Cargo monitor apparatus and method

Cargo container monitoring system comprising:

• a plurality of transmitter units each adapted for separate attachment to different cargo containers which have a door having a metal surface through which cargo is adapted to be loaded and unloaded and each comprising:
• a radio frequency oscillator and antenna radiation system having a plurality of elements of conductive material in spaced planes substantially parallel to each other and to the metal surface of said door, and the transmitted radio frequency signal has a wavelength which is approximately four times the width or height of the metal surfaces of a container wall having said door;
• means for generating a mid-frequency signal and for modulating the signal from said radio frequency oscillator;
• circuit means operating at a low frequency up to about 1000 Hz connected to momentarily interrupt said mid-frequency generator signals and including a plug-in device for controlling frequency of said interruptions with sufficient precision as to enable a transmitter unit to be distinguished from other transmitter units in the system;
• a sensing element connected to cause initiation of a transmitter signal;
• and
• a receiver station having an antenna adapted to receive signals from a transmitter antenna and comprising:
• a radio frequency section connected to said antenna;
• circuit means including a bandpass filter for passing said mid-frequency signal connected to receive an output signal from said radio frequency station;
• a circuit including a frequency selective plug-in unit connected to receive an output signal from said bandpass filter responsive to the interruption rate of said mid-frequency signal for selectively producing a signal;
• and
• an alarm circuit connected to be actuated in response to the signal from said frequency selective unit.

A cargo container monitoring system characterized by having separate transmitter units attached to different cargo containers with each cargo container having a door with a metal surface through which cargo is adapted to be loaded and unloaded, and each of said transmitter units having a radiation system including a plurality of elements of conductive material mounted in spaced planes substantially parallel to the metal surface of said door, and an output signal power less than about one watt at about 27 MHz to have a wavelength approximately four times the width or height of the metal surface of a container wall having said door, said transmitter units being normally de-energized, a motion responsive unit for each transmitter unit for energizing the transmitter unit, means for limiting the duration of each transmitted signal from said transmitter unit to a period of between about 12 and 30 seconds after cessation of a motion which caused energization of said transmitter unit, and a central station located in the vicinity of but remotely from said containers having a receiver with an incoming radio frequency signal level detector responsive to signal at said about 27 MHz, and means for detecting jamming of the system including a jamming alarm connected to a signal level detector actuated in response to the detection of radiation energy at said about 27 MHz having a signal level in excess of that produced by a nearby one watt transmitted signal over an interval of time greater than about 3 seconds.

A method of maintaining mobile cargo containers under surveillance while stored in a transportation terminal having a central station including a console with a plurality of alarm devices by use of a wireless radio system comprising:

• removably attaching a battery powered transmitter unit to the door of the cargo container while the cargo container is located in the transportation terminal;
• positioning a transmitter antenna element along a central portion of the container metal side wall having a dimension of approximately nine feet;
• providing a transmitter circuit in said transmitter unit having a maximum power output of less than about 1 watt;
• operating said transmitter circuit to generate a radio frequency signal having a wavelength approximately four times greater than said dimension of said container sidewall to thereby utilize the cargo container as a radio frequency radiating element for the transmitter unit;
• and
• removing said transmitter unit from the cargo container when the cargo container is to be no longer under surveillance.

A method of maintaining mobile cargo containers under surveillance when stored in a transportation terminal by use of a wireless radio system comprising:

• removably attaching a battery powered transmitter unit to a cargo container under surveillance while the cargo container is located in the transportation terminal adjacent a central monitoring station;
• energizing the transmitter circuit in response to a variation of a condition being monitored for a brief period of at least 10 seconds and otherwise maintaining the transmitter circuits in a non-operating condition;
• modulating a carrier signal which for all the transmitters in the system is at the same radio frequency with a mid-frequency signal that is also substantially the same for all transmitted signals for the transmitters in the system;
• interrupting the mid-frequency signal at a predetermined low frequency less than about 1000 Hz generated by a first circuit means controlling said low frequency to be within about .+-.3Hz of a predetermined value;
• detecting at the receiver station the interruption at the predetermined low frequency rate of the mid-frequency signal by a second circuit means substantially identical to said first circuit means;
• generating an alarm at the central station only in response to the detection of the interruption at the predetermined low frequency rate of the mid-frequency signal for a preselected minimum time interval to thereby prevent false alarm signals from other carrier waves containing said mid-frequency signal;
• and
• removing said transmitter unit from the cargo container when the cargo container is to be no longer under surveillance.

A method of maintaining mobile cargo containers having a metal sidewall with a predetermined dimension under surveillance when stored in a transportation terminal by use of a wireless radio system comprising:

• removably attaching a battery powered transmitter unit near a center portion of the metal side wall of a cargo container under surveillance while the cargo container is located in the transportation terminal adjacent a central monitoring station;
• utilizing the cargo conainer as a radiating element for the transmitter carrier wave;
• energizing the transmitter circuit to generate a carrier wave frequency having a wavelength that is approximately four times greater than said container sidewall dimension in response to a variation of a condition being monitored for a brief period of at least 10 seconds and otherwise maintaining the transmitter circuits in a non-operating condition;
• modulting a carrier signal which for all of the transmitters in the system is at the same radio frequency with a mid-frequency signal that is also substantially the same for all transmitted signals for the transmitters in the system;
• interrupting the mid-frequency signal at a predetermined low frequency rate;
• detecting at the receiver station and generating an alarm at the central station in response to the detection of the interruption at the predetermined low frequency rate of the mid-frequency signal for a preselected minimum time interval to thereby prevent false alarm signals from other carrier waves containing said mid-frequency signal;
• and
• removing said transmitter unit from the cargo container when the cargo container is to be no longer under surveillance.