[Rastabart]

Hi again all....I have a question for those who have Sirius Satellite installed, how often do you guys get signal drop outs ?? I'm a little puzzled by the reception that I get at times, occaisonally while going on or off highway exit ramps my signal will drop out for just a moment(I'm aware that if you go under a bridge/tunnel or ramp this will happen but the ramps I am talking about are all clear in the open) Also going down my street(lot's of trees) I also occaisonally loose my signal, is this normal ?? Anyone else experience things like this ?? (Just in case anyone is wondering I did NOT do the hidden antenna installation)....

2005 MO SL AWD...Loaded

 

[jaak]

Dense foliage will attenuate the signal causing dropouts. Unless there's something tall next to the on/off ramp or lots of trees, that one doesn't make a lot of sense.

When trying to imagine what your system has to go through, think about being able to see a specific pair (for XM) or three (Sirius) objects, like the moon in the sky, towards the south. If at some time, you can not see the "moons", you aren't getting light from them.

The satellite radio is similar. Put things in the way of your line of sight and you lose the signal. When looking at the moon through a tree, some light will make it through, but the denser the foliage, the less it's possible.

XM has a "moon" in the eastern sky and a "moon" in the western sky.

Sirius has three moons in the center where they do a little waltz up from the southern horizon towards an area overhead and then back down to the southern horizon.

If you're on the west coast, they will appear more to east and the east cost, more to the west. Being in the south center of the US is the best place for Sirius. (TX is good!)

 

[MO Dale]

I've also noticed this issue cropping up recently. Music stops for no more than a second and the screen continues to display the proper song ID. I'm noticing it two or three times a week, sometimes a couple of times a day. Weather has been clear. I mostly drive on a "rural" 4-lane with no obstructions.

Searched some sat forums for "momentary signal loss." Other Sirius users also have the problem. XM users also seem to have the issue, but get "no signal" messages. Most seemed to agree it was due to antenna location or damaged antenna wires, bad connections, satellites low on the horizon (like Jaak said). FM interferrence was lightly mentioned as a possible cause.

Maybe it's just dropouts, just like satellite tv or any other medium, originating from the broadcaster.

 

[jaak]

While it normally should not be significant, if you are near another transmitter that has very high power, Broadcast or otherwise, it's possible for the front end of the satellite receiver to be swamped by it and not be able to "hear".

Imagine trying to have normal conversation, why some guy is screaming in another language. The effect is the same.

Most Broadcast services are so far away in frequency, that they would be unlikely to cause an issue. But if the signal's strong enough, it can still cause problems.

 

[Rastabart]

MO Dale, That's EXACTLY what I am getting !! A momentary drop out, sometimes it's long enough to where I get the "acquiring" signal message....Jaak, you're right the drop outs on/off the exit ramps makes no sense to me either !! no tall structures or anything around, at first I thought maybe something was loose in my system and the act of going around those sharp corners on/off the exit ramps was causing my system to move around but that's not the case, I'm at a loss on this one, but as MO Dale points out it's just a momentary drop out....

 

[hfelknor]

Drop outs?
More consistent at off ramps?

You wouldn't be taking these ramps "at speed" would you?
In other words you wouldn't be loading the Gs to one side?
Which, if you had a loose antenna connection somewhere, would tend to make the connection bad until you straightened out.............
Or, for that matter any type of sideways movement causing any kind of Antenna disruption.......open OR short.


Just a thought.

Homer

 

[Rastabart]

"Homer"...No I have not been taking the ramps at "speed" ...I've actually been babying this car since I got it, at the same time that theory(loose connection) still wouldn't hold water as the same thing should happen when I go over bumpy roads right ??
I'm going to keep track of this a little bit closer since I seem to recall it may only be occuring at certain off ramps(one in particular I am thinking about but as I stated it's out in the open, no over hanging trees/buildings or any other "visible" obstructions...I'm just puzzled by this that's all and wanted to see if anyone else has been having this problem, could it be the area I live in ??? hmmmm ??? don't know, doubt it, I'm in Maryland by the way if anyone else besides MO Dale has experienced this...

 

[jaak]

Stab in the dark... Any cellular transmitters nearby?

