Patent ID: 7050817

Claim:
A method for determining a location of a digital radio transmitter comprising: detecting a digitally encoded radio signal radiated from the transmitter by at least three spatially separated receivers; receiving, at each of the receivers, a common synchronizing pulse; parsing, at each of the receivers, a received radio signal into data block samples; time stamping, at each of the receivers, each data block sample with a time stamp offset from the common synchronizing pulse; detecting, at each of the receivers, potential bit transitions in the data block samples; generating, at each of the receivers, bit transitions from the potential bit transitions; recognizing, at each of the receivers, a desired pattern of bit transitions; determining, at each of the receivers, a respective time of acquisition for at least some of the bit transitions comprising the desired pattern; transmitting, from each of the receivers, an indication of the times of acquisition to a central processor; determining, at a central processor, time of arrival differences among receivers from differences in the respective times of acquisition for common bit transitions in a desired pattern received from each of the receivers; calculating, at the central processor, the location of the transmitter from the time of arrival differences, further comprising, for each receiver: associating a first set of times of acquisition with bit transitions comprising the binary forms “01” and “10;” comparing bit sequences among respective data strings in a repetitively transmitted data string forming at least a portion of the desired pattern to determine if a bit error exists in the data string; associating a second set of times of acquisition with bits in the data string having “good” calculated time stamps; determining an intersection of the first set and the second set of times of acquisition; generating a bit mask corresponding to the intersection; generating an absolute time of acquisition for at least one bit transition in the signal using an offset value for the common synchronizing pulse; generating offset times of acquisition for a remainder of bit transitions in the signal using offset values from the absolute time of acquisition; and generating an indication of times of acquisition comprising the bit mask, the absolute time of acquisition, and the offset times of acquisition.