Intelsat To Start Refueling Satellites In Orbit 79
mangu writes "Intelsat has signed a contract with Canadian MDA to refuel satellites in geostationary orbit. The $280 million contract will buy half of the 2000kg fuel carried by the space servicing vehicle. Besides refueling aging satellites, the vehicle will also be able to tow failed satellites away from the geostationary orbit."
Re:Who's into satellite TV? (Score:5, Informative)
Symbol rate is the speed at which the bytes are transferred. It's usually in Baud, so if you have ever played with a classical modem (14K4 for example, it means 14400 Baud) you should know something about it.
Channels are multiplexed into a transponder. For example the Astra 1 at 19.2 E satellite (default here in NL) has a transponder 97 (12344 MHz, horizontal) with 25 channels on it (13 TV and 12 radio channels). These channels all have a different addresses (I believe they are called PID's). Video signals have even 2 (one for audio and one for video). There is some system that tells the receiver what addresses are taken at the moment and some system that creates a start (an adress 0). From the start point each channel is send when it's time slot is there. The receiver simply waits until the correct time slot and puts the data into the input buffer.
Most satellites nowadays use Mpeg 2 encoding to compress the data. Due to this there is a lot of spare space on the satellite, although Astra 1 contains about 700 channels (radio + TV).
What most people (including me) refer to as a satellite is usually a bunch of satellites. They are positioned in a geostationary orbit within an angle of 0.1 degree. The receiving disk people use is to small to distinguish between them, so it appears as one sat. The opening angle of a 60 cm dish is about 2 degrees. This determines the effective resolution. I believe communication satellites (sattelite groups) are never spaced less than 3 degrees apart, so it's quite easy to distinguish the sat. With a bigger disk (80 cm, 1 meter) you have a smaller opening angle, so you receive less noise and thus effectively a stronger signal. A bigger disk also has a larger area, and thus the absolute signal strength is higher as well. Off course you should not increase to a disk with an opening angle of less than 0.1 degree, or you won't be able to receive the complete group.
The Astra 1 satellite (or group of satellite's if you will) sends at 10 to 12 GHz, with two polarisations (Horizotal and vertical antenna's give different signals). The frequency is way to high to send over a cheap cable, so it's downconverted. This is done by the LNBC, the small box on the receiving disk. This thing does a couple of tasks:
This LNBC is quite an interesting thing. It's a high frequency device (up to 12 GHz) but it is cheap (you can have one for less than EUR 20). Most of the parts are etched into the PCB.
The signal is send over a relatively cheap (like 1 euro per meter) to the set-top box. There is a great variety in these: simple ones, versions with recording harddisks, versions that can display two different channels (PIP or different outputs), versions for HD signals. Even versions with Linux as main operating system (the Dreambox).
In the receiver is usually a smartcard with encryption data. This can be directly into the receiver, but sometimes there is a PCMCIA-like "sleeve" (a module) in the receiver with the card in that. The receiver (or the module) decrypts the signal with the data from the smartcard. Both ways usually work
There is a strange thing with brands: While Phillips and Nokia make satellite set-top boxes (sometimes called receivers) the best brands (IMHO) are not very well known in other fields (Topfield is a good brand. I have not heard of a non-"digital set-top box" product from them. They do have quite good cable receivers.). I am not sure why this is. Phillips receivers in the Netherlands are very locked-in devices and a Nokia receiver is something you
Re:Who's into satellite TV? (Score:4, Informative)
Symbol rate is the speed at which the bytes are transferred. It's usually in Baud, so if you have ever played with a classical modem (14K4 for example, it means 14400 Baud) you should know something about it.
DVB Satellite signals are specified in Megasymbols/sec, not baud. A DVB carrier is specified by the a few parameters:
Center frequency (either in the actual downlink frequency from the satellite or in L-Band after the LNB)
Symbol rate (In kilo or megasymbols/sec)
Modulation (BPSK, QPSK, 16PSK, 32PSK)
FEC rate (1/2, 3/4, 7/8)
Once you lock onto the stream, then you can dig out the various PIDs.
Channels are multiplexed into a transponder.
Not multiplexed onto a transponder, but multiplexed into a carrier. A transponder can have multiple carriers, each carrier can have mutliple channels (separated by PIDs). Transponders are just a chunk of raw spectum on the satellite, each usually either 36 or 72MHz wide.
Most satellites nowadays use Mpeg 2 encoding to compress the data.
Technically, it's not the "satellite" that encodes the signal. The satellite is just a "radio bent pipe" in space. The ground station is what encodes the signal, the satellite just retransmits what it gets. MPEG-2 is the prevalent digital compression mode, but more services are going to MPEG-4, especially for HD video and on DVB-S2.
Not trying to be pedantic, just making sure the right terms are used. Having been in the satellite industry for 10+ years now, those things annoy me just as much as someone saying "I've got 250GB of memory in my computer"