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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 the
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. Background Description
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, there sometimes
is a need to know the location of the goods while in transit. 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, requiting 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.
Most present-day 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 to 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 mobile tracking unit used in the present invention 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.
The present invention obviates any need for a directional antenna and
minimizes power requirements. Since both the navigation set and the
emitter are devices which, when energized, require a large portion of the
total electrical power consumed by the mobile tracking unit, it is
desirable to control the respective rates at which such devices are
activated so as to minimize power consumption by the mobile tracking unit.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method and
protocol to locate and track multiple movable assets using a two-way
satellite link.
Another object of the invention is to provide an asset tracking system
which links proximate assets in a mobile local area network in a manner
which conserves power and bandwidth.
In according with the invention, there are two modes of communication for
the asset tracking units. The first of these modes is carried out between
a central manager or station and the individual tracking units. This
communication usually takes place through a satellite link. The second
mode, carried out by the local area network, is referred to as the
"mutter" mode, or mode in which a subset of tracking units communicate
with each other in a mobile, dynamically-configured local area network
(LAN).
The first of these modes is the primary communication link for tracking the
assets. Mutter mode communication is used as a secondary communication
mechanism to conserve power. The main requirements met by the mutter mode
are the following:
1. Energy conservation. This is a key issue in asset tracking, since the
individual tracking units have no external sources of energy. Mutter mode
communication requires much less energy for local communication between
the tracking units as opposed to direct satellite communication with the
central station.
2. High reliability. Mutter mode leads to increased reliability of an asset
tracking system by obtaining location information from asset tracking
units which have batteries too weak to provide communication with the
central station but which are capable of providing communication in mutter
mode. Mutter mode also enables finding of tracking units which are not
communicating with the central station due to a fault in their primary
communication equipment or to some other inhibiting condition.
The invention specifies a protocol for mutter mode communication. The prime
requirement of any protocol is that it be simple for implementation
purposes and at the same time be robust under different failure modes. The
protocol for the mutter mode as set forth herein makes use of the fact
that a two-way communication channel exists between the tracking units and
the central station. The central station includes a fairly powerful
computer, allowing the processing power of that computer to be used in
setting up and maintaining the mutter mode network. This enables the
mutter mode protocol to be kept simple and reduces the complexity at
individual tracking units whose numbers may be in the hundreds of
thousands. Moreover, in conjunction with the protocol for the central
station communication, the protocol for mutter mode communication is very
similar. The frame structure developed for the central station
communication protocol can be used for the mutter mode communication as
well, further simplifying the implementation of mutter mode communication.
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 mobile tracking units in accordance with the present invention;
FIG. 2 is a block diagram showing in further detail a mobile 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 flow diagram of the process employed at a master tracking unit,
of adding a slave tracking unit to a mutter mode LAN;
FIG. 5 is a flow diagram of the process, employed at an autonomous tracking
unit, by which that unit transitions to a slave mode; and
FIG. 6 is a flow diagram of the process, employed at a master unit, for
polling a slave tracking unit.
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 pseudorandom
sequences which allow users to determine location on the surface of the
earth (within approximately 100 ft), 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 word-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 location. 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 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 being that 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 supplies e.g., shipping
containers, railcars used for carrying freight, truck trailers, etc. For
example, 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, the GPS receiver 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 suitable transceiver 52 functionally
independent from navigation set 50. Transceiver 52 is optional depending
on the particular design implementation for the tracking unit. Moreover,
if the navigation set comprises a transceiver, then transceiver 52 would
be redundant. Both communications transceiver 52 and navigation set 50 are
actuated by a controller 58, which receives clock 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 and maintain high reliability and
functionality of such network. As shown in FIG. 2, in addition to a power
source 62 (which comprises 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. When a train has
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 exchange
information. The information exchanged allows the cars to recognize that
they are all part of the same train. Because each microprocessor is
interfaced to its own power sources, respectively, the status of available
power for each tracking unit can also be exchanged. Once this information
is available, the tracking unit with the most available power (i.e., most
fully charged batteries) becomes the designated master, the other tracking
units being slaves. The master tracking unit performs the GPS position and
velocity reception and processing function, assembles these data along
with the identification (ID's) 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.
To implement the protocol according to the invention, a two way
communication link between the satellite and the asset tracking units is
first established. This depends upon the access method, which is chosen to
be a Time Division Multiple Access (TDMA) protocol. The TDMA protocol
requires each tracking unit to transmit in an assigned time-slot during
which no other unit transmits. A TDMA system requires tracking units to be
time-synchronized to prevent communication collisions. This can be done
using GPS absolute time as a reference, or it can be initiated by the
central station using a broadcast control channel which is
time-synchronized to the traffic channel used by the asset tracking units.
