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5 Terrestrial LF-systems
During and after the Second World War, radio navigation has gone through great developments. The necessity to know precisely the position, under all-weather conditions, has led to widely spread applications of hyperbolic localization. The latter choice has been mainly forced by the relatively poor electronic means of those old days together with the lack of small-sized computers.
In order to acquire sufficient range and sufficient localization accuracy in the pre-satellite era, people had to stick to the use of relatively low frequencies. One has been aiming at a compromise between feasible range, stability of propagation velocity and simplicity of creating electro-magnetic fields of sufficient power. The magnitude of the desired received signal depends on the presence of the atmospheric noise, the presence of interference and on the required signal-to-noise ratio at the navigation-receiver’s input. Three systems have been developed, viz.
- Decca
- Loran-C/Chayka (100kHz-band; distance range 200 km to 2000 km)
- Omega (10-20 kHz-band; worldwide coverage)
The coming of satellite systems (GPS) with worldwide coverage and with a higher feasible accuracy has strongly contributed to the disappearance of the Decca and the Omega system. Therefore, the latter systems are not covered in this course. The only surviving system is Loran-C/Chayka due to its importance for localization by submarines and due to its increasing importance for Eurofix (see chapter 11) applications.
Here, we confine ourselves to the technical aspects of Loran-C (Europe, North America, East Asia) and, inherently, also of Chayka (Russia).
5.1 Loran-C / Chayka
Although not worked-out here, because the Omega and Decca systems are out of service now, the use of continuous-wave (CW) signals and the difference in propagation velocity between ground wave (atmosphere) and sky wave (ionosphere) have always been the main reason for an unavoidable accuracy problem. In contrast to CW, the use of separate, not-modulated, pulses enables the rejection of ionosphere reflections for a major part. The relatively slow ground-wave signals will generally reach the receiver earlier than the faster sky-wave signals because the traveled distance is shorter. In most cases, this pulse-shape character enables the receiver to separate the ground wave and the sky wave sufficiently. After that, the preserved ground wave will be used for distance measurements. This technique is used for Loran-C, and for its Russian variant named Chayka.
Short pulses with a carrier frequency of exactly 100 kHz are transmitted, where a single pulse is mathematically represented by (see Figure 5-1):
2 exp 2 1 cos p p
e t A t^ t t PC t t
w
Ê ˆ È^ Ê ˆ˘
= ◊ ÁÁ ˜˜ ◊ Í ÁÁ - ˜˜˙◊ +
Ë ¯ ÍÎ^ Ë ¯˙˚
where e ( t ) = instantaneous value of the electric field A = constant t = time in seconds tp = 65 ms w = 2p·100 kHz PC = phase code ( 0 of p)
Loran-C distance measurement is based on a coarse arrival-time measurement and on a precise arrival-time measurement. The accurate measurement takes place by a phase- measurement of the 100 kHz carrier. As becomes obvious from Figure 5-1, the Loran-C pulse is built up from several 100 kHz periods. Consequently, this means that the phase measurement is not unambiguous. Therefore, a coarse measurement has to be executed first in order to know which 100 kHz period has to be used for the phase measurement. The coarse measurement, also named “envelope tracker”, is based on the accurate knowledge of the shape of the pulse envelope. Therefore, Loran-C transmitters continuously monitor and control the accurately defined envelope shape of transmitted pulses. Then, by correlation of the received pulse-envelope shapes with the “known” reference-pulse-envelope shape, the receiver is capable to derive which 100 kHz cycle has to be used for the phase measurement.
All Loran-C transmitters use the same carrier frequency of 100 kHz. The transmitters are organized in chains consisting of one master with two to five secondaries. In order to receive the station pulses separated from each other, time-division multiple-access (TDMA) is applied. The master starts with
Figure 5-1 Loran-C pulse with a phase code of 0 radians.
Figure 5-2 Timing organization of Loran-C positioning
that the signal is transmitted in-phase (PC = 0), while a down-arrow represents a phase inversion (PC = π).
The insensitivity for sky waves is reached by the exclusive use of the first part of the received signals. The ionosphere height and the distance to the transmitter determine which part of the pulse can be used without sky-wave pollution. Figure 5-4 makes clear that for distances over 300 NM the sky wave appears only 40 μs later than the ground wave when the ionosphere height is 50 km. In that case, we are not allowed to sample the incoming signal later than 40 μs after the beginning of the ground- wave pulse.
