UHF SATCOM Satellite System
Geostationary SATCOM communication satellites are located in orbit at an approximate distance of 36.000 km from the earth above the Equatorial Line. Due to its long distance from Earth, a UHF SATCOM satellite covers almost a hemisphere as its broadcast area, thus the communicating units can be spread over a very wide area. Due to the same fact, the round-trip delay is quite high varying between 239-279 ms. depending on the location of the unit in the coverage area. In a UHF SATCOM network, no control information is kept on the satellite; the satellite only performs channel separation, amplification and signal relaying. As the satellite bandwidth is limited, effective use of resources is required.
STANAG aims to perform the resource management dynamically by means of a central control unit on earth, which keeps track of connection requests and allocates resources accordingly. Each unit connected to the network listens to and participates in the control communication on a pregiven frequency and time slot on the satellite. The control communication takes place over the so-called order-wire (OW) service.
Tactical Field Satellite Communication Network
Dynamic Resource Management
Dynamic resource management is addressed by two different methods. The first is the use of pre-planned services. In this method, continuous services can be planned by assigning resources before deployment of the system. The resource assignment can also be changed and distributed during the operation phase. It is possible to end unused services, add/remove new services, or add/remove new members to existing services; but in this method terminals cannot request a service that is not predefined. Whenever an unplanned communication service is needed, the ad hoc services and DAMA protocol come into play.
Whereas an SFOW service is used for scheduled pre-assigned services, DFOW and DROW services are used for DAMA protocol. The SFOW service is located in the Master Channel, the DFOW service is located in the DAMA Control Channel. DROW, UCOM (user communication) and Ranging services can take place on all channels.
Sharing of a satellite channel by multiple communicating units is accomplished by employing a TDMA protocol. A single access communication mode is also supported in which some satellite channels are dedicated for exclusive use of certain units.
Channel Controller (CC) and Alternate Channel Controller (CC)
The Channel Controller (CC) is the central unit that manages satellite resources. The terminals join the network and request resources from the CC while the Alternate Channel Controller (ACC) stands by and watches the control communication in order to replace CC and take control of the network when necessary. Apart from performing resource assignments, CC also monitors if the allocated resources are used adequately and takes necessary steps to reassign unused resources to new services. ACC, on the other hand, listens to all control transmissions in the air in order to form its own database of ongoing services as accurately as possible so that it can replace the CC when required.
Timing Requirements and Ranging
All terminals are time synchronized to the transmissions of the CC. The clocks of different units may however differ due to synchronization errors, doppler shift or phase noise. Terminals also need to correct for this variation and provide synchronization to other units’ data rates. Transmissions of all units, including the CC, must fit into their assigned time slots with reference to the satellite’s time frame. The operations carried out to achieve time synchronization are called Ranging. The range, which is the terminal’s distance to the satellite, can be calculated passively using GPS location and time information, or be measured actively by transmitting a dummy signal in a preassigned time slot, receiving back the same signal and calculating the time difference.
Three types of user services are defined by the STANAG. The Circuit Service provides fixed-rate voice and data transfer, while the ADT Service is designed to provide fixed or variable rate data transfer. The Block Assignment Service, on the other hand, permits sub-division of an assigned time slot by higher layer protocols for further multi-access purposes. This is an optional feature.
The satellite network provides many modern communication services such as point-to-point calls, conference calls, joining an existing call, leaving a call, pausing a call, queuing service requests, waiting on a busy terminal and prioritizing services. As a military system, features such as encryption or joining a network as listener-only (silent mode) are also supported.
Variable Data Rates and Error Detection/Correction
The physical layer is designed to accommodate various terminals with different modulation capabilities under varying link quality conditions. A wide range of data rates is supported through various modulation types. The signaling protocol provides means to the target and source units to agree on mutually supported transmission parameters suitable for the actual channel quality. Control data fields are protected with CRC or RS codes.