The tracking units can synchronize to the broadcast control channel and
hence derive synchronization to the traffic channel. The various tracking
units are also assigned transmission frequencies and time slots by the
central station.
When utilizing the TDMA profile, the tracking units transmit on the
assigned frequency and respective time slots. The data sent comprises the
tracking unit ID, its location (as derived from GPS or equivalent) and
battery strength. The tracking unit can decode the GPS data and forward
location information with its ID and battery strength. Alternatively, the
tracking unit can avoid decoding the GPS data and forward the raw received
data. In the latter case, the raw data together with the unit ID and
battery strength are sent. This latter mode may be viewed as a store and
forward mode. However, in order to maintain data integrity, a fairly high
oversampling rate must be used which will increase the data rate at a cost
of employing more power on the satellite link. Thus the trade-off is
between the GPS processing power required at the tracking unit and the
power required over the satellite traffic channel. The main advantage of
the store and forward mode is that it requires less hardware in the asset
tracking unit.
The central station receives the data from the different tracking units,
and decodes and stores the information in a table. Each row of the table
has at least four entries; namely, tracking unit ID, location, battery
strength and signal quality. Signal quality here can be defined as
received signal strength, bit error rate measured over a known sync word,
or carrier-to-interference ratio. The table is sorted by location, and all
asset tracking units within a predetermined proximity are grouped
together. These asset tracking units form a "mutter" group. Next, a "best"
tracking unit is chosen from each group, and the central station assigns
that unit its new role. The best unit serves as the master tracking unit
and collects data from each of the members of its group which have
likewise been assigned their new role by the central station via the
satellite link. The collected data are then transmitted by the master
tracking unit via satellite link to the central station. This saves power
as other tracking units in the group, especially ones with low battery
power, do not have to transmit except at relatively long intervals.
The method by which the master tracking unit is chosen is as follows. From
each of the groups in the table, the tracking units are sorted by battery
strength and signal quality to determine the unit with not only the best
battery strength but also the one with the best propagation path to the
satellite. Use of the signal quality measure in selecting the best
tracking unit helps overcome effects detrimental to signal quality, such
as "shadowing", and provides inherent diversity. If shadowing is present,
that is, if the tracking unit with the highest battery strength is under a
canopy (e.g., foliage, ice or awning), then there is no point in choosing
that tracking unit to forward the group's data as that unit will need to
expend substantial power to overcome the attenuation of the canopy.
Because very little data must be transferred between a slave tracking unit
and a master tracking unit, this communication link can be designed to be
very low in power. Simple message repetition with differentially encoded
modulation and differential detection can be used for the mutter mode.
The protocol for mutter mode according to the present invention is a simple
polling scheme in which the master tracking unit sequentially polls the
other tracking units in the group. The tracking units which do not
communicate with the central station directly, but in fact report to the
master tracking unit, are referred to as slave units. Since direct
communication with the central station is the primary mode of
communication, tracking units operate in the mutter mode only at the
command of the central station, through a process known as the mutter mode
handover process. The main steps of the handover protocol are as follows.
Each tracking unit must be enlisted with the central station. Through the
central station communication protocol, the tracking unit registers
itself. Hence, the central station has information as to the position of
the tracking unit and the status of its various components, for instance,
the health of its battery and its various sensor readings. As long as the
tracking unit is in direct communication with the central station, it is
continually being polled by the central station.
Based on the information at the central station, the central station might
form a mutter mode network in a vicinity where there are multiple tracking
units. Whether or not to form such network can be a function of battery
levels at the tracking units.
In forming a mutter network in a vicinity, the central station selects one
or more master tracking units which will be reporting for other tracking
units in the vicinity. These master tracking units are selected because
they have a strong battery and a good communication link with the central
station. They also perform polling of the slave tracking units.
When the central station initiates a change in the status of a tracking
unit to a slave unit through the central station communication channel, it
commands the slave unit to receive a polling command from a particular
master unit in the mutter mode channel. At the same time, it commands the
master unit to poll the slave unit. If the polling is successful, the
slave unit is assigned to that particular master unit. The success or
failure of the mutter mode poll is relayed to the central station by the
individual tracking units. If the polling is successful, the slave station
is assigned to the master station in the mutter mode network and the
central station no longer directly polls the slave unit. The slave unit
now communicates with the master unit in the mutter mode. If the polling
in the mutter mode is not successful, either the master or the slave or
both of them provide this information to the central station. The central
station then assigns another master station to the slave unit and the same
procedure repeats. This happens until there is a successful poll in the
mutter mode or until all the masters in a predetermined vicinity are
exhausted.
