Reverse locator

Abstract

A remote locator (RL) continuously transmits multi-frame pings in a slow ping mode. The user activates a transponder/micro-transponder (MT) to receive at least a portion of the multi-frame ping and transmits a reply to the RL. The RL calculates a distance between the RL and the MT using the time-of-flight between the transmission of the ping and the receipt of the corresponding reply. The RL continues to send pings to the MT, where the ping includes distance measurements encoded therein. The user initiates sending a message from the MT to the RL to change to a fast ping mode, where the RL transmits pings at an increased rate. The MT includes a compass to capture readings while receiving and replying to pings. The MT determines a directional location for the RL with the collected measurements and received information and can provide a distance and directional readout to the user.

Description

RELATED APPLICATION

formaka a.k.a. the car), a directional indicator is illuminated on a display of the MT device as shown in FIG. 11B. Also shown in FIG. 11B, the distance between the RL and the MT is displayed indicating that the car is located 227 feet away towards the front and right. As shown in FIG. 11C, the person then walks in the direction of the arrow on the display of the MT to locate the car (indicated as the shaded car in the back). As the person approaches the Car, the distance measurement will be updated to indicate that they are getting closer and closer. Once the person is within a close proximity (e.g., 10 feet) of the car, the MT can provide a short audible indicator, visible indicator, or vibrating alert prior to activating a sleep mode.

Ping Modes

FIG. 12A is an example diagram illustrating single ping mode, slow ping mode, and fast ping mode. As previously described a “ping” corresponds to a complete transmission by the RL to the MT, such as a complete set of the three frame transmission sequence. Similarly a “reply” corresponds to a complete set of frames from that are transmitted from the MT to the RL. In FIG. 12A, each block designated as Px is intended to indicate a time of transmission for a ping that includes a complete set of frames, while Rx is intended to indicate a time of transmission for a reply that also includes a complete set of frames.

In the slow ping mode, the RL is arranged to continuously transmit a series of single pings (P1, P2 . . . PN) (P1′, P2′ . . . PN′) to the MT. Each subsequent ping is separated in time by a ping interval (T1) as illustrated. The MT receives each ping when it is located within a transmission range of the RL for proper reception, and transmits a corresponding reply (R1, R2 . . . RN) (R1′, R2′ . . . RN′) for each ping that is properly recognized as coded for the particular MT.

In the fast ping mode, the RL is arranged to continuously transmit a series of single pings (P1′, P2′ . . . PN′) (P1″, P2″ . . . PN″) to the MT. Each subsequent ping is separated in time by a ping interval (T2), which is significantly shorter in time than ping interval T1. The MT receives each ping when it is located within a transmission range of the RL for proper reception, and transmits a corresponding reply (R1′, R2′ . . . RN′) (R1″, R2″ . . . RN″) for each ping that is properly recognized as coded for the particular MT.

It is important to note that the MT may not always be able to properly receive a particular ping from an RL in even though it is properly coded for recognition by the MT. Environmental conditions such as noise, buildings, and other electronic interferences may inhibit a ping (e.g., ping P2) P2′) from reaching the intended MT. Similarly, environmental conditions may cause prevent a reply (e.g., reply R3′ R3″) from reaching the intended RL.

Power Conservation and Signal Interference Reduction by Hollowing Transmission Frames

The described system performs distance measurement by round trip time measurements. The reverse locator is arranged to provide regular communications between the RL and the MT without excess energy consumption or spectral pollution. After a transponder and locator have exchanged signatures, they share very precise mutual clock rate information. The accuracy of this clock rate information, absent any Doppler shift, is one part per billion or better. As time elapses between transmissions, the unit time bases, which aren't perfectly steady, will drift with respect to each other. By calibrating the low-speed sleep mode oscillator against the high-speed clock, so that a given sleep period can be accurately enumerated as a known number of high-speed clock periods, it is possible to accurately measure periods of several minutes without actually operating the high-speed clock. However, a long initial baseline for frequency determination is necessary to initially synchronize the clocks between the MT and the RL. Once synchronized/calibrated, excessive accumulation of reply transmissions from the MT is not necessary since precise timing is known.

To reduce power consumption and reduce interference from other devices, the transmission frames can be “hollowed out” or thinned. The benefit of hollowing out the transmission frames is that there is a drastic reduction of radio noise, allowing other equipment or other locators to operate in the same local area. The benefit of hollowing the MT and RL transmissions is a huge reduction in energy consumption and a reduction in interference with other devices.

