fig1 illustrates a floor 100 of a building which has been fitted with a real time location system embodying the invention . the floor 100 is divided into a number of separate spaces 101 to 108 . 101 is a stairwell with stairs leading up and / or down to floors above and / or below . 102 to 107 are various rooms and 108 is a corridor . in particular , room 105 is a monitoring room containing a central computer 130 which may be monitored by an operator 134 . it should be appreciated however that the computer 130 may be located elsewhere within the building or completely off site , connected to the system through a direct connection , or over a network and / or the interne . fig2 schematically illustrates a mobile tag 140 and a base station 110 in communication with each other . the base station 110 is fixed to a static structure of the building such as a wall , floor or ceiling . it has a receiver 220 and a transmitter 222 . these may be any kind of wireless transmitter and receiver , but most conveniently ultrasound or radio frequency ( rf ) electromagnetic communication are used . ultrasound is advantageous in care environments as electromagnetic waves can interfere with other equipment in the building . ultrasound also does not penetrate the walls and so interference between base stations or from background noise is reduced compared with electromagnetic transmissions . combinations of ultrasound and electromagnetic communication may be used . the base unit 110 includes an air pressure sensor 224 and a temperature sensor 226 . these are illustrated separately , but in practice the two sensors could be combined on a single sensor circuit . the base unit 110 also includes a processor 228 and memory 230 . the processor 228 controls the transmitter 222 and receiver 220 for transmitting and receiving data and also reads data from the pressure and temperature sensors 224 , 226 . memory 230 may be used during the processing procedure and may also store historical data and / or preloaded values such as predetermined values or thresholds for use in calculations and / or database structures ( which may be populated or unpopulated with data ). all of these data values may be updated during the course of operation . the mobile unit or tag 140 is mounted on a strap 202 which can be used to attach the tag 140 to a person or object . in some preferred embodiments , the strap 202 is an arm band . the tag 140 has a transmitter 204 and a receiver 206 . as with the base unit 110 , these may be ultrasound transducers or rf antennae or combinations of both . the tag 140 also has an air pressure sensor 208 and a temperature sensor 210 similar to the base station 110 . again , the transmitter 204 , receiver 206 , pressure sensor 208 and temperature sensor 210 are all operated by a processor 214 which again has a memory 216 available for use . additionally , a motion sensor 212 is provided ( also connected to the processor 214 ) for use as described below . for the pressure sensors and temperature sensors 208 , 210 , 224 , 226 the units 110 , 140 may use the scp1000 from vti and / or the ms5607 from meas switzerland . these devices are low power , relatively inexpensive and can provide a height measurement accurate to within 10 cm . both units also incorporate a temperature sensor . first the overall infrastructure will be described with reference to fig1 . the description below is given in relation to a care environment where the tags 140 are attached to patients and the system is arranged to detect emergency situations when a patient collapses . a number of tags 140 a - d are shown in different locations within the floor . one or more base stations ( base units ) 110 are provided per floor 100 of a building , each of which contains an air pressure sensor 224 ( shown in fig2 ). the base stations 110 are statically located , i . e . they are at known fixed positions . the height above the floor of each base station 110 is known . the base station 110 samples the air pressure at regular time intervals . the sampling interval may depend on whether the unit 110 is battery powered or externally powered . a tag ( mobile unit ) 140 is attached to a patient , e . g . via an arm band 202 . the tag 140 also contains a pressure sensor 208 ( shown in fig2 ). the air pressure at the tag 140 is transmitted , either via ultrasound ( us ) or via radio frequency electromagnetic radiation ( rf ), and the system calculates the difference in air pressure between the tag 140 and the base station 110 . t is the temperature in kelvin , p is the pressure , p 0 is a reference pressure at a fixed height , g =− dt / dh is the negative of the temperature gradient , r is the specific gas constant , and g is the acceleration of gravity . it can be assumed that the temperature within the vicinity of a pressure base unit is constant , or alternatively the system can compensate for it as a function of height based on the fact that hot air rises to the ceiling . if all pressure base units are at the same height , that height and the corresponding pressure may be used as the reference height and pressure . in general , if the base units are at different heights , each unit must convert its measured pressure to a reference pressure at a common reference height ( e . g . at the floor ). the temperature gradient may also be measured by using temperature sensors at different heights , e . g . at the floor and at the ceiling ( or at the highest relevant height for the installation ). for the calculation of height differences within rooms , the above formula can be approximated as : where p is the reference pressure and ap is the pressure difference between the mobile unit and the base unit . equation ( 2 ) is a low power variant of equation ( 1 ) which may be simpler to implement in mobile units . hence , the air pressure measurements in the tag 140 and in the base station 110 provide a measure of the height difference between the two . knowing the height of the base station 110 above the floor , the height of the tag 140 above the floor can be deduced . in particular , it can be determined whether the tag 140 ( and the patient ) is possibly lying on the floor or not . the air pressure measurements are associated with considerable noise . for