Backscatter profile reporting

Several data messages can contain the two-way attenuated backscatter profile. Each value in the profile consists of five ASCII characters—not raw binary. Each character is one of "0–9", "a–f", or "A–F", and represents 4 bits of data, even though it is stored as an 8-bit ASCII byte. Thus, 5 characters × 4 bits = a 20-bit value, which includes the sign. These values are encoded as signed two's complement integers. As a result, values greater than 2¹⁹–1 represent negative integers. Each group therefore represents a signed 20-bit integer in the range of –2¹⁹ to +(2¹⁹–1), rather than the range of 0 to (2²⁰–1), which would apply to an unsigned 20-bit integer.

To correct the decimal value, apply the following two-stage process: Convert the five-character hexadecimal string to a decimal number. If the result is greater than 1,048,575 (2²⁰ – 1), subtract 1,048,576 (2²⁰) to account for two's complement representation of negative values. To convert the result into a backscatter coefficient (units of sr⁻¹·m⁻¹), the sensor multiplies the corrected decimal value by a factor of 10⁸. The values are scaled by the Attenuated_SCALE parameter, see Summary of the terminal mode commands available.