Prospects for Application and Further Development of Technical Modules on the Base of DOMESTIC Chipsets
According to preliminary analysis, LNS-Progress owns a flexible architecture and adjustable parameters, carries out receiving in synchronous and asynchronous mode which makes possible both active and passive navigation. In case of using PRS as operating signal, there is a possibility of choosing its parameters:
length starting from 1024 symbols — PRS1024;
permissible deviation of PRS1024 frequency from nominal value ±10kHz;
operating range of 0.1–2.5GHz;
clock frequency of 1MHz
modulation — BPSK;
interval of PRS1024 arrival measurements in LNS — 1ms;
instrumental error of PRS1024 arrival measurement in the mode of navigation does not exceed 1ps, in asynchronous mode — less than 10ps.
In case of increase in signal duration, particularly for PRSN1024, permissible frequency mismatch for input signal relative to rated frequency decreases by a factor of N, if interference tolerance increases by the same factor of N. LNS-Progress includes measurement/navigation channel and handshaking channel. In particular, in case of avionics it is signal with GFSK modulation in 25kHz band. Apart from handshaking, it may be used for creating aviation self-organizing networks AZN-В. The presence of micro-controller and 1MB RAM within K1917ВС014 makes it possible to use digital HC for generating PRS with the length of at least 2200 and for solving navigation problems.
It is known that largest problems arising in the course of local navigational network creation are related to the presence of the statistically uneven boarder with repeated reflections of navigation and communication signals from it. On the one hand, this affects the accuracy of navigation, while transmission capacity of communication ground line mismatch with the rate of data entry in LNS results in the information distortion in communication ground line and in LNS reset. For this reason in some missions it is necessary to reduce the receiving data flow intensity. It can be done, for example, by means of restricting the amplitude. For matching throughput capacity of ground communication line with the rate of incoming data flow it is possible to make use of conversion time — spatial coordinate, and then to reduce the rate of transmitted reduced message down to the value, which is matched with the communication channel. Expenses of the proposed matching method are attributed to frequencies instability at LNS reception centres, with rate distortions of information sequence transmission from one centre to another, and the presence of multipath propagation interferences (MPPI), but with considerable restriction of data transmission rate it is possible to completely remove MPPI (which is one of the reduction purposes).
As a rule, LNS structure is an external perimeter that forms navigation signal. Fig. 1 shows generalized LNS block-scheme. It is assumed that navigation field of LNS enables specifying subscriber’s coordinates relative to the certain coordinates system. In most general form, LNS is based on elevated above the ground surface centres with allocated transmitting equipment: surveying rods, buildings, aerostats, airships, aircraft (AC), unmanned aircraft (UFA), piloted AC (PAC), satellites (low-orbit) with stable orbits, in particular geosynchronous. The range of LNS action, D, in case of unequipped territories is restricted by radio horizon specified by approximate formula (1):
where D(km) is the range of reliable signal reception in kilometers; К = 4/3 — refractive index; h1 — altitude of LNS reference station antenna in meters; h2 — altitude of LNS user station antenna in meters.
It is obvious that due to servicing region echelonment, LNS is restricted only by the ability to form the area of its operation and by nothing else.
Among LNS most exotic are covert valuable cargo searching and rescuing systems. The following scheme (see Fig. 2) presents the formation (navigation field) and interaction of LNS with “valuable cargo (VC)”.
Aircraft emits PRS and transmits its current coordinates and time.
On VC side this information is received.
After reception of the values from at least 3–4 AC path points, VC computes mutual AC-VC coordinates in AV coordinates system.
Then VC directs PRS signal at AC, transferring its own coordinates and its own accurate time.
Having received this data, AC heads for VC, takes it away and thereby rescues it.
The specifics of the developed LNS is in that it exists virtually (all the points which are used for computing VC coordinates are known only in the past), that is LNA is synthesized by permissible dimensions, and does not actually exist. One might direct four AC at VC detection and rescue, but in real conditions it is very expensive, so in practice an acceptable solution is the one based on construction and application of synthesized LNS.
Characteristic of Local Navigational Systems
It is possible to consider as LNS any system determining its own coordinates by observed navigation field parameters, in which it is submerged. Thus, the LNS concept is associated with the following two essences:
Navigation signal (field);
Navigation field (NF) sensor — sensor.
Global navigation satellite systems (GNSS) stand out against LNS, though on the other hand, they differ from LNS only in the boundaries of the system forming area.
A distinction is made between:
LNS with fixed base;
LNS with mobile base;
LNS also differ in the application environment: ground, underground, above-water, underwater, space etc. Sometimes it is possible to give up LNS characteristic as a network, focusing only at navigational aspects.
