cobradav

XM Satellite Radio
XM Radio uses two Boeing HS 702 satellites, appropriately dubbed "Rock" and "Roll," placed in parallel geostationary orbit, one at 85 degrees west longitude and the other at 115 degrees west longitude. Geostationary Earth orbit (GEO) is about 22,223 miles (35,764 km) above Earth, and is the type of orbit most commonly used for communications satellites. The satellit appears to be at a fixed position compared to the earths surface. The first XM satellite, "Rock," was launched on March 18, 2001, with "Roll" following on May 8. On 28 February XM Radio launched a third HS-702 satellite. With its popularity growing and new subscribers steadily signing up, XM Satellite Radio successfully deployed its third broadcasting spacecraft Monday night using a sea-launched rocket from a platform floating in equatorial waters of the Pacific. In the coming days, the spacecraft will maneuver itself into geostationary orbit 22,300 miles above the equator where it can match Earth's rotation and appear parked above 85 degrees West longitude. The XM 3 spacecraft is a revised model from its two predecessors, which were deployed in 2001 before engineers discovered a crippling design flaw that degrades the power-generating capacity as the satellites age. The manufacturer -- Boeing Satellite Systems -- removed the faulty hardware and lengthened XM 3's twin solar arrays to overcome the power problem.

Sirius Satellite Radio
Unlike XM, Sirius does not use GEO satellites. Instead, its three SS/L-1300 satellites form an inclined elliptical satellite constellation. Sirius says the elliptical path of its satellite constellation ensures that each satellite spends about 16 hours a day over the continental United States (NA), with at least one satellite over the country at all times. This means the satellites have a constantly changing altitude that is at times much lower than XM satellites but will also be higher at times. However, when the Sirius satellite is higher it will be out of view of NA and you are actually getting your signal from another of the three birds that is within your view and is again at a much lower altitude as it passes over NA. Sirius completed its three-satellite constellation on November 30, 2000. A fourth satellite will remain on the ground, ready to be launched if any of the three active satellites encounter transmission problems.

I do not have the exact orbital parameters of the Sirius and XM satellites right this minute but will have them early next week if anybody is interested.

More general info here on satellite orbits.

http://www.radio-electronics.com/info/sate...lite_orbits.php

Rudy

cobradav, since you're a engineering/science guy, do you have an opinion on which method is better for overall coverage? I thought I read somewhere that Sirius' layout was better because you were usually withing range of two of the three birds.

I have XM in my wife's car and Sirius in mine. I get only a handful of dropouts with both. Around the downtown Pittsburgh area, I know for sure that Sirius is using terrestrial repeaters and, because of that, I actually am able to receive a signal almost all the way through a tunnel that I pass through twice a day (can't get AM of FM in the tunnel.) I assume XM is using repeaters as well since I think they kind of rely on them for better coverage.

UUronL

I've read that the Sirius birds are up over central Canada when they're in coverage range, and yes - the design goal is to have two provide coverage at all times. The orbit of Sirius' birds peak at 29,000 miles, so it's quite a bit higher than XM's, although I've not seen enough information to know how low they get during the periods of the day when providing coverage.

The further north you are for XM, the more chance you have of natural obstructions. They say Seattle receives XM signal 35 degrees above the horizon which is pretty easily blocked. I guess you can think of it like winter sun vs. summer sun.

Merzbow

I get constant dropouts when I use Sirius too. Sirius does not have enough repeater stations around the Bay Area.

cobradav

Ok, lets do the MAPS

http://www.tiger-usa.com/sirius-xmorbitanim.gif

U can see by the Sirius trajectories that there will be overlapping coverage on the 3 Sirius satellites as they spend approximately 16 hours over US and more so when Canada is included, The map shows both systems.

Both companies are using repeaters in large cities where tall buildings interfere with Line-of-sight (LOS). Since XM started first they are a little ahead of Sirius in this regard. I do not have the status of the cities they have covered as I am in a rural area and not impacted by this aspect.

TitaniumGray5

I've spent more time comparing FM to Sirius and I agree with the others now...Sound quality is no better than FM...pretty disapointing, because clearly the technology is there for better sound, but Sirius chooses to over compress to fit in more channels. Overall very happy with the Logic 7 sound system.

TitaniumGray5

I've heard talk of a buffer before...but don't see how that would work. To store up a buffer, the sound would have to be delayed at startup and it isn't. I don't think there is any buffer. Cutouts simply depend on orientation of satellite at that specific time to your car and the obstruction.

kscarrol

This may be true, but for me it was never a question of sound quality but instead was a content issue. If I'm really concerned about sound quality, I'm switching over to a CD, not FM or Sirius. But I do have to agree that Logic 7 rocks, at least to my laymen's ears...

gwf545

I've heard talk of a buffer before...but don't see how that would work. To store up a buffer, the sound would have to be delayed at startup and it isn't. I don't think there is any buffer. Cutouts simply depend on orientation of satellite at that specific time to your car and the obstruction.
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I've been looking for an explanation of how they actually can use a buffer to reduce or eliminate drop-outs. I think this link to an article may help explain how they do that: http://www.techreview.com/articles/02/01/w...ltzel012502.asp

It basically says there is a 5 second buffer and two data streams for each channel. I can only assume that one stream lags the other by a few seconds so they simply build the buffer from whichever stream that is recieved best at any point in time. I understand that there also may be a stream from a ground based repeater involved in some cases. In the end, what you hear is built up from one or all of these sources.

300TTto545

Just to chime in on the dropout issue. Sirius is borderline unacceptable in my area - Raleigh/Durham, NC. The trees are the problem but so are 1/3 overpasses. Also near the airport for a mile on the interstate there are dropouts. I will not renew even though I paid a lot for the module. It is no fun to have the music just completely drop out when you are listening at high volumes.

No question - my most regretted option. I've listened and liked it but as I think about it, I haven't listened once in the last 2 weeks because the dropouts are so annoying.

Does anyone else have this problem? I am considering going short on Sirius stock because I expect it to tank as people realize this annoying behavior.