A
system to determine position, frequency and
clock offsets over a network utilizing signals of opportunity transmitted by one or more transmitters with known locations, the
system includes a base
receiver with a
clock and a known position that determines ranges to the transmitters, takes a series of samples of the signals of opportunity and time tags the series with times of
receipt, calculated times of transmission based on the calculated ranges, or both. The base
receiver transmits the time tagged series and, as appropriate, computed ranges to the remote receivers. A given remote
receiver saves and time tags samples of the signals of opportunity, correlates the time-tagged series with the saved samples, and calculates a time offset as a
time difference of the times of
receipt at the remote receiver and either the time of
receipt at the base receiver or the time of transmission calculated at the base receiver. The remote receiver calculates position based on the time offsets, and as appropriate, the ranges provided by the base receiver. The elevations of the remote receivers may be calculated as part of the position calculations, determined iteratively based on constraining the
Z coordinate to an average elevation, or determined from differences in air
pressure sensor readings at the base and remoter receivers.