Home' RTCA Documents for Review : DO-262D Contents Appendix D
© RTCA, 2018
Call processing in the Iridium Satellite Network consists of Acquisition, Access,
Registration and Auto-Registration, Telephony, and Handoff. This is managed
through the Iridium ATS Network which is co-located with the Iridium Gateway location
currently located in Tempe, AZ. The Iridium ATS Network manages all aspects of
acquisition, access, registration, telephony and Handoff. See Iridium Reference G –
“Iridium ATS Operator Guide”.
Acquisition is the first step in obtaining service from the Iridium Satellite Network. It is
the process of establishing a communication link between a satellite and the SDU.
Acquisition by an SDU is necessary for registration, call setup, answering call
terminations, or to initiate any service on the Iridium Satellite Network.
To enter the Iridium Satellite Network, a subscriber unit must go through an
Acquisition sequence. The first steps in Acquisition are to achieve frame timing
alignment, determine the correct downlink time slot, and detect the Doppler shift of the
received signal. Then the SDU must pre-correct the transmitted signal so the received
signal, at the satellite, arrives during the correct receive time window and has, at most,
a small Doppler offset.
To acquire the system, an SDU turns on its receiver and acquires the satellite
Broadcast Channel transmission for the beam in which the SDU is located. The Ring
Channel includes the broadcast time/frequency for each beam, and the SDU can use
this to determine which channel to use. The decoded satellite broadcast (Broadcast
Acquisition Information message) indicates to the SDU if Acquisition is permitted; this
is done via the Acquisition Class control. Acquisition denial might occur as a result of
network capacity or some other system constraints. If the network permits Acquisition,
the SDU extracts the beam ID and selects a random Acquisition Channel.
The SDU estimates Doppler offset and predicts uplink timing based on beam ID. It
pre-corrects its timing and frequency and then transmits a ranging burst (Acquisition
Request message) to the satellite on the Acquisition Channel. Upon receipt of the
Acquisition Request message from the SDU, the satellite calculates the time and
frequency error of the received signal. It then sends a Channel Assignment message
to the SDU along with time and frequency corrections.
After each transmission on the uplink Acquisition Channel, the SDU decodes the
Broadcast Channel and checks for an acknowledgment of its request (Channel
Assignment message) and makes sure its acquisition class is still allowed on the
system. Receiving no acknowledgment after a request, the SDU repeats its request
after a random time interval (Slotted Aloha) and on a random Acquisition Channel.
This minimizes the number of collisions between the acquiring SDU and other SDUs
attempting to use the Acquisition Channel.
The SDU, upon receiving the Channel Assignment message, immediately transitions
to the new Sync Channel and acknowledges the change by sending a Sync Check
message to the satellite. The satellite measures the time and frequency offset error of
the received burst and responds with a Sync Report message. The Sync Report
message contains a Sync Status information element. The satellite will set Sync
Status to “Sync OK” if the time and frequency errors are within the tolerance for Traffic
Channel operation. If the satellite sends a Repeat Burst in the Sync Status
information element, the SDU adjusts its timing and frequency and retransmits a Sync
Check message. If the satellite sends “Sync OK” in the Sync Report message, the
SDU acknowledges by sending a Sync Check message and waits for a Sync/Traffic
Switch message from the satellite. Upon receipt of the Sync/Traffic Switch message,
the SDU exits the Acquisition process and initiates the Access process. The satellite
then switches the Sync Channel to a Traffic Channel.
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