Tracking a radio beacon indoor

Abstract

A method and devices are disclosed, for tracking a radio beacon at a mobile device, using short range RF signals, emitted by the beacon and detected at the mobile device, typically indoor, requiring no GNSS service. According to the disclosed method, the beacon is configured to broadcast short bursts, at a carefully structured difference in time of emission (DTOE), while at the tracking device, the difference in time of arrival (DTOA) of said signals is measured, and along with the DTOE, used to accurately determine the location of and direction to the beacon relatively to the tracking device. According to a preferred embodiment of the present invention, said short range RF signals are associated with Bluetooth advertising, configured to broadcast in 1-way, requiring no pairing and no connection from the tracking device, and practically not requiring a receiver at the beacon, therefor saving battery energy.

Claims

1. A method for determining a line of position (LOP) on which a radio beacon is placed, at a mobile device, comprising the steps of: a. at the radio beacon transmitting periodic signals, comprising at least a first signal and a second signal, configuring the difference in time of emission of said signals as: DTOE.sub.12=advinterval+m*t, wherein m is an integer number pseudo-randomly generated, advinterval and t are fixed time durations; b. communicating to said device t; c. at the device, at a first location, recording a time of arrival of said first signal (TOA.sub.1); d. moving said device from said first location to a second location, measuring direction and distance from said first location to said second location (vector.sub.12); e. at the device, at said second location, recording a time of arrival of said second signal (TOA.sub.2), and determining the difference in time of arrival of first and second signals: DTOA.sub.12=TOA.sub.2TOA.sub.1; f. at the device, determining a line of position (LOP.sub.12) on which said radio beacon is placed, based on: DTOA.sub.12, vector.sub.12 and DTOE.sub.12; wherein said LOP is a hyperbola, defined in a local coordinate system by two foci at both ends of said vector.sub.12, and C*|DTOA.sub.12DTOE.sub.12| being the difference in distances between the beacon and each of said foci, and C is the speed of light.

2. The method according to claim 1, wherein vector.sub.12 is determined at the device by an accelerometer measuring the direction and magnitude of the movement between said first and second locations.

3. The method according to claim 1, further comprising the steps of: a. at the beacon, encoding in each of said signals the transmission power level thereof; b. at the device, decoding said encoded transmission power level; c. at the device, measuring the receiving signal strength indication (RSSI) of said signals; d. at the device, estimating the range between beacon and device, based on said decoded transmission power level and said measured RSSI, and determining a LOP based on RSSI associated with the first signal (LOP-RSSI.sub.1), and a LOP based on RSSI associated with the second signal (LOP-RSSI.sub.2); e. at the device, determining the beacon position, in said local coordinate system, at a crossing point of: LOP.sub.12, LOP-RSSI.sub.1, and LOP-RSSI.sub.2, accounting for measurement inaccuracy.

4. The method according to claim 1, with at least an additional third signal emitted by the beacon, and at the device, determining at least another LOP and estimating the position of the beacon.

5. The method according to claim 1, at the device, further displaying the LOP along with the device self-position, and an estimated direction and distance to the beacon.

6. The method according to claim 1, at the device further measuring an ambient barometric pressure, and comparing said pressure measurement with at least one of: another pressure measurement made at the device, or a pressure measurement encoded in the beacon signal; and using said pressure comparison to improve the accuracy in determining LOP, self-position and beacon position, in said local coordinate system.

7. The method according to claim 6, at the beacon further measuring an ambient barometric pressure, and encoding said pressure measurement in the transmitted signal.

8. The method according to claim 1, said signals being part of a Bluetooth advertising event.

9. A mobile device for tracking a radio beacon, said device comprising: a receiver an accelerometer a controller, and a display; said device configured to determine the difference in time of emission (DTOE.sub.12) between a first signal and a second signal emitted by the beacon, and measure the difference in time of arrival (DTOA.sub.12) thereof, said first signal arriving while the device is at a first location, and said second signal arriving while the device is at a second location; and measure the distance and direction between said two locations (vector.sub.12), and determine a line of position (LOP.sub.12) on which the beacon is placed, based on DTOA.sub.12, vector.sub.12 and DTOE.sub.12; wherein said LOP is a hyperbola, defined in a local coordinate system by two foci at both ends of said vector.sub.12, and C*|DTOA.sub.12DTOE.sub.12| being the difference in distances between the beacon and each of said foci, and wherein C is the speed of light; wherein DTOE.sub.12=advinterval+m*t, m being a pseudo-random integer number, advinterval and t are fixed time durations, and communicating to said device t.

10. The device according to claim 9, configured to determine vector.sub.12 using the accelerometer to measure the direction and magnitude of the movement between said first and second locations.

11. The device according to claim 9, further configured to decode in said signal the transmission power level thereof, and measure the receiving signal strength indication (RSSI) thereof, and estimate the range between beacon and device, based on said decoded transmission power level and said measured RSSI; and determine a LOP based on RSSI associated with the first signal (LOP-RSSI.sub.1), and a LOP based on RSSI associated with the second signal (LOP-RSSI.sub.2), and determine the beacon position, in said local coordinate system, at a crossing point of: LOP.sub.12, LOP-RSSI.sub.1, and LOP-RSSI.sub.2, accounting for measurement inaccuracy.

