Radio frequency signal transmission detector and locator
View Patent ↗The present invention is a system and method for detecting and locating the transmission of radio frequency signals from within a defined geographical area. The system uses statistical confidence limits to detect outliers caused by transmissions in the defined geographical area. The source of the transmission can then be located with triangulation.
1. A system to detect and locate a suspected radio frequency signal transmitter and to cause an action to occur when the suspected radio frequency signal transmitter is detected, the signal transmitter operating within the boundaries of a pre-defined geographical area, the system comprising:
a plurality of at least two radio frequency signal receivers, each signal receiver equipped with a plurality of direction finding antennas, each direction finding antenna designed and constructed to receive radio frequency signals in a predefined range of frequencies, the at least two radio frequency signal receivers positioned near the predefined geographical area in an arrangement that allows the signal receivers to receive radio frequency signals from approximately every origin point inside the predefined geographical area, the attributes of the radio frequency signals comprising at least the signal time of arrival and the signal frequency and the signal receiver identification and the direction finding antenna identification and the signal power level, and each radio frequency signal receiver designed and constructed to determine the estimated direction of each received signal; and
a memory, the memory for storing and for retrieving the radio frequency signal attributes;
an alarm, the alarm activated when a suspected radio frequency signal transmitter operating within the predefined geographical area is detected; and
a memory controller, the memory controller operationally connected to the memory, the memory controller operationally connected to the alarm, the memory controller in communication with the signal receivers, the memory controller programmed for:
(I) performing a training process, the training process having a predetermined frequency of occurrence, the training process having a predefined duration of time, the training process consisting of two sub-processes, the first sub-process comprising receiving radio frequency signal attributes on the signal receivers and the building of sample populations of power levels in each classification of signal, the classifications consisting of, at least, the signal receiver identification, the directional antenna identification and the frequency of the signal and the second sub-process comprising the calculation of the mean and variance of the sample populations, and the fitting of a probability distribution to the sample populations, and the calculation of a one tail confidence limit of a predetermined amount for each sample population; and
(II) performing a monitoring process, the monitoring process having a predetermined frequency of occurrence, the monitoring process having a predefined duration of time, the monitoring process comprising the steps of:
(i) receiving radio frequency signal attributes on the signal receivers; and
(ii) calculating the estimated direction of the received signal on the signal receiver; and
(iii) retaining the signal attributes of the direction finding antenna with the maximum power level in each signal receiver and discarding the remaining signal attributes in the signal receiver; and
(iv) discarding all signal attributes with power levels below the one tailed confidence limit, leaving a second set of surviving signal attributes, and
(v) matching the signal attributes, from the second set of surviving signal attributes, across all combinations of signal receivers by time interval and frequency interval, the matched signal attributes forming a matched pair, and
(vi) discarding the signal attributes that are not in at least one matching pair leaving a third set of surviving signal attributes, and
(vii) calculating the intersection points of the two lines radiating from each signal receiver having a matched pair with another signal receiver, in the direction of the estimated direction of the received signal, the intersection point giving a possible location of a radio frequency signal transmitter; and storing the intersection points in a first set of intersection point attributes;
(viii) discarding the intersection point attributes that are not in the predefined geographical area leaving a second set of surviving intersection points; and
(ix) determining whether the second set of surviving intersection point attributes in the predefined geographical area are indicative of a suspected signal transmitter, and discarding the intersection point attributes that are not indicative of a suspected signal transmitter leaving a third set of surviving intersection point attributes; and
(x) activating the alarm to report the suspected signal transmitter locations as indicated by the third set of surviving intersection points;
whereby the retained intersection points are the suspected radio frequency transmitters operating within the predefined geographical area, whereafter the alarm is activated.
2. The system of claim 1 , further comprises at least one display which shows the predefined geographical area and shows the locations of the suspected radio frequency signal transmitters operating within the boundaries of the pre-defined geographical area.
3. The system of claim 1 , further comprises at least one camera, the camera for monitoring the predefined geographical area and for verifying the location of any suspected radio frequency signal transmitters operating within the boundaries of the pre-defined geographical area.
