Dumb Question- Why no rain fade with Sirius?

[angiodan]

Ok, so I'm driving home from work yesterday in a downpour, and I started to wonder why my signal was still coming in strong. I know that my Dish signal would have gone out for sure! (Unfortunately, the downpour was over by the time I got home to check)

Knowing the technical geniuses we have on this forum, is there an explanation?

 

[avediswolf]

Dish Network downlinks from the satellite in the DBS Ku spectrum at about 12GHz
Sirius downlinks from the satellite in the S Band at about 2GHz

The 12GHZ signal can get blocked by raindrops easier than a 2GHz signal.

 

[seandudley]

And, in addition to being able to pick up the audio from satellite, in cities, the audio is also sent by towers which can be picked up if you are unable to get the audio from the satellite.

 

[angiodan]

Thanks guys, I learned something new today!

 

[Krapola]

The combination of the Downlink Transmission in the 2Gig range and the Transmit EIRP starting around 55DbW is your answer for a lack of Rain Fade..It is not out of the question for Rain Fade, but substantionaly reduced...

 

[VO1ONE]

EIRP is greater than what's used for DBS services, but the SDARS antenna is also omnidirectional with a lot less gain than a 18+" dish. The biggest reason why precipitation and atmospheric moisture effects DirecTV/Dish network/ExpressVu/etc and not Sirius/XM AS MUCH is because of the frequency. As the frequency increases, the amount of attenuation also increases, and the difference in this attenuation between the SDARS band and the Ku DBS band are great. S-band is low enough to where precipitation doesn't attenuate the signal that much. Ku band, Ka band and the sort are heavily attenuated by precipitation which if heavy enough will lead to loss of signal. Here's a chart which shows how different frequencies are affected by atmospheric attenuation (i.e. precipitation).
https://ewhdbks.mugu.navy.mil/rf_absor.htm


Scroll down to Figure 2. Sirius runs in the S band at a frequency of 2320
- 2332.5 MHz, and although frequencies below 3GHz aren't shown, you can
pretty reliably extrapolate what the readings would be from the data
that's already there. I did find a chart at one time showing down to DC,
but couldn't find it within 2 minutes of searching this time around ;-)
The DBS portion of the Ku band is 12.224 GHz to 12.661 GHz. The new Ka
band, currently being used by Wildblue 2-way satellite internet, and to be
used by DirecTV and Dish Network in the future to provide even more HD
channels, is located between 20 and 30GHz. As you can tell by the chart,
to achieve the same amount of attenuation that DirecTV gets from a rather
light 2mm/hr rainfall on Sirius, you'd have to have a tropical downpour of
100mm/hr!! A light rainfall is still probably not enough attenuation to
make DirecTV cut out, but what would be a heavy enough rainfall to make
DirecTV cut out would be a storm of biblical proportions to cause the same
amount of attenuation on Sirius to make them cut out! Think about a giant
12 foot C band dish. Rain has no perceivable affect to reception on C
band. C band is not that far off from S band and has the same
characteristics WRT attenuation by precipitation.

 

[miller5]

Great comparisons, and relatively easy to understand considering the subject matter. Thanks for the explanation VO1ONE!

 

[tvdxer]

The S-band frequency used by Sirius is less subsceptible to rain attenuation than the much higher Ku-band one used by Dish Network and DirecTV. In addition, I'm pretty sure the Sirius and XM sats transmit with much more power (although the larger antennas on DBS services might compensate on the downlink).