Method and system for finding the position- of mobile terminals.
BACKGROUND OF THE INVENTION
The invention relates to a method for finding the position- of mobile terminals capable of setting up a link with base stations of a cellularly set up transmission network, a terminal measuring the field strength of its adjacent base stations and recording the combinations of identifier and measured field strength of the nearest base stations according to the field-strength measurements. The invention also relates to a system for carrying out the method.
In known systems, on the basis of the field strengths measured by a terminal of a number (e.g., six) base stations and the (known) positions of said base stations, it is estimated by a central server what the position of the terminal is. The drawback of the known method is that the result is no more accurate than about plus or minus 500 metres. The cause of said inaccuracy is particularly the disturbance of the field-strength image excited by the local base stations as a result of landscape obstacles (forests, hills) and architectural obstacles (flats, electricity masts, factories, offices) , as a result of which the field-strength image deviates far from the theoretical ("ideal") field-strength image.
SUMMARY OF THE INVENTION
The invention proposes a method and system with which it is possible to achieve a considerably more accurate result, namely, location finding with an accuracy of approximately plus or minus 5 metres .
The method according to the invention provides for the terminal passing on the recorded identifier-field-strength combinations to a location-finding server, comprising a data base having stored therein the positions of a fine-meshed geographic matrix - having meshes of, e.g., 5x5 metres - with associated identifier-field-strength combinations measured in situ in advance, in which location-finding server the identifier-field-strength combinations transmitted by the terminal are compared to the identifier-field-strength combinations stored in the data base and the position having the best-matching identifier-field-strength combination is determined. The system according to the invention, suitable for implementing the above method, comprises the position-finding server and data base having the identifier-field-strength combinations measured in situ as already referred to, in which the identifier-field-strength combinations transmitted by the terminal are matched with those stored in the data base.
As referred to in the foregoing, the terminal is capable of receiving, from at least one of the adjacent base stations, rough position-finding data (having an accuracy of about 500 metres) . Said rough data may be passed on, if so desired, by the terminal to the position-finding server and be used by the position-finding server for verifying the outcome of the matching process. Due to this, the result will even gain in reliability. The position of the terminal, determined and possibly verified by the position-finding server, may be returned to the mobile terminal whose position was determined, but said position, calculated by the position-finding server, may also be transmitted to another terminal or system, e.g., a monitoring system for mobile terminals of a transportation company. The method and the system according to the invention in practice may be implemented without exorbitant investments if, as a data base for the position-finding server, there is utilised a data base, usually already present in a cellular wireless transmission system, which primarily serves for planning and managing the geographic position, field strength etc. of the base-station system of the cellular transmission network.
The results of the position finding by the position-finding server may still be improved if, in the calculation of the terminal position, preceding position findings are taken into account, e.g., by calculating the speed and direction of motion and extrapolating said data.
EXEMPLARY EMBODIMENTS
FIG. 1 schematically shows an exemplary embodiment of a system for finding the position of the mobile terminals 3, which are capable of setting up a link with base stations 2 of a cellularly set up transmission network 1. A terminal 3 comprises means (not shown) for measuring the field strength of its adjacent base stations 3 and for recording the combinations of identifier and measured field strength of the nearest base stations, according to the field-strength measurements. To the network 1, there is connected a position- finding server 4, to which the termnal 3 passes on the recorded identifier-field-strength combinations. The position-finding server 4 comprises a data base 5 having stored therein the positions of a fine-meshed geographic matrix having associated identifier-field- strength combinations measured in advance in situ. The position- finding server 4 compares the identifier-field-strength combinations transmitted by the server with the identifier-field-strength combinations stored in data base 5 and determines the position with the best-matching identifier-field-strength combination. In order to clarify all this, FIG. 2 shows a geographic map of the area in which the terminal 3 is located at a specific point in time. Terminal 3 sets up a link with the network 1 shown in FIG. 1 and the position-finding server 4 by way of the base station 2. Terminal 3 measures the field strength of the nearest base stations 3, in FIG. 2 - based on the relatively high field strength - the base stations denoted by G, I and J. Terminal 3 records the combinations of identifier (in this case "G", "I" and "J") and measured field strength for each of said base stations. Suppose that the terminal records the following combinations: "G36 131 J69", in which the letters denote the station identifier and the numbers against them denote the field strength measured by the terminal.
