Sensing with active electronic tags - Patent Review 5892441

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BACKGROUND OF THE INVENTION

This invention relates to managing assets equipped with active electronic tags.

Asset management systems attempt to monitor the existence, location and status of items being transported or stored. The systems include two main types: barrier systems and continuous systems.

Barrier systems determine whether an asset is located within a cell delimited by a set of stationary interrogators. The interrogators detect the asset as it crosses a boundary of the cell, but are incapable of determining a position of the asset within the cell.

Typical barrier systems fix a bar code to each asset, and use bar code readers as interrogators. Examples include systems used by car factories and mail carriers, such as Federal Express and United Parcel Service.

Another type of barrier system attaches a transponder to each asset and uses radio transceivers as interrogators. The transponders are passive, that is, they respond when activated by an interrogator but are incapable of initiating communications or other functions. An example includes a railroad system developed by AMTECH, in which each railroad car on a train is equipped with a transponder. Interrogators include transceivers that are placed along a railway.

Continuous systems determine the position of an asset anywhere within a monitoring area. An example, implemented by LOJACK, equips cars with an active transponder. The transponder is active because it initiates its own functions, for example, it monitors a particular hailing frequency. If the car is stolen, LOJACK uses special transmitters to establish communication with the transponder at the hailing frequency. Receivers provided to police cars analyze the communications to locate the car.

SUMMARY OF THE INVENTION

In general, in one aspect, the invention features a method for monitoring an object in a moving vehicle by attaching an electronic tag to the object. An electronic device detects the presence of the object by communicating with the tag while the object is being moved by the vehicle.

Preferred embodiments of this aspect of the invention include the following features.

The absence of the object is reported to a user or a computer accessible to the user when the device is unable to communicate with the tag. The absence may be reported periodically to either an owner of the object or a possessor of the object.

A location of the object is determined by establishing communications between the tag and a sensor storing location information. The location of the object is compared to a planned route, and any discrepancy between the location and the planned route is reported to the user. The discrepancy may be reported through a computer. The user determines a new route for the object and notifies the elective device of the new route. The user may determine the new route using a computer.

The object may be disposed in a container to which is attached an electronic tag. Communication is established between the tag on the object and the tag on the container. The tag on the object communicates with the electronic device through the tag on the container. The container may also be placed in a second container to which is attached an electronic tag. The tag on the first container communicates with the tag on the second container.

Each tag stores a unique identification for the object, a description of the object, the object's shipper and destination and a signature of each possessor of the object. The signatures stored by the tag are forwarded to the electronic device when the tag reaches the limit of its memory capacity or when the object to which the tag is attached reaches the end of its monitoring period. The signatures are then deleted from the tag memory.

In general, in another aspect, the invention features a system for monitoring and affecting actions on behalf of a group of objects. A first electronic tag attached to an object includes circuitry for communicating with a second electronics tag, the circuitry communicating at least information pertaining to an object in the group. The first tag also includes a memory connected to the circuitry and capable of storing the information. The second electronic tag attached to another object includes circuitry for communicating with the first tag and a memory connected in an analogous manner.

Preferred embodiments of this aspect of the invention include the following features.

The first and second tags each include a controller connected to the memory and the communication circuitry. The information communicated between the tags includes a distance between the tags. The first tag has a time measuring device connected to the controller, the device measuring a time duration required for a signal to travel between the first tag and the second tag. The device in the first tag includes a clocked counter which has a first count corresponding to the time of transmission of the signal and which subsequently has a second count corresponding to the time of recognition that the signal has been received. The device uses the product of the clock period and the difference between the counts to determine the time duration.

The second tag may also include a delay circuit connected between the circuitry and the controller, the delay circuit causing the second tag to delay returning the signal to the first tag by a given time period. The memory in the first tag includes a program causing the controller to execute a function when the controller reads the memory, the function causing the controller to determine the distance based on the time duration.

The second tag includes a second counter connected to the controller in the second tag. The counter in the first tag has a mechanism for adjusting a frequency of the first-mentioned counter, the mechanism being coupled to the controller in the first tag. The program in the first tag causes the controller to employ the mechanism to offset the frequency of the first-mentioned counter from a frequency of the counter in the second tag based on the frequency of the counter in the second tag.

The program in the memory of the first tag also causes the controller to enable the circuitry in the first tag to transmit a number of symbols at an intersymbol period such that a number of clock cycles of the first tag during a transmission and receipt of the symbols is substantially equal to a number of clock cycles of the clock of the second tag within an offset (n), where n and the number of symbols have no common factors other than one.

