Slashdot Mirror
DirecTV Plans 1500 HiDef Channels by End of 2007
doormat writes "DirecTV plans on launching four Ka-band satellites by 2007. This means local HiDef channels over satellite for the biggest markets by the end of 2005, with room for 500 HD channels. Plus 1000 more HD local channels and 150 national HD channels by the end of 2007. Thats a total bandwidth of 34Gbit/s, which is about 10 times the bandwidth they currently have in the Ku band (the band they use now for direct-to-home TV service). The bandwidth crunch for satellite providers is over, and the Ka band is the answer."
I still haven't bought a satellite or digital cable subscription. Partly because I am cheap, but also in large part because MPEG fragments drive me up the wall. I mean, I'll deal with it when it's a uhh... legally downloaded movie I'm watching on my computer, but when I'm watching shows on my TV, I don't want them to be skimping on the bandwidth. If I can tell that you're using compression, then your bitrate is too low! Lord help the people with HDTVs, paying a boatload more for a better TV and HDTV channels and still getting MPEG fragments? Come on people, it's 2004.
But the 1500 HD channels is going have a majority of the channels devoted to local channels that you will only get in your respective local area. So, you won't have 1500 channels show up on your program guide, only the local channels all broadcast in HD plus the 200+ satellite-only HD channels.
Let's not forget that these sats are geosynchronous, which puts them out at roughly 25K miles. The shadow cast by the Earth is much smaller than near the Earth. And, of course, these sats won't necessarily be in the path of that shadow at all.
So, I think it's fair to assume that they spend most of their time in the light soaking up power. Also, solar cell panels on large expensive satellites are usually computer guided. They deploy and then track the Sun so they'll get most of the power most of the time.
The link you gave only mentions the three existing satellites. They generate 4.3kW of power. Those sats, however, are almost 10 years old now. The article doesn't say, but I would guess that these new sats generate even more power (more efficient and/or bigger cells).
Elrond, Duke of URL
"This is the most fun I've had without being drenched in the blood of my enemies!"-Sam&Max
C band hardware doesn't have a problem with rain fade because the dish is over a meter wide.
Actually, the primary reason for C band's superior performance w.r.t. rain fade is the reduced atmospheric attenuation associated with lower frequencies, in general, and, in particular, with C band's frequencies vs. K band's frequencies. The atmosphere has different effects at different frequencies. The reduced attenuation at C band allows for greater link margin and, therefore, greater link robustness vs. rain.
The gains of a 1.8m dish at C band and a DirecTV dish at K band are similar. (Higher frequencies require smaller dishes for the same gain.)
If you aimed a 1.8m dish at one of the DTV birds, you wouldn't have a problem with rain fade either. (you'd have a bigger problem keeping it properly aimed, btw.)
Very true. Of course, the dish (antenna, in general) would have to be designed to operate at K band frequencies. It's not a given that you can just swap reflectors around. Antenna design at microwave frequencies is complex.
Solar cells are small. They are arrayed to create solar arrays, or solar panels. These can be quite large, depending on the power requirements of the satellite.
Nominal power ratings for satellites assume sun-normal orientation of the solar arrays, which is actively maintained by the satellite. The satellite receives 100% illumination by the sun during most of the year, the exceptions being the spring and fall eclipse seasons, when the satellite transits the Earth's penumbral and umbral regions for up to a couple of hours per day. During these events, solar array power is augmented with battery power. Bus voltage drops and current draws increase, but transmitted powers generally stay the same. Yes, over the life of the satellite (10-15+ years) batteries degrade somewhat, though battery reconditioning techniques are employed to mitigate this. With today's designs, running out of fuel is usually what limits mission life.
The reason terrestrial radio stations require the power levels they do is that they typically transmit more or less omni-directionally (or at least toroidally), as opposed to how geo satellites use highly-directional (high gain) antennas for CONUS (or whatever) coverage. The effect of the differences between these two antenna types (tens of dB in gain) far outweighs the 20 dB power difference you mention (1 kW vs. 100 kW). The high gain antennas for DBS allow multiple channels of high bandwidth at reduced power vs. their terrestrial brethren. They're really two totally different kettles of fish.
Finally, thermal management is an important part of modern satellite design. Heat pipes, thermal radiators (mirrors), finishes, and other techniques are all used to collect, distribute, and reject heat. The effectiveness of these techniques can limit a design, and how capable a company is at dealing with thermal problems can determine the capabilities of its offerings relative to those of its competitors.
BTW, the current commercial satellite models offered by Boeing are based on the 702 bus, which supercedes the 601. Both of these designs were the product of Hughes Space and Communications Co. (part of the old Hughes Aircraft Company), now Boeing Satellite Systems (Boeing bought HSC in 2000).