Figure 5-5 and Figure 5-6 depict the Loran-C signals received from the Sylt transmitter (in the German-Danish Bow) in the time domain as well as in the frequency domain. In the latter domain we see much interference originating from, among others, Decca and FSK transmitters (meanwhile Decca has become obsolete). Therefore in a Loran-C receiver interference (and also general channel noise) has to be removed as much as possible from the
Figure 5-6 Loran-C signals received in Delft from the Sylt transmitter. The upper plot is the time domain; the lower plot is the frequency domain.
Figure 5-5 Loran-C radio spectrum of 60 - 140 kHz as received in Delft.
Figure 5-4 Delay of the sky wave signal relative to the ground wave signal as function of the distance between transmitter and receiver for three different ionosphere heights.
antenna signal by means of a band-pass filter. The bandwidth of this filter should be wide enough to ensure pulse-shape preservation. This requires a filter pass band of at least 20 kHz wide. A narrower filter band will affect the pulse shape too much. Such a filter will also introduce a significant extra delay of the pulse envelope. Both effects represent the penalty for a good interference suppression. As usual for a technical solution, a good compromise has to be found.
Figure 5-7 shows the envelope response of a 5-pole Butterworth band-pass filter of 20 kHz on a logarithmic scale. The dotted lines represent the amplitude of the ground wave and of a sky wave with an ionospheric delay of 37.5 ms and with an amplitude of +12 dB above the ground wave.
In order to acquire a good signal-to-noise ratio, we like to sample the burst as close as possible to the maximum amplitude. When we accept a ground-wave-to-sky-wave ratio of 20 dB, then a phase- measuring error occurs of at most 5.7°, which will introduce a distance error of 5.7/360·3000 = 48 m. Figure 5-7 shows that it is allowed to sample the antenna signal till about 37.5 ms after the start of the pulse. The pulse amplitude of the ground wave is at that time 2 dB below the peak value of the pulse.
The solid lines represent the response of the ground wave and the sky wave after being processed by the band-pass filter. When we follow the 20 dB threshold, we notice that our measuring time is limited to about 65 ms. At that point, we see that the ground-wave amplitude level is 12 dB below the top of the pulse. Thus, the band-pass filter introduces a loss in the signal amplitude of about 10 dB. We see that bandwidth-limiting countermeasures only partly make sense. It would be more effective to exploit the properties of the relatively wide Loran-C spectrum (about 20 kHz) and of the narrow- band (few Hz) interference. This offers the possibility to apply narrow-band notch filters, which are capable to suppress the interference, if well tuned, without noticeably affecting the Loran-C spectrum.
Since the relatively wide Loran-C spectrum is constructed from a number of discrete components, the effects of interference are frequency and GRI dependant. The frequency components have a smallest mutual distance of ½GRI and, consequently, may vary between 5 to 12.5 Hz. Figure 5-8 shows some of these frequency components. Around these components, the frequency responses are depicted derived from the phase-lock-loop devices. When an interfering component coincides with a Loran-C spectral component, a stable amplitude and phase distortion in the latter component will originate.
Figure 5-7 Signal-envelopes of ground- en sky waves before and after the band-pass filter. The sky wave is +12 dB greater than the ground wave and has a delay of 37.5 ms. The band-pass filter is a 5-pole Butterworth configuration with a –3 dB bandwidth of 20 kHz.
components. Therefore, the cycle identification is a complex operation, in which we have to cope with all kinds of effects.
As seen from the transmitter’s point of view, the Loran-C system, as it is currently realized in major parts of Europe, is an “absolute” (and not a hyperbolic) localization system. This means that the absolute timing relative to UTC is checked in the monitoring stations (and not the time differences or hyperbolic values). This offers great advantages for the integration of Loran-C with other systems such as e.g., GPS or Glonass. The simplicity of the current atomic frequency standards makes the realization of absolute positioning systems hardly more difficult than the realization of an “old- fashioned” hyperbolic system. A drawback of absolute systems is that in local areas, such as the access to the “Nieuwe Waterweg” as the gateway to Rotterdam, there is no simple compensation for propagation changes. On the other hand, it is now much simpler to apply corrections for the whole local area by using differential methods (see section 9.2). But the user is still free in his choice to calculate the position by using the hyperbolic technique.
Although the Loran-C system exists already for a very long time, we see nowadays a revival of the system in Europe, Russia and Asia. The most important reasons for this revival are the fact that these countries do not want to become solely dependant on satellite systems (e.g., GPS, see section 6) and that the 100 kHz Loran-C signal penetrates better in urban areas than the 1.5 GHz satellite signals.