12. The device according to claim 9, further configured to: detect at least an additional third signal emitted by the beacon, and determine at least another LOP, and update the estimated position of the beacon.

13. The device according to claim 9, further configured to display the LOP along with the device self-position, and an estimated direction and distance to the beacon.

14. The device according to claim 9, further comprising a barometric pressure sensor, and configured to: measure an ambient barometric pressure, and compare said pressure measurement with at least one of: another pressure measurement made at the device, or a pressure measurement communicated in the beacon signal; and use said pressure comparison to improve the accuracy of determined LOP, self-position and beacon position, in said local coordinate system.

15. The device according to claim 9, said signals being part of a Bluetooth advertising event.

16. A radio beacon trackable by a mobile device, said beacon comprising: a transmitter, and a controller; said beacon configured to: transmit periodic signals, comprising at least a first signal and a second signal, separated in time of emission by DTOE.sub.12, wherein DTOE.sub.12=advinterval+m*t, m being an integer number pseudo-randomly generated, advinterval and t are fixed time durations, encode in the signal its transmission power level, and communicate t to said device; enabling at a distanced mobile device decoding said transmission power level, determining DTOE.sub.12 and measuring the difference in time of arrival of said signals (DTOA.sub.12) while the device is placed in two different locations, correspondingly, and further enabling the device measuring the distance and direction between said two different locations (vector.sub.12); then enabling the device determining and displaying a line of position (LOP.sub.12) on which the beacon is placed, in a local coordinate system, based on said DTOA.sub.12, vector.sub.12 and DTOE.sub.12, wherein said LOP.sub.12 is a hyperbola, defined by two foci at both ends of said vector.sub.12, and C*|DTOA.sub.12DTOE.sub.12| being the difference in distances between the beacon and each of said foci, and C being the speed of light; and enabling at said distanced mobile device measuring a Received Signal Strength Indication (RSSI) associated with the beacon transmission, and estimating the range between beacon and device, based on said decoded transmission power level and said measured RSSI and determining a circle LOP-RSSI based on said estimated range, and determining the beacon position, at a crossing point of LOP-RSSI and LOP.sub.12.

17. The beacon according to claim 16, further configured to transmit at least another third signal, encoded with its transmission power level, wherein DTOE is associated with the third signal and a previously transmitted signal, enabling the device determining at least another LOP, and estimating the position of the beacon.

18. The beacon according to claim 16, further comprising a barometric pressure sensor, and configured to measure an ambient barometric pressure, and encode said pressure measurement in the signal.

19. The beacon accoding to claim 16, said signals being part of a Bluetooth advertising event.

20. The beacon according to claim 19, wherein DTOE.sub.12 is a time between consecutive Bluetooth advertising events.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other characteristics and advantages of the invention will be better understood through the following illustrative and non-limitative detailed description of preferred embodiments thereof, with reference to the appended drawings, wherein:

(2) FIG. 1 illustrates a prior art method for Tracking a radio beacon outdoor using DTOA according to U.S. Pat. No. 9,709,656. A helicopter is shown, flying from right to left along the X-axis of a 2D Cartesian coordinate system, visiting three points: R.sub.1(x.sub.1, y.sub.1), R.sub.2(x.sub.2, y.sub.2), R.sub.3(x.sub.3, y.sub.3), searching for a beacon marked by a small triangle at point T(x, y). The beacon is shown to be placed on a crossing point of two hyperbolas, whose formulas are written, inside rectangular frames, further below, based on the difference in time of emission (DTOE) of the signals transmitted by the beacon, and the difference in time of arrival (DTOA) of said signals, as measured at the tracking device assumed to be installed onboard the helicopter. According to U.S. Pat. No. 9,709,656, the position at R.sub.1, R.sub.2 and R.sub.3 is acquired by GNSS, so also the beacon coordinates (x, y) are defined in a global coordinate system, typically WGS-84.

(3) FIG. 2 illustrates a prior art Angle of Arrival (AoA) Method to geolocate a radio beacon. The beacon (Transmitter) is shown at the bottom of the figure, coupled to an assumedly omnidirectional antenna. Above the transmitter, two receivers are shown, side by side, each coupled to an array of four antennas, enabling determining the angle of arrival of the signal arrived from the beacon, even if radiated omnidirectionally. The receivers are coupled to one location engine, enabling determining the beacon position, in 2D, based on the two measured AOAs, and knowing the location of the two receivers.

(4) FIG. 3 illustrates a prior art method for Tracking a radio beacon from a mobile device based on RSSI. On the X-axis of a 2D Cartesian coordinate system, a mobile device is depicted at two different locations: a first location (x.sub.1, y.sub.1) and second location (x.sub.2, y.sub.2). Somewhere above the X-axis, a beacon is depicted at a location with coordinates (x, y), where two circles cross each other: a first circle centered at (x.sub.1, y.sub.1) and second circle centered at (x.sub.2, y.sub.2). The range from the beacon to each of said device locations, is marked, obviously as the radius of said each circle. The figure shows that the two circles have two common points: one is the true position of the beacon, at (x, y), and the other is mirrored over the X-axis, at (x, y) and marked still possible solution of beacon position.

