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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to asset tracking and, more particularly, to
tracking of assets, including goods and vehicles, using the Global
Positioning System (GPS). While goods are an example of assets that need
to be tracked, the cargo containers, container trucks and railcars in
which the goods are shipped are themselves assets which need to be
tracked.
2. Description of the Prior Art
Goods shipped from a manufacturing plant, warehouse or port of entry to a
destination are normally tracked to assure their timely and safe delivery.
Tracking has heretofore been accomplished in part by use of shipping
documents and negotiable instruments, some of which travel with the goods
and others of which are transmitted by post or courier to a receiving
destination. This paper tracking provides a record which is completed only
on the safe delivery and acceptance of the goods. However, during transit,
there sometimes is a need to know the location and position of the goods.
Knowledge of the location of goods can be used for inventory control,
scheduling and monitoring.
Shippers have provided information on the location of goods by tracking
their vehicles, knowing what goods are loaded on those vehicles. Goods are
often loaded aboard shipping containers or container trucks, for example,
which are in turn loaded aboard railcars. Various devices have been used
to track such vehicles. In the case of railcars, passive radio frequency
(RF) transponders mounted on the cars have been used to facilitate
interrogation of each car as it passes a way station and supply the car's
identification. This information is then transmitted by a radiated signal
or land line to a central station which tracks the locations of cars. This
technique, however, is deficient in that while a particular railcar
remains on a siding for an extended period of time, it does not pass a way
station. Moreover, way station installations are expensive, requiring a
compromise that results in way stations being installed at varying
distances, depending on the track layout. Thus, the precision of location
information varies from place to place on the railroad.
Recently, mobile tracking units have been used for tracking various types
of vehicles, such as trains. Communication has been provided by means of
cellular mobile telephone or RF radio link. Such mobile tracking units are
generally installed aboard the locomotive which provides a ready source of
power. However, in the case of shipping containers, container truck
trailers and railcars, a similar source of power is not readily available.
Mobile tracking units which might be attached to containers and vehicles
must be power efficient in order to provide reliable and economical
operation. Typically, a mobile tracking unit includes a navigation set,
such as a Global Positioning System (GPS) receiver or other suitable
navigation set, responsive to navigation signals transmitted by a set of
navigation stations which may be either space-based or earth-based. In
each case, the navigation set is capable of providing data indicative of
the vehicle location based on the navigation signals. In addition, the
tracking unit may include a suitable electromagnetic emitter for
transmitting to a remote location the vehicle's location data and other
data acquired from sensing elements on board the vehicle. Current methods
of asset localization require that each item tracked be individually
equipped with hardware which determines and reports location to a central
station. In this way, a tracked asset is completely "ignorant" of other
assets being shipped or their possible relation to itself. In reporting to
the central station, such system requires a bandwidth which scales
approximately with the number of assets being reported. The aggregate
power consumption over an entire such system also scales with the number
of assets tracked. Further, since both the navigation set and the emitter
are devices which, when energized, generally require a large portion of
the overall electrical power consumed by the mobile tracking unit, it is
desirable to control the respective rates at which such devices are
respectively activated and limit their respective duty cycles so as to
minimize the overall power consumption of the mobile tracking unit.
Most current asset tracking systems are land-based systems wherein a radio
unit on the asset transmits information to wayside stations of a fixed
network, such as the public land mobile radio network or a cellular
network. These networks do not have ubiquitous coverage, and the asset
tracking units are expensive. A satellite-based truck tracking system
developed by Qualcomm Inc., known as OMNITRACS, is in operation in the
United States and Canada. This system requires a specialized directional
antenna and considerable power for operation, while vehicle location,
derived from two satellites, is obtained with an accuracy of about
one-fourth kilometer. A rail vehicle positioning system described in U.S.
Pat. No. 5,129,605 to Burns et al. is installed on the locomotive of a
train and uses, to provide input signals for generating a location report,
a GPS receiver, a wheel tachometer, transponders, and manual inputs from
the locomotive engineer. The rail vehicle positioning system of Burns et
al. is not readily adaptable to use of battery power and, therefore, is
unsuitable for applications which are not locomotive-based.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a local area
network of tracked assets which utilizes minimal power and bandwidth so as
to allow a large number of assets to be tracked in a practical manner.