 

[pbuelow]

Remember, it's on the horizon. Pretty steep on the horizon depending on where you're at. The farther north you go, the steeper you look. It's also like previously posted, XM is southeast and southwest, so it might be clear above you, but what's closer to the horizon? Since it's so repeatable in a specific area, I'd look geographic as a likely cause. I can guaruntee three locations on my way to work where signal drops for a few seconds. One of them is a tad odd too, but I know exactly why. And finally, there is a one or so second processing delay. The signal gets to the decoder a little more than a second before it gets to your radio. So, when it drops, it's actually just after an object just obstructed you if you're flying down the highway for instance. Hope this helps. And a last note, all of my receivers will occaisionally just drop signal for a second or so for no apparent reason. That's three car units (one's a roadie), and a boombox unit. The spectrum they use _should_ be immune to these types of issues (but hates trees), but seemingly, you get random interference occasionally. C'est la Vie.

 

[CP-Mike]

XM's satellites are near the horizon, especially if you are in the northern US. However, Sirius' satellites are not.

Check out this animation showing the Sirius satellite paths as well as the locations of the XM satellites: http://www.cashflowbusiness.net/siri_orbit.htm

And here for actual numbers regarding the orbits as well as to see where each satellite is right now:
http://www.heavens-above.com/orbitdisplay.asp?satid=26390
http://www.heavens-above.com/orbitdisplay.asp?satid=26483
http://www.heavens-above.com/orbitdisplay.asp?satid=26626

The orbit paths ensure that there are always two satellites over the continental US at all times, and they tend to loiter above Minnesota/the Dakotas. Also, XM's choice to use geostationary orbits (the satellites remain over fixed locations on the equator at all times) means that their satellites are at lower altitudes than Sirius', which are in geosynchronous orbits. These two things together mean that Sirius satellites provide much higher angles of elevation from the horizon than does XM for most locations in the US, which means that trees and hills are less likely to cause signal dropouts (unless the trees are really tall and close to you).

Since the satellites move around, it also means that if you're not getting signal where you are right now, the odds are good that you will get signal there at some point. With XM, if you don't get signal somewhere, you probably will never get signal at that location. However, this also means that even though you have good signal somewhere most of the time, it is possible that you will lose the signal from time to time.

In terms of overall signal coverage and dropouts, I think XM and Sirius are more or less comparable, although I have heard reports of ex-XM customers in northern states who live in valleys that had crappy signal from XM but report excellent signal from Sirius.

For good info about Sirius or XM, check out their forums at www.siriusbackstage.com/forum/ or www.xmfan.com

 

[jaak]

XM works well here, but my latitute in Toronto is already a third of the way into the US, from the northern most part of the lower 48.

Since the XM satellites are relatively fixed, I expect the antennas are higher gain, than the constantly moving Sirius satellites, so the ERP may be higher on the XM birds. The Satellite designers would be able to focus the transmitter power using a more directional antenna, as the target coverage area is not shifting relative to the satellite. This may overcome the greater path loss of XM relative to Sirius, but I've not looked into it.

Also, Sirius will suffer from doppler frequency shift, but again, I've not looked into the technology to determine how significant the impact to the receiver is, with this happening. As the Sirius birds "swoop" up towards you, the apparent frequency of operation will shift up, and as it departs away, it will shift down. XM will be static at one frequency. (Your Murano's speed is insignificant, unless it's been put into the bay of a shuttle.)

Of course, throw into this, the terrestrial repeaters, and coverage from the system can be dramatically altered.

 

[CP-Mike]

jaak, good call, I forgot about doppler shift that must occur to the Sirius signal. Sirius must have written their transmitter/receiver software to take this into account, because on my home unit (which obviously stays stationary and has the antenna pointed in a fixed direction) I get constant signal with only 20% fluctuation in signal strength (according to the signal strength bar graph, however accurate that is, heh) and only an occasional two-second drop out once every week or so.

I doubt antenna gain has much to do with overcoming the path loss, except through foliage. Both signals still require line of sight. Now the terrestrial repeaters are a whole 'nuther story. Definite plus to XM (in urban areas).

I don't know or really care which service has better signal, I just think it's frickin cool that they both somehow figured out how to get such good uninterrupted (for the most part) signal when both the transmitter and the receiver are moving towards/away from each other at constantly changing angles, with such small antennas and with different satellite / orbit / transmission technologies.

 

[jaak]

Yeah, I'm with you, I don't care either. I actually like the coverage concept of Sirius. Because the system is designed to tolerate satellites moving, they can add more into the system, if they wanted. Like two more sets of waltzing birds, one over the east coast, one over the west. And they'll do a better job of urban areas, if one of the satellites is high in the sky, in your location. (Which is why XM needs terrestrial repeaters in urban areas.)