(5) FIG. 4 depicts LOP on which a beacon is placed drafted indoor based on DTOA. On the X-axis of a 2D Cartesian coordinate system, a mobile device is depicted at two different locations: a first location (x.sub.1, y.sub.1) and a second location (x.sub.2, y.sub.2). Somewhere above the X-axis, a beacon is depicted at a location with coordinates (x, y), on the left hyperbola branch, which formula is indicated below, in a rectangular frame, noting that also the right branch of the hyperbola is a valid geometrical representation of this formula. In this formula, actually an equation, DTOA.sub.12 is the difference in time of arrival of two signals transmitted by the beacon, at a difference in time of emission of DTOE.sub.12, assuming that a first of these signals arrives at the mobile device while visiting the first location, and a second of these signals arrives at the mobile device while visiting the second location. The distance between said first location and said second location is marked as vector.sub.12, and it is actually the basis for the local coordinate system X-Y, since the direction of vector.sub.12 defines the X-axis, and the center of vector.sub.12 defines the origin (0, 0) of the local coordinate system. Thus, the Y-axis is perpendicular to vector.sub.12, and the first location and the second location are equally distanced from the Y axis.

(6) FIG. 5 shows a method for determining LOP based on DTOA+LOP based on RSSI. On the X-axis of a 2D Cartesian coordinate system, a mobile device is depicted at two different locations: a first location (x.sub.1, y.sub.1) and a second location (x.sub.2, y.sub.2). The distance between said first location and said second location is marked as vector.sub.12, and it is actually the basis for the local coordinate system X-Y, since its direction defines the X-axis, while the Y-axis is erected perpendicularly to vector.sub.12, at its center, such that the first location and the second location have the same distance from the Y axis. Somewhere above the X-axis, a beacon is depicted at a location with coordinates (x, y), on a hyperbola branch, drafted in a wide solid line, marked as DTOA LOP selected. At the right side of the Y-axis, another hyperbola branch is drafted in dashed line and marked as DTOA LOP abandoned. Further, two circles are depicted, representing LOP based on RSSI, one centered at the first location and another centered at the second location, and the circles radii are accordingly marked range based on RSSI at 1.sup.st location and range based on RSSI at 2.sup.nd location. It is shown that the selected DTOA LOP branch and both two circles have two common points: one is the true position of the beacon, at (x, y), and the other is mirrored over the X-axis, at (x, y) and marked another possible resolution of beacon position.

(7) FIG. 6 illustrates a method for selecting moving direction on LOP based on RSSI. On the X-axis of a 2D Cartesian coordinate system, a mobile device is depicted at two different locations: a first location (x.sub.1, y.sub.1) and a second location (x.sub.2, y.sub.2). Somewhere above the X-axis, a beacon is depicted at a location with coordinates (x, y), on a hyperbola branch at the left side of the Y-axis, marked as DTOA LOP. A smaller icon depicting same mobile device, is shown on the LOP, above the X-axis, from which two dashed lines are shown, one upward and the other downward the LOP, illustrating two different directions of movement on the LOP. Accordingly, text boxes indicate that the upward movement is associated with increasing RSSI measured at the mobile device (since the distance to the beacon becomes shorter), while the downward movement is associated with decreasing RSSI measured at the mobile device (since the distance to the beacon becomes larger).

(8) FIG. 7 illustrates a Display at mobile device tracking a beacon according to P t embodiment. A hand-held mobile device is illustrated, on which a dotted curved line is displayed; at the bottom side of this line a generic person icon is shown, while a beacon icon is shown at the upper side of the line. Asides the dotted line, appears a text indicating the distance and elevation to the beacon.

(9) FIG. 8 illustrates a Block Diagram of beacon according to the present invention. The main block, indicating inside Bluetooth system on chip (SOC), comprises two inner solid-line blocks, one marked Controller and the other marked Transmitter, and one dashed-line block, marked barometer, which is optional. Further, an antenna is shown coupled to the transmitter. It should be noted that the Bluetooth transmitter is practically a transceiver.

(10) FIG. 9 illustrates a Block Diagram of tracking device according to the present invention. The main block, indicating inside mobile phone, comprises five inner blocks, from which the center block is marked Controller, coupled to other four blocks: Receiver, display, accelerometer and barometer (optional, in dashed-line). Further, an antenna is shown coupled to the receiver.

(11) FIG. 10 illustrates the beacon signal according to Pt embodiment. At the upper side of the picture, three groups of three signals each are depicted by vertical rectangles marked by 37, 38, and 39. Each group of three signals defines an advertising event according to the Bluetooth standard, wherein the time difference between two consecutive advertising events is marked as advinterval+advDelay. Further down, the time between the start of first and second events is marked as DTOE.sub.12, and the time between the start of second and third events is marked as DTOE.sub.23. At the bottom side of the picture, three bullets specify some aspects of the Bluetooth advertising, including: the frequency of transmission associated with each of the channels 37, 38 and 39; advinterval timing restrictions; advDelay timing restrictions.

