SE468452B - Position-indicating system and position station for a position-indicating system - Google Patents

Abstract

A movable position station 1 in an aircraft contains a satellite receiver 4 which supplies position data for the station in addition to time signals in UTC time. The time signals control a time base with sufficient accuracy for a large population of stations to have access to well- defined time slots in a radio channel at a VHF or UHF frequency. All members of the population transmit identity and position signals, as a result of which each station can keep track of the positions of the others. A ground station at an airport can, by polling, control the otherwise spontaneous traffic when the aircraft is in its area. <IMAGE>

Description

468 452 2 participants around the world can be very large. The congestion on the radio channel that then arises requires special solutions.

The indicated problems are solved according to the invention by a position indicating system according to the characterizing part of claim 1 and by position stations presenting the features stated in the characterizing part of claim 8. ß A first condition for remedying the congestion in the air that must result from a large number of participants in the same channel around the world is that the range of each mobile station is limited by using a frequency without significant ground wave (VHF or UHF). The range is then limited to the line of sight distance, which for aircraft can be 400 km, for ships about 50 km. However, this is not enough, as a large number of mobile stations will accumulate locally. In aviation operations, for example, there may be 500 planes or more within a radius of 400 km.

A second necessary condition according to the invention is therefore that the radio channel used must be able to be tightly packed in time distribution, so that at least if necessary it must be possible to offer very frequent position determinations.

A wish according to the invention is then that mobile stations, which are located outside accumulation areas, should reduce their position-indicating frequency to a minimum, in favor of others.

According to the invention, these necessary conditions are not sufficient. A very strict division of time between the stations is also required, so that enough people can be accommodated in the same radio channel.

According to the invention, this time division is based on a precise time base common to all participants, from which useful time slots are calculated locally. For more local systems, special time signals from fixed stations can be used, - K 468 452 3 but with satellite navigation (GPS) a time base signal is automatically obtained, since each of the satellites transmits UTS time, so a mobile station can expect to have true UTS time with an accuracy of about 100 ns. Obvious military reasons mean that the time signals from the satellites are coded in a timed manner, but, as will be shown below, the accuracy is still sufficient for the purposes of the invention, in particular because everyone individually possesses such a time base.

Instead of such a true time standard, it is also possible to have one of the units in the population by lottery become a time master, so that all stations in the vicinity follow this, in terms of the distribution of time slots. However, given that the GPS system leaves such a good standard time, it is preferable to build on it.

A third condition according to the invention is that it must be guaranteed that two moving stations do not unknowingly interfere with each other. Even if a time channel was initially used as vacant, a moving station approaching may have seized the same time channel. None of them can then detect this, because they can not receive at the same time as they send. Special precautions are taken according to the invention to eliminate this risk.

Thus, according to the basic principle of the invention, in each mobile station time slots defined by a common time base are used, and each mobile station seeks to select time slots which are not occupied by any other mobile station. At the same time, each station listens to the predetermined radio frequency and calculates at least the relative position of the nearest neighbors in relation to its own position. Based on this, the own station chooses how often it will broadcast itself. An aircraft over the "empty sea" may send out its position island once a minute, while an aircraft in very heavy traffic may need to give its position at very short intervals. 4r> (p.

CO 452 4 In accordance with another approach to polling, the mobile stations can be subordinated to a central office, which orders them to no longer "freely" and spontaneously select time slots but only send on orders. In the vicinity of an airport, for example, nearby aircraft can be assigned a separate number and then ordered to transmit in numerical order in their respective successive time slots, which provides maximum density and optimal utilization. The air traffic control then has, in view of the high accuracies in the position indications, good opportunities to keep track of a much larger traffic intensity with computer support than is currently possible, observation. with only radar and visual The invention is not limited to use in the air and on the seas, but can also be used on land, to keep track of, for example, taxis or even animals, with appropriate equipment. provided In some cases and according to a particular embodiment, polling may go beyond just requesting a position indication for the moving station. Each mobile station has stored in its memory the positions of the other mobile stations which it itself has received. It is then possible to request transmission of this memory content from the exchange to the exchange, whereby stations can be located, whose own transmissions may not have been received by the exchange.

Based on the principle that all moving stations in the system have an accurate time base, the assignment / recourse of time slots can take place in many ways, as well as how long they should be, and the applied signaling speed and thus the bandwidth can vary.

Depending on the circumstances, it exchanges information. for each unit 150-200 bits. With a transmission speed of 9600 baud, such a message takes a maximum of just over 20 ms. ¿Switching from reception to transmission takes time, up to 1 ms, and since the station must listen to all time slots where it does not transmit, this dead time is preferably placed first in the time slot, whereby any next time slot message from another station also has time to listen. A suitable length of the 468,452 time slot can then be 25 ms, so that each minute comprises 2400 time slots. For practical reasons, it is appropriate to let a certain period, for example one minute, constitute a kind of maxi frame, which can conceivably be divided into mini-frames of, for example, 6 seconds, each comprising 240 time slots.