In accordance with the invention, a mobile local area network (LAN) is
established among a plurality of mobile tracking units in close proximity.
Assets are approximately located according to their connection in the
mobile LAN where the exact location of at least one network node is known.
Each tracked asset may have the capability to independently determine and
report its location to a central station, and each asset also has the
capability to communicate locally with other cooperative assets via the
LAN. Because of the inherent mobility of the tracked assets, the LAN is a
wireless network preferably using low power spread spectrum transceivers.
The LAN is dynamically reconfigurable so that as other cooperative assets
come into proximity, they can join the network, and as others move away,
they can leavet the network.
Within the network, a protocol is established which assigns one of the
assets to be the "master" and all others to be "slaves". The master asset
takes the responsibility of determining its own exact geographical
position. This may be done via LORAN, OMEGA, Global Positioning System
(GPS) or other navigational aid. When operating in a LAN, slave assets
report their identification (ID) to the master asset according to the
local protocol and do not determine their own location in order to
conserve power. The master reports to the central station its location and
ID, as well as the ID of each of the other assets in the LAN. The central
station can then know that the assets associated with each ID are within
the communication range of the geographical position reported by the
master. Uncertainty in the location of a slave asset is limited by the
known possible geographical extent of the LAN, which is known a priori.
In one preferred embodiment of the invention, a railcar location and
tracking system is comprised of independent mobile tracking units affixed
to railroad freight cars. These tracking units are battery powered and
have Global Positioning Satellite (GPS) receiving and communication
transmitting capabilities. The units have an extremely low power radio
data link between units on the freight cars which are in close proximity
(about 1 km). This radio link allows units which are part of the same
train to share information. Sharing information allows use of a single GPS
receiver and a single communications transmitter. Since the GPS receiving
and communications transmitting functions are the most power consuming
tasks performed by the units, this sharing of information reduces the
average power consumed by the units on the train. System reliability is
significantly enhanced by allowing units with inoperative GPS receivers or
communications transmitters to continue to provide location and tracking
information through the LAN.
BRIEF DESCRIPTION OF THE DRAWINGS
The features of the invention believed to be novel are set forth in the
appended claims. The invention, however, together with further objects and
advantages thereof, may best be understood by reference to the following
description taken in conjunction with the accompanying drawing(s) in
which:
FIG. 1 is a block diagram of an exemplary asset tracking system which
employs independent mobile tracking units in accordance with the present
invention;
FIG. 2 is a block diagram showing in further detail a tracking unit as used
in the tracking system shown in FIG. 1;
FIG. 3 is a block diagram illustrating organization of the mobile local
area network implemented by the present invention;
FIG. 4 is a timing diagram showing organization of time used for
mutter-mode communications;
FIG. 5 is a flow diagram showing the process employed at a master unit for
polling a slave unit;
FIG. 6 is a flow diagram showing the process employed at a slave unit for
transmitting information to the master unit and for changing state to
autonomous;
FIG. 7 is a flow diagram showing the process of merging two established
local area networks; and
FIG. 8 is a flow diagram of the process employed at a master unit for
transferring the role of master to slave.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
FIG. 1, illustrates mobile tracking units which employ navigation signals
from a GPS satellite constellation, although, as suggested above, other
navigation systems can be used in lieu of GPS. A set of mobile tracking
units 10A-10D are installed in respective cargo-carrying conveyances, such
as vehicles 12A-12D, which are to be tracked or monitored. A communication
link 14, such as a satellite communication link through a communication
satellite 16, can be provided between each mobile tracking unit
(hereinafter collectively designated as 10) and a remote central station
18 manned by one or more operators and having suitable display devices and
the like for displaying location and status information for each vehicle
equipped with a respective mobile tracking unit. Communication link 14 can
be conveniently used for transmitting vehicle conditions or events
measured with suitable sensing elements. Communication link 14 may be
one-way (from mobile tracking units to remote central station) or two-way.
In a two-way communication link, messages and commands can be sent to the
tracking units, thereby further enhancing reliability of the
communication. A constellation of GPS satellites, such as GPS satellites
20A and 20B, provides highly accurate navigation signals which can be used
to determine vehicle location and velocity when the signals are acquired
by a suitable GPS receiver.