DETAILED DESCRIPTION

(12) The above examples and description have of course been provided only for the purpose of illustration, and are not intended to limit the invention in any way. As will be appreciated by the skilled person, the invention can be carried out in a great variety of ways, employing more than one technique from those described above, all without exceeding the scope of the invention.

(13) The present invention discloses a method for determining a line of position (LOP) on which a radio beacon is placed, at a mobile device, comprising the steps of: a. at the radio beacon transmitting periodic signals, comprising at least a first signal and a second signal, configuring the difference in time of emission of said signals as: DTOE.sub.12=n*t, wherein n is an integer number and t is a fixed time duration; b. communicating to said device, at least one of: DTOE.sub.12 or t; c. at the device, at a first location, recording a time of arrival of said first signal (TOA.sub.1); d. moving said device from said first location to a second location, measuring direction and distance from said first location to said second location (vector.sub.12); e. at the device, at said second location, recording a time of arrival of said second signal (TOA.sub.2), and determining the difference in time of arrival of first and second signals: DTOA.sub.12=TOA.sub.2TOA.sub.1; f. at the device, determining a line of position (LOP.sub.12) on which said radio beacon is placed, based on: DTOA.sub.12, vector.sub.12 and DTOE.sub.12; wherein said LOP is a hyperbola, defined in a local coordinate system by two foci at both ends of said vector.sub.12, and C*|DTOA.sub.12DTOE.sub.12| been the difference in distances between the beacon and each of said foci, and C is the speed of light.

(14) FIG. 4 depicts LOP on which a beacon is placed drafted indoor based on DTOA. On the X-axis of a 2D Cartesian coordinate system, a mobile device is depicted at two different locations: a first location (x.sub.1, y.sub.1) and a second location (x.sub.2, y.sub.2). Somewhere above the X-axis, a beacon is depicted at a location with coordinates (x, y), on the left hyperbola branch, which formula is indicated below, in a rectangular frame, noting that also the right branch of the hyperbola is a valid geometrical representation of said formula. In this formula, actually an equation, DTOA.sub.12 is the difference in time of arrival of two signals transmitted by the beacon, at a difference in time of emission of DTOE.sub.12, assuming that a first of these signals arrives at the mobile device while visiting the first location, and a second of these signals arrives at the mobile device while visiting the second location. The distance between said first location and said second location is marked as vector.sub.12, and it is actually the basis for the local coordinate system X-Y, since the 3D direction of vector.sub.12 defines the X-axis, and the center of vector.sub.12 defines the origin point (0, 0) of the local coordinate system. Thus, the Y-axis is perpendicular to vector.sub.12, and the first location and the second location are equally distanced from the Y axis.

(15) As appreciated by a skilled person, FIG. 4 provides a 2D LOP based on C*|DTOADTOE| and vector.sub.12, which is a hyperbola, while a 3D representation thereof is a hyperboloid.

(16) According to a first embodiment of the present invention, the beacon, is a small battery powered tag attached to an object which geographical position is desired to be determined remotely, such as: person (e.g. dementia/ill/injured person taken care of; soldier or policeman on duty; visitor or employee at a highly secured or dangerous site; athlete participating in competition), animal (wild or domestic), vehicle (e.g. bicycle, car in large parking lot), suitcase or briefcase, personal object (e.g. keys, bags, apparel, small electronic device), goods, mailed/delivered item, weapon or ammunition, toy, to name a few. The mobile device tracking this beacon is preferably a personal mobile device, such as mobile phone, smartphone (iphone or Android), tablet (such as iPad), laptop, etc.

(17) Further according to a first embodiment of the present invention, the beacon comprises a Bluetooth transmitter or transceiver (possibly among other communication capabilities), and the tracking device is Bluetooth enabled, such that Bluetooth signals are used to track the beacon. Furthermore, the Bluetooth transmitter at the beacon is configured to broadcast periodically short signals of Bluetooth advertising, once per 1-2 seconds or so, such that the data packages been sent indicate that no Bluetooth connection is required, enabling a one-way communication between beacon and tracking device. So, the beacon signals are preferably part of a Bluetooth advertising event, and the tracking device is configured to detect Bluetooth advertising event signals, avoiding the need of pairing and connection according to Bluetooth.

(18) FIG. 10 illustrates the beacon signal according to 1st embodiment of the present invention. At the upper side of the picture, three groups of three signals each are depicted by vertical rectangles marked by 37, 38, and 39. Each group of three 37-38-39 signals defines an advertising event according to the Bluetooth standard, wherein the time difference between two consecutive advertising events is marked as advinterval+advDelay. Further down, the time between the start of first and second event is marked as DTOE.sub.12, and the time between the start of second and third event is marked as DTOE.sub.23. At the bottom side of the picture, three bullets specify some aspects of the Bluetooth advertising, including: the frequency of transmission associated with each of the channels 37, 38 and 39; advinterval timing restrictions; advDelay timing restrictions.