The hollowing out of the MT reply signal can be done when the MT and the RL are relatively close to one another since the MT's low power signal is normally integrated by the RL. In reverse locator mode, the RL is arranged to monitor the distance of a proximate MT such that power conservation by thinning or hollowing is possible.

FIGS. 12B and 12C are an illustration for example of thinned or hollowed out transmission frames. As previously described, a ping is transmitted from the RL to the MT, where the ping consists of a multi-frame transmission such as the three frames illustrated in FIG. 12B. By way of example only, the transmission frames described for the RL are substantially similar to that described with respect to FIG. 3, but need not be so limited. As illustrated, frame 1 includes a sequence of pattern A, frame 2 includes a sequence of pattern B and the cyclically rotated versions thereof (e.g., B′, B″ . . . B′^N), and frame 3 includes a sequence of repeated pattern C encoded with any command and control instructions. The relative timing illustrated in this example indicates that the “A” frame extends about 2.2 s, while the “B” frame extends about 0.3 s to 2.5 s and the “C” frame extends about 20 ms to 2.52 s.

When the MT is located very close to the RL, the RL has a very high signal-to-noise ratio (SNR) and the frames provided between the MT to the RL can be hollowed out since long integration times are not necessary. As shown in FIG. 12C, at the closest proximity between the RL and the MT, a hollowed frame can be utilized that includes a single instance of pattern A and optionally a single instance of pattern C. It is important to note that pattern B is not transmitted since the MT and the RL continue to have accurate timing. As illustrated, the ping with a single pattern A has a transmission time of about 157 us μs.

As the MT travels further from the RL, the SNR degrades slightly and the RL and MT can increase the energy being transmitted. As shown in FIG. 12C, the number of patterns that are transmitted is increased as the distance between the RL and the MT increases. This process can allow for continuous monitoring of the MT for periods of a year or more. As illustrated, two A patterns are transmitted initially, and another A pattern is transmitted at time of approximately 627 uS μs. When the MT travels still further from the RL, the SNR continues to degrade and additional A patterns are transmitted. FIG. 12C illustrates three A patterns (about 471 us μs) repeating at a time of about 1.1 ms.

Since the low-speed clock is calibrated as a precise number of high-speed clock ticks, the transponder can sleep for short periods of time and resume with confidence regarding the approximate timing of upcoming signals, if needed. The problem of searching through A & and B for frame and packet timing information can be avoided in these instances. By allowing the transmitter to remain active, although quiet, the transmitter can later resume and the carrier and data phase timing are preserved. As a consequence of these factors, a synchronized transmitter and receiver can be arranged to operate as though a constant transmission is occurring, while ignoring the quiet times. Both the RL and the MT remain quiet during these portions of time while maintaining their respective high speed clocks active.

In general, when recent timing information is available and future sessions can be scheduled with good accuracy (˜a few baud, or +/−200 nS ns or so), packets of sequence “A” can be reliably captured at the known times. These times can be close together or spaced apart in time. Receipt of the “A” transmission portion can be utilized to regain fine carrier timing since the gross timing from the “B” transmission is already known.

In situations such as reverse locator mode, after the initial full capture sequence is used to acquire gross and fine timing, subsequent captures maybe be as few as four “A” packets in a burst (600 uS μs), or several individual “A” packets spread out over a few mS ms. If “C” modulated data is needed, then the “C” packet can be sent. In some applications where the coarse and fine timing have been acquired and the RL and MT have not lost communications between one another, as few as zero “B” or “C” packets might be sent. In other situations where the RL and the MT lose contact between one another and the sequence position is lost (e.g., the uncertainty is approximately 50 uS μs of drift for sequences that are 157 uS μs long), the “B” pattern is required to bring the timing back into proper operation.

Example Operation of the RL in Search and Locate Modes

FIGS. 13A-13B are example flow charts for search and locate modes of an example remote locator (RL) arranged according to at least one aspect of the present disclosure.

Initially, the RL is in a search mode where the slow ping mode is active. The RL transmits pings that are coded for the MT, indicating the last distance that was calculated from the round-trip time between ping and reply. The RL also listens for replies from the MT. When a reply is received, the RL calculates the distance between the RL and the MT and evaluates the reply message from the MT for any mode change information that is requested. When a mode change is not requested, the RL continues transmitting pings to the MT that are encoded with pertinent distance information. When a change mode is requested, processing continues to FIG. 13B.

As described in FIG. 13B, the change mode request is analyzed by the RL. When the mode corresponds to a bookkeeping mode, a bookkeeping process is activated. A synchronization pairing procedure could also be employed, as well as any other desired mode.