example , the pressure in the region of the sensor will be continually varying due to the constant movement of air due to movement of people , opening of doors , circulation of air caused by fans , etc . signal conditioning such as averaging over multiple samples , low - pass filtering , or kalman filtering may be applied in order to get a robust measure of the height above the floor . the air pressure sensor readings may inherently drift slowly over time . using multiple base stations 110 with air pressure sensors 224 on the same floor 100 , this drift can be averaged out to provide a reference air pressure for the floor ( assuming the drift is of a random nature , the drifts from each base station will cancel out when averaging over several base stations ). drift in the air pressure sensors 208 in the tags 140 can be compensated in a number of ways . some tags 140 include a motion sensor 212 that can be used to make the tag 140 enter a sleep mode when the tag 140 is stationary . in this mode , the air pressure can be sampled occasionally to compensate for drift . the process of air pressure calibration and drift compensation can be preformed in the background even though the mobile unit is stationary ( in the case where it has a motion sensor and would otherwise be asleep to save power ). in some embodiments , when the tag is stationary ( as indicated for example by the motion sensor ), the tag can perform the role of a base station , communicating with the other base stations and improving the reference pressure estimation and saving cost in the infrastructure . another way to compensate for drift in the sensors of the mobile tags 140 is to assume a height band when the system detects motion from room to room . for example , a tag 140 attached to a patient &# 39 ; s arm can be assumed to be in a rather narrow height band if one can determine that the patient is walking such an arrangement is illustrated in the corridor 108 of fig1 where mobile unit 140 d is passing along the corridor . it should be noted that although two base units 110 are shown in the corridor 108 , only one base unit 110 is required for this method of calibration . when the unit 110 detects that the tag 140 d is in close proximity ( e . g . within a threshold distance ), it determines that the patient wearing tag 140 d is in the corridor in the vicinity of the base unit 110 and is therefore almost certainly walking . it can therefore assume that the tag 140 d is at some predetermined height . the predetermined height will vary from patient to patient ( according to the patient &# 39 ; s height ) and is preferably stored in a database on the central computer 130 . the unit 110 can contact the central computer 130 ( either through a direct connection or over a network and either wired or wirelessly ) to request the predetermined height for that tag 140 d . each tag has a unique identifier which can be read by base unit 110 and sent to the central computer 130 . the central computer looks up the identifier in its database and returns the appropriate predetermined height to base unit 110 . alternatively all calibration calculations can be performed on the central computer 130 which can then return calibration values ( pressure and / or height ) to the base station 110 . if the tag is capable of receiving data , base unit 110 can send calibration data ( e . g . a current height and / or a corrected pressure reading ) to the tag 140 so that the tag 140 can recalibrate itself . in an alternative arrangement , the tag 140 could have the predetermined height programmed into it , e . g . stored in memory 216 ( which could be random access memory , flash memory or similar ). an on board processor unit 214 can perform the recalibration calculations . if the tag 140 is not set up to receive incoming data or instructions then the unit cannot be instructed to recalibrate . instead , the tag 140 d sends its pressure data as normal to the base unit 110 and the base unit 110 ( or the central computer 130 ) determines an error between this reading and the reading that would be expected for the predetermined height . this determined error can be stored ( e . g . in a database on central computer 130 ) alongside the unique identifier for tag 140 d and used to apply a correction to all data received from tag 140 d . every time the tag 140 d passes through a recalibration zone , the calculation can be performed again and the database can be updated with corrected data . another way to compensate for drift is to take advantage of zones instrumented for full 3d positioning to calibrate the air pressure reading from the tag 140 . such zones do not rely on any pressure data in order to determine an accurate height for the tag 140 . these zones can be deployed for the purpose of general 3d positioning as illustrated in room 104 of fig1 which has three base units 110 attached to the walls and preferably has a further base unit 110 mounted in the ceiling or floor to provide an accurate z - coordinate ( i . e . height above the floor ). calibration can be performed in a manner similar to that described previously . where the tag 140 is constrained to move through portals , e . g . doors , narrow corridors , etc , height estimation infrastructure may be deployed specifically to provide an accurate height measure . for example a series of receivers ( which could be a base unit 110 ) arranged in a vertical array could simultaneously detect the proximity of a tag 140 as it passes by . the receiver which receives the strongest signal is determined to be the receiver at the closest height to that of the tag 140 thereby providing a simple yet accurate height detection method . if the tag is fitted with two - way communication , the tag can be instructed to enter a frequent update mode to provide accurate position estimation , for example to catch the exact moment when the tag 140 moves through a door . fall detection ( e . g . the collapse of a patient ) can , in the simplest embodiment , be detected simply on the basis of the height of a tag 140 being detected at or near the floor . however , there may be other reasons for a tag being at or near the floor , such as a patient bending down to pick something up or to access a low drawer or cupboard . therefore a system relying solely on the current height of a tag is liable to trigger false positive alarms , i . e . the system could raise an alarm when the situation does not require it . such a system is however reasonably robust to false negatives , i . e . it should reliably detect most situations where a patient has collapsed to the floor and correctly raise an alarm . in a slightly more advanced system , fall detection can also be triggered by a sudden drop in height , independent of the reference pressure in the base station 110 . the mobile unit 140 can raise the alarm itself based solely on its own data . however , the base station 110 can provide extra information about the starting height and finishing height of the tag . for example , the base station 110 can confirm that the tag actually ended up on the floor . an intelligent system can combine these pieces of information and raise an alarm based on the sudden drop in height together with the information that the finishing height is at or near floor level . in yet more advanced systems , the indoor real time location system may additionally have zoning within rooms . for example , as shown in room 104 of fig1 , the system can identify different areas within the room . such areas may include areas containing beds , sofas , chairs , stairs , etc that can be associated with typical patient / tag heights , and can provide additional filtering information for providing reliable alarms . as shown in fig1 , two special zones have been identified ( and programmed into the system ) in room 104 . the first zone 120 is a zone containing a bed 122 . when a patient is lying on the bed 122 , a tag 140 can be expected to be within a predictable height band and can be expected to remain at that height for some time . a second zone 124 contains a table 126 and four chairs 128 . again , tags 140 within this zone 124 can be expected to remain mostly within a certain predictable height band , but will be prone to sudden rises and falls as people sit down or get up . the heights of the tags 140 should not however end up at floor level . the system may also combine time information with the spatial zones , for example time periods may be identified when a person is expected to be in bed 122 or dining at table 126 . as shown in room 107 of fig1 , an intelligent building may be equipped with a number of other sensors which can feed data into the position detection system and can be used in determining alert situations . room 107 contains a motion sensor 150 ( e . g . an infrared , ultrasound or radar motion detector such as may be used in a burglar alarm system ), a light switch sensor 152 which indicates the current state ( on / off ) of the light , and a door sensor 154 which indicates whether the door is open or closed . it will be appreciated that these are just examples of sensors which could be used and many others could just as easily be incorporated into the system . with all of this sensor data available to the system , substantial logic can be built involving all available sensor readings , e . g . air pressure ( at the tag 140 and base units 110 ), ultrasound and / or rf signals from the tags 140 and other indicators such as telephone activity , motion evidence from a burglar alarm , operation of light switches , opening of doors , and other sensors in the intelligent building . it will be appreciated that the fall detection alarm needs to be reliable . in a home care setting , there is a high cost of false positives as emergency procedures will be initiated when the alarm is set off . likewise , false negatives may be disastrous for the persons not getting the proper attention . in general , the more information that is available , the more reliably the situation can be determined . it will be appreciated that indoor real time location systems can provide additional information like motion detection via doppler shift that can also be used to verify the fall detection alarm . in some situations , the logic for determining an alert situation can be significantly different , depending on the location . in particular , there could be zones for stairs with dedicated logic such that there will be no alarm when the tag descends towards ( and through ) the floor level . at the same time , within a stair zone , an alarm should be issued if the height remains constant , regardless of the height ; or if it reduces too quickly . in general , the air pressure sensors can be used to allow three dimensional positioning in an infrastructure deployed for two dimensional positioning . this set up is not only useful for the fall detection case outlined above , but can be used in all situations where three dimensional positioning is useful . the cost benefits of using the pressure sensors in place of additional ultrasound or rf base units still apply in such situations . it will also be appreciated that the above principle of using an air pressure sensor to detect the height of a mobile unit 140 can be combined advantageously with any rtls independent of the type of positioning principle deployed . further , the tags 140 and base stations 110 described above can interact easily with existing tags ( without pressure sensors ) or with two dimensional infrastructure , simply without using the pressure information . similarly , existing two dimensional infrastructure can be upgraded easily and conveniently simply by adding pressure sensors to existing base units or by replacing old base units with new ones . such upgrades need not involve installation of additional units to provide full three dimensional positioning . although this invention has been shown and described with respect to the detailed embodiments thereof , it will be understood by those skilled in the art that various changes in form and detail thereof may be made without departing from the spirit and the scope of the invention .
a monitoring system is provided that includes a three dimensional position determination system for determining the three dimensional coordinates of one or more mobile units . the system includes at least one zone in which the system is arranged to raise an alarm when the rate of change of a vertical coordinate of a mobile unit is less than a predetermined value .