Each master tracking unit, being responsible for a list of slave tracking
units, sequentially polls the slave units of the list at fixed intervals.
The frame structure for master-slave and slave-master communication is
illustrated below:
##STR1##
where SYNCH is the synchronization preamble to establish carrier
synchronization and symbol boundaries, DEST ADDR is the destination
address of the tracking unit to which the packet is destined and may be a
broadcast address in case the packet is addressed to all tracking units,
SOURCE ADDR is the address of the source unit, C is a control field
designating the message type, DATA is the main information in the message,
and FEC is a forward error correction for errors formed over DEST ADDR
field through DATA field. Bit stuffing or bit escaping is used to avoid
inadvertent flag creation. The number (or designation) above each segment
of the packet indicates the number of bits that make up the respective
segment.
FIG. 4 is a flow diagram of the process at an autonomous or master tracking
unit for adding a slave tracking unit to either an existing LAN or to form
a LAN. The mode is initially set at step 401 to either master or
autonomous, depending on whether a LAN exists or is to be formed. The
autonomous or master unit remains in low power or "sleep" operation until
occurrence of its TDMA assigned slot at step 402. At this time, the unit
"wakes up" and transmits a packet to the satellite at step 403. A test is
then made at step 404 to determine if a command has been received by the
autonomous or master unit to poll a slave unit. If not, the process loops
back to step 402 to await the next TDMA assigned slot. However, if a
command to poll a slave unit has been received, the autonomous or master
unit polls the slave tracking unit at a specified time at step 405. A test
is then made at decision step 406 to determine if the poll was successful.
If not, the failure is stored at step 407 before the process loops back to
step 402. If the poll is successful, the slave is added to the master's
slave list at step 408. A test is next made at decision step 409 to
determine if the number of slave units is greater than or equal to one. If
not, the process loops back to step 402; otherwise, the mode of the unit
is set to master before the process loops back to step 402. Thus, if the
original mode was autonomous, the addition of a slave unit to the slave
list results in the mode of the unit being changed to master.
FIG. 5 is a flow diagram illustrating the process at an autonomous tracking
unit by which it transitions to a slave mode. Initially, the mode of the
unit is autonomous at step 501. The unit remains in low power or "sleep"
operation until occurrence of its TDMA assigned slot at step 502. At this
time, the unit "wakes up" and transmits a packet to the satellite at step
503. A test is then made at decision block 504 to determine if a command
has been received to listen for a poll. If not, the process loops back to
step 502 to await the next TDMA assigned slot. However, if a command to
listen for a poll has been received, the unit wakes up at the specified
time and awaits a poll at step 505. A test is then made at decision step
506 to determine if a poll was received at the specified time. If not, the
failure is stored at step 507 before the process loops back to step 502.
If the poll is received, the master information and time interval to the
next poll is stored at step 508, and the mode of the unit is set to slave
at step 509 before the process loops back to step 502.
In the case of master-slave communication, the destination address is the
address of the slave tracking unit, whereas in the case of slave-master
communication, it is the address of the master tracking unit. A predefined
central station address can be used by the master unit to multi-cast
messages to multiple slave units as in the case of time and data
information update. A control field designates the message type. The
various message types can include polling for slave status information,
specifying to a slave unit a new master unit, acknowledgment of apparent
error-free message reception, request for retransmission, and the like.
For master-to-slave communication, the data can contain the time at which
the slave unit should transmit its information and the time of the next
polling epoch for that slave unit. In the case of slave-to-master
communication, the data field will contain status information from the
slave unit.
FIG. 6 is a flow diagram illustrating the process at a master tracking unit
for polling one slave tracking unit. For each slave unit, the master unit
goes through a similar logic. The mode is set to master at step 601. The
master unit "sleeps" until the slave unit polling time at step 602. At
that time, the master unit "wakes up" and transmits a poll to the slave
unit at step 603. A test is then made at decision step 604 to determine
whether a reply was received from the slave unit. If so, the slave unit
information received is saved at step 605, the failure count for that
slave unit is set to zero at step 606, and the process loops back to step
602. If, however, a reply is not received, the failure count for that
slave unit is incremented at step 607. A test is made at decision step 608
to determine if the failure count for that slave unit is equal to six. If
not, the process loops back to step 602; otherwise, the slave unit is
removed from the master's slave list at step 609. Next, a test is made at
decision step 610 to determine if the number of slave units on the
master's slave list is equal to or greater than one. If so, the process
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