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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the tracking of assets, including goods and
vehicles, using the Global Positioning System (GPS), and more particularly
to a protocol and mechanism for centralized asset tracking communications.
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 various
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. 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 hack 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 whenever 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. 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 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., kuown 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. U.S. Pat. No. 5,129,605 to Burns et al. describes a
rail vehicle positioning system for installation on the locomotive of a
train and which uses, to provide input signals for generating a location
report, a GPS receiver, a wheel tachometer, transponders, and manual
inputs from the locomotive engineer.
In an asset tracking system disclosed in U.S. application Ser. No.
08/484,950, entitled "Local Communication Network for Power Reduction and
Enhanced Reliability in a Multiple Node Tracking System" by Welles et al.
and in U.S. application Ser. No. 08/487,272 U.S. Pat. No. 5,588,505
entitled "Protocol and Mechanism for Primary and Mutter Mode Communication
for Asset Tracking" by Ali et al. assigned to the instant assignee and
incorporated herein by reference, a tracking system based on a "mutter"
mode local area network is used to generate dam which are transmitted to a
central station. In this asset tracking system, there are two modes of
communication. One mode is communication between the central station and
the tracking units, which is usually via satellite: The second mode is a
local area network, referred to as the "mutter" mode, between tracking
units. One of the tracking units, denoted the master unit, communicates
with the central station.
One of the chief challenges in using the first mode of communication is to
devise a protocol for the communications that will provide efficient use
of the communication facilities and respect the special sensitivities of
the reporting scenario. Such protocol should meet the following
guidelines:
1. The protocol should be two-way, thereby supporting transmission to and
from a central station.
2. The protocol must accommodate a large number of assets and be scalable
so that assets can be added and deleted without impacting normal service.
3. The protocol must accommodate variable length messages. The variable
length may arise from a number of considerations; for example, the
individual asset may have extra sensor data to report in addition to its
location.
4. The protocol must have a chatter suppression feature to allow selective
turn-off of a specific malfunctioning asset's transmitter.
5. The protocol must function efficiently if used over an extremely long
path such as is implied by use of a geostationary satellite.
6. The protocol must allow encryption or a privacy feature to be added
later without significantly impacting the capacity.
7. The protocol must be sufficiently robust to allow an asset to enter the
system at any time without knowledge that cannot be gleaned following its
entry into the system, and must tolerate occasional transmission errors
and not be unstable but degrade gracefully under additional load.
8. The protocol must not require the assets to be receiving all the time
but accommodate a duty cycle significantly less than 100% for periods of
monitoring communication frequencies.
The protocol must be designed to be easily adjusted and nominally
reprogrammable to allow presentation of its efficiency as the operational
scenario matures.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a protocol and mechanism
for centralized asset tracking communications which meets the above
guidelines.
According to the present invention, the protocol and mechanism for
implementation of the above guidelines is based on a control/polling
forward channel; i.e., a narrow-band channel from the central station to
the assets, a narrow-band return or service channel used by the assets to
transmit to the central station to aid the central station in efficient
scheduling of asset reporting, and a plurality of narrow-band back
channels that are appropriately multiplexed and used for conveyance of
data from the asset tracking units to the central station.
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 and operates in accordance with the method
of the present invention;
FIG. 2 is a block diagram showing in further detail a mobile tracking unit
as used in the asset 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; and
FIG. 4 is a flow diagram showing the functioning logic of the protocol
according to the invention.
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 vehicles 12A-12D, which are to
be hacked 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 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 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 shows 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 the 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 measurements, 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 whereby location
of the vehicle is simply determined at the central station by range
measurements to the vehicle and the central station from two satellites
whose position in space is known. In each case, the power consumed by the
navigation set imposes a severe constraint for reliable and economical
operation of the mobile tracking unit in vehicles which do not have power
supplies, such as shipping containers, railcars used for carrying cargo
and the like. For example, typical present-day GPS receivers generally
require as much as two watts of electrical power. In order 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 to generate a navigation
solution. A key advantage of the present invention is the ability to
substantially reduce the energy required 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, during
times when the vehicle is stationary, the activation rate for the
navigation set is reduced, then the energy requirement of the mobile
tracking unit can be substantially reduced, for example, by a factor of at
least about one hundred.
Mobile tracking unit 10 also includes a communications transceiver 52
functionally independent from navigation set 50. If the navigation set
comprises a transceiver, then the function of transceiver 52 can be
performed by the transceiver of navigation set 50. Both, transceiver 52
and navigation set 50 are activated 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 GPS
receiver and the transceiver can be conveniently integrated as a single
unit for maximizing efficiency of installation and operation. Art 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 conveniently used for both GPS signal
acquisition and satellite communication.
A low power, short distance radio link permits joining the nearby mobile
tracking units in a network to conserve power and maintain high
reliability and functionality of such network. In addition to a power
source 62 (which may be charged from 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 local area network (LAN), herein after called 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 these tracking units as indicated in FIG. 3, all of these
units will exchange information. 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, then the 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 tracking unit performs the GPS location and
velocity reception function, assembles these data along with the
identification (IDs) of all other tracking units on the train, and
transmits this information periodically in a single packet to a central
station 84 via a communication satellite 86.