Each station listens to the traffic on the band and registers in its memory which channels are free and registers position etc. for the stations whose signals are perceived.

This information is processed automatically to determine how close the neighbors are and determine how often the transmission should take place and keep track of available channels. Those who do not have close neighbors do not have to send as often, for example only once a minute.

The information can also be prepared to be displayed on a monitor.

On a suitable scale, surrounding stations can then be displayed on a screen, with a vector whose length indicates the speed, and with a numerical display of the height. In contrast to what is obtained with radar, this image is also attributed to a fixed coordinate system ("plotted"). Consequently, a picture of possible collision risks is obtained in good time. - In some cases, for example to keep track of servicing vehicles at an airport, you can exclude the monitor in the vehicle. It is enough then if the management has the overview.

Each station also performs transmission, either autonomously or in polling mode.

In autonomous transmission mode, the transmitter selects a time slot that is not perceived as occupied by another station. To prevent more than one station from accessing and blocking the same time slot, something neither of them can detect, as they cannot receive when they transmit, a time slot is systematically changed, each time or at least often using a random number generator (pseudo -random-algorithm). For example, you can jump a random number of slots to a new free 468 452 6 slot each time. This eliminates the risk of any systematic mutual disturbance.

Polling is normally performed from a fixed station, for example from the air traffic control at an airfield. The fixed station has the same time base and can, in a free time slot, call mobile stations, which are assigned serial numbers and switch to stop with autonomous broadcasting and to send only on orders. A transmission order from the fixed station then polls these moving stations to transmit its information in succession in successive time slots, whereby the fixed station has the possibility to obtain position indications at any rate. Such group polling can be done extremely efficiently. In addition, time slots used are limited to 75% of the total number, but even with that limitation, in the case of example, 1,800 time slots per minute remain. If you then want to keep track of 50 planes, they can be polled 36 times per minute, ie at intervals of an average of 1.7 seconds. This should be compared with modern radar systems, whose antennas cannot be swept around at speeds greater than 6-8 seconds per revolution. Air traffic control can also view traffic on a monitor, on an appropriate scale, and maintain far better monitoring than has been possible so far.

For air traffic, the invention entails several advantages. A first advantage is that one can dispense with the hitherto used, on radio beacons building the flight corridors, which unnecessarily clogged the traffic and sometimes extended the flight path between airports. A second advantage is that the airline's ability to keep track of arriving and departing aircraft is improved. In many cases, the capacity of a runway can be increased. In calm winds, strong air vortices remain over the runway for one or another minute after landing, but normally these are removed very quickly by the wind, so even at moderate crosswinds the capacity can be significantly increased, as the safety distance between successive landing or take-off aircraft can be reduced without that the risk of collision is increased. When it comes to shipping, the big problem is that many heavily trafficked trails offer major problems, especially in bad weather, and furthermore collisions can have very serious consequences. Here too, therefore, the invention is valuable. In relation to radar, the advantage is achieved that the direction of the speeds on the monitor can without problems plot directly in the absolute coordinate system and not, as directly on radar, relative to the own vessel, something which in practice has proven to be misleading and accident-causing.

If both shipping and aviation are equipped according to the invention, each with its own operating frequency, an additional advantage is possible. In the event of a sea emergency, a distressed ship can be allowed to break into the frequency of the flight and issue an emergency message (SOS, MAYDAY), which is then probably perceived by an aircraft station, taking into account the high altitude. The flying station can then break into the ship frequency and there reach other ships, which cannot be reached by position signals from the distressed ship.

The following is an exemplary embodiment, which is intended to explain but not limit the invention. Fig. 1 shows a block diagram of a station according to the invention. Fig. 2 shows its communication processor.

Example A mobile station according to Fig. 1 comprises a unit 1 which keeps track of and manages the traffic, a presentation computer 2 and a monitor 3, on which a pilot can follow the traffic and visually control it. The unit 1 comprises a satellite receiver 4 for the GPS system, which receives signals from several satellites, which comprise time signals and path elements. From these, both local time signals and the position of the station are calculated, which information is available to the communication processor 5, which in turn is connected to a 452 8 transceiver. The satellite receiver 4 operates at 1.4 GHz, while the transmitter receiver operates at 438 MHz. Satellite receivers of a suitable type can today be obtained commercially.

The communication processor is shown in more detail in Fig. 2.

The following information is transmitted during each broadcast. l. Start flag and code for independent broadcasting or polling broadcasting. 2. Identity of the station. 3. Position in longitude and latitude. 4. Height and direction.

. Speed. 6. Time when this data was valid (0-60 seconds). 7. Check amount. 8. Final flag.

As shown in Fig. 2, the communication processor 5 includes a microprocessor 10, a RAM 11, a program memory PROM 12 and a timer circuit 13, all cooperating via a data bus 14 and an address bus 15. For connection to other units there is a serial communication circuit 16 and, for transmission and reception, an asynchronous communication coprocessor 17.

