Technology challenges for satellite communications on-the-move

by Mark Gouker

Army transformation and the accompanying changes in the way the Army thinks, trains and fights will increase demand for satellite communications. The very nature of this new mobile, agile and fast-paced battlefield � coupled with the goal of strategic dominance across the entire operations spectrum � demands a way for warfighters to communicate beyond-line-of-sight while actively moving.

SATCOM systems today do a remarkable job of facilitating global communications quickly and accurately. However, technology limitations make the goal of SATCOM on-the-move unattainable right now. That�s changing. New technology is emerging that will bring this tool on-line, providing BLOS access to command, control, intelligence, surveillance and reconnaissance information.

The process of achieving SOTM capability is intricate and difficult. There are two fundamental technology challenges that must be overcome in developing SOTM, using bands in super-high frequency and extremely high frequency. The first challenge is overcoming signal blockage when a vehicle goes under a bridge or behind a building or tree. The second challenge is ensuring the antenna is properly and precisely aligned to access the satellite even when the vehicle is in motion.

Signal blockage

SATCOM systems that operate at EHF and SHF rely on highly directional antennas to establish connection with the satellite. Directional antennas are needed at higher frequencies to overcome signal loss during propagation and to support higher data rates than can be supported at ultra-high frequency. One of the consequences of using a directional antenna is that most objects (trees, buildings) coming between the antenna and the satellite will interfere with signal reception to the ground terminal.

Thus, the SOTM link is characterized by periods when the signal is stable and strong enough for good communications and periods of poor reception caused by signal blockages. The frequency and duration of good and poor reception will depend on the moving vehicle�s surroundings.

In most situations, periods of signal blockage can last many seconds. So how can we provide reliable communications even if the basic radio link is unreliable?

Where there�s a will, there�s a computer

Increasingly, SATCOM terminals are tied to computers and routers to provide range extension for terrestrial data networks. It�s possible to use these computers to filter and recondition poor signal reception to make the links more reliable. One approach, being developed by the Massachusetts Institute of Technology�s Lincoln Laboratory, is to use an automatic-repeat-request protocol in the network-protocol stack�s link layer.

Data coming to the terminal from the terrestrial network is placed in packets for transmission over the satellite. As the packets are transmitted to the other side of the link, a copy of each is placed on a "waiting list" inside the transmitting terminal. If the packet is received on the other side of the link, an acknowledgment is returned to the transmitting terminal and the packet is removed from the waiting list. However, if an acknowledgment isn�t received within a predetermined period of time, the packet is taken from the waiting list and retransmitted. As the packet is retransmitted, another copy is placed on the waiting list. This cycle continues until the packet is acknowledged or until the packet has reached the maximum number of retransmission attempts.

The ARQ protocol is particularly well suited to SOTM because it adapts quickly and efficiently to changing signal conditions (open/blocked). When the radio link is open, packets arriving at the other side of the link are acknowledged and removed from the waiting list before they are retransmitted. The size of the acknowledgment is small and thus doesn�t use too much of the available throughput. When the channel is blocked, copies of the packets are correctly retransmitted from the waiting list, providing a reliable means to deliver data to the link�s other end.

Even though the ARQ protocol is useful for SOTM, there will be situations where the local surroundings are simply too dense to permit steady, reliable communications. It�s possible, however, to stop the vehicle in a location that provides an open channel to the satellite by using a signal-strength indicator. Since the terminal is already set up and logged onto the satellite system, little time is wasted. This technique, known as SATCOM-at-the-pause, will be an effective backup to SOTM.

Antenna alignment with the satellite

Antenna alignment, or precisely pointing the antenna towards the satellite, is the second technology challenge. There are commercial-off-the-shelf antenna positioners that are used on boats and recreational vehicles to receive direct-broadcast television from satellites. However, these products are designed for vehicles on improved roads where turns in the road or maneuvers by the vehicle are the most stressing motion. These positioners weren�t designed for vehicles traveling off-road where traversing ruts, potholes and other obstacles are common occurrences.

Proper construction of a reliable antenna positioner is determined by how much fluctuation can be tolerated in aligning the antenna to the satellite. The dilemma is that increasing the allowable "mispointing" will decrease the network�s data-throughput efficiency. The most conservative approach is to specify that an antenna must point well enough that very little signal strength is lost due to antenna mispointing. It�s possible to develop positioners that meet the stringent alignment requirements, but their complexity and sophistication would come with a high price tag.

Another approach, however, is to realize that antenna mispointing will cause a loss in signal strength much like a signal blockage. Thus the ARQ protocol will also overcome some amount of antenna mispointing. The degree of acceptable mispointing is still being investigated, but it�s pretty clear the ARQ protocol designed for blockage events will have the added benefit of permitting more relaxed antenna alignment requirements. The underlying question in this approach is can enough antenna mispointing be permitted to allow a less sophisticated antenna positioner, making it less costly and able to be more widely deployed.

SOTM demonstrations planned

Preparations are underway to demonstrate SOTM with the Milstar system in March or April 2002. Communications-Electronics Command is leading this effort, with the assistance of MIT Lincoln Laboratory and Harris Corporation. MIT Lincoln Laboratory is developing the ARQ protocol and performing system integration. Harris Corporation is developing the antenna positioner.

Even though this program is expected to demonstrate SOTM, the main emphasis is on gathering data and developing solutions to address the technical challenges described. The outcome of the demonstrations and the information gleaned from the results of this work will aid in designing future prototype and production SOTM terminals and in meeting the Army�s transformation requirements.

Dr. Gouker received a doctorate in electrical engineering from the Georgia Institute of Technology in 1991. Since then, he has worked for MIT�s Lincoln Laboratory, a federally funded research-and-development center dedicated to assisting the Defense Department. Currently he�s an assistant group leader in the tactical-communications systems group and leads the program to develop SOTM.

Acronym QuickScan
ARQ � automatic repeat request
BLOS � beyond-line-of-sight
EHF � extremely high frequency
MIT � Massachusetts Institute of Technology
SATCOM � satellite communications
SHF � super-high frequency
SOTM � satellite communications on-the-move

dividing rule

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