When the locate mode is activated, the RL activates the fast ping mode. During the fast ping mode, the RL transmits pings to the MT that are coded with the last calculated distance and correlator phase information that was determined for a successful ping and reply pair. The RL evaluates any replies from the MT and determines if the MT has requested to stop the locate mode (e.g., stop the fast ping mode). When the fast ping mode continues, the distance calculations are updated between the RL and the MT, and the correlation phase information is identified for the received reply, both being coded into the next ping transmission to the MT. When the fast ping mode is terminated, processing resumes at FIG. 13A, where the slow ping procedure is initiated again. An optional error trap can be arranged to switch from fast ping mode to slow ping mode the communication link between the RL and the MT is lost over a timeout interval during the fast ping mode.

Example Operation of the MT in Search and Locate Modes

FIGS. 14A-14C are example flow charts for search and locate modes of an example micro-transponder (MT) arranged in accordance with at least one aspect of present disclosure.

As shown in FIG. 14A, the transponder (MT) is initially inactive. The user initiates a wake-up such as by pressing a button on the MT and a slow ping mode is activated in the MT device such as described previously. When the transponder and the remote locator have been in communications recently, then the transponder will have a coarse and fine timing that is sufficiently accurate so that communication can be commenced. However, in some instance where the coarse and fine timing of the MT and the RL no longer match relative to one another (e.g. the MT is out of range from the RL), communications are not immediately possible. When a ping is not detected after activation, the MT initiates a search for a communication signal from the RL. During the search, the transponder captures samples over a predetermined time interval that is sufficiently long so that an expected transmission from the RL can be correlated. Thus, during the search, the RL and the MT communicate with one another sufficient to establish coarse and fine timing in the MT for further communications. When the ping has not been detected from the RL over a timeout period, the MT activates a sleep mode since the RL is likely out of range.

When a ping is detected from the RL, the MT transmits a reply to the RL and evaluates any coded messages or commands that are communicated in the ping, and extracts any distance information that is encoded in the messages. The MT alerts the user that communication has been established and updates the display as distance measurements are identified. User inputs are evaluated to determine if he the user desires to change to the locate mode. When the locate mode is activated, the MT sends a reply message to the RL to change to locate mode and the MT activates the fast ping mode. Otherwise, the slow ping mode continues and the MT sends a reply to the RL acknowledging that communication is active.

Locate mode is described with reference to FIG. 14B. During the locate mode, the user initiates a spin around procedure such as that previously described, and the RL transmit transmits pings at a more frequent interval such as illustrated by time interval T2 in FIG. 12A. Compass readings are captured by the MT while the MT is “listening” for pings from the RL. When pings are not detected by the MT for a timeout interval, the MT activates a sleep mode since the RL is likely out of range. When pings are detected by the MT, distance and phase measurements are extracted from the transmission. The MT then evaluates the distance measurements and compass readings to determine if the target has been located. When the target has not been located, the MT sends a reply message to the RL acknowledging receipt of the distance information. When the target has been located the user is alerted that the target is located. A reply message is then transmitted to the RL and the slow ping mode is activated. Processing continues in FIG. 14C.

As shown in FIG. 14C, the user can begin walking towards the RL after the target location is identified. The RL transmits a slow ping, where each subsequent ping is spaced apart in time such as is illustrated by time interval T1 in FIG. 12A. Compass readings are captured and the distance measurements and current direction indicators are updated each time a ping is received from the RL. The MT continues to transmit replies to the RL for each received ping, acknowledging receipt and continually synchronizing their time bases. The desired direction to locate the MT is also indicated on the MT so that the user can monitor if they are walking in the proper direction or not. Once the MT is within a prescribed range such as 10 feet, for example, the MT provides an alert indication (e.g., a sound, a flashing light, a vibrating alert, etc.) that the target has been found and the MT goes to a sleep mode. A timeout detection and sleep mode activation can be employed for cases where the MT loses communication with the RL once the slow ping mode is enabled.

The presently described system, apparatus, and methods take advantage of the acquired frequency knowledge to allow for synthesis of a time and phase coherent response to accurately determine location with a low-power MT. Although the preceding description describes various embodiments of the system, the invention is not limited to such embodiments, but rather covers all modifications, alternatives, and equivalents that fall within the spirit and scope of the invention. For example, the positioning of the various components may be varied, the functions of multiple components can be combined, individual components may be separated into different components, or components can be substituted as understood in the art. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention is not limited except as by the appended claims.