4. The system of claim 1 , wherein the estimated direction is selected from a group consisting of the centerline angle of the directional antenna in the signal receiver with the maximum power and the power weighted average of all directional antenna centerline angles in the signal receiver.
5. A method to detect and locate a suspected radio frequency signal transmitter and to cause an action to occur when the suspected radio frequency signal transmitter is detected, the signal transmitter operating within the boundaries of a predefined geographical area, the method comprising:
providing a plurality of at least two radio frequency signal receivers, each signal receiver equipped with a plurality of direction finding antennas, each direction finding antenna designed and constructed to receive radio frequency signals in a predefined range of frequencies, the at least two radio frequency signal receivers positioned near the predefined geographical area in an arrangement that allows the signal receivers to receive radio frequency signals from approximately every origin point inside the predefined geographical area, the attributes of the radio frequency signals comprising at least the signal time of arrival and the signal frequency and the receiver identification and the direction finding antenna identification and the signal power level, and each radio frequency signal receiver designed and constructed to determine the estimated direction of each received signal; and
providing a memory, the memory for storing and for retrieving the radio frequency signal attributes; and
providing an alarm, the alarm activated when a suspected radio frequency signal transmitter operating within the predefined geographical area is detected; and
providing a memory controller, the memory controller operationally connected to the memory, the memory controller operationally connected to the alarm, the memory controller in communication with the signal receivers, the memory controller for:
(I) performing a training process, the training process having a predetermined frequency of occurrence, the training process having a predefined duration of time, the training process consisting of two sub-processes, the first sub-process comprising receiving radio frequency signal attributes on the signal receivers and the building of sample populations of power levels in each classification of signal, the classifications consisting of, at least, the signal receiver identification, the directional antenna identification and the frequency of the signal and the second sub-process comprising the calculation of the mean and variance of the sample populations, and the fitting of a probability distribution to the sample populations, and the calculation of a one tail confidence limit of a predetermined amount for each sample population; and
(II) performing a monitoring process, the monitoring process having a predetermined frequency of occurrence, the monitoring process having a predefined duration of time, the monitoring process comprising the steps of:
(i) receiving radio frequency signal attributes on the signal receivers; and
(ii) calculating the estimated direction of the received signal on the signal receiver; and
(iii) retaining the signal attributes of the direction finding antenna with the maximum power level in each signal receiver and discarding the remaining signal attributes in the signal receiver; and
(iv) discarding all signal attributes with power levels below the one tailed confidence limit, leaving a second set of surviving signal attributes, and
(v) matching the signal attributes, from the second set of surviving signal attributes, across all combinations of signal receivers by time interval and frequency interval, the matched signal attributes forming a matched pair, and
(vi) discarding the signal attributes that are not in at least one matching pair leaving a third set of surviving signal attributes, and
(vii) calculating the intersection points of the two lines radiating from each signal receiver having a matched pair with another signal receiver, in the direction of the estimated direction of the received signal, the intersection point giving a possible location of a radio frequency signal transmitter; and storing the intersection points in a first set of intersection point attributes;
(viii) discarding the intersection point attributes that are not in the predefined geographical area leaving a second set of surviving intersection points; and
(ix) determining whether the second set of surviving intersection point attributes in the predefined geographical area are indicative of a suspected signal transmitter, and discarding the intersection point attributes that are not indicative of a suspected signal transmitter leaving a third set of surviving intersection point attributes; and
(x) activating the alarm to report the suspected signal transmitter locations as indicated by the third set of surviving intersection points;
whereby the method will detect and locate a suspected radio frequency signal transmitter operating within the boundaries of the pre-defined geographical area and cause the alarm to be activated.
6. The method of claim 5 , further comprises providing at least one display, the display for showing the predefined geographical area and for showing the locations of the suspected radio frequency signal transmitters operating within the boundaries of the pre-defined geographical area.
7. The method of claim 5 , further comprises providing at least one camera, the camera for monitoring the predefined geographical area and for verifying the location of any suspected radio frequency signal transmitters operating within the boundaries of the pre-defined geographical area.
8. The method of claim 5 , wherein the estimated direction is selected from a group consisting of the centerline angle of the directional antenna in the signal receiver with the maximum power and the power weighted average of all directional antenna centerline angles in the signal receiver.