenna beams are directed from an airplane to ground. The speed of the plane is obtained from the measured Doppler shifts. The position of the plane is calculated by integrating the velocity vector.
Hyperbolic systems, satellite systems, and systems used in aviation are treated in this section. In addition to these systems, radio beacons, broadcasting stations, and base stations of mobile phone systems are used for navigation. By taking a bearing to two beacons, the position of the vehicle can be solved. The null in the radiation pattern of a loop or Adcock antenna can be used for bearing.
12.5.1 Hyperbolic Radionavigation Systems
In a hyperbolic system, differences in distances to the base stations are measured. The difference in distance for a pair of stations corresponds on a plane surface to a hyperbola, on which the vehicle is located. The position is obtained from the intersection of two such hyperbolas.
Both pulsed and continuous signals are usable: The difference of the arrival times of pulses, Dt , or the phase difference of continuous waves, Df , from two stations determine a hyperbola. The transmitted signals from
322 Radio Engineering for Wireless Communication and Sensor Applications
Loran-C consists of chains, which have one master station and two to five slave stations. The stations are located about 1,000 km apart. There are about 30 chains around the world.
The carrier frequency of Loran-C pulses is 100 kHz. Transmitted pulse power is typically 400 kW. The shape of the pulse is well defined, as shown in Figure 12.8. Both the envelope of the pulses and the phase of the carrier are used to determine time differences. The transmission sequence of a chain starts with nine pulses from the master station. The pulses are 1 ms apart, except the ninth pulse, which is 2 ms apart from the eighth pulse. Then, after a given delay, each slave station transmits eight pulses. Because of the delay, signals are always received from the stations in the same order in spite of the location of the receiver. The length of the pulse pattern is 30 to 100 ms. Figure 12.8 shows an example of a received pulse pattern. Adjacent chains have different repetition frequencies, which helps in the identification of the chain.
The signals of Loran-C system propagate to a distance of about 2,000 km as surface waves. Reflections from the ionosphere come at least 30 m s later than the surface waves and do not disturb if only the beginning of the pulse is used for timing. Navigation accuracy is about 250m at a distance
Figure 12.8 Pulse of Loran-C, and pulse pattern of the Norwegian Sea chain received in Helsinki.
Applications |
323 |
of 1,000 km. Reflections from the ionosphere can be used for navigation at distances greater than 2,000 km but with inferior accuracy.
12.5.2 Satellite Navigation Systems
A global navigation system is realized best by positioning the stations on satellites. Satellite navigation has several advantages:
•Vast areas can be covered with a few satellites, which makes the maintenance of the system economical.
•Navigation accuracy is better because the radio frequency used can be higher than in terrestrial systems.
•Three-dimensional navigation is possible.
In addition to navigation, satellite systems are used for locating, surveying, and transferring time signals.
Transit and GPS are briefly described below. Many other satellitebased navigation and location determination systems are in operation. The Global Navigation Satellite System (GLONASS) and Tsikada are the Russian counterparts of GPS and Transit. Cospas-Sarsat is a worldwide rescue system, which can locate rescue transmitters.
Transit was the first satellite navigation system and was operating between 1964 and 1997. It was initially maintained by the U.S. Navy. Six satellites in polar orbits at the altitude of 1,075 km transmitted at frequencies of about 150 and 400 MHz. The use of two frequencies made it possible to correct the effect of the ionospheric delay. Transit navigation was based on the Doppler effect. The position of a receiver was determined from the measured Doppler shift, f D , as illustrated in Figure 12.9. The longitude was obtained from the rate of change in f D , and the latitude from time of passage when f D = 0. The location accuracy for an immobile vehicle was 50m.
The Global Positioning System (GPS) is maintained by the U.S. Department of Defense. Full operational status was reached in 1995. GPS includes 24 satellites, of which three are spare. Satellites are placed on six orbits at the height of about 20,000 km (orbiting time: 12 hours), as shown in Figure 12.10. At least five satellites are high enough above the horizon for any location on the globe.
GPS satellites transmit at two carrier frequencies, L1 = 1,575.42 MHz and L2 = 1,227.60 MHz. Using both frequencies, the ionospheric delay can be taken into account. The signal contains information about the orbit and
324 Radio Engineering for Wireless Communication and Sensor Applications
Figure 12.9 Principle of Transit satellite navigation. The elevation is the maximum angle from the horizon during a passage.
Figure 12.10 Orbits of GPS satellites.
clock correction, and general notices. Timing of signals is based on accurate atomic clocks.
GPS is based on the measurement of the distances between the user and satellites. The distance is determined from the time of propagation from
Applications |
325 |
the satellite to the receiver. The coordinates of the receiver can be calculated from distances to three satellites and from the coordinates of the satellites. The clock error of the receiver can be eliminated by using four satellites.
The GPS signal is PSK modulated with two pseudorandom codes, precise, or P, code and coarse/acquisition, or C/A, code, which have rates of 10.23 and 1.023 Mbit/s, respectively. The received code is compared to a similar code generated in the receiver, as illustrated in Figure 12.11. The delay of the generated code is adjusted until the correlation is at maximum. The transit time from the satellite to the receiver is the sum of the delay and a multiple of the length of the code. Figure 12.11 also shows the autocorrelation function of a pseudorandom code. Correlation is near zero if the phase difference is more than one bit. An error of one-hundredth bit period in the peak of the autocorrelation function corresponds to errors of 30 cm and 3m in the distance to the satellite with P and C/A codes, respectively.
The uncertainties of the clocks on the satellites and in the receiver, the uncertainties in orbital parameters, the propagation delays in the atmosphere and receiver, and multipath propagation are sources of error that impair navigation accuracy. The absolute accuracy of position is typically some tens of meters. The relative accuracy, that is, the difference in position for two receivers, is better, about 2m to 5m, because most of the errors are the same for both receivers. This is utilized in the differential GPS (DGPS), in which fixed stations monitor their positions and send correction signals to nearby GPS receivers. In surveying, the accuracy can be further improved by using the phase of the carrier and a long measurement time. The accuracy may be as good as about 1 cm.
Figure 12.11 Determination of the transit time from a satellite to the receiver, and the autocorrelation function of a pseudorandom code. The code length is n bits.