Home' RTCA Documents for Review : DO-262D Contents Appendix D
© RTCA, 2018
As the inter-satellite geometry changes, radio channels must be reallocated among
the beams to avoid interference. This process can cause an SDU to be handed off to
a different channel in the same beam. This is called Intra-beam handoff. An SDU can
also request an Intra-beam handoff to reduce interference. If the Iridium system
detects an allocation change coming up where it will not have enough channels to
support the number of current users, the satellite will ask for volunteers to handoff into
other beams so calls will not have to be dropped when the resource change takes
place. Handoffs made under these conditions are called Volunteer handoffs.
Volunteer handoffs may result in one of two situations requiring handoff, namely Inter-
satellite or Intra-satellite, but are initiated by the SDU (at the request of the Iridium
system) rather than by the Iridium system itself.
Voice and Data Traffic Channel
Traffic channels provide two-way connections between space vehicles and subscriber
equipment that support Iridium services. These channels transport the system voice
and data services along with the signaling data necessary to maintain the connection
and control the services.
The uplink and downlink Traffic Channels use identical burst structures. Each burst is
8.28 ms long and contains 414 channel bits. The bursts are divided into four major
data fields: Preamble, Unique Word, Link Control Word and Payload Field. The
preamble and unique word are used in the receiving demodulator for burst acquisition.
The preamble and unique word patterns are different for the uplink and downlink. The
Link Control Word provides a very low data rate signaling channel that is used to
support link maintenance, the associated control channel and handoff. The payload
field furnishes the primary Traffic Channel that carries the mission data and signaling
The Link Control Word field provides a low rate signaling channel used for control of
the subscriber link. The uplink and downlink Traffic Channels use the same Link
Control Word format. The Link Control Word is used to support link maintenance,
handoff and the ACK/NAK of the associated control channel transmission protocol.
The Link Control Word field is protected by FEC code.
The Traffic Channel payload field provides the primary Traffic Channel. This field
carries the mission data and mission control data. This field supports a channel bit
rate of 3466.67 bps. Typically error correction coding, error detection, and other
overhead functions provide a nominal information throughput on this channel of
Mission data may be either vocoded voice data or data services. For voice service,
the proprietary Iridium vocoder uses FEC to achieve an optimal MOS of 2.5 to 2.7
depending on AES type (based on mean opinion score for a basic telephony voice
call, where 1 is bad and 5 is toll quality). For data service, the L-band transport
employs a frame check sequence to provide essentially error free data transport
Iridium data services also provide additional service specific interfaces to facilitate
user access. In summary, the Iridium communication channel appears to the end
users as an efficient and reliable data transport.
Iridium Data Services – RUDICS and SBD
Iridium RUDICS Service
The Iridium RUDICS service “Iridium Router-based Unstructured Digital Information
Connectivity Solution” is an enhanced gateway termination and origination capability
for circuit switched data calls across the Iridium Satellite network. RUDICS offers an
optimized data connection service for various end to end data applications or
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