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Section 4.0 Calibration and Configuration
4.1 Calibration Overview
Reference: FAR 91.217; FAA Advisory Circular 43-6B
FAR 91.411; FAR 43-Appendix E and F
FAA TSO-C88a; EASA ETSO-C88a SAE AS8003
This procedure will allow adjustment to the calibration curve of the SSD120-(XX)N-RS5 or
SSD120-(XX)NE-RS5 as an aide in matching the digitizer output to a primary flight
altimeter or NIST traceable pressure standard. The maximum allowed error between the
primary flight altimeter and the altitude digitizer is ±125 feet as required by TSO-C88a and
ETSO-C88a. All Trans-Cal digitizers are calibrated to within ±50 feet of a NIST traceable
pressure standard; however, the error allowed on altimeters at higher altitudes could lead
to a combined error in excess of ±125 feet. When the altitude digitizer is installed in an
aircraft for use as the transponder's source of mode "C" information the digitizer must be
recalibrated for correspondence to the aircraft's primary flight altimeter, as required by FAR
91.217 and 91.411. Model SSD120-(XX)N-RS5 and SSD120-(XX)NE-RS5 are designed
to be field calibrated to meet this requirement, as per the procedure described in either
§4.4 or §4.8. The correspondence required for altitude digitizers is fully addressed in SAE
Aerospace Standard AS8003 §3.11. The correspondence described by the SAE standard
requires the digitizer to report altitude within ±125 feet of the primary flight altimeter's
reading when the pressure datum is set to 29.92 In. Hg., (1013 MB) absolute. The SAE
standard also requires a transition accuracy of ±75 feet of the nominal transition point for
that altitude. A transition is defined as the point at which the digitizer changes from one
altitude to the next, either increasing or decreasing altitude. The nominal transition point of
the ICAO code occurs 50 feet prior to the altitude in question. See Figure 1.
There are two different methods used to change the calibration of this device. The
technician need only perform the method that is best suited for the application in question.
There is no need to perform both methods. The digitizer may be adjusted using two
potentiometers, which affect the span and reference of the pressure transducer. This
device may also be adjusted utilizing an externally addressable EEPROM, which is
configured to accept an alternate error curve entered to the digitizer via an IBM compatible
PC.
The Span Adjust calibration (§4.4) is normally used in applications where only a slight
modification is required to bring the altitude digitizer curve up or down.
The Dynamic Calibration procedure (§4.8) is an alternate method used to match the
altitude digitizer to the primary flight altimeter or NIST standard. It assumes the digitizer
and altimeter are connected as shown in the Dynamic Calibration Block Diagram and
the technician may adjust the input pressure to run the digitizer and primary flight altimeter
to the same altitude and then enter this altitude into an IBM compatible computer, which
will transmit the correction to the digitizer's EEPROM. This calibration procedure differs
from the Span Adjust procedure in that the adjustments are made at every 1000-foot
interval and the Digitizer is adjusted at the 0 foot mark
the ICAO data nominal
NOT
transition point.
Page 14 of 57
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