Briefly, the GPS was developed by the U.S. Department of Defense and
gradually placed into service throughout the 1980s. The GPS satellites
constantly transmit radio signals in L-Band frequency using spread
spectrum techniques. The transmitted radio signals carry pseudo-random
sequences which allow users to determine location on the surface of the
earth (within approximately 100 feet), velocity (within about 0.1 MPH),
and precise time information. GPS is a particularly attractive navigation
system to employ, being that the respective orbits of the GPS satellites
are chosen so as to provide world-wide coverage and being that such
highly-accurate radio signals are provided free of charge to users by the
U.S. government.
FIG. 2 is a block diagram of a mobile tracking unit 10 which includes a
navigation set 50 capable of generating data substantially corresponding
to the vehicle position. Choice of navigation set depends on the
particular navigation system used for supplying navigation signals to any
given mobile tracking unit. Preferably, the navigation set is a GPS
receiver such as a multichannel receiver; however, other receivers
designed for acquiring signals from a corresponding navigation system may
alternatively be employed. For example, depending on the vehicle location
accuracy requirements, the navigation set may comprise a Loran-C receiver
or other such less highly-accurate navigation receiver than a GPS
receiver. Further, the navigation set may can conveniently comprise a
transceiver that inherently provides two-way communication with the
central station and avoids the need for separately operating an additional
component to implement such two-way communication. Briefly, such
transceiver would allow for implementation of satellite range measurement
techniques whereby the vehicle location is determined at the central
station simply through use of range measurements to the vehicle and the
central station from two satellites whose position in space is known. The
need for power by either such navigation set imposes a severe constraint
for reliable and economical operation of the mobile tracking unit aboard
vehicles which typically do not carry power sources, e.g., shipping
containers, railcars used for carrying freight, truck trailers, etc.
Typical GPS receivers currently available generally require as much as two
watts of electrical power for operation. For the GPS receiver to provide a
location fix, it must be energized for some minimum period of time in
order to acquire sufficient signal information from a given set of GPS
satellites so as to generate a navigation solution. A key advantage of the
present invention is the ability to substantially reduce the energy
consumed by the mobile tracking unit by selectively reducing the
activation or usage rate for the navigation set and other components of
the mobile tracking unit. In particular, if, while the vehicle is
stationary, the activation rate for the navigation set is reduced, then
the energy consumed by the mobile tracking unit can be substantially
reduced, for example, by a factor of at least one hundred.
Mobile tracking unit 10 includes a communications transceiver 52
functionally independent from navigation set 50. If the navigation set
comprises a transceiver, the function of transceiver 52 can be performed
by the transceiver of navigation set 50. Both transceiver 52 and
navigation set 50 are actuated by a controller 58 which, in turn, is
responsive to signals from a clock module 60. Transceiver 52 is capable of
transmitting the vehicle location data by way of communication link 14
(FIG. 1) to the central station and receiving commands from the central
station by way of the same link. If a GPS receiver is used, the
Transceiver and the GPS receiver can be conveniently integrated as a
single unit for maximizing efficiency of installation and operation. An
example of one such integrated unit is the Galaxy InmarsatC/GPS integrated
unit which is available from Trimble Navigation, Sunnyvale, Calif., and is
conveniently designed for data communication and position reporting
between the central station and the mobile tracking unit. A single, low
profile antenna 54 can be used for both GPS signal acquisition and
satellite communication.
A low power, short distance radio link permits joining the nearby tracking
units in a network to minimize power consumption and maintain high
reliability and functionality such network. As shown in FIG. 2, in
addition to a power source 62 (which may comprise a battery pack that can
be charged by an array of solar cells 66 through a charging circuit 64), a
GPS receiver 50, a communications transceiver 52, and various system and
vehicle sensors 68A-68D, each tracking unit includes a low power local
transceiver 70 and a microprocessor 72. Microprocessor 72 is interfaced to
all of the other elements of the tracking unit and has control over them.
Transceiver 70 may be a commercially available spread spectrum transceiver
such as those currently utilized in wireless local area networks. Spread
spectrum transceiver 70 is equipped with its own low profile antenna 74.
