Skip to main content
Message Font: Serif | Sans-Serif
 
No. of Recommendations: 8
There has been a lot of talk about SIRI and XMSR. Let me explain one clear difference and why important.

The SIRI system uses three satellites that are in low earth orbit. They are NOT in geosync. orbit. This means they are constantly traveling across the USA. This is important because the receivers in cars must switch between the strongest satellite. That makes the chipsets or brains for these radios VERY complex and difficult to produce.

The XMSR system uses two satellites that are parked in geosync. orbit just like Direct TV. The receivers in the cars pick from the strongest satellite and actually use what is called a diversity system. In other words, they use the best signal.

No one from the company has yet to say that the chipsets in the receivers work. Listen to their last conference call carefully. They say the satellites work. I don't doubt that the satellites are broadcasting something. However, do the receivers work?

The reason that SIRI is rolling out service in 4 large towns is because they use ground antennas to support their service where large buildings block the satellite antennas.

HERE IS THE BIG QUESTION: When SIRI customers drive into another city and into the country on Feb. 14, will the satellites deliver music to their cars? I don't think they will.

No one with the company has shown they work. The Consumer Electronics Show was in Los Angeles where there are ground antenna.

Someone needs to ask the important question. This is the million dollar question.

Print the post Back To Top
No. of Recommendations: 1

This is the million dollar question.

Actually, for SIRI this is a matter of life or death. That makes it more like a billion dollar question. :-)

Print the post Back To Top
No. of Recommendations: 5
The SIRI system uses three satellites that are in low earth orbit. They are NOT in geosync. orbit. This means they are constantly traveling across the USA.

First, SIRI's satellites are in geosynchronous orbit. They are not in geostationary orbit. Satellites in geosynchronous orbit orbit Earth once every 24 hours. Satellites in geostationary orbit orbit Earth once every 24 hours over Earth's equator.

SIRI uses three satellites in a style of geostationary orbit called a Molniya orbit. They orbit the earth every 24 hours, but are inclined so that at its apogee (farthest and slowest part of the orbit) it is high over the United States. The period of which the satellite is in the best position to transmit is approximately eight hours, requiring three satellites to cover the United States 24 hours per day.

This is important because the receivers in cars must switch between the strongest satellite. That makes the chipsets or brains for these radios VERY complex and difficult to produce.

The XMSR system uses two satellites that are parked in geosync. orbit just like Direct TV. The receivers in the cars pick from the strongest satellite and actually use what is called a diversity system. In other words, they use the best signal.


Correct me if I'm wrong, but, from what you said, SIRI's chipset is far more complicated than XMSR's because it has to choose the strongest signal from three signal sources instead of two?

Since SIRI satellites time-share the United States, at most, only two satellites would be visible at any time, since the third would be over the other hemisphere.

The reason that SIRI is rolling out service in 4 large towns is because they use ground antennas to support their service where large buildings block the satellite antennas.

HERE IS THE BIG QUESTION: When SIRI customers drive into another city and into the country on Feb. 14, will the satellites deliver music to their cars? I don't think they will.


XMSR does the same thing. In fact, this is where SIRI should have an advantage. Their use of Molniya-orbiting birds is supposed to result in far less signal shadow becuase, during the eight-hour period that each satellite is overhead, they are much higher overhead than XMSR's satellites.

In addition, to the best of my knowledge, XMSR is using far more repeaters than SIRI to fill in the gaps in signal. One person said (I can't remember who) once said while SIRI is a satellite radio system using repeaters to fill in gaps, XMSR is a ground system using satellites to fill in gaps. That's surely an exxageration, but it does illustrate the differences in signal shadow each system might have.

All of this might be moot, since XMSR's reception seems to be very good, but I would have bet SIRI would have XMSR beat in terms of reception.

Craig
Print the post Back To Top
No. of Recommendations: 3
First, SIRI's satellites are in geosynchronous orbit. They are not in geostationary orbit. Satellites in geosynchronous orbit orbit Earth once every 24 hours. Satellites in geostationary orbit orbit Earth once every 24 hours over Earth's equator.

SIRI uses three satellites in a style of geostationary orbit called a Molniya orbit. They orbit the earth every 24 hours, but are inclined so that at its apogee (farthest and slowest part of the orbit) it is high over the United States. The period of which the satellite is in the best position to transmit is approximately eight hours, requiring three satellites to cover the United States 24 hours per day.

There are several errors above. First you claim that SIRI's satellite's are geosynchronous and in the next paragraph you claim they are geostationary. If they were in a geo orbit, the correct term would be geosynchronous, because the orbit is inclined. However...

You then claim that the satellites are in a molniya orbit. Molniya orbits are NOT any kind of geo-orbit. A geo-orbit has a period of 24 hours. A molniya orbit has a 12 hour period.

XMSR does the same thing. In fact, this is where SIRI should have an advantage. Their use of Molniya-orbiting birds is supposed to result in far less signal shadow becuase, during the eight-hour period that each satellite is overhead, they are much higher overhead than XMSR's satellites.