To the network 1 shown in FIG. 1, there is connected a position- finding server 4, to which the terminal 3, by way of a base station 2 (G, I or J) passes on the recorded identifier-field-strength combinations. The position-finding server 4 comprises a data base 5 having stored therein the positions of a fine-meshed geographic matrix having associated identifier-field-strength combinations measured in advance in situ. Part of the contents of data base 5 looks, e.g., as follows. From left to right, one sees, of the boxed area in FIG. 3, the identifier-field-strength combinations measured in the various areas (in situ): "p05 G15 105 J70", e.g., indicates that area p05 was the measured field strength of base station G, 25 was the field-strength unit, the field strength of station I was 5 units and the field strength of station J was 70 units. Only the field strengths greater than zero are noted, and therefore not, e.g., the field strengths of the base stations A, B, C etc., which are too negligibly small in said areas.
p03 - - - p04 - - - p05 G15 105 J70 p06 G25 110 J75 p07 G 0 120 J70 p08 G50 130 J60 p09 G60 140 J50 plO G€O 150 J40 pll G50 170 J30 pl2 - - - pl3 - - -
q03 - - - q04 - - - go5 G20 105 J80 q06 G25 110 J90 q07 G30 120 J80 qOΘ G35 130 J70 q09 G40 140 J50 qlO G35 150 J40 qll G 0 160 J30 ql2 - - - ql3 - - -
r03 - - - r04 - - - r05 G10 105 J95
r07 G25 120 J90 r08 G30 135 J75 r09 G35 150 J60 no G30 170 J 0 rll G25 180 J30 rl2 - - - rl3 _ - -
It should still be noted that, in FIG. 3, the "iso-field-strength lines* (for simplicity's sake) are drawn circularly. Such would only be the case, however, if there were no obstacles in the vicinity of the base stations 2, such as (schematically shown in the figures 2, 3 and 4) forests, buildings, hills etc. Such obstacles cause field- strength disturbances, as a result of which the field-strength course in practice is significantly more erratic (non-circular "iso-field- strength lines") . A great advantage of the invention is that, however erratic the field-strength course is, it is always stored in the data base 5; after all, the data base 5 is a reflection of the real, non-ideal local field-strength image measured in situ. In fact, this is the reason why the system according to the invention provides such a great accuracy in position finding. Of course, it is a condition that in the data base 5 the ID-field-strength combinations stored therein be kept up to date by, in the event of any change in the local terrain which may affect the field-strength image, having the manager of the system carry out new field-strength measurements in situ and having the measurement results entered into the data base.
The position-finding server 4 compares the identifier-field- strength combinations transmitted by the terminal, "G36 131 J69", with the identifier-field-strength combinations stored in the data base 5 - see example - and determines - by way of a matching algorithm - the position having the best-matching identifier-field- strength combination "best match". In the above example, this is "q08 G35 130 J70", which signifies that terminal 3, based on the field strengths measured by the terminal on the one hand, and on the other hand the previously measured field strengths stored in the data base 5, would be located within the area that is denoted by the coordinates q08. FIG. 4 demonstrates that the terminal is indeed located within area q08.
As the data base 5, there is preferably used a system data base which is already in use and is kept up to date for planning and managing the geographic position, field strength etc. of the base- station system of the cellular transmission network. Inter alia it is calculated, using said data base, where new base stations must be placed. In the event of modifications in local situations, e.g., in the event of the construction of architectural works which may affect the effective range of base stations (more attenuation) , there are carried out new measurements at such locations and the data base is updated based on the results thereof. When in specific areas the field strength of the adjacent base stations is too low (this is often pointed out by terminal users and passed on to the network manager) , as a rule there will be added a new base statio .
Subsequently, in this area field-strength measurements are carried out once again and the results thereof are entered into the data base 5.
It may be that, in looking through the data base, there is not found one, but two or more good matches, as a result of which it is unclear at which position the terminal is located.
In order to increase the score even more, use is preferably made of the option that the terminal receives rough position-finding data from the nearest base station, the base station having the greatest field strength (in our example, station J having a field strength of 69) - by way of which the link to the network runs at that point in time. Said position-finding data indicates the estimated distance to the base station J with an accuracy of about 500 metres. Apart from the identification-field-strength combinations, the terminal now also passes on the rough distance indication from station J to the position-finding server 4.
Position-finding server 4 now verifies whether the co-ordinates of the "best-match" location correspond to the rough Indication with respect to base station J.
The position of the terminal determined and possibly verified by the positions-finding server may be returned to the mobile terminal whose position has been determined. It is also possible, however, that the terminal position determined by the position-finding server is passed on to another - mobile or nonmobile - terminal or to a terminal-monitoring system with which, e.g., a fleet owner may monitor where his lorries are located.
The calculation algorithm of the position-finding server may still be refined by, in the calculation of the terminal position, involving preceding position determinations. Thus, the position- finding server 4 may calculate, from consecutive position calculations, the speed and direction of motion of the terminal and extrapolate said data in the calculation of the new terminal position. Based on said new estimated terminal position, the "best- match" result may then once again be verified or corrected.