RT_INS_Heading_Gradient_2: INS Heading and Gradient
RT_INS_Heading_Gradient_2: 142, 21 (0x8E1520 + unit id) (0x395)
Byte 0: Validity
Byte 1: Accuracy heading
Bytes 2-3: INS heading (degrees in the range 0 to 360)
Byte 4: Accuracy gradient
Bytes 5-7: INS gradient (degrees)
Resolution of heading and gradient is degrees/100.