Operation | 9
SBAS satellites that are in view.
Receiver Autonomous Integrity
Monitoring (RAIM)
RAIM is a special software algorithm in the MX521
program which gives the operator timely warnings
when the GPS system accuracy is questionable. This
feature requires a minimum of five GPS satellites to
operate properly. If the position solution error exceeds
a preset limit a “RAIM Unsafe (R-)” or ”RAIM Caution
(R?) alarm will be indicated in the MX CDU. This means
that the accuracy of the position cannot be guaranteed
to be very accurate. The operator is advised to use the
GPS cautiously for navigation until the RAIM indicator
switches to (R+) denoting safe RAIM condition. Position
errors may be caused by unhealthy satellites, incorrect
pseudoranges, poor satellite geometry, excessive
atmospheric interference and problems at particular
reference stations.
GLONASS Overview (MX521B only)
GLONASS is a global satellite navigation system
developed by the Soviet Union, providing real-time
position and velocity determination for military and
civilian users. The GLONASS satellites are located
in orbits at 25,510 km altitude with a 64.8 degree
inclination. GLONASS’ orbit makes it especially suited
for use in high latitudes (north or south), where getting
a GPS signal can be problematic. The constellation
operates in three orbital planes, with 8 evenly spaced
satellites on each plane. A fully operational constellation
with global coverage consists of 24 satellites. To get a
position fix the receiver must be in the range of at least
four satellites.
Space Segment
There are 21 satellites that are being used for navigation
with 3 spare satellites. All GLONASS satellites are
operating using the same code but on slightly different
frequencies (L1 signals at 1598.0625 to 1609.3825
while the L2 at 1242.9375-1251.6875 MHz). This is
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