(19) Further according to a first embodiment of the present invention, the beacon is configured to set the time between consecutive advertising events to a sum of a constant time interval (advinterval) and a pseudo-random time interval (advdelay), wherein advdelay=m*t, m been an integer number with pseudo-random distribution and t a fixed time duration. Further according to said first embodiment, advinterval=1 s, and advdelay=m*1 ms, wherein m is an integer number between 0 to 10, with a pseudo-random distribution. Thus, for example, the 1.sup.st advertising event can be 1.001 s long, the 2.sup.nd advertising event can be 1.005 s long, then 1.003 s, 1.009 s, 1.000 s, etc. so following this example and FIG. 10, the time duration of the 1.sup.st advertising events is DTOE.sub.12=1.001 s, and the time duration of the 2.sup.nd advertising events is DTOE.sub.23=1.005 s.

(20) Then, in the Bluetooth advertising PDU (packet data unit) payload broadcast by the beacon, 1 byte is allocated to report t, in 10 ns resolution, so t=1 ms is represented by 100 [dec]=0110 0100 [bin].

(21) Accordingly, at the tracking device, when for example measuring DTOA.sub.12=1.001000003 s, and decoding t=1 ms, it is assumed that DTOE.sub.12=1.001 s, so |DTOA.sub.12DTOE.sub.12|=3 ns. Accordingly, C*|DTOA.sub.12DTOE.sub.12|=300,000 Km/s*3 ns=9 m.

(22) Further according to said first embodiment of the present invention, at the tracking device, vector.sub.12 is determined by an accelerometer measuring the direction and magnitude of the movement between different locations. For example, let's assume that the first and second locations where the first and second signals were monitored, i.e., the two end points of vector.sub.12, are located on a leveled floor, at 10 m from each other. Then, referring to FIG. 4, the magnitude of vector.sub.12 is 10 m, and its direction defines the X-axis, on the floor, such that the coordinates of the first position are (x.sub.1, y.sub.1)=(5, 0), the coordinates of the second position are (x.sub.2, y.sub.2)=(5, 0), and assuming that the beacon is also more or less placed on that floor, at the unknown yet coordinates (x, y), the hyperbola formula is: (x+5).sup.2+y.sup.2|[(x5).sup.2+y.sup.2]|=9 [m], allowing two branches mirrored over the Y-axis.

(23) Possibly, from time to time, the user may calibrate the accelerometer reading, along a precisely measured 10 m line on the floor. Actually, according to a third embodiment of the present invention, vector.sub.12 is determined based on a precise line on the ground whose length is precisely measured (e.g., with length measuring tape or laser distance meter), wherein the tracking device is placed on one side of this line (the first location) to monitor a first signal emitted by the beacon, then placed on the other side (second location) of the line, to monitor a second signal emitted by the beacon.

(24) Further according to a first embodiment of the present invention: a. at the beacon, encoding in each of said signals the transmission power level thereof; b. at the device, decoding said encoded transmission power level; c. at the device, measuring the receiving signal strength indication (RSSI) of said signals; d. at the device, estimating the range between beacon and device, based on said decoded transmission power level and said measured RSSI, and determining a LOP based on RSSI associated with the first signal (LOP-RSSI.sub.1), and a LOP based on RSSI associated with the second signal (LOP-RSSI.sub.2); e. at the device, determining the beacon position, in said local coordinate system, at a crossing point of: LOP.sub.12, LOP-RSSI.sub.1, and LOP-RSSI.sub.2, accounting for measurement inaccuracy.

(25) FIG. 5 shows a method for determining LOP based on DTOA+LOP based on RSSI according to a first embodiment of the present invention. On the X-axis of a 2D Cartesian coordinate system, a mobile device is depicted at two different locations: a first location (x.sub.1, y.sub.1) and a second location (x.sub.2, y.sub.2). The distance between said first location and said second location is marked as vector.sub.12, and it is actually the basis for the local coordinate system X-Y, since its direction defines the X-axis, while the Y-axis is erected perpendicularly to vector.sub.12, at its center, such that the first location and the second location have the same distance from the Y axis. Somewhere above the X-axis, a beacon is depicted at a location with coordinates (x, y), on a hyperbola branch, drafted in a wide line, marked as DTOA LOP selected. At the right side of the Y-axis, another hyperbola branch, drafted in dashed line and marked as DTOA LOP abandoned. Further, two circles are depicted, representing LOP based on RSSI, one centered at the first location and another centered at the second location, and the circles radii are accordingly marked range based on RSSI at Pt location and range based on RSSI at 2 nd location. The figure shows that the two circles and the selected hyperbola branch (DTOA LOP selected) have two common points: one is the true position of the beacon, at (x, y), and the other is mirrored over the X-axis, at (x, y) and marked another possible resolution of beacon position.

(26) Further according to a first embodiment of the present invention, an additional third signal is emitted by the beacon, detected by the tracking device at a third location, enabling the device determining another LOP, and estimating the position of the beacon.