Claims

1. A remote locator device that is tagged to an object, wherein the remote locator device is arranged to communicate with a portable transponder device that is operated by a user such that the user can determine a relative direction and at least a distance to the object from the portable transponder device, the remote locator device comprising:

a time control circuit that is arranged to provide timing control signals according to an internal clock of the remote locator device;

a transmitter means that is arranged to transmit a structured multi-frame transmission to the portable transponder device when activated such that the structured multi-frame transmission has a transmit cadence and frequency that is determined by the internal clock of the remote locator device, wherein the structured multi-frame transmission is coded with an identifier recognized by the portable transponder device;

a receiver means that is arranged to capture samples when activated with an array of capture buffers;

a circular correlator that is arranged to identify a correlation and a correlation phase in response to captured samples from the receiver means;

a processor means that is arranged in cooperation with the time control circuit, the transmitter means, the receiver means and the circular correlator, wherein the processor means is arranged to initialize the remote locator device in a slow ping mode, wherein in the slow ping mode the processor means is arranged for:

initiating the transmission of a first structured multi-frame transmission to the portable transponder device at a first time;

capturing samples with the array of capture buffers and the receiver means over a first predetermined time interval that is sufficient to accumulate values for multiple transmissions of a first sequence from the portable transponder device;

detecting a first reply from the portable transponder device at a second time when the captured samples in the array of capture buffers correlates with an expected first reply transmission for the first sequence from the portable transponder device;

calculating a distance between the remote locator device and the portable transponder device based on a difference between the second time and the first time;

encoding the calculated distance between the remote locator device and the portable transponder device in a second structured multi-frame transmission; and

initiating the transmission of the second structured multi-frame transmission to the portable transponder device at a third time such that the portable transponder device can extract the encoded calculated distance from the second structured multi-frame transmission upon receipt.

2. The remote locator device of claim 1, wherein in the slow ping mode the processor means is further arranged for:

capturing samples with the array of capture buffers and the receiver means over a second predetermined time interval that is sufficient to accumulate values for multiple transmissions of a second sequence from the portable transponder device;

detecting a second reply from the portable transponder device at a fourth time when the captured samples in the array of capture buffers correlates with an expected second reply transmission for the second sequence from the portable transponder device;

identifying a message to change to a fast ping mode from second reply from the portable transponder device; and

initiating a change to the fast ping mode in response to the identified message.

3. The remote locator device of claim 2, wherein in the fast ping mode the processor means is arranged for repeatedly:

capturing samples with the array of capture buffers and the receiver means over a third predetermined time interval that is sufficient to accumulate values for multiple transmissions of a third sequence from the portable transponder device;

detecting a third reply from the portable transponder device at a fifth time when the captured samples in the array of capture buffers correlates to an expected third reply transmission for the third sequence from the portable transponder device;

calculating a second distance between the remote locator device and the portable transponder device based on a difference between the second time and the first time;

encoding the calculated second distance between the remote locator device and the portable transponder device in a third structured multi-frame transmissions; and

initiate initiating the transmission of the third structured multi-frame transmission to the portable transponder device at a sixth time such that the portable transponder device can extract the encoded calculated distance from the third structured multi-frame transmissions transmission upon receipt.

4. The remote locator device of claim 3, wherein in the fast ping mode the processor means is further arranged for:

extracting compass measurements from the portable transponder device that is encoded in each third reply;

pairing each compass measurement with an associated calculated distance;

determining the relative direction of the remote locator device from the portable transponder device from at least the compass measurements and the calculated distances;

encoding the relative direction in a fourth structured multi-frame transmission; and

initiating the transmission of the fourth structured multi-frame transmission to the portable transponder device at a seventh time such that the portable transponder can extract the relative direction from the fourth structured multi-frame transmission upon receipt.

5. The remote locator device of claim 3, further comprising:

a first antenna located in a first region of the remote locator device;

a second antenna that is located in a second region of the remote locator device;

wherein the receiver means comprises a first receiver means configuration with coupled to the first antenna;, and a second receiver means coupled to the second antenna

a second receiver means configuration with the second antenna, wherein in the fast ping mode the processor means is further arranged for:

evaluating captured samples associated with the first receiver configuration and the second receiver configuration; and

determining the relative direction of the remote locator device from the portable transponder device based on the evaluated captured samples from the first receiver configuration and the second receiver configuration;

encoding the relative direction in a fourth structured multi-frame transmission; and

initiating the transmission of the fourth structured multi-frame transmission to the portable transponder device at a seventh time such that the portable transponder can extract the relative direction from the fourth structured multi-frame transmissions transmission upon receipt.

