Hemisphere GPS MBX-4, User Manual

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MBX-4 User Guide
5
DGPS Format
For manufacturers of GPS equipment, commonality is essential to
maximize the utility and compatibility of a product. The governing
standard associated with GPS is the Interface Control Document,
ICD-GPS-200, maintained by the US DoD. This document provides the
message and signal structure information required to access GPS.
Like GPS, DGPS data and broadcast standards exist to ensure
compatibility between DGPS networks, and associated hardware and
software. The Radio Technical Commission for Maritime Services
Special Committee 104 has developed the primary DGPS standard
associated with radiobeacon DGPS, designated RTCM SC-104 V2.2.
Various broadcast standards may exist for the beacon networks
installed internationally, controlled by their respective operating
authority. The United States Coast Guard maintains a broadcast
standard that is referenced in the Further Reading section of this
manual.
Radiobeacon Range
The broadcasting range of a 300 kHz beacon is dependent upon a
number of factors including transmission power, free space loss,
ionospheric state, surface conductivity, ambient noise, and atmospheric
losses.
The strength of a signal decreases with distance from the transmitting
station, due in large part to spreading loss. This loss is a result of the
signal’s power being distributed over an increasing surface area as the
signal radiates away from the transmitting antenna.
The expected range of a broadcast also depends upon the conductivity
of the surface over which it travels. A signal will propagate further over
a surface with high conductivity than over a surface with low
conductivity. Lower conductivity surfaces such as dry, infertile soil,