(27) According to a first embodiment of the present invention, the transmission power level is encoded in [dBm], allocating a single byte in the advertising packet, covering a range between 128 dBm to 127 dBm. So, for example, assuming a transmission power of 10 dBm, and RSSI measurement of 70 dBm, and assuming 0 dB antenna gain at the beacon and tracking device, the path loss is 80 dB; then, considering the Bluetooth frequency of 2.4 GHz, the calculated distance [d] is 100 m, according to the free space path loss (FSPL) formula: (received EIRPtransmitted EIRP)=80 dB=20 log(4df/C).

(28) So, with three signals monitored at the tracking device at three different locations, even in the 3D context, three spheres can be defined, having two common crossing points, as a possible resolution of the beacon position. However, the accuracy in determining the radius of these spheres is expected to be poor, particularly since the RSSI cannot distinguish between signal attenuation due to range, and signal attenuation due to obstacles, such as concrete walls, between the beacon and the tracking device. Nevertheless, as shown in FIG. 5, the RSSI measurements could serve to remove the two-branch ambiguity of the hyperbola (2D or hyperboloid (3D).

(29) On the other side, the method of LOP determination based on time measurement disclosed here is much more accurate, since the traveling time of the signal between beacon and tracking device is practically insensitive to such obstacles; furthermore, LOP determination based on time-difference measurement is also practically immune to clock drifting, and oscillator aging, and difference between transmitter clock and receiver clock. So, with 3 beacon signals been monitored at the tracking device, 2 hyperboloids can be defined, in the 3D context, having a LOP in common, while the RSSI measurements can be used to remove the redundant LOP defined by the DTOA measurements. Four such signals already provide a single and accurate resolution of the beacon position.

(30) The searching process can be further improved when the user is moving towards the estimated position of the beacon, as illustrated in FIG. 6.

(31) FIG. 6 illustrates a method for selecting moving direction on LOP based on RSSI. On the X-axis of a 2D Cartesian coordinate system, a mobile device is depicted at two different locations: a first location (x.sub.1, y.sub.1) and a second location (x.sub.2, y.sub.2). Somewhere above the X-axis, a beacon is depicted at a location with coordinates (x, y), on a hyperbola branch at the left side of the Y-axis, marked as DTOA LOP. A smaller icon of mobile device is shown on the LOP, above the X-axis, from which two dashed line arrows are shown, one upwards and the other downwards the LOP, illustrating two alternative directions of movement on the LOP. Accordingly, text boxes indicate that the upward movement is associated with increasing RSSI measured at the mobile device (since the distance to the beacon becomes shorter), while the downward movement is associated with decreasing RSSI measured at the mobile device (since the distance to the beacon becomes larger).

(32) So, according to a preferred embodiment of the present invention, with 3 signals monitored at three different locations, a LOP can be determined, even in the 3D context, then, the user is prompted to move along this LOP, selecting the right direction according to RSSI (maybe with a short trial-and-error), as illustrated in FIG. 6. In the 2D context, such as when the tracking device moves on a constant level at which the beacon is placed, (e.g., searching for car on parking floor, bicycle at train station, briefcase at specific office floor), the 3 signals provide not only a LOP but actually two specific points on this LOP, at which the beacon could be placed, as illustrated in FIG. 5, then the selection between the two points of FIG. 5, can be done moving along the LOP, as illustrated in FIG. 6.

(33) Further, according to a first embodiment of the present invention, at the tracking device, a LOP is displayed along with the device self-position, and an estimated direction and distance to the beacon, as illustrated in FIG. 7.

(34) FIG. 7 illustrates a Display at mobile device tracking a beacon according to Pt embodiment. A hand-held mobile device is illustrated, on which a dotted curved line is displayed; at the bottom side of this line a person icon is shown, while a beacon icon is shown at the upper side of the line. Asides the dotted line, appears a text indicating the distance and elevation to the beacon.

(35) Such, the user is prompted to approach the beacon moving along the displayed LOP. Obviously, the user may decide to deviate from the displayed LOP, for example due to physical obstacles onsite, then the tracking device is configured to update the LOP and display per FIG. 7, from time to time, upon monitoring further signals periodically emitted by the beacon, and the determining the relative self-position of the tracking device as sensed by the internal accelerometer.

(36) According to a second embodiment of the present invention, the tracking device comprises also a barometric sensor, also known as altimeter, configured to measure the ambient barometric pressure, from time to time. The beacon also comprises a barometric sensor, and is configured to measure the ambient barometric pressure, from time to time, and encode said pressure measurement in the transmitted signal.

(37) So further according to said second embodiment, a byte is allocated at the Bluetooth advertising PDU payload to encode the barometric reading. Actually, to save bit count, the difference from the nominal MSL atmospheric pressure of 1013 mbar (=101.3 KPascal) can be encoded, in [mbar] or in [Pascal] from 128 to +127. Alternatively, an altitude above MSL may be reported instead of air pressure.