The first tag further includes a power source coupled to the controller and supplying power to the circuitry in the first tag. The program in the memory of the first tag causes the controller to adjust the power supplied to the controller to that minimal level which allows communication between the circuitry in the first tag and the circuitry in the second tag.

The system also includes a set of tags neighboring the first tag and including the second tag. A program in the memory of the first tag causes the controller to identify the set of tags by enabling the circuitry in the first tag to establish communications with each tag in the set. The controller determines the distance between each pair of tags in the set and the relative locations of each tag in the set based on the distances. The controller then selects a particular tag located on the periphery of the set for communicating outside the set.

In some embodiments, the information communicated between the tags includes a distance of the first tag from three different tags in the set and the absolute locations of the three different tags. The absolute locations may be derived from a sensor communicating its location to one of the three different tags. A program in the memory of the first tag causes the controller to determine the absolute position of the first tag from the information. The controller also minimizes an error in the absolute position of the first tag.

A program in the memory of the first tag also causes the controller to enable the circuitry in the first tag to communicate information with each tag in the set and analyze the information from the circuitry to determine the presence of a new tag in the set. The controller then causes the circuitry to communicate with the new tag to obtain a distance between the first tag and the new tag and a location of the new tag. The controller refines the absolute position of the first tag based on the location of the new tag and the distance between the first tag and the new tag.

In some embodiments, the information communicated by the tags includes the distance of the first tag from the second tag at a number of different times, and the absolute locations of the second tag at each of the times. A program in the memory of the first tag causes the controller to determine the absolute position of the first tag from the information.

A program in the memory of the first tag causes the controller to select tags in the set as relay tags, the circuitry in the relay tags being designated to route messages between a pair of tags in the set. The relay tags provide communications between each pair of tags in the set with a minimum number of communication links. The controller performs a Delaunay triangulation to determine a topology of the tags in the set and selects the relay tags based on the topology.

In some embodiments, the system includes a user-operated device for communicating with the circuitry in the first tag. The information communicated between tags includes an identity of a tag in the set capable of long range communication and a change in an identity of a tag in the set. The program in the memory of the first tag configures the controller to enable the circuitry in the first tag to notify the device of the change in the identity of a tag in the set.

A program in the memory of the first tag also causes the controller to divide the set of tags into subsets when a number of tags in the set increases above a threshold value. The memory in the first tag further stores an identification code for the first tag, and the program causes the controller to truncate the code and employ the circuitry in the first tag to communicate the truncated code to a second tag within a subset to minimize energy consumption.

The information communicated between tags may also include an identification of a tag belonging to a different set. A program in the memory of the first tag causes the controller to merge the first-mentioned set with the different set when a number of tags in the first-mentioned set falls below a selected threshold value.

The system may also include a time sensor which communicates a time to the circuitry in the second tag. The controller enables the circuitry in the first tag to communicate with the second tag to obtain the time, and sets the counter in the first tag using the communicated time.

A sensor may also communicate location information to the first tag, and the memory in the second tag may store a transportation route for the object attached to the first tag. The controller employs the circuitry in the first tag to query the second tag regarding the transportation route and to query the sensor regarding the tag's location. The controller then derives a present time from the counter in the first tag, determines an expected position of the object at the present time based on the route and an actual position of the object based on the location. The tag then decides whether the actual position deviates from the expected position. The controller notifies the user-operated device of a deviation from the expected position by causing the circuitry to communicate with the device.

The user-operated device includes a processor connected to a device communication circuitry. A device memory is coupled to the processor and includes a program configuring the processor to perform a called-for function when the processor reads the device memory. The function can cause the processor to determine a new route for the object attached to the first tag based on the deviation, and causes the processor to employ the device communication circuitry to communicate the new route to the first tag. The processor also employs the device communication circuitry to receive communications indicating a change in the route from at least one of the following: a second device, a tag and a system operator.

In some embodiments, the first tag has a detector connected to the controller. The detector notifies the controller when the first tag is detached from the object to which it had been attached. The controller enables the circuitry in the first tag to notify the user-operated device in response to notification from the detector that the first tag is detached from the object.

The circuitry in the first tag includes ports for infrared, radio frequency and acoustic communications. The port for radio frequency communications includes circuitry for communicating over a number of channels. A program in the memory causes the controller to select one channel for RF communication over the port and to code the information with an encryption code before enabling the tag's circuitry to communicate the information to the second tag.

