The SAT-C system is a simple store-and-forward messaging system.
SAT-C has four operational channels:
NCS Common Channel (SAT-C-TDM)
Ø Continuous TDM
Ø 8.64 s frame
Ø 1200 symbols/s
Ø Scrambled, encoded and interleaved
Ø One bulletin board per frame
Ø Signaling, Enhanced Group Calls (EGC), polls (see below)
LES TDM (SAT-C-TDM)
Ø Continuous TDM or demand assigned by NCS
Ø 8.64 s frame
Ø 1200 symbols/s
Ø Scrambled, encoded, interleaved
Ø One bulletin board per frame
Ø Signaling and LES-mobile messages
MES Signaling Channel
Ø Slotted Aloha random access, 28 slots/frame, 120 bits/slot
Ø 1200 symbols/s
Ø Scrambled, encoded
Ø Distress and normal calls, data reporting, NCS log-in, log-out (registration)
MES Message Channel (SAT-C-TDMA)
Ø TDMA
Ø 1200 symbols/s
Ø Scrambled, encoded, interleaved
Ø Mobile-LES messages
Services
SAT-C supports several services:
Store-and-forward messaging
Allows messages to be exchanged between a mobile station and a user connected to the fixed network (telex, modem connection, X.25 or Internet email). Only messages that are received completely error-free are forwarded to the destination.
Distress calls
Distress calls are sent from a mobile (only maritime) to a LES. Distress priority messages may be sent in both directions. Maritime SAT-C forms part of the Global Maritime Distress and Safety System (GMDSS).
Enhanced Group Calls (EGC)
A broadcast service which may be addressed to individual units as well as groups, either geographical or logical. Two EGC services are available:
Fleet NET for commercial traffic and Safety NET for maritime safety traffic, e.g., navigational warnings, weather forecasts or distress call relays.
EGC traffic is transmitted on the NCS Common Channel.
Data reporting
Lets MES send short data reports such as position information, sensor data etc. which are transmitted via the MES Signaling Channel. Two types of data reporting are supported: reserved and unreserved.
Reserved access is controlled from the LES by poll messages which contain instructions on starting time and duration of the reporting and also the type of data report required. A poll message may be interpreted as a programming macro by the MES.
Unreserved access is initiated by the MES itself.
Polling
Used by base stations to initiate transmissions from a MES of text messages or data reports. Polls may be directed to individual stations, to groups or to geographical areas.
VMS (Vessel Monitoring Systems) and SSAS (Ship Security and Alarm Systems)
The C system is also used for the VMS operated by many states for fisheries control and the mandatory SSAS used for pirate and terror alerts.
Operation
A NCS controls each active satellite and the network within the satellite coverage area. Whenever a mobile enters or leaves an ocean region it has to register with the NCS using the MES Signaling Channel. The NCS updates its tables accordingly and informs the other NCSs and LESs within its region. These stations also keep updated tables of MES status.
A NCS Common Channel TDM is transmitted by each NCS - when idle, a MES is always tuned to this channel. A Common Channel frame always has a bulletin board as the first information. This board contains network information for all LES capabilities, channel information and other system information.
Each LES has at least one TDM assigned to it. The assignment may be permanent or on a demand basis.
A LES having a message for a mobile unit advises the NCS, which transmits a call announcement packet on the Common Channel containing the ids of the MES and LES, which LES TDM will be used, call direction and priority. The MES retunes to the assigned LES TDM. The LES TDM also carries a bulletin board with information on which MES Signaling Channel slots are free. The MES finds a free slot and transmits a call announcement response on a MES Signaling Channel. It then returns to the LES TDM. The LES then transmits a channel assignment packet containing message length information, time slot number (Logical Channel Number, LCN) and local routing information. The succeeding message packets are all labeled with the LCN and have also a sequence number and a check sum. The MES must send an assignment acknowledgement packet before message transfer is initiated. The last LES packet is a request for acknowledgement. The MES acknowledge packet will contain a list of packets received in error and the LES will retransmit these packets until all packets have been received by the MES. Then the LES releases the LCN and the MES reverts to idle state.
Message transfer from a mobile station to a LES is initiated by the MES finding the LES TDM frequency information in the NCS bulletin board. It then retunes to this TDM and synchronizes to the TDM frame. The MES finds a free signaling channel slot by reading the LES TDM bulletin board information and transmits an assignment request to the LES. The LES informs the NCS that the MES is busy. It then sends an assignment packet to the MES containing, message channel frequency, slot number and LCN. The MES retunes and starts transmitting its message.
SAT-C is mainly used in maritime communications, but is also widely used for long distance truck fleet management where the trucks are polled and then return position information from a GPS receiver. The system also finds its use in the remote control of pumping stations or remote data acquisition.