6. The remote locator device of claim 3, wherein in the fast ping mode the processor means if is further arranged for: identifying correlator phase information associated with detected replies, and encoding correlator phase information in the third structured multi-frame transmission for receipt by the portable transponder device.

7. The remote locator device of claim 2, wherein the fast ping mode is terminated and the slow ping mode activated when the processor determines that a timeout condition is satisfied without detecting replies from the portable transponder device.

8. The remote locator device of claim 1, wherein the processor means comprises at least one of: a micro-processor, a micro-controller, a complex instruction set computer (CISC) processor, a reduced instruction set computer (RISC) processor, an application specific integrated circuit (ASIC), a programmable logic device (PLD), a mode control logic, a firmware, a software, a storage circuit, a memory circuit, a non-volatile memory (NVM), and a read-only memory (ROM).

9. The remote locator device of claim 1, wherein the receiver means is arranged to accumulate captured signals to form an integrated captured sequence as the captured samples such that the circular correlator can identify the correlation when the captured samples from the integrated captured sequence correlates with a structured reply transmission from the portable transponder device.

10. A portable transponder device that is operated by a user and arranged to communicate with a remote locator device that is tagged to an object such that the user can determine at least a relative direction and distance to the object from the portable transponder device, the portable transponder device comprising:

an input device that is arranged to accept user initiated input;

a time control circuit that is arranged to provide timing control signals from a high speed internal clock, wherein the time control circuit is arranged to deactivate the high speed internal clock when the portable transponder device is in a low power sleep mode, and enable the high speed internal clock when a wake-up is initiated;

an indication means that is arranged to report information to the user;

a transmitter means that is arranged to transmit a structured multi-frame reply transmission to the remote locator device when activated such that the structured multi-frame transmission has a transmit cadence and frequency that is determined by the internal clock and the structured multi-frame transmission is coded with an identifier recognized as from the portable transponder device;

a receiver means that is arranged to capture samples when activated;

a correlator that is arranged to identify a correlation and a correlation phase in response to captured samples from the receiver means;

a processor means that is arranged in cooperation with the input device, the time control circuit, the indication means, the transmitter means, the receiver means and the correlator, wherein the processor means is arranged for:

initializing the portable transponder device in a low power sleep mode;

detecting user initiated inputs;

activating a slow ping mode when the user initiates a wake-up, wherein in the slow ping mode the processor is arranged for:

capturing samples with the receiver means over a first predetermined time interval sufficient to capture a first expected transmission of a first sequence from the remote locator device;

detecting a first ping from the remote locator device at a first time when the captured samples correlate with the expected first transmission from the remote locator device;

initiating the transmission of a first structured multi-frame reply transmission to the remote locator device at a second time,

capturing samples with the receiver means over a second predetermined time interval sufficient to capture a second expected transmission of a second sequence from the remote locator device;

detecting a second ping from the remote locator device at a third time when the captured samples from the second predetermined time interval correlate to the second expected transmission from the remote locator device; and

extracting the distance to the object from the detected second ping.

11. The portable transponder device of claim 10, wherein in the slow ping mode the processor means is further arranged for:

encoding a first message to the remote locator device in a second structured multi-frame reply transmission, wherein the message includes a request to change to a fast ping mode; and

initiating the transmission of the second structured multi-frame reply transmission in response to user initiated input at a fourth time.

12. The portable transponder device of claim 11, wherein in the fast ping mode the processor means is arranged for repeatedly:

capturing samples with the receiver means over a third predetermined time interval sufficient to capture a third expected transmission of a third sequence from the remote locator device;

detecting a third ping from the remote locator device at a fourth time when the captured samples correlate to the third expected transmission from the remote locator device; and

extracting the distance to the object from each detected third ping.

13. The portable transponder device of claim 12, wherein in the fast ping mode the processor means is arranged for repeatedly:

monitoring a rotational position of the portable transponder device for each detected third ping;

initiating the transmission of a third structured multi-frame reply transmission to the remote locator device at a fifth time; and

determining the relative distance and direction to the object.

14. The portable transponder device of claim 12, wherein in the fast ping mode the processor means is arranged for repeatedly:

capturing samples with the receiver means over a fourth predetermined time interval sufficient to capture a fourth expected transmission of a fourth sequence from the remote locator device;

detecting a fourth ping from the remote locator device at a fifth time when the captured samples correlate to the fourth expected transmission from the remote locator device; and

extracting the relative direction of the object from the detected fourth ping.