Utilizing local transceiver 70, microprocessor 72 communicates with all
other tracking units within communications range, forming a dynamically
configured LAN, hereinafter denominated a "mutter network". Such mutter
network is generally shown in FIG. 3. When a train includes multiple
freight cars 82.sub.1, 82.sub.2, . . . , 82.sub.n equipped with tracking
units of the type shown in FIG. 3, all of these units will exchange
information. Because each microprocessor is interfaced to its own power
source, respectively, the status of available power for each tracking unit
can also be exchanged. Once this information is available, then the
tracking unit with the most available power (i.e., most fully charged
batteries) will become the designated master, the other tracking units
being slaves. The master unit performs the GPS position and velocity
reception function, assembles these data along with the IDs of all other
tracking units on the train, and transmits this information periodically
in a single packet to a central station 84 via communication satellite 86.
Because one GPS receiver among all of the tracking units is turned on at a
time (as well as only one communications transceiver), total system power
is reduced. Moreover, this function also increases reliability for each
tracking unit because it automatically reduces the power consumed by a
unit which has a degraded or partially functional power source. Thus,
while a unit with weak batteries cannot perform the GPS receiving or
information transmitting and command receiving functions, which are the
most power consuming functions in the tracking unit, a tracking unit with
damaged solar cells or a battery which can not hold a full charge can
still be fully functional when it is part of a train with fully functional
tracking units.
In each tracking unit the GPS receiver (or navigation set) and the
satellite transceiver and their antennae are major, complex modules, so
that a failure of any of these modules would render its tracking unit
inoperative if no alternative communication system existed. Using low
power spread spectrum transceiver 70 shown in FIG. 2 allows a tracking
unit with this malfunctioning module to operate when it is part of a train
with fully operational tracking units, thereby increasing the tracking
system reliability and the reliability of the tracking unit. Another
reliability feature is that the malfunctioning tracking unit can report
its faulty status along with its location so that repairs can be
scheduled.
An additional reliability feature allows location of a malfunctioning
tracking unit that is not part of a train carrying a properly functioning
tracking unit. A solitary railcar with a malfunctioning tracking unit (or
a malfunctioning tracking unit which is the only tracking unit on a train)
will monitor or "listen" on the low power transceiver at a low duty cycle
(to conserve power). If the malfunctioning tracking unit comes within
communication range of a properly functioning tracking unit (which
continuously broadcasts ID requests to other tracking units), the
malfunctioning unit will send out its own ID and status. This information
will be passed on to the central station where data are collected. In this
fashion, a solitary tracking unit with a malfunction in the power source,
GPS receiver, satellite transmitter or antenna will still be reported each
time it comes within range of a functional tracking unit.
The ability to exchange the roles of master and slave among the tracking
units provides transmission diversity which enhances link quality and
received data integrity. This occurs because one of the two units (i.e.,
the one with the most charged batteries) could experience severe
attenuation of its transmitted signal due to shadowing loss resulting from
an obstruction in the line of sight to the satellite. Selecting between
the two units can mitigate this effect. Inclusion of more units in the
selection procedure improves link quality at the cost of averaging power
over a large number of tracking units. Currently, the GPS function
consumes the most power and, in this case, the transmission selection is
restricted to two tracking units.
If the satellite has dual channels (i.e., two frequencies or two time
slots) for transmitting information on the reverse up-link (railcar to
satellite) and on the forward down-link (satellite to ground station),
then the two tracking units with the most charged batteries can be used to
provide receive diversity. In this scheme, the ground station will poll
the two transmissions and detect them either by combining the signals or
by selecting between the two signals. This form of diversity reception
reduces the link power budget, implying that both transmissions can be of
lower power.
If there is a two-way link between the ground station and railcar tracking
units, then it is possible to use the return link to control which of the
two tracking units to use for transmission. This is useful when the unit
with the stronger battery is more heavily shadowed, and can help to
conserve the battery power.
To make use of the mutter mode, a protocol is provided which allows certain
operations to occur. These operations include the following:
1. Forming a network from two or more independent tracking units and
determining which unit is master of the network.
2. Maintaining a network with regular communication between master unit and
all slave units.