Common Timing System (CTS)
The next LNS characteristic is temporal instability of its components and user’s sensors. Common timing system is responsible for time in all LSN points: at reference stations. If the user knows his own time in CTS scale, then by determining the time of signal arrival from reference station (RS), the user may specify the distance from that station. In plane geometry (ground LNS) it is sufficient to use two reference stations to determine their own in-plane coordinates, in 3D LNS — three reference stations. This is a very important provision, because it makes it possible to sufficiently reduce computing complexity and to improve the accuracy of coordinates computing, spending the same time as the counterpart. If the user time is not synchronized with LSN time, then, by measuring pseudo-ranges to three reference stations and current time, it is possible to restore (for plane LNS) user’s coordinates and system time in CTS scale. Measuring four pseudo-ranges to reference stations and time enables restoring their own coordinates and time in 3D LNS.
Another important issue is related to the presence (absence) of communication system embedded into LNS. The absence of embedded communication system implies prior knowledge of LNS geometry and schedule of emitters operation at reference stations. Contrariwise, communication system permits informing users and other constituting parts of LNS on these parameters, which makes it possible to implement any type of local navigation system. It is natural to require that the communication system does not disclose the fact and the modes of LNS operation, meaning that its signals should have the same format as LNS signals in the mode of navigation.
LNS synchronization depends on its assumed accuracy and imposed requirements for cyber security. For example, LNS “Lokata” (issued in 2013, developed in USA) has an open synchronization system with central signal generator and receivers located at reference stations, by whose signals reference stations are synchronized. Examples of such synchronizing systems are well known for many decades, e.g., Russian station “Mayak”. It is impossible to assert that it possesses high stability to natural interferences and, moreover, to jamming. On broken terrain overgrown with vegetation, the phase of signal arriving at the reference station depends on a sum of partial constituents that arrived by various paths. In case of radiolocation such constituents are called interferences of multipath propagation. Accordingly, even in the absence of jamming, an open synchronization system may fail to provide LNA coherence. Phase hopping at reference stations attributed to wind-dried instability results in the appearance of significant errors in the open type system. Jamming, false or simulated echo in the synchronization system easily disable such LNS system. But in the absence of natural interferences and jamming, navigation by its own signals makes it possible to provide error of its own coordinates detection up to the phase fractions, that is 5–10cm (averaged at one second interval).
Types of Signals Used in LNS
Originally, LNS used rectangular signals, whose generation is rather simple and well utilized. However attempts to improve the accuracy of direction finding, for example, in aviation and neighbouring fields [3–5], in the presence of constrained type signals, enforce the system designers to examine other types of signals.
Instrumental Error of LNS-Progress
The results of LNS technologies experimental investigations into determination of correlation function of spikes position measurements instrumental error were kindly granted by Tatarchuk I. А. and Grigoriev I. D. Fig. 3 presents spikes of correlation function (CF), and Fig. 4 presents the respective image magnified 20 times. CF’s interval is 20ms, CF length is 1024ms, clock frequency is 1MHz, root-mean-square error of CF spike position detection in case of single measurement is 0.1ns.
It follows from the diagrams that CF spikes reduce amplitude from one function to another, that is they respond to variations of transmitter frequency relative to receiver frequency. It also follows that averaged at one second interval root-mean-square error equals 3.3ns.
Illustration of Synthesized Local Navigation Systems
Main task: “Detection and rescue of objects, particularly human being, at uncontrolled territory”.
As noted above, the complexity of its solution lies in that several parties (not always friendly to one another) may be simultaneously involved in object detection. The object is interested in a positive result of only one party. That is why the main question is: how to construct the scheme of detection and rescue that will make it possible to covertly inform its own location to a friendly party. Aircraft (AC) is used for searching, and detection itself is conducted in radio frequency band with the aim of assuring both all-weather operation and covertness . Most suitable is multi-positional aviation system (MPAS) [7, 8], enabling one to determine with high accuracy coordinates of radiocontrast object. But as a rule, the number of AC in MPAS, directed at detection and rescue operation, does not exceed one (otherwise it is expensive), and signal for rescue may be sent only once, for the same reasons — to assure covertness. That is why it is necessary to use pseudo-random signal (PRS). The scheme itself may look the following way (see Fig. 5): The aircraft follows a certain trajectory and scans using pseudo-random signal the ground surface within the region of supposed location of the object being searched. Each PRS is marked by the time when it was emitted (in time scale of LNS) and by coordinates of AC location. The object being searched receives a part of signals emitted from the aircraft, and knows the time when PSR was emitted as well as AC coordinates assigned to the time when the signal was emitted. It measures the time of PRS receipt in its own time scale.
The object measures the time of PRS arrival, knows the time when they were emitted and computes the difference of PRS time arrival, as if all these PRS were emitted simultaneously.
Since coordinates of 1 ÷ N points are known (have been reported), the object computes its own position coordinates relative to geometrical figure defined by 1, 2, 3, …, N points, that is now it knows the aircraft position relative to itself. If it has a compass, it can determine the direction to the aircraft and after that it transmits by directional antenna its own PRS to the aircraft, reporting its own coordinates, computed relative to the system of 1 ÷ N points. The aircraft receives its PRS, determines the time of its arrival, and computes the distance to the object. Then it specifies its location. And the task is completed.
Fig. 6 illustrates the second part of search and rescue task (signal transmission to the AC).
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