Forward and reverse (tracking unit to central station) channels are used
for communication between the tracking units and the central station. In
the protocol according to the present invention, flags that occur in the
data are not used. This is ensured by using bit stuffing (or bit
escaping). This increases the traffic load by a factor of approximately
63/62. The preferred protocol for the forward channel frame structure is
as follows:
##STR1##
In the above frame structure,
F is an 8-bit flag.
ADDR is an identification number of an addressed unit comprising 20 bits,
19 for the address with one bit reserved. FC/C is a frame counter for
forward control link. A first bit denotes presence of the counter. A zero
indicates no counter is present, while a one indicates that the next
twenty bits are the counter bits. C is a control field which specifies the
message type; e.g., a zero specifies a polling message and it is
understood that no control field bits follow a zero, while a one specifies
another type message specified in the next three bits.
DATA specifies the future time for the addressed unit to start its response
transmission. This could be keyed to GPS time or it could be keyed in
another way, such as to a counter based on the end flag epoch of a
correctly received forward control frame.
CHNL is the narrow band channel on which the addressed unit will respond.
The channel field contains eight bits. Bits 1-7 are used to specify the
channel number. Bit 8 is reserved. It is normally zero. If the system is
to expand beyond 128 channels, then bit eight can be set to a one and the
field interpreted as extended by a present number of bits.
EC is an error detection code formed over the ADDR through CHNL fields.
As a quick check on the feasibility of such control system, assume that
there are A assets, that the forward channel is running in just the
sequential polling mode, that the FC/C counter is not used, that the DATA
field is twenty bits, that the CHNL field is eight bits, and that the
error checking field is sixteen bits long. The time in minutes, T, to
complete a sequential poling, is then approximately:
##EQU1##
assuming that ten kilobits per second can be passed over the forward
control link. If A is on the order of 100,000, then T is on the order of
fifteen minutes.
The asset tracking unit receivers need not continuously monitor the forward
control link; rather, they can extrapolate to the next minimum time to the
repeat of interrogation and listen at just before that epoch. If there has
been much traffic other than polling, the asset tracking unit receiver can
determine, from what the polling number is, whether to stay on or go back
into standby or "sleep" mode until just before the minimum time to poll
from that point.
The preferred protocol for the return channel has the following frame
structure:
##STR2##
In the above frame structure, SYNC is a synchronization preamble to
establish carrier synchronization, symbol boundaries, and epoch via a
unique word of low autocorrelation sidelobes.
ID is the asset tracker identification field.
C is the control field which designates message: type. If the first bit is
zero, then the message is conveying length in response to a polling
message on the forward link. The length of the message is coded in binary
from MSB to LSB (most significant bit to least significant bit). The
number of bits need not be fixed as the number can be determined by
counting backwards from the ending flag. FEC is an optional forward error
correcting field. It is not present if the first bit is zero.
EC is an error detection code formed over the ID field through the FEC
field.
The protocol functions as illustrated in the flow diagram of FIG. 4, to
which reference is now made. The forward control channel is run ahead of
the reporting asset tracking units. The forward control channel
determines, from responses received, which asset tracking unit is prepared
to transmit and how much that asset tracking unit will transmit. This
could be a wide range. For example, the mutter control master tracking
unit might transmit all the data for all of its constituent tracking units
when it is itself polled. This would save resynchronizing. The process
begins by the central station polling the tracking units in the narrow
band forward channel at step 401. The tracking units answer on the narrow
band return or service channel in fixed frame format at their assigned
slot at step 402. The central station receives the responses from the
tracking units at step 403 and determines which of the tracking units is
prepared to transmit dam and how much data those tracking units will
transmit. Based on the list generated at step 403 regarding the amount of
data to be sent and by which tracking units, the central station assigns a
report-back channel and a time to begin transmission. The scheduled time
and report-back channel are transmitted to the tracking units on the
forward channel at step 404. There may be a plurality of narrow band
report-back channels which may be appropriately multiplexed among the
tracking units transmitting data to the central station to conserve
frequency spectra. When a scheduled time for report-back by a tracking
unit occurs as determined at decision step 405, the central station
monitors the assigned report-back channel at step 406. If the central
station must pause or wait before proceeding with scheduling, it may send
repeated flags on the forward channel as an accepted interframe flag-fill
mode. After each tracking unit in the list reports, a check is made at
decision step 407 to determine if all the tracking units which are on the
list to report have reported and, if not, the process loops back to
decision step 405. When all data to be sent have been received, the
process ends.
While only certain preferred features of the invention have been
illustrated and described herein, many modifications and changes will
occur to those skilled in the art. It is, therefore, to be understood that
the appended claims are intended to cover all such modifications and
changes as fall within the true spirit of the invention.
* * * * *
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