15. The portable transponder device of claim 10, further comprising a means for determining a rotational position associated with the portable transponder device about an axis that is approximately centered about the user of the portable transponder device.

16. The portable transponder device of claim 15 wherein the means for determining the rotational position comprises at least one of: an analog compass sensor device, a digital compass sensor device, and an analog-to-digital converter that is arranged to work with the analog compass sensor device.

17. The portable transponder device of claim 12, wherein in the fast ping mode the processor means is further arranged for: extracting correlator phase information associated with the remote locator from the detected third ping, and utilizing the correlator phase information to synthesize the a high frequency clock for the transmitter means.

18. The portable transponder device of claim 12 13, wherein the fast ping mode is terminated and the slow ping mode is initiated after the relative distance and direction to the object is determined.

19. The portable transponder device of claim 10, wherein the processor means comprises at least one of: a micro-processor, a micro-controller, a complex instruction set computer (CISC) processor, a reduced instruction set computer (RISC) processor, an application specific integrated circuit (ASIC), a programmable logic device (PLD), a mode control logic, a firmware, a software, a storage circuit, a memory circuit, a non-volatile memory (NVM), and a read-only memory (ROM).

20. The portable transponder device of claim 10, wherein the processor means is further arranged to initiate a user notification via the indication means when the direction associated with the location of the remote locator relative to the portable transponder device is determined.

21. The portable transponder device of claim 10, wherein the indication means is arranged to provide a visual indicator of the determined direction.

22. The portable transponder device of claim 10, wherein the visual indicator comprises at least one of: a graphical distance indication, an alphanumeric distance indication, a graphical direction indication, and an alphanumeric direction indication.

23. The portable transponder device of claim 10, the input device comprising an interrupt signal, a wake-up timer, a keyboard device, a keypad device, a button, a key, a touch-screen, a touch-panel, a joystick device, a joy-pad device, a mouse device, a pointing device, a touch-pad device, a pressure sensitive input device, another processor, or an input generated by a software program.

24. The portable transponder device of claim 10, the input device comprising an audio input processor that is arranged to process sound as the user input.

25. The portable transponder device of claim 24, the audio input processor comprising either an analog to digital-converter (ADC) circuit or a coder-decoder (CODEC) circuit.

26. The portable transponder device of claim 24, the audio input processor comprising a voice input means.

27. The portable transponder device of claim 10, the indication means comprising an audio output circuit that is configured to provide an audible alert to the user, wherein the audio output circuit includes an audio output processor that is combined in function with the audio input processor.

28. The portable transponder device of claim 10, the indication means comprising an audio output circuit that is configured to provide audible information to the user.

29. The portable transponder device of claim 28, wherein the audio output circuit comprises either: an audio output device, an audio output processor, or a combination of the audio output device and the audio output processor.

30. The portable transponder device of claim 28, wherein the audio output circuit comprises either a speaker device, a piezo device, or an audio output port that is accessible by the user.

31. The portable transponder device of claim 28, wherein the audio output circuit comprising either an analog to digital-converter (ADC) circuit or a coder-decoder (CODEC) circuit.

32. The portable transponder device of claim 28, the audio output circuit is arranged to playback sounds from either a previously recorded sound or a user recorded sound.

33. The portable transponder device of claim 28, wherein the audio output circuit is arranged to synthesize previously selected sounds.

34. The portable transponder device of claim 10, the indication means comprising a visual output circuit that is configured to provide visual information to the user.

35. The portable transponder device of claim 34, wherein the visual information comprises at least one of: a graphical distance indication, an alphanumeric distance indication, a graphical direction indication, and an alphanumeric direction indication.

36. The portable transponder device of claim 34, the visual output circuit comprising at least one of: an LED type display, an LCD type display, an active display, a passive display, a black and white display, a monochromatic display, a color display, a discrete arrangement of LEDs, a seven segment display, and a light emitting device.

37. The portable transponder device of claim 34, wherein the visual output circuit and the input device are combined in a touch screen device.

38. The portable transponder device of claim 10, wherein the processor means is further arranged for determining an elapsed time since the last successful communication and computing an approximation of a time uncertainty associated with the portable transponder device based on the elapsed time.

39. The portable transponder device of claim 38, wherein the portable transponder device is arranged to re-establish communications with the remote locator device by evaluating the approximation of the time uncertainty associated with the portable transponder device and scanning the relevant time range such that communication are re-established with minimal impact on power consumption.