BACKGROUND OF THE INVENTION Systems combining a device for determining the position of an object with a transmitter arranged to send the position of the object to a central unit at regular intervals are today com- mercially available. The device for determining the position may for example be a GPS-receiver. GPS-receivers receives short radio sequences from a number of satellites intended for this purpose, and can from the time difference between the re- ception times for the sequences from the different satellites calculate the position with a high degree of accuracy. The transmitter may, for example, be a GSM-based mobile phone, or an ORBCOMM-transmitter. The GPS-receiver states the calculated position of the object and the GSM-transmitter sends this to a central unit.

The system regularly sends position information to the central unit, but if the system is outside the area of radio coverage, the GPS-unit can no longer calculate the position of the sys- tem. If a vehicle with such as system transmits an alarm, or if the owner of a rental car or some other institution, such as police or medical services has a reason to find the vehicle quickly, they only have access to information regarding the latest position of the vehicle. If the vehicle is in a multi- storey car park, or if a person carrying such a device is in a

multi-storey building, the effort needed to find the system may be considerable and the time required correspondingly long.

DISCLOSURE OF THE INVENTION It is a purpose of the invention to provide a system, a device and a method, with which the disadvantage that only informa- tion regarding the latest position of the vehicle is available if the device is outside the radio coverage area. This and other purposes are achieved with a system, a device and a method according to the characterising portions of the inde- pendent claims. Hereby one of, or several of, the advantageous features and functions below are achieved.

According to the invention a system is achieved, which based on the latest stated position of the GPS-device, and using an altitude meter can calculate at least the present vertical po- sition of the device, even though the device is out of reach of radio coverage from GPS-satellites. From the information regarding the vertical position of the device, the storey at which the device is can be calculated.

According to another aspect of the invention a device is pro- vided, which may be manually or automatically activated to send an alarm, comprising the position of the device, to a central unit.

In addition, an improved device according to the invention is provided, which may be manufactured at a low cost compared to the cost of a system according to prior art, and with a rea- sonably low technical development effort.

OVERVIEW OF THE DRAWINGS

Figure 1 schematically shows an overview of the total system.

DESCRIPTION OF AN EMBODIMENT Figure 1 schematically shows the whole system, comprising at least one object with a mobile unit 2, the GPS-satellites 8, the central unit 1 and customer units 6. It is further shown that the mobile unit 2 comprises a positioning device 4, a transmitter 3 and unit for altitude measurements 5, and that each mobile unit 2 is provided with an ID-number 7. The posi- tioning unit 4, which typically is a GPS-unit, receives radio signals from the satellites 8, and can from these signals cal- culate the position of the mobile unit in three dimensions.

The positioning unit 4 can detect that the satellite signal has disappeared, for example when the object with a mobile unit is in a garage or multi-storey building. When the satel- lite signal disappears, the transmitter 3 is arranged to store the latest calculated position in the horizontal plane and calculate the high above the ground by using information from the altitude meter 5. In order to give a usable result from the method, it is assumed that the system has got out of the radio coverage area inside a building which essentially ex- tends vertically, such as a multi-storey car park. It can then be assumed that the system is inside the building where the signal disappeared and based on information from the altitude meter the storey at which the system is can be concluded.

The information from the transmitter, which may consist of an SMS-message, an E-mail or some other means of transferring the information, is transmitted to the central unit 1. The central unit 1 stores received information for each object, and can forward it to a customer unit 6, or the central unit 1 may be situated at the customer and thus constitute the only customer unit.

The altitude meter 5 may be embodied in many different ways, as an example an air pressure meter can estimate the height in relation to the starting point where the GPS-receiver got out- side the area of satellite radio coverage, if the air pressure in the building changes as a function of the elevation in the same way as it does outside buildings. This is fulfilled by an open car park or another open building. In an at least par- tially closed building, particularly a heated one, the pres- sure as a function of the height above the ground for the building may be different and other kinds of measuring devices may be a better choice. In such a case can, for example, meas- uring results from an accelerometer, which gives the accelera- tion in the vertical direction, be integrated and a relative altitude be derived from this value, alternatively other kinds of gauges can be used, such as sensitive gravitation gauges.

In an alternative embodiment of a system according to the pre- sent invention, which is specifically intended to function as a mobile alarm system, the invention is also provided with an alarm switch 9, which directly or indirectly can be manually activated by the user of the system. The manual alarm function can be activated with an immediate response to the user's ac- tions, or alternatively when the user does not deactivate an automatic alarm function which reoccurs regularly with a cer- tain time interval, or as a result of some other stimuli, such as for example that a detector senses a drastically worsened physical state of the user of the device. The last example may be applicable for carriers of the device that sufferers from a heart disease, diabetes or some other disease that may demand that precise information regarding the position of the carrier rapidly reaches a hospital or some other authority. When the alarm function is activated a particular message is transmit- ted from the transmitter 3 to the central unit 1, which is

forwarded, when applicable, to the correct receiver of alarms, in the figure denoted as customer units 6, which in addition to the normal information regarding position, time and the identity of the unit, states that the alarm has been acti- vated.

An alternative to an internal altitude meter in the system is to place local transmitters in the building that transmits a simulated satellite signal, which gives the position inside the building by radio signals which are interpreted by the re- ceiver as originating from satellites.

Even if the invention has been described in connection with a preferred embodiment, it should be understood that a plurality of modifications may be done without departing from the scope of the invention. The positioning unit 4 many for example cal- culate its position by using signals from GSM base stations, instead of signals from GPS-satellites.