On Jul 21, 6:14*am, "Mike Schumann" mike-nos...@traditions-
nospam.com wrote:
*If there is a legitimate
concern that the VFR encoder is not accurate, the logical conclusion would
be to make sure you have extra separation vertically between the IFR
aircraft than you otherwise might.

In a system where the transponder and encoder are separate units, the
encoder to transponder altitude interface is typically implemented by
multiple ground/open discretes. Something as simple as a dirty
contact may result in a discrete being assumed open instead of ground
state. A single bit error may result in an encoder reporting an
altitude several thousands of feet in error. The Gilham Grey code
used for altitude encoders has no parity check and, with few
exceptions, no other means of error checking except correlation with
the pilot's altitude report. One exception is transponders that
display the reported altitude and allow the pilot to check it.
Nevertheless the controller has no way to know the reported altitude
is accurate unless verified against a pilot altitude report.

In this context accurate does not mean plus/minus 200 feet (the
resolution is only 100ft) but perhaps plus/minus 5000ft or more.

Of course the same non error checked, low integrity, transponder/
encoder systems are the basis for TCAS conflict resolution.

ref http://www.airsport-corp.com/dot_faa_ct-97_7.pdf

"The results of this study indicate that most of the transponders
carried
by GA aircraft fail to meet all of the performance criteria specified
in national
standards documents, and that a number of these failures may be
serious enough
to significantly affect their performance with secondary surveillance
radar
systems and TCAS collision avoidance equipment. In addition, the data
showed
that performance failures on key transponder parameters were unrelated
to the
time that had elapsed since a transponder had received its last
biennial
inspection."

Anyone ready for ADS-B yet.

Andy