Satellites - RCA DRD102RW Installation Manual

12 Technical Overview
Each data packet is 147 bytes long. The first two bytes (a byte is made up of 8 bits)
of information contain the SCID and Flags. The SCID (Service Channel ID) is a
unique 12bit number from 0 to 4095 that uniquely identifies the packet's data channel.
The Flags are made up of 4 bits used primarily to control whether or not the packet is
encrypted and which key to use. The third byte of information is made up ofa 4 bit
Packet Type indicator and a 4 bit Continuity Counter. The Packet Type identifies the
packet as one of four data types. When combined with the SCID, the Packet Type
determ'mes how the packet is to be used. The Continuity Counter increments once for
each Packet Type and SCID. The next 127 bytes of information consists of the
"payload" data which is the actual usable information
sent
from the program provider.
(See figure 3.)
Satellites
2 BYT_.S 1BYTE
,=, BYTES I 17 BYTES I
I I PAYLON) FORWARDERROR
SCID & FLAGS PACKET TYPE & CORRECTION
CONTINUITY COUNTER
Figure 3, Data Packet
Two high power KU-band satellites provide the DSS signal for the receiver. The
satellites are located in a geostationary orbit in the "Clarke" belt, 22,247 miles above
the equator. They are positioned less than .5° apart from each other with the center
between them at 101° W. longitude. This permits a fixed antenna to be pointed at the
101° slot and receive signals from both satellites. The downlink frequency is in the
K4 part of the KU-band at 12.2 GHz to 12.7 GHz. The total transponder channel
bandwidth is 24 MHz per channel with channel spacing at 14.58 MHz. Each satellite
has sixteen 120 watt transponders. The satellites have a life expectancy of 12
years.(See figure 5.)
Unlike C-band satellites that use horizontal and vertical polarization, the DSS
satellites use circular polarization. The microwave energy is transmitted in a spiral-
like pattern. The direction of rotation determines the type of circular polarization
(Figure 4). In the DSS System, one satellite is configured for only right-hand circular
polarized transponders and the other is configured for only left-hand circular polar-
ized transponders. This nets 32 total transponders between two sateliites.
RightHandCircularly P oladzed Wave LeftHandCircularly P oladzed Wave
Figure 4, Right-hand and Left-hand Circular Polarization