The first tag can also include a power source connected to the controller and a timer connected to both the power source and controller. The controller loads a time period in the timer, and the timer causes the power from the power source to be removed from the controller and restored after the time period elapses.

A detector connected to the controller and the power source provides a detection signal to the controller indicating whether the power source has deteriorated beyond a threshold level. Another detector connected to the controller provides a detection signal to the controller indicating whether the tag has been tampered with. This detector includes a transmitter, a receiver, and an optical fiber connecting an output of the transmitter to an input of the receiver. The transmitter forwards a signal over the optical fiber to the input of the receiver. The receiver notifies the controller if the signal does not arrive at the input of the receiver.

The system may also include a sensor detecting the condition of an object and communicating the condition to the circuitry in the first tag. The circuitry in the first tag subsequently communicates the condition to the second tag. The memory in the first tag includes a stored set of thresholds values related to the condition, each threshold being associated with a time. A program in the memory configures the controller to compare the condition to the threshold corresponding to a given time and to employ the circuitry to report a deviation from the threshold to the second tag. Similarly, the stored thresholds may be associated with a location of the first tag.

The sensor may be an analog sensor, such as a temperature sensor or a fuel level sensor for a vehicle. Alternatively, the sensor may be a digital sensor. If the object is disposed in a vehicle, the digital sensor may detect whether a door of the vehicle is open or closed, or whether a vehicle brake has been released.

In some embodiments, the system includes a computer having circuitry in communication with the circuitry in the first tag. A processor is connected to the circuitry in the computer, and a port for communicating with a user is coupled to the processor. A computer memory linked to the processor includes a stored threshold and a program configuring the processor to perform a selected function when the processor reads the computer memory. The function can cause the processor to enable the circuitry to receive information from the first tag, compare the information to the threshold, and enable the port to notify the user if the information exceeds the threshold.

The system can also include a module comprising a port for communicating with the circuitry in the first tag. A processor is connected to the module port. A module memory coupled to the processor includes a program configuring the processor to perform a selected function when the processor reads the module memory. The function causes the processor to employ the port to receive an activating signal and to provide information regarding an object in the system in response to the activating signal. The port includes circuitry for receiving a wireless activating signal and circuitry for responding to the activating signal with an acoustic signal. The circuitry transmits the acoustic signal at a carrier frequency identical to (or different from) a carrier frequency of the activating signal.

In some embodiments, the system has a device with an information exchange port configured to communicate with the circuitry on the first tag. The port enables the device to store information in the memory of the first tag and to retrieve information from the memory of the first tag. The device also includes circuitry for long-range wireless communications with a wireless component.

In general, in another aspect, the invention features a module including a first tag having a port. Circuitry in the first tag is coupled to the port and configured for communicating with the second tag. A second tag in the module includes a mechanism for attaching the second tag to the first tag. A port disposed on the second tag is adjacent the port on the first tag when the second tag is attached to the first tag. A circuit in the second tag is coupled to the port in the second tag, the circuit being configured for communicating with the circuitry in the first tag.

Preferred embodiments of this aspect of the invention include the following features.

The first tag includes a controller connected to the circuitry and a power connection coupled to the controller. The second tag includes a power source coupled to its circuit and a second power connection. The second power connection couples to the first power connection when the second tag is attached to the first tag. The power source may include a connection to a power source external to the second tag.

The second tag has an interrupt source coupled to a second port on the second tag. The first tag further includes an interrupt port coupled to its controller. The second port on the second tag is coupled to the interrupt port on the first tag when the second tag is attached to the first tag.

The second tag includes a second circuit for communicating with a third tag and a switch connecting the second circuit to the first-mentioned circuit in the second tag. The first tag includes a memory coupled to its controller which stores a program configuring the controller to execute a selected function when the controller reads the memory. The selected function causes the controller to communicate with the second tag over the first-mentioned port and causes the second circuit in the second tag to communicate with the third tag.

The memory may also cause the first tag's controller to monitor the second circuit in the second tag over the first-mentioned communication port for the receipt of an activating signal. The controller causes the second circuit to respond with voice to the activating signal.

In general, in another aspect, the invention features a method for determining a topology of a set of spatially distributed nodes. The method includes the step of causing each node in the set to find its neighboring nodes by performing a Delaunay triangulation. Each node then stores an identification of each of its neighboring nodes.