(38) As known in the art, there is a mathematical relationship (with some variations) between barometric pressure and altitude above MSL, at same atmospheric conditions. Typically, the altitude calculations are based on the measured pressure (p), the equivalent MSL pressure to compensate for local weather conditions (OFF_H). Pressure [p] is given in Pascals [Pa], altitude [h] given in meters [m], as following:

(39) $h=44330.77\left\{1-{\left(\frac{p}{p_0}\right)}^{0.1902632}\right\}+\mathrm{OFF\_}H$

(40) Where: p0=sea level pressure (101,326 Pa);

(41) As known to persons skilled in the art, there are many types of small low power and low-cost pressure sensor chips on the market, by many manufacturers, including: Bosch, ST, NXP, Honeywell. For example, NXP provides MPL3115A2I2C precision pressure sensor with altimetry, which outputs the air pressure in [Pascal] from 20 to 110 kPa (1000 mbar=100 kPa), and also the altitude (calibrated to a specific MSL air pressure), in [meter], between 698 to 11,775 m.

(42) Typically, these devices obtain a good differential reading, sensing a change of 1-2 meters in elevation.

(43) However, there might be a difference in reading the same air pressure, by different sensors, due to tolerance, drift, aging and other factors, which do not disturb the differential air pressure/altitude reading at the tracking device while moving from point to point, yet could introduce errors when comparing the reading at the beacon to the reading at the tracking device.

(44) To address this potential error source, according to a second embodiment of the present invention, the tracking device is configured to adjust the barometric pressure reported by the beacon, according to an adjustment parameter stored at the tracking device. This adjustment parameter is determined in advance, when the beacon is close to the tracking device (typically administered by the user), and comparing at the tracking device the beacon reported air pressure to its self-measured air pressure.

(45) The barometric or altitude reading can enhance the accuracy of position determination, both of the beacon and tracking device, at the tracking device, as described below.

(46) In FIG. 4, the DTOA LOP is defined in 2D: [(xx.sub.2).sup.2+(yy.sub.2).sup.2]|[(xx.sub.1).sup.2+(yy.sub.1).sup.2]=C*|DTOA.sub.12DTOE.sub.12|; and representing that in 3D: [(xx.sub.2).sup.2+(yy.sub.2).sup.2+(zz.sub.2).sup.2]||[(xx.sub.1).sup.2+(yy.sub.1).sup.2+(zz.sub.1).sup.2]=C*|DTOA.sub.12DTOE.sub.12|; Then, assuming that X-Y is the floor level on which the tracking device moves, i.e., z.sub.1=z.sub.2=0, while the beacon is placed above this floor, i.e., z>0, then the 3D DTOA LOP equation is written as: [(xx.sub.2).sup.2+(yy.sub.2).sup.2+z.sup.2]||[(xx.sub.1).sup.2+(yy.sub.1).sup.2+z.sup.2]=C*|DTOA.sub.12DTOE.sub.12|; wherein z can be estimated at the tracking device based on the difference between barometric pressure at the beacon and at the tracking device, leaving only 2 unknowns (x, y) in the 3D DTOA LOP equation.

(47) For example, if x.sub.1=5 m, x.sub.2=5 m, y.sub.1=y.sub.2=0, and |DTOA.sub.12DTOE.sub.12|=3 ns, then the LOP equation is: [(x+5).sup.2+y.sup.2+z.sup.2]||[(x5).sup.2+y.sup.2+z.sup.2]=9; wherein x and y are in [m].

(48) The present invention also discloses a mobile device for tracking a radio beacon, said device comprising: a receiver, an accelerometer (also known as IMUInertial Measurement Unit), a controller and a display; said device configured to determine the difference in time of emission (DTOE.sub.12) between a first signal and a second signal emitted by the beacon, and measure the difference in time of arrival (DTOA.sub.12) thereof, said first signal arriving while the device is at a first location, and said second signal arriving while the device is at a second location; and measure the distance and direction between said two locations (vector.sub.12), and determine a line of position (LOP.sub.12) on which the beacon is placed, based on DTOA.sub.12, vector.sub.12 and DTOE.sub.12; wherein said LOP is a hyperbola, defined in a local coordinate system by two foci at both ends of said vector.sub.12, and C*|DTOA.sub.12DTOE.sub.12| been the difference in distances between the beacon and each of said foci, and wherein C is the speed of light, as illustrated in FIG. 4.

(49) FIG. 9 illustrates a Block Diagram of tracking device according to the present invention. The main block, indicating inside mobile phone, comprises five inner blocks, from which the center block is marked Controller, coupled to other four blocks: Receiver, display, accelerometer and barometer (in dashed-line). Further, an antenna is shown coupled to the receiver.

(50) According to a first embodiment of the present invention, the device is a personal mobile phone or tablet, wherein said receiver is a Bluetooth transceiver.

(51) Further according to a first embodiment of the present invention, the accelerometer is a chip accelerometer, for example manufactured by one of: Analog devices, NXP, Bosch, ST, Freescale, particularly: Bosch BMI160small, low power inertial measurement unit.

(52) Further according to a first embodiment of the present invention, the tracking device is configured to determine vector.sub.12 using the accelerometer to measure the direction and magnitude of the movement between said first and second locations.