I would tend to hypothesize the opposite. The molniya orbit is only 5000 km higher at apogee (40080 km vs 35790). In addition, it is not at that height for the entire 8 hours. It is only higher for approximately 4.5 hours of the 12 hour orbit.
Print the post Back To Top
No. of Recommendations: 1
There are several errors above. First you claim that SIRI's satellite's are geosynchronous and in the next paragraph you claim they are geostationary.

Oops! I certainly goofed. I meant "SIRI uses three satelllites in a style of geosynchronous orbit..." Glad I included the definitions in the previous paragraph. :-)

You then claim that the satellites are in a molniya orbit. Molniya orbits are NOT any kind of geo-orbit. A geo-orbit has a period of 24 hours. A molniya orbit has a 12 hour period.

You are correct. Molniya orbits do have 12 hour periods. SIRI's orbits have been called Molniya-like because they use highly elliptic and and highly inclined like Molniya-orbiting sats have, but they are not in Molniya orbits. My mistake.

Far anyone interested, here's a link to a site with great graphics illustrating the differences in orbits. They include geosynchronoous and Molniya orbits, but not specifically Siri's sats:

http://www.geocities.com/CapeCanaveral/Hangar/1668/orbits.htm

SIRI's sats do indeed have 24 hr periods, not 12 hour periods that Molniya-orbiting sats have.

http://spaceflightnow.com/proton/sirius3/001130sirius.html

>during the eight-hour period that each satellite is overhead, they are
>much higher overhead than XMSR's satellites.

I would tend to hypothesize the opposite. The molniya orbit is only 5000 km higher at apogee (40080 km vs 35790). In addition, it is not at that height for the entire 8 hours. It is only higher for approximately 4.5 hours of the 12 hour orbit.


What I meant by "higher overhead" is that it is at an angle higher above the horizon than geostationary sats. I should have worded that more clearly.

From the previous link, SIRI's satellites are useful when they are north of the equator, and each satellite is above the equator for 16 out of 24 hours. Since the satellites are spaced 8 hours apart, as satellite #1 drops below the equator, satellite #2 is at its apogee (highest angle above the horizon) and satellite #3 is just rising above the equator. Pretty cool the way it works out...

Here's another article detailing the orbit scheme of SIRI:

http://www.spaceandtech.com/digest/flash-articles/flash2000-090.shtml

You bring up a good point though about altitude: If SIRI and XMSR use geosynchronous orbits and SIRI's orbit is elliptical, its apogee must be higher than XMSR's sats. This can be seen in the previous link: SIRI's apogee is about 47,000 km, vs XMSR's 36,000 km. I'm sure this extra distance doesn't help SIRI, but I really don't know enough to say by how much.

Thanks for the critique -- its keeping me honest. :-)

Craig




Print the post Back To Top
No. of Recommendations: 3
You bring up a good point though about altitude: If SIRI and XMSR use geosynchronous orbits and SIRI's orbit is elliptical, its apogee must be higher than XMSR's sats. This can be seen in the previous link: SIRI's apogee is about 47,000 km, vs XMSR's 36,000 km. I'm sure this extra distance doesn't help SIRI, but I really don't know enough to say by how much.

Electromagnetic signals lose power in proportion to 1/(distance squared). So you can see the received signal has very low power for both in general.

It's not all about the satellites. You have to have good engineering in the antennas and receivers, too. It's amazing how much talk there is about the satellite paths when that is only part of the picture of signal reception. If you've cut too many corners and built a crappy receiver or too "noisy" (in electrical terms) of an antenna, you get a big loss in signal quality.

Though I don't know anything about the comparative quality of the receivers for XM and SIRI. For now it seems that most people who have subscribed to XM are pretty happy with their reception.

e-liz
Print the post Back To Top
No. of Recommendations: 0
""You bring up a good point though about altitude: If SIRI and XMSR use geosynchronous orbits and SIRI's orbit is elliptical, its apogee must be higher than XMSR's sats. This can be seen in the previous link: SIRI's apogee is about 47,000 km, vs XMSR's 36,000 km. I'm sure this extra distance doesn't help SIRI, but I really don't know enough to say by how much.
Electromagnetic signals lose power in proportion to 1/(distance squared). So you can see the received signal has very low power for both in general. ""


Wow, you people are smart. I understood about five words of those paragraphs (and those were the conjunctions and verbs). Whether an ounce of this is true... I have no idea (way beyond my scope), but it sounds really impressive nonetheless!

Yours in geophysics (right),
doddfj
Print the post Back To Top
No. of Recommendations: 0
SIRI satellites are higher in orbit, thus less shadow and blockage from earthbound objects. The fact that SIRI satellites are moving doesn't make them any harder to get signal from that it does for a car to get XMSR signal while the XMSR receiver is being driven across the country, in fact, there is more signal overlap with SIRI so it should be easier and cause few "transition edges".
Print the post Back To Top
No. of Recommendations: 0
Several people posting seem to have made the connection of the satellites to the difficulty that Sirius has had with developing a reliable chipset. What do you think?
Print the post Back To Top
No. of Recommendations: 0
Somebody set me straight if I am misguided, but WHY would a company give millions of dollars of financing to another company without knowing if the production of their product (chipsets) were reliable?

Is this question simply naive? Am I missing something?

And Why the jump in volume at 3pm? I do not see any news. Is it spontaneous investment for the launch date?

Thanks
Print the post Back To Top