3. Removing one or more units from a network when they are moved out of
communication range from the master unit.
4. Adding one or more units to a network when they are brought into
communication range with the master unit.
5. Merging two or more networks when the network master units come within
communication range of each other.
6. Transferring the role of master unit from a master unit with weak
battery power to a slave unit with a stronger battery.
The above operations must be performed in a manner that conserves battery
power, which implies providing a minimal amount of transmitted data from
any tracking unit and minimizing the time during which the unit's receiver
must be on. These objectives must be met within realistic economic and
technological constraints such as limited individual clock accuracy and a
communication channel which has a finite error rate.
Certain characteristics and parameters must be defined for a description of
the "mutter mode", or mode by which a subset of tracking units communicate
with each other in a mobile, dynamically-configured LAN. As shown in the
timing diagram of FIG. 4, all slave units in a network communicate with
the master unit of that network during a report period. Shorter report
periods provide better time resolution of asset movements, while longer
report periods use less power. The report period is divided into several
sub-periods. Shorter sub-periods allow more message retransmissions for
more reliability, while longer sub-periods promote fewer message
collisions and accommodate more assets per network. As indicated in FIG.
2, each asset tracking unit includes a moderately accurate local clock 60.
This clock has a short term relative clock accuracy measured over one
report period and a long term absolute clock accuracy measured over
several days. The long term clock accuracy is corrected during any
communications with the GPS satellite communication system, or any other
tracking or communication system with which the tracking unit is in
periodic contact. Greater accuracy reduces system power consumption, while
lesser accuracy lowers system hardware costs.
Messages sent between tracking units in the mutter mode have a transmission
time which includes any preamble and synchronization bits, the data bits
and error checking bits. A poll-acknowledge bit pair occupies a time slot,
including all guard bands and turnaround times. Shorter transmission times
reduce power consumption, while longer transmission times increase message
transmission success rate. Tracking units can transmit and/or receive on
two different channels. These channels may be different frequencies, or
they may be different sequences in a PN sequence spread spectrum code. The
channels will be identified as channel 1 and channel 2. In this protocol,
it is also possible to use the same channel for all communications, but
interfaces to protocols in other applications may make use of these two
channels.
The communications actions of a tracking unit depend on what state or mode
it is in. A tracking unit can be in one of the following modes:
1. Autonomous Mode--In this mode, a tracking unit communicates with the
central station directly. It is not networked to a mutter mode network.
2. Orphan Mode--In this mode, the tracking unit (i.e., "orphan" tracking
unit) is unable to communicate with the central station and is not
included in a mutter network. The central station does not have
information about the current location of the orphan tracking unit. The
orphan unit may or may not have data identifying its current location.
3. Master Mode--The master tracking unit communicates with the central
station directly. It is also networked to other tracking units in the
mutter network and transmits information about the networked tracking
units to the central station.
4. Slave Mode--Each slave tracking unit does not communicate with the
central station directly, but is included in a mutter network and sends
its sensor data and, optionally, its location data, to a master tracking
unit. The master tracking unit in turn transmits the data from the slave
tracking unit to the central station.
When a mutter network has been formed, the following protocol is used when
no tracking units are joining or leaving the network. The times given are
by way of example only. Time is given as minutes:seconds for a one hour
report period, and the master tracking unit and slave tracking unit are
respectively referred to as "master" and "slave".
00:00-00:05--The network master is tuned to receive on channel 1 during
this interval except for the short (50 msec) transmission time during
which the master transmits a "CALL FOR NEW MEMBERS" on channel 1.
The transmission time is randomly placed within the center second of the
five second period. A different random position is chosen for each
sub-period. During steady state (no slaves or masters join the network),
no data will be received.
01:00-01:39--This period is reserved for adding new tracking units to an
existing network. It is not used in steady state operations.
01:40-01:59--This period is reserved for forming new networks among two or
more autonomous tracking units. It is not used in steady state operations.
02:00-18:00--This period is divided into 16.times.60/0.1=9600 time slots
(each slot is 100 msec long) for poll-acknowledge pairs.
Each slave tracking unit has a time slot assigned by the master tracking
unit. The master unit randomly distributes the assigned time slots among
the 9600 availabl | | |
