Here is how it used to work.......
As shown in Hexagram's excellent illustration above, there were 3 satellites in a 'tundra' orbit. At any one time 2 of those were above North America and were active. The other was keyed off to conserve power.
Sirius used 3 frequencies. 1 for the terrestrial repeaters which were fed from AMC-6 and 2 for the satellites. These were/are centered at 2322 & 2330.5 MHz. Since there were an odd number of satellites, at each pass over N. America each satellite changed frequency. In addition to all of that, Sirius did a time offset between the two frequencies to provide an error correction advantage.
Graphic from Radio Magazine July 1, 2001.
So each Sirius receiver was actually 3 receivers. One receiving the terrestrial signal and two others receiving the satellite signal. Although the system worked with only one of the three signals, best error correction and concealment happened when there were two receivable signals.
So now we have an additional satellite (FM-5) parked in the traditional arc over the equator. I've got to wonder what frequency is this thing on? Or maybe Sirius has been secretly manufacturing receivers that can handle 4 signals. I sure would like to know how all of this works.
btw, still no improvement here in the Washington, DC area with my Sirius OEM car radio.