Raim; Gps Xpress™ Card 8-Channel Receiver - Honeywell BENDIX/KING KLN 94 Pilot's Manual

KLN 94 Pilot's Guide

RAIM

Receiver Autonomous Integrity Monitoring (RAIM) is a function that every
IFR-certified GPS receiver must continuously perform to assure an accu-
rate position. One characteristic of the GPS Space and Control Segments
is that they can't instantly fix a satellite if it begins sending misleading
positioning information. For this reason, a GPS receiver can't rely on the
GPS satellites to tell it when there is a possible error. The words "receiver
autonomous" indicate that the GPS receiver, on it's own, can detect an
inaccuracy in the GPS system. It does this by using distance measure-
ments from as many satellites as possible to check the consistency of the
position (this is called an over-determined position solution).
If there are not enough satellites in view to assure the position integrity,
the KLN 94 notifies the pilot. Five measurements with good geometry (rel-
ative positions of the satellites) are needed to provide RAIM. This means
that at least five satellites or four satellites and a pressure altitude input
from the aircraft's altimeter are required to assure RAIM availability.
Likewise, if this check shows some inconsistency in the position, the KLN
94 notifies the user that other navigation sources should be used to cross-
check the position. Additionally, sometimes the receiver is unable to
assure the position to enough accuracy for the phase of flight, due to the
location of the satellites at that instant.
RAIM availability is very seldom a difficulty when in the en route and termi-
nal phases of navigation. The allowable limits are much tighter when a
non-precision approach is performed with a KLN 94. For this reason, the
KLN 94 Auxiliary (AUX) 3 page allows the pilot to enter a time and loca-
tion, and the GPS receiver will predict if RAIM will be available to shoot
the non-precision approach. See section 6.2.14 for more information
about the RAIM prediction capability.
GPS XPRESS™ CARD 8-CHANNEL RECEIVER
The KLN 94 and KLN 94 use a Honeywell Bendix/King GPS receiver
known as the GPS Xpress™ card. It was dubbed this way because it is
identical in size to an everyday credit card, and its faster acquisition time
than previous single-channel designs. The GPS receiver has eight paral-
lel channels, which means each channel can continuously track a satellite,
for continuous tracking of up to eight GPS satellites. The parallel receiver
design has several advantages:
• Excellent performance during high dynamic conditions (high velocity
and/or acceleration).
• Improved position acquisition time (also known as time-to-first-fix)
over single-channel designs.
F-3
Appendix F GPS Primer