A preferred embodiment of this aspect of the invention includes the following feature. A new node is added to the set and a Delaunay triangulation is performed at the new node to find the neighboring nodes of the new node. An identification of each of the neighboring nodes is stored at the given node. Each neighboring node of the new node stores an identification of the new node.

In general, in another aspect, the invention features a method for verifying a measured distance between a pair of objects in a set of spatially distributed objects. The method includes the steps of choosing a group of three objects and measuring the distances between each pair of objects in the group. The method estimates an error for each distance and determines whether the distances correspond to three sides of a triangle, taking into account the estimated errors.

Preferred embodiments of this aspect of the invention include the following features.

The method further includes the steps of choosing a group of four objects from the set and repeating the measuring and estimating steps for each pair of objects in the group of four objects. The method then determines whether the distances correspond to the sides of a planar tetrahedron sharing a common side, taking into account the estimated errors.

The method may also include choosing a group of five objects from the set and repeating the measuring and estimating steps for each pair of objects in the group. The method then determines whether the distances correspond to the sides of a pair of planar tetrahedrons sharing a common triangle, taking into account the estimated errors.

In general, in another aspect, the invention features a method of measuring a distance between two objects. The method includes the steps of transmitting from one of the objects a plurality of symbols, the transmissions occurring in a first series in which consecutive transmissions are separated by a first time interval which is a multiple of a first clock period; receiving each of the symbols at the other of the objects and re-transmitting each of the symbols back to the one of the objects, the re-transmissions occurring in a second series in which consecutive re-transmissions are separated by a second time interval which is different from the first time interval and which is a multiple of a second clock period which is different from the first clock period; measuring a round-trip time corresponding to detection of receipt from the other of the objects of each of the symbols, each round-trip time being a multiple of the first clock period; and determining the distance between the objects using a velocity of the symbols, the clock periods, at least one of the time intervals, and the round-trip times.

Preferred embodiments of this aspect of the invention include the following features. One clock period may be approximately equal to the product of the other clock period and a fraction, the numerator of the fraction may be equal to the sum of two terms, the first term of the sum may be the product of the number of symbols transmitted and the number of first clock periods corresponding to the first time interval, the second term of the sum may be a whole number that shares no multiplicative factors, other than 1, with the number of symbols transmitted, and the denominator of the fraction may be equal to the product of the number of symbols transmitted and the number of first clock periods corresponding to the first time interval. The measured round-trip times may vary and may have a variation range equal to one first clock period.

In another aspect, the invention features another method of measuring a distance between two objects. The method includes the steps of transmitting from one of the objects a plurality of symbols, the transmissions occurring in a series in which consecutive transmissions are separated by a time interval which is a multiple of a first clock period; returning each of the symbols from the other of the objects back to the one of the objects; measuring a round-trip time corresponding to detection of receipt from the other of the objects of each of the symbols, each round-trip time corresponding to a multiple of a second clock period; and determining the distance between the objects using a velocity of the symbols, the clock periods, at least one of the time intervals, and the round-trip times.

Preferred embodiments of this aspect of the invention include the following features. The method may further include a step of detecting each transmission using the second clock period, wherein the round-trip times correspond to time intervals between detections of receipt and transmission of the symbols. One of the clock periods may be approximately equal to the product of the other clock period and a fraction, the numerator of the fraction may be equal to the sum of two terms, the first term of the sum may be the product of the number of symbols transmitted and the number of first clock periods corresponding to the first time interval, the second term of the sum may be a whole number that shares no multiplicative factors, other than 1, with the number of symbols transmitted, and the denominator of the fraction may be equal to the product of the number of symbols transmitted and the number of first clock periods corresponding to the first time interval. The measured round-trip times may vary and may have a variation range equal to one first clock period.

In another aspect, the invention features another method of measuring a distance between two objects. The method includes the steps of transmitting from one of the objects a first symbol; transmitting from the one of the objects a second symbol, the transmission of the second symbol being separated from the transmission of the first symbol by a time interval which is a multiple of a first clock period; receiving the first symbol at the other of the objects and re-transmitting the first symbol back to the one of the objects; receiving the second symbol at the other of the objects and re-transmitting the second symbol back to the one of the objects, the re-transmission of the second symbol being separated from the re-transmission of the first symbol by a second time interval which is a multiple of a second different clock period; measuring a first round-trip time corresponding to detection of receipt from the other of the objects of the first symbol, the first round-trip time being a multiple of the first clock period; measuring a second round-trip time corresponding to detection of receipt from the other of the objects of the second symbol, the second round-trip time being a multiple of the first clock period; if the round-trip times are not different by at least one first clock period, repeating the transmitting, the receiving, the re-transmitting, and the round-trip measuring of the second symbol until the round-trip times are different by one first clock period; and determining the distance between the objects using a velocity of the symbols, the clock periods, the time intervals, and the round-trip times.

