First off, please don’t send me to HowStuffWorks, the answer’s not there (unless it’s the simple ‘ground station’ answer). I searched the forums too, but forgive me if I missed something.
I got Sirius for Xmas and promptly installed it in car and at home. I’m aware that the antenna needs line of sight to the satellite to receive the signal. So, I think to myself, I shouldn’t be receiving the signal if I drive under an overpass. Happily, most of the times I drive under an overpass, the signal isn’t lost. Why?
I’m thinking it could be two reasons:
The signal isn’t sent in real time. Somehow, perhaps through the wonders of the radio “delay” (the DJs actually said what they said 7+ seconds ago), the satellite is compressing the data and I’m getting more than 1 second of audio for every second of reception. So when I quickly pass under the bridge the receiver has a little bit of signal backed-up and ready to play. When I emerge from underneath, the radio syncs up again.
I am actually receiving the signal from one of the metropolitan area ground repeater stations. I live in Northern NJ (Denville) outside of NYC and drive around NNJ and NYC regularly. Where are my local ground repeater stations? What’s their range?
To further muddy the waters, sometimes I DO lose signal going under an overpass. Is this because:
The overpass was at the right angle overhead to block the signal for long enough to ‘eat up’ the backed-up audio…or,
The overpass blocked both the satellite AND the ground station(s).
There is some buffering of audio in the receiver, but no more than a few seconds.
The NYC area is also one of the places that Sirius has decent repeater coverage. They have far fewer repeaters than XM, yet the Sirius satellite setup would greatly benefit from more repeater coverage.
I’ve been playing with my new XM radio, and I’m pretty sure that “line-of-sight” is not an absolute term with satellite radio, like it would be with a laser beam or something.
I can get a steady signal inside the house even if I’m somewhat away from a south-facing window - just not too far away. There’s clearly some penetration/scattering going on, just not a whole lot of it.
Repeater locations and coverage maps aren’t available from either company directly, as they want to discourage vandalism (same reason cell phone companies don’t disclose tower locations.) There are some incomplete lists available on the web, as well as maps created by listeners (at least for XM.)
I don’t know Sirius receivers that well, but I’d imagine they have a diagnostic mode similar to the 2-0-7-XM mode on XM receivers that displays repeater reception as well as the satellites.
That’s probably true. IANARE (radio engineer,) but it stands to reason that sat radio, like other radio, is working with radiation that has a longer wavelength than visible light, which means that it has a slightly higher tendency to either diffract around obstacles or pass through thin barriers.
All electromagnetic radiation (light, radio waves, gamma rays, etc.) has a wavelength. And waves have a tendancy to more or less ignore obstacles smaller than their wavelength. For obstacles about the same size as the wavelength, the waves tend to scatter and bend around it, rather than just being stopped. And obstacles much larger than the wavelength tend to just stop the waves.
So, visible light, for instance, has wavelengths much shorter than most things we humans deal with. So most things we think of as obstacles will just stop visible light dead. Radio waves, though, have much longer wavelengths. For cell phones, say, it’s a few meters or so. So the waves will go right past a small obstacle. A narrow bridge, they might bend around, since the bridge isn’t too much bigger than the wavelengths. A big hill in the way, though, will stop them, since it’s much bigger than the wavelength. So when folks say “line of sight”, mostly what they mean is no big obstacles like hills in the way.
All that said, I’m not sure what wavelength (or equivalently, frequency) satellite radio typically uses.
The satellite radio spectrum allocation is in the microwave region, around 2.3 GHz( a little below your wireless network/phones). This puts the wavelength a little over 10 cm. I don’t know how much power they put out.
Here’s a handy Calculator to tell you where the satellite is in relation to you (click the drop-down to find one of the Sirius satellites).
<minor nitpick>
Cell phones operate in the 800-900Mhz range. Wavelength (in Meters) = Freq/300. Cell phones operate in wavelengths just a bit over 1/3 of a meter.
Remember that not only will objects of the same size block radio signals, but multiples as well. You lose signal in some underpasses when you enter the “shadow” created by the obstruction. Near the edges, you get some bending, which allows you to still recieve, but the signal drops as you move away from the edge of the obstruction towards the middle.
You still can recieve terestrial FM signals for some distance, as the wavelength is prone to “bouncing,” and the time differences of the bounce are negligible.
Radio waves do some pretty strange stuff too. I’ve seen wireless networks that get no coverage on a hospital floor, but great coverage in the stairwells, even on “non-wireless” floors, as the stairwell’s shape acts as a waveguide, often for many levels up/down. I’ve seen signals blocked by pine needles (high frequency 6-10Ghz microwave links), steel mesh behind plaster walls (900Mhz, 1.2Ghz, and 2.4Ghz ISM band products), and a host of other odd propigation issues. It all depends on the size and orientation of the obstruction.