(53) According to a first embodiment of the present invention, the device is configured to decode the transmission power level of the signal, encoded at the beacon, and measure the receiving signal strength indication (RSSI) thereof, and estimate the range between beacon and device, based on said decoded transmission power level and said measured RSSI; and determine a LOP based on RSSI associated with the first signal (LOP-RSSI.sub.1), and a LOP based on RSSI associated with the second signal (LOP-RSSI.sub.2), and determine the beacon position, in said local coordinate system, at a crossing point of: LOP.sub.12, LOP-RSSI.sub.1, and LOP-RSSI.sub.2, accounting for measurement inaccuracy, as illustrated in FIG. 5.

(54) In a preferred embodiment of the present invention, the tracking device is further configured to detect an additional third signal emitted by the beacon, and determine another LOP, and update the estimated position of the beacon.

(55) Further, according to a first embodiment of the present invention, the tracking device is configured to display the LOP on which the beacon is placed, along with the device self-position, and an estimated direction and distance to the beacon, as illustrated in FIG. 7.

(56) According to a second embodiment of the present invention, the tracking device comprises a barometric pressure sensor, as illustrated in FIG. 9, and configured to measure an ambient barometric pressure, and compare said pressure measurement with at least one of: another pressure measurement made at the device, or a pressure measurement communicated in the beacon signal; and use said pressure comparison to improve the accuracy of LOP, self-position and beacon position, in said local coordinate system.

(57) Further according to a second embodiment of the present invention, the barometer is a small low power and low-cost chip, such as NXP MPL3115A2I2C precision pressure sensor with altimetry.

(58) The present invention further discloses a radio beacon trackable by a mobile device, said beacon comprising: a transmitter and a controller; said beacon configured to transmit periodic signals, comprising at least a first signal and a second signal, separated in time of emission by DTOE.sub.12, wherein DTOE 12=n*t, n been an integer number and t a fixed time duration, and encode in the signal its transmission power level, and at least one of: DTOE.sub.12 or t; enabling at a distanced mobile device decoding said transmission power level, and at least one of: DTOE.sub.12 or t, and measuring the difference in time of arrival of said signals (DTOA.sub.12) while the device is placed in two different locations, correspondingly, and further enabling the device measuring the distance and direction between said two different locations (vector.sub.12); then enabling the device determining and displaying a line of position (LOP.sub.12) on which the beacon is placed, in a local coordinate system, based on said DTOA.sub.12, vector.sub.12, DTOE.sub.12, and the received signal strength (RSSI) associated with said signal, wherein said LOP is a hyperbola, defined by two foci at both ends of said vector.sub.12, and C*|DTOA.sub.12DTOE.sub.12| been the difference in distances between the beacon and each of said foci, and C been the speed of light.

(59) FIG. 8 illustrates a Block Diagram of beacon according to the present invention. The main block, indicating inside Bluetooth system on chip (SOC), comprising two solid-line blocks, one marked Controller and the other marked Transmitter; outside the main block, another block is depicted in dashed lines, marked barometer, which is optional. Further, an antenna is shown coupled to the transmitter. It should be noted that the Bluetooth transmitter is practically a transceiver.

(60) According to a first embodiment of the present invention, the beacon is based on a Bluetooth Low Energy (BLE) system on chip (SOC), as illustrated in FIG. 8. There are many off the shelf Bluetooth SOC on the market (e.g., by TI, Silabs, Nordic, Onsemi, ST), comprising Controller and Transmitter (practically a transceiver), with free memory onboard enabling Bluetooth stack configuration (for example configuring advertising event parameters, such as advinterval and advdelay) and embedding some application code, as persons skilled in the art are aware of.

(61) So, according to a first embodiment of the present invention, the beacon is a Bluetooth Low Energy device, emitting signals that are part of a Bluetooth advertising event.

(62) Further according to a first embodiment of the present invention, the beacon is configured to perform periodic Bluetooth advertising, as illustrated in FIG. 10, with 1-2 seconds between consecutive advertising events, and specifically configure advinterval with a constant time duration, for example advinterval=1 s, and advdelay=m*1 ms wherein m is an integer number with pseudo-random distribution between 0-10, and encode in the advertising PDU payload: t and transmission power level.

(63) Also according to said first embodiment, the beacon is further configured to transmit another third signal, encoded with its transmission power level, and t, enabling the device determining at least another LOP, and estimating the position of the beacon.

(64) According to a second embodiment of the present invention, the beacon comprises also a barometric pressure sensor, as illustrated in FIG. 8, and is configured to measure the barometric pressure from time to time, and further encode in the advertising PDU payload also the barometric pressure reading.

(65) Further, according to a second embodiment of the present invention, the beacon further comprises a barometric pressure sensor, as illustrated in FIG. 8, and configured to measure an ambient barometric pressure, and encode said pressure measurement in the signal. The barometer, also known as altimeter, is preferably a small low power and low-cost chip, such as MPL3115A2I2C precision pressure sensor with altimetry, by NXP.

(66) In many cases, the place of implementation described herein is merely a designer's preference and not a hard requirement. For example, functions disclosed as implemented at the tracking device may alternatively be partially implemented at access points. Given the rapidly declining cost of digital signal processing and other processing functions, it is easily possible, for example, to transfer the processing or a particular function from one of the functional elements described herein to another functional element, such as the so called cloud, without changing the inventive operation of the system.