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ArcLight Hub User’s Guide
General System Description
1-24 1081006 Rev. 001
1.5.2.1.3 Login Packet
On receipt of a correct Bulletin Board transmission, the Modulator
attempts to transmit Login request in the form of an IP packet with a
destination address corresponding to the RtNMS IP address.
The VMT initially transmits the Login information at the nominal
Return Channel center frequency using the designated “Robust Return
Channel” data rate.
1.5.2.1.4 Frequency Hypothesis
After initial transmission of the Login packet, the VMBT monitors the
Outlink for a Configuration Message from the RtNMS.
If a Configuration Message is not received within 5 seconds, the VMT
offsets it’s transmit frequency by +N Hz from the nominal Return
Channel center frequency, and repeats the transmission. If a
Configuration Message is not received within 5 seconds of this
transmission, the VMT offsets its transmit frequency by –N Hz from the
original nominal frequency and retransmit the Login information. The
VMT continues to repeat this procedure, each time increasing the
frequency offset by integer multiples of N Hz in alternating directions
from the initial center frequency until the Configuration Message is
received, or a (configurable) maximum frequency offset limit is reached.
If the maximum frequency offset limit is reached, the VMT repeats the
entire Frequency Hypothesis procedure, beginning again at 0 Hz offset
from the nominal center frequency.
1.5.2.1.5 Configuration Message
On successful receipt of a VMT’s Login information, the Real-Time NMS
transmits one or more Configuration Packets directed to it.
On receipt of a Configuration packet, the VMT implements the specified
parameter adjustment and shall provide a positive acknowledgment of
the receipt of this packet.
After the receipt of this initial Configuration Packet, Login is considered
to be complete.
1.5.2.2 Frequency Correction Protocol for Moving VMTs
The system employs a VMT frequency correction protocol to provide
system frequency accuracy in the airmobile environment. The worst
case Doppler is defined as the aircraft moving at Mach 1 in the
direction of the satellite. The airborne system provides the velocity of
the aircraft moving in the direction of the satellite.
The frequency correction protocol allows a “worst case” VMT to
successfully enter the network without prior frequency correction. For
purposes of this requirement, a worst case VMT is defined as a VMT
performing at the limit of the VMT reference oscillator requirements,
assuming a 10 year aging.