The HGA transmits data to Earth on two frequency channels (the downlink). One, at about 8.4 gigahertz (8,400 million cycles per second), is the X-band channel and contains science and engineering data. For comparison, the FM radio band is centered around 100 megahertz. The X-band downlink science data rates are as high as 7.2 kilobits per second. The other channel, around 2.3 gigahertz, is in the S-band, and contains only engineering data on the health and state of the spacecraft at the low rate of 40 bits per second. S-band has not been used since the last planetary encounter.