Home' RTCA Documents for Review : DO-262D Contents 7
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GEO constellations consist of satellites at an orbital altitude of 35,800 km. At this
altitude, the orbital period is 24 hours, and the ground track of the satellite varies only
slightly with time. From the viewpoint of a user aircraft, the satellite is nearly
stationary. Typical GEO constellations consist of only four or five satellites in
equatorial orbits, spaced nearly uniformly in longitude. The AMS(R)S/AMSS
communications described in the current version of DO-210D MOPS utilize such a
constellation of GEO satellites. This document specifically allows for the possibility of
future GEO constellations that comply with the more generic requirements of NGSS,
vice the original requirements detailed in Chapter 4 SARPs and in DO-210D.
Satellite System Coverage
The coverage area of a satellite system is determined by the aggregation of all
service-link beams formed by all satellites in its constellation. In current AMSS
systems, coverage of an individual beam can range from that portion of the Earth’s
surface within line of sight of a given satellite (roughly one-third of the Earth’s surface
for a GEO satellite) to a portion of the Earth’s surface as small as 800 km in diameter.
These beams can be stationary (as in GEO system) or can be continuously moving
over the Earth’s surface (as in a LEO/MEO system).
This document defines coverage area as that airspace over a delineated portion of the
Earth's surface, within which all service/performance criteria are met by a given
Satellite Subsystem. It is the responsibility of the Satellite Subsystem service provider
to define, declare, and verify its coverage areas. Information on the service area
provided by a specific NGSS system can be found in the applicable technique-specific
appendix to this MOPS.
The Aeronautical Telecommunication Network (ATN) is being implemented on a
world-wide basis to provide a common set of interfaces and protocols for aeronautical
data communications (see also Section 1.4 .1). The requirements of this document are
based on the assumption that NGSS data services will be constituent subnetworks of
The ATN is based on the Open System Interconnect (OSI) data communications
model. The OSI Reference Model defines seven such functional layers with
responsibilities ranging from control of data transfer on the physical media (radio, wire,
fiber, etc.) to control of the application user interface. The seven layers are named:
(1) Physical, (2) Data Link, (3) Network, (4) Transport, (5) Session, (6) Presentation
and (7) Application. Each layer provides communication services to the layer above it.
The particular protocol for a given layer may be optimized for the environment in which
it operates; e.g ., a satellite communications subnetwork must operate in a medium
prone to errors, has long propagation times, and has limited bandwidth.
Each layer incorporates well-defined operations designed to optimize control and
information flow across the layer boundaries. To carry out its function, each layer may
add protocol control information fields to the service-data-unit supplied by the layer
above. However, each layer leaves the control information added by previous layers
intact, treating it as data to be passed on unaltered. The aircraft avionics and its
counterpart of the ground station implement either the lowest two or the lowest three
layers of the OSI model to provide packet-mode data services.
This document recognizes two potential modes of data operation. These modes are
referred to as Data-2 and Data-3, to be consistent with existing industry standards
(see RTCA/DO-210D). Data-2 operation provides transparent data communications
by enveloping the packets received from a higher-layer entity (HLE) without using
internal network layer protocols. Data-3 operation provides to the user a packet-
switched subnetwork that implements the lowest three layers of the OSI model
(physical, data link and network) and is intended for use with the Aeronautical
Telecommunications Network (ATN). One proven means of providing an OSI
compliant interface for ATN data is by using the standard interface defined in
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