The timer circuit 13, which keeps track of the time distribution, is fed from the GPS unit 4 (Fig. 1) with time-synchronized signals in UTC via a line 18, whereby a time signal per second together with additional time information is received from the GPS system.

The drive units 19 and 20 complement for suitable adaptation.

A directory of all received signals from other stations is stored in the RAM 11, so that the identity and position are stored and updated. All audible participants will be questioned within the maximum frame, and, so that obsolete participants do not fill the memory, will be removed if they are not questioned again within a certain time. Furthermore, information is stored regarding which channels are available. The communication processor also determines the transmission depending on the traffic density or after polling. 468 452 9 The presentation computer 2 retrieves its data from the directory in the communication processor's memory and prepares the information, partly for the monitor and partly to be able to draw the pilot's attention to any necessary measures. Since during long-haul flights the pilot usually experiences only rare stimuli, it is difficult to keep full attention, and it is therefore very valuable to be able to emit clear signals according to certain criteria (another transmitter close, on the way to one's own station, etc.). calls for attention.

For the system's function, it is very important that all stations have access to a time base with significant security. This is achieved in the example in the timer circuit 13, which on the basis of time pulses obtained from the GPS unit can synchronize the clock frequency of the processor once per second within the required accuracy, i.e. so that assigned or resorted time slots are contained. It is also possible, if for some reason the GPS system should fail, to take help of the signals received by adjacent stations. The own position can still be known, for example by death count (inertial navigation) but with lower accuracy. Suitably, information sent out then also includes information that the position indication is less accurate.

Of course, in order for the invention to be applicable in a worldwide system, it is required that a standardized protocol is agreed, both for frequencies and more detailed execution of, for example, time slots and frames. What has been described here can therefore only be regarded as a non-limiting example of how the inventive concept can be applied.

Claims (10)

IG 468 452 10 15 20 25 30 35 Patent claim Position indicating systems with a population of simultaneously operating moving stations, each of which knows its geographical position by receiving signals from a number of geometrically distributed transmitters with known positions, each participating station having a transmitter for transmission in a common radio channel of signals indicating its own geographical position, and memory means are provided for memorizing information obtained from other participating stations, characterized in that each said moving station has a) a time base (13) which is accurately controlled by time signals from said number of geometrically distributed transmitter, and which time base defines time slots that are standardized, countable and form a common, accurate, predetermined and repeating maximum frame, and b) means for accessing an unoccupied time slot in each maximum frame and for autonomously transmitting a position signal therein in the common radio channel. Position indicating system according to claim 1, characterized in that the common radio channel has a frequency so determined that its detection distance is substantially limited to the line of sight distance and said number of geometrically distributed transmitters comprises satellites which transmit time signals and each participating mobile station has means for calculating from the time signals its geographical position and an absolute time for updating its time base using the said transmitted time signals. Position-indicating system according to Claim 2, characterized in that the movable stations are arranged in airplanes which can be distributed around the earth. Position indicating system according to claim 3, characterized by means in each movable station for sensing a transmission order signal from a ground station and for terminating said autonomous transmission to instead transmit in a ground station ordered mode, in time slots indicated by said ground station. Il / 10 15 20 25 30 35 468 452 ll Position indicating system according to claim 3, characterized in that each movable station is provided with means for systematic replacement of used time slot, and means comprising memory means for registering free time slots in said maximum frame, in which no transmission from other stations are detected, the means for systematically changing the time slot being arranged to select a new free time slot at predetermined time intervals for its transmission in the maximum frame. Position indicating system according to claim 5, characterized by means in each moving station, which are arranged that when no free time slot is detected, access a time slot which is in use of a geographically remote moving station. Position-indicating system according to claim 1, characterized in that each mobile station has means for determining its geographical distance to the nearest mobile neighboring station, and means for determining as a function of said distance the number of time slots in each maximum frame to be transmitted, for limiting the load on the common radio channel. Position station for a position indicating system, which station comprises a GPS satellite receiver arranged to deliver the geographical position, and a transceiver for transmitting this geographical position in a radio channel, characterized in that it comprises a communication processor (5) connected to the satellite receiver (4). ) and to the transmitter, and which radio channel has a frequency with a physically limited range, and which communication processor has a time base (13), a time synchronizing connection between the time base and the satellite receiver, a microprocessor (10), a RAM memory (11) for collecting position signals received by the transceiver in time slots determined by the time base, a program memory (12), a data bus (14), an address bus (15), means for recording position messages received from adjacent stations and retrieved from the RAM, and means for transmission by auto- A 10 f. UOU 452 12 nomt position of position signals to the transceiver for their transmission ing in time slots not occupied by other stations. Positioning station according to claim 8, characterized in that said range of the radio channel is substantially limited to the aiming distance. Positioning station according to claim 9, characterized by means for sensing ground station order signals arriving from a ground station and for transmitting position signals in the radio channel only upon receipt of such an order signal only according to orders from the ground station instead of autonomously. '91