Home' RTCA Documents for Review : DO-343B Contents Appendix C
© RTCA, 2018
that an Inmarsat transmit burst must occur during a (long term) Iridium message and
must specifically occur during a TDD downlink burst. If both conditions are not satisfied,
there is no interference event.
Frequency Coincidence: The victim aircraft must also be receiving on a frequency
that is affected by the particular type of source emission under consideration, at a
sufficient signal level to cause interference.
Spatial Coincidence: The victim (Iridium) aircraft must be close enough to and in the
proper angular relationship to the source (Inmarsat) aircraft. If this condition is not
satisfied, then the free space path loss between source and victim aircraft will be
sufficient to completely mitigate any interference events.
If all of these conditions are satisfied, then the victim receiver may experience repeated
events that could extend for the longer of the period of geographic coincidence (long
term A) or the recovery operation (long term B), and, therefore, might result in the loss
or significant delay of a safety-related message.
C.3.1 Basic Source-Path-Victim Link Budgets
The first step in any interference analysis is the preparation of source-path-victim link
budget tables that capture the key parameters of the source system, the propagation
path, and the victim system. These tables provide a convenient location to detail
assumptions and provide references for system performance. We divide the Inmarsat
equipment into two types: single-carrier and multicarrier. DO-262B, Change 1,
Appendix E defines AES4 for single carrier equipment and AES6 as the larger radiated
power of multicarrier equipment. 47 Table through Table provide the basic source-path-
victim tables for AES4 and AES6 equipment as the source and Iridium Block 1
equipment as the victim. The tables also provide explicit references to the source
material in DO-262B, Change 1 and details of the actual computations performed.
Table and Table provide the simple source-path-victim scenarios for the AES4
equipment that is expected to dominate the Core Europe airspace. For undesired
wideband/HSN emissions from Inmarsat AES4 terminals, the negative margins (Table
, Item 17) indicate some concern about interference in terminal (approach and landing)
airspace and below, but indicate a reasonable separation value of 1.4 nmi for airborne
aircraft. That is, if aircraft are separated by more than 1.4 nmi, then the free space path
loss between the source and victim aircraft is sufficient to reduce the Inmarsat
emissions at the Iridium receiver to a level below the Iridium susceptibility floor. In
particular, the positive margin for high altitude and oceanic airspace, required aircraft
separation is greater than even the conservative “on-the-ground” assumptions,
indicating that aircraft at the same altitude will be sufficiently separated to permit
simultaneous independent operation on both systems without interference. For Iridium
aircraft operating above Inmarsat aircraft, the separation distances are small, which will
result in a low probability of spatial coincidence, as shown in Section 0 below.
47 Inmarsat AES7 equipment is also multicarrier capable, but operates with a lower antenna gain. Therefore,
AES6 is taken as representative of multicarrier terminals. Any differences in detailed analysis are more than
compensated for by the assumption that every AES6 transmission induces IM products.
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