FLARM signals between two aircraft are only possible in a line of
sight; there can be no signal between two aircraft on opposite
sides of the same mountain.
Position Determination
To operate correctly FLARM must be constantly aware of its own
current position, for which reason it will only operate if there is
good, three-dimensional GPS reception. GPS reception is greatly
influenced by the correct installation and position of the GPS
antenna and aircraft attitude. It also requires that the US GPS
system is in full and unrestricted use. Especially when flying in a
turn, close to hills or mountainsides, in areas where reception is
known to be unreliable, or if the antenna installation is poor, the
GPS signal quality may be degraded; this also causes rapid
deterioration of height calculation. FLARM operates correctly
again when the GPS signal quality is restored.
The movements registered by a GPS relate to a fixed system of
terrestrial coordinates. When the wind is strong, the aircraft
heading deviates from Track over Ground, and this has an effect
upon the collision threat calculated. If the wind speed is one
third of True Airspeed (TAS) and the aircraft heading is at 90° to
the wind and with no drift, then the display has an error of e.g.
18°. If the wind is very strong the Track over Ground can deviate
as much as 180° from the Heading. If the aircraft is circling, the
calculation and thus collision warning threat are unusable.
In the event of poor GPS reception, for aircraft at close distance
and at similar heights, the angle offset from the vertical is
imprecise and irregular.
Flight Path Calculation
FLARM calculates its own predicted flight path for about the next
20 seconds. The prognosis is based upon immediate past and
current vectors, plus a movement model that has been optimised
for the respective user. This prognosis incorporates a number of
errors that increase with the duration predicted. There is no
guarantee that the aircraft will fly along the projected flight path.
For this reason, a prediction may not be accurate in every case.
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