A preferred embodiment of this aspect of the invention includes the following feature. The first and second round-trip times may be original round-trip times, the original round-trip times being different by one first clock period, and the method may further include the steps of (a) equating the clock periods; (b) temporarily changing one of the clock periods for a period of time equal to a fraction of the time interval; (c) determining a new round-trip time for a symbol; (d) comparing the new round-trip time to at least one of the original round-trip times; and repeating (a) through (d) for a plurality of iterations, the fraction being different for each iteration.

In another aspect, the invention features another method of measuring a distance between two objects where one of the objects is able to measure time with a resolution of a minimum period of time. The method includes the steps of transmitting a plurality of symbols from the one of the objects; receiving each of the symbols at the other of the objects and re-transmitting each of the symbols back to the one of the objects after a delay period, the delay period being different for each symbol and being a multiple of a constant delay unit, the delay unit being approximately equal to the minimum period divided by the number of symbols transmitted; measuring a round-trip time corresponding to detection of receipt of each symbol from the other of the objects, each round-trip time corresponding to a multiple of the minimum period; and determining the distance between the objects using a velocity of the symbols, the minimum period, and at least one of the round-trip times.

In another aspect, the invention features another method of measuring a distance between two objects, where one of the objects is able to measure time with a resolution of a minimum period of time. The method includes the steps of transmitting a plurality of symbols from the one of the objects; after a first delay period, detecting each transmission, the first delay period being different for each transmission and being a multiple of a constant delay unit, the delay unit being approximately equal to the minimum period divided by the number of symbols transmitted; returning each of the symbols at the other of the objects back to the one of the objects; after a second delay period, detecting the receipt of each of the symbols from the other of the objects, the second delay period being different for each receipt and being a multiple of the delay unit; measuring a time difference between detections of transmission and receipt of each of the symbols, each time difference being a multiple of the minimum period; and determining the distance between the objects using a velocity of the symbols, the minimum period, and the time differences.

In another aspect, the invention features another method of measuring a distance between two objects, where one of the objects is able to measure time with a resolution of a minimum period of time. The method includes the steps of transmitting a first symbol from the one of the objects; receiving the first symbol at the other of the objects and re-transmitting the first symbol back to the one of the objects; measuring a first round-trip time corresponding to detection of receipt from the other of the objects of the first symbol, the first round-trip time being a multiple of the minimum period; a) transmitting another symbol from the one of the objects; b) receiving the another symbol at the other of the objects and re-transmitting the another symbol back to the one of the objects after a delay period, the delay period being a multiple of a delay unit, the delay unit being approximately equal to a fraction of the minimum period, the number of delay units in the delay period being equal to the number of symbols previously re-transmitted; c) measuring another round-trip time corresponding to detection of receipt from the other of the objects of the another symbol, the another round-trip time being a multiple of the minimum period; if the first round-trip time and the another round-trip time are equal, repeating a) through c) until the round-trip times are different by one minimum period; and determining the distance between the objects by using a velocity of the symbols, the minimum period, delay unit, and the round-trip times.

A preferred embodiment of this aspect of the invention includes the following feature. The method may further include a step of reducing the delay unit to a smaller fraction of the minimum period and repeating a) through c).

In general, in another aspect, the invention features apparatus for measuring a distance between two objects. The apparatus includes a transmitter having a first clock for transmitting from one of the objects a plurality of symbols, the transmissions occurring in a first series in which consecutive transmissions are separated by a first time interval which is a multiple of the period of the first clock; a transponder having a second clock for receiving each of the symbols at the other of the objects and re-transmitting each of the symbols back to the one of the objects, the re-transmissions occurring in a second series in which consecutive re-transmissions are separated by a second time interval which is different from the first time interval and which is a multiple of the second clock's period which is different from the period of the first clock; a clock counter for measuring each round-trip time corresponding to detection of receipt from the other of the objects of each of the symbols, each round-trip time being a multiple of the first clock period; and a calculator for determining the distance between the objects using a velocity of the symbols, the clock periods, at least one of the time intervals, and the round-trip times.

