Data encoding in a low frequency magnetic field

A method of transmitting data from a sonde, comprising:

• measuring parameters associated with the sonde with sensors in the sonde;
• encoding the parameters into a data frame, the data frame having a sequence of bits, the data frame including a separator followed by a synchronization field and one or more data fields separated by separators;
• determining a sequence of data symbols to represent the data frame, each of the sequence of bits in the data frame being represented by transitions between adjacent data symbols in the sequence of data symbols, the data symbols each being formed by K repetitions of a first state or formed by K repetitions of a second state, where the first state includes M/2 cycles of a nominal frequency with a high frequency signal at a high frequency and M/2 cycles of the nominal frequency with a low frequency signal at a low frequency, the high frequency being higher than the nominal frequency and the low frequency being lower than the nominal frequency such that the average signal is at the nominal frequency;
• and
• transmitting a magnetic signal formed from the sequency of data symbols.

A sonde, comprising:

• one or more sensors to measure parameters associated with the sonde;
• an antenna configured to transmit a magnetic signal;
• a driver coupled to drive the antenna to transmit a magnetic signal according to input signals;
• and
• a processor coupled to the one or more sensors and the driver, the processor configured to
• receive parameters associated with the sonde from the one or more sensors;
• encode the parameters into a data frame, the data frame having a sequence of bits, the data frame including a separator followed by a synchronization field and one or more data fields separated by separators;
• determine a sequence of data symbols to represent the data frame, each of the sequence of bits in the data frame being represented by transitions between adjacent data symbols in the sequence of data symbols, the data symbols each being formed by K repetitions of a first state or formed by K repetitions of a second state, where the first state includes M/2 cycles of a nominal frequency with a high frequency signal at a high frequency and M/2 cycles of the nominal frequency with a low frequency signal at a low frequency, the high frequency being higher than the nominal frequency and the low frequency being lower than the nominal frequency such that the average signal is at the nominal frequency;
• and
• communicate the input signal corresponding to the sequence of data symbols to the driver.

A method of receiving digital data from a magnetic signal transmitted by a sonde, comprising:

• receiving a magnetic signal transmitted by the sonde, the magnetic signal having a nominal frequency and being formed of a first signal having a high frequency and a second signal having a low frequency, the high frequency being higher than the nominal frequency and the low frequency being lower than the nominal frequency, the nominal frequency being the average frequency of the magnetic signal;
• digitizing the magnetic signal to provide a digitized magnetic signal;
• and
• processing the digitized magnetic signal to recover a bit stream, where each bit in the bit stream is represented by a transition between adjoining data symbols, each of the data symbols is formed of K repetitions of one of a first state or a second state, where the first state of the pair of states includes M/2 cycles of the nominal frequency with a signal at the high frequency and M/2 cycles of the nominal frequency with a signal at the low frequency, and where a second state of the pair of states includes signals complementary to the first state.

A receiver, comprising:

• one or more antennas, each of the one or more antennas producing one or more signals related to a magnetic signal generated by a sonde;
• an analog front end that receives and digitizes each of the one or more signals from each of the one or more antennas;
• and
• a digital processor configured to receive the digitized signals from the analog front end and recovering digital data modulated onto the magnetic field generated by the sonde,
• wherein the magnetic signal is modulated according to a bit stream, the magnetic signal having a nominal frequency and being formed of a first signal having a high frequency and a second signal having a low frequency, the high frequency being higher than the nominal frequency and the low frequency being lower than the nominal frequency, the nominal frequency being the average frequency of the magnetic signal, and
• wherein the bit stream is modulated onto the magnetic signal by encoding the bit stream into the magnetic signal, where each bit in the bit stream is represented by a transition between adjoining data symbols, each of the data symbols is formed of K repetitions of one of a first state or a second state, where the first state of the pair of states includes M/2 cycles of the nominal frequency with a signal at the high frequency and M/2 cycles of the nominal frequency with a signal at the low frequency, and where a second state of the pair of states includes signals complementary to the first state.