N.S. Khokhlov1

1 JSC «Russian Institute of Radionavigation and Time» (Saint Petersburg, Russia)

1 hohlov_ns@rirt.ru

Currently, pulse-phase radio navigation systems (PPRNS) of long-wave range are considered as backup positioning, navigation and timing systems in absence of global navigation satellite systems signals [1,2]. Therefore, development of these ground-based systems is of great practical importance. PPRNS in addition to positioning, navigation and timing measurements (PNTM) provide the possibility of information transmission [3–5]. Currently, the information transmission rate over this channel ranges from 50 to 100 bps. However, in order to meet the increasing requirements of consumers, it is necessary to ensure the information transmission rate of at least 1000 bps. The purpose of this paper is to evaluate the existing problems of increasing throughput capacity of the information channel of PPRNS, to review the used and known methods of information transmission through this channel, to identify possible ways of overcoming existing problems.

Throughput capacity of the PPRNS information channel is limited by the following: permitted bandwidth (determined by the regulatory documents) [6]; features of the radio signal generation in the transmitter (the transmitter operates on a resonant load, switched mode methods are used to generate the radio signal); requirement to provide PNTM. The resonant load (antenna circuit) has a rather high quality factor, which enables high power of the radiated radio signal, but leads to narrow bandwidth: from 2,5 to 5,0 kHz. The switched mode methods [8–10] used to generate the radio signal allow to provide the specified envelope parameters and phase stability of the radio pulse, which are of fundamental importance for accurate PNTM. In addition, they allow to provide a high efficiency of the transmitter (80–90%). The information transmission should not interfere with PNTM. In this regard, the use of time and frequency resources occupied by them for the information transmission is limited. Thus, the issue of organizing the information channel is to identify a free resource and its fullest use.

Let us determine the potentially possible throughput capacity. Suppose that only the first area of the radio pulse with a duration 40 μs is used for PNTM. Then, with the number of pulses in the group 10 and group repetition interval (GRI) 40 ms, the time interval used for 1 s for PNTM is 10 ms. Other time can be used for information transmission. Then, at signal-to-noise ratio 5 and bandwidth of the channel 5 kHz, throughput capacity is 12790 bps.

The overview of the used and known modulation methods is given in Table 2 [3–5,11–15]. The comparison of the methods presented in Table 2 by information transmission rate is shown in Fig. 4. It is shown that the highest information transmission rates (up to 540 bps) are achieved by methods that use time intervals that are not occupied by PNTM, and attempts to implement even higher rates (up to 2 500 bps) indicate the desire of developers to use the time-frequency resource most effectively (in this case radical changing the used signal-code constructions does not stop them).

The possibility of implementing UNB 50 kHz method using known models of radio transmitters, simultaneously with generating the main radio signal, has not been considered by the authors of the article [13] and requires additional research. This method assumes a change in the phase of the signal by a jump of 180°, which is not possible in known models of radio transmitters.

Loran-5G method [14,15] assumes the generation of 222 radio signals simultaneously by a radio transmitter at orthogonal subcarrier frequencies. Moreover, the accuracy of the generation of a radio signal with a given frequency should be at least 15 Hz, the transition to another frequency should be carried out (in general) every 166,67 ms. In addition, the method assumes a 90° jump in the phase of the signal. The implementation of these modes in known models of radio transmitters is not possible. Meanwhile, the authors of the article [14] and the patent [15] did not consider the issues of building a radio transmitter.

The most perspective modulation method for increasing throughput capacity of the information channel is phase (frequency) modulation (during time intervals that are not used for PNTM), in this case the phase change of the current in the antenna circuit should be continuous. This modulation can potentially be implemented by switched mode methods.

The obtained results confirm the existence of a potential possibility to increase throughput capacity of the information channel of PPRNS, identify the main limitations, summarize scientific and technical experience and allow to identify the most perspective ways of overcoming existing problems.

Khokhlov N.S. Problems of increasing throughput capacity of the information channel of long-wave radio navigation systems. Achievements of modern radioelectronics. 2023. V. 77. № 12. P. 81–89. DOI: https://doi.org/10.18127/j20700784-202312-10 [in Russian]

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