In another aspect, the invention features another apparatus for measuring a distance between two objects. The apparatus includes a transmitter having a first clock for transmitting from one of the objects a plurality of symbols, the transmissions occurring in a series in which consecutive transmissions are separated by a time interval which is a multiple of the period of the first clock; a transponder for returning each of the symbols from the other of the objects back to the one of the objects; a counter having a second clock for measuring a round-trip time corresponding to detection of receipt from the other of the objects of each of the symbols, each round-trip time corresponding to a multiple of the period of the second clock; and a calculator for determining the distance between the objects using a velocity of the symbols, the clock periods, the time interval, and the round-trip times.

In another aspect, the invention features another apparatus for measuring a distance between two objects. The apparatus includes a transmitter for transmitting from one of the objects a first symbol and transmitting from the one of the objects a second symbol, the transmission of the second symbol being separated from the transmission of the first symbol by a time interval which is a multiple of a first clock period; a transponder for (a) receiving the first symbol at the other of the objects and re-transmitting the first symbol back to the one of the objects and (b) receiving the second symbol at the other of the objects and re-transmitting the second symbol back to the one of the objects, the re-transmission of the second symbol being separated from the re-transmission of the first symbol by a second time interval which is a multiple of a second different clock period; a clock counter for (a) measuring a first round-trip time corresponding to detection of receipt from the other of the objects of the first symbol, the first round-trip time being a multiple of the first clock period, and (b) measuring a second round-trip time corresponding to detection of receipt from the other of the objects of the second symbol, the second round-trip time being a multiple of the first clock period; a comparator for (a) testing if the round-trip times are not different by one first clock period, and (b) initiating repeating of the transmitting, the receiving, the re-transmitting, and the round-trip measuring of the second symbol until the round-trips are different by one first clock period; and a calculator for determining the distance between the objects using a velocity of the symbols, the clock periods, the time intervals, and the round-trip times.

In general, in another aspect, the invention features another method of measuring a distance between two objects. The method includes the steps of transmitting from one of the objects a plurality of symbols, the transmissions occurring in a first series in which consecutive transmissions are separated by a first time interval which is a multiple of a first clock period, the symbols having a constant velocity; receiving each of the symbols at the other of the objects and re-transmitting each of the symbols back to the one of the objects, the re-transmissions occurring in a second series in which consecutive re-transmissions are separated by a second time interval, the second time interval being different from the first time interval, the second time interval being a multiple of a second clock period which is different from the first clock period, each re-transmission thus occurring after a delay, the delay being different for each re-transmission and constituting the period of time between arrival of each symbol at the other of the objects and re-transmission of each symbol, the delay having a nominal length which is a multiple of a delay unit, the delay unit being equal to the product of the number of first clock periods corresponding to the first time interval and the difference between the clock periods, the delay unit also being equal to the second clock period divided by the number of symbols in the plurality of symbols, the delay having an actual length which is within one delay unit of the nominal length; measuring a round-trip time corresponding to detection of receipt from the other of the objects of each of the symbols, each round-trip time being a multiple of the first clock period; and determining the distance between the objects by multiplying the velocity of the symbols by half of the difference between the average of all of the round-trip times and the first clock period.

Preferred embodiments of this aspect of the invention include the following features. The delay may further include an extra delay which is a multiple of the second clock period; and the distance may be determined by multiplying the velocity by half of the difference between (a) the average of all of the round-trip times and (b) the sum of the extra delay and the first clock period. The method may further include a step of offsetting the average of the round-trip times, wherein the offset is an amount of time equal to the second clock period divided by twice the product of (a) the number of symbols and (b) the number of first clock periods corresponding to the first time interval.

In another aspect, the invention features another method of measuring a distance between two objects. The method includes the steps of transmitting from one of the objects a plurality of symbols, the transmissions occurring in a series in which consecutive transmissions are separated by a time interval which is a multiple of a first clock period, the symbols having a constant velocity; detecting the transmissions using a second clock period, the second clock period being different from the first clock period, each transmission detection thus occurring after a delay, the delay being different for each detection and constituting the period of time between each transmission and the detection of each transmission, the delay having a nominal length which is a multiple of a delay unit, the delay unit being equal to the product of the number of first clock periods corresponding to the time interval and the difference between the clock periods, the delay unit also being equal to the second clock period divided by the number of symbols in the plurality of symbols, the delay having an actual length which is within one delay unit of the nominal length; returning each of the symbols from the other of the objects back to the one of the objects; detecting the arrivals of the returning symbols back at the one of the objects using the