Trees? At that frequency? Not much you can do but wait until the fall...

I agree, gain is not a problem when the satellite is in sight, but the free space path loss from a Sirius satellite might be less, when it's overhead. I haven't checked in to how high either one's orbit is, or the ERP of either. I might be interesting, just to see, as that would help determine how much foliage either one could tolerate.

Of course, if you're in a position to see at least two satellites at once, this could help. (Depending on receiver design.)

Neither service discloses much about their technology, however. Even the suppliers of test equipment for them, are very tight with detailed information. Rohde & Schwarz, for example, can emulate the satellite constellations for XM and Sirius, in a lab, so that receiver designs can be tested. Or if one satellite is good enough, they can do that cheaper.

It's very high level, but there's some insight into the structure. They talk about Sirius being two satellites. Effectively, that's true, because one of them is always in the southern hemisphere.

http://www.rohde-schwarz.com/WWW/Publicat.nsf/article/n175_smiq03s/$file/n175_smiq03s.pdf

Woa, we sure did dive deep into the systems, but the first step in truly understanding dropouts, is to understand the system's tolerance to poor signal conditions and how they get created.

Remember, the dropout you hear, occured about a second before you hear it. So the off ramp, may be the result of going under a bridge, just before it.

 

[jaak]

Oh, one more thing. A leaky microwave oven will wipe out your sat radio signal reception. So try not to drive through the kitchen, when making popcorn.

I suspect a strong 802.11 (WIFI) signal could potentially desense the sat receiver, and there are also people using that frequency band for data links between buildings, so that could trash the reception if the signals' great enough.

 

[jaak]

Oh, one other source for a dropout, is multipath fading. This is where you receive the signal directly (or indirectly) and you receive another signal that is destructive, due to it's delay in time. e.g. a reflection off an apartment building or office tower. While moving, this is not a problem, because the data for the audio signal is interleaved in time, and the fading goes through constructive and destructive peaks and valleys.

An example of this, most of us already know. You stop at a stoplight with your FM radio on. The signals distorted and weak sounding. You move the car forward a foot or two and the signals back. You move it forward another foor or so, and it's distorted and weak again. This is fading from multipath.

It's possible to suffer a dropout on a satellite radio system, while stopped, from the same problem. If you can only 'see' one satellite and the signal is destroyed like this, no more data to reconstruct, and no more sound.

You would have to remain stopped for a second or so (same delay as going under a bridge) as the data in the buffer plays out.

 

[pbuelow]

Yeah, you're right, they aren't _on_ the horizon, but if going down a ramp and there's a two story building in the way, you could be low enough as compared to an object you don't expect to be in the way. Given the fact that it happens every time, it's safe to say some obstruction exists even though it's not expected. Jaak's microwave could be possible too, although usually anything above the horizon at that power level is too directional and I don't know if it would be a sig impact. And if someone has an 802.11 that is strong enough to impact bordering spectrum, then any cell operator in the area using the 2100 Mhz spectrum would have complained to the FCC by now. The multipath would be as good an explanation as any, as it's possible there's a constant multipath that is driven through every time, causing the receiver to get confused. However, digital signal decoding is very good at multipath after so much engineering work has gone into it (still dry as anything though, sit through a couple of classes on CDMA chips and bits and antenna's and signal, blah blah, and try to stay awake!). There's a whole team of engineers here at work that just do multiplath and other signal degradation work so your cell calls aren't always bad (just sometimes). I suspect that Sat operators have solved all but the worst cases for this.

 

[jaak]

Tell them to look at the OS while they're at it. My V600 just did something my Timeport (CDMA) used to do. Phone's sitting in charging cradle SPN4997A (yeah I cut it to let the V600 fit in), the phone rings, I pick it up and answer, and the audio is choppy and messed up.

Surprised me that a GSM phone had the same bug as a CDMA phone.

I've not looked into XM and Sirius modulation, with any depth, but I seem to remember the modulation was fairly simple, it's the data that's interleaved in time, so a dropout won't trash all of it, for section of audio, just certain parts of a longer reconstructed time period in the hopes that the subscriber will not detect it, or if they do, it's not as severe as a drop out.

Yeah, the microwave oven was an RF joke. Possible, but highly unlikely.

If I lived nearer, I'd go do a site survey. Speaking of FCC Engineers, I used to be an Industry Canada, Spectrum Management Officer. The Canadian equivalent of an FCC Engineer.