The target airplane does not show up on the newer (Mode Select) FAA ground radar, nor the airborne TCAS (traffic collision avoidance system) airplane based radar, regularly reliably, because the FAA mis-specified required radar transponder operation.  This happened in 1973, and absolutely nobody in the FAA seems to have noticed the error.  They have responded after we showed them the error (called it to their attention many times).

The interrogation, to which the transponder must reply, consists of four pulses, called P1 then following P2 maybe (or maybe not) then following P3 then following P4 (if the newer Mode Select ground radar or TCAS aircraft radar interrogation).  There is no P4 in the older FAA ground radars interrogation.  There may be some in the FAA that know this.

The FAA specifies that the transponder must comply with the following Technical Standards Order C74c, and the transponder manufacturers must state compliance with this specification:

2.6  Decoding Performance.
c.  Side-lobe Suppression.  The transponder must be suppressed for a period of 35 ±10 microseconds following receipt of a pulse pair of proper spacing and suppression action must be capable of being reinitiated for the full duration within 2 microseconds after the end of any suppression period.  The transponder must be suppressed with a 99 percent efficiency over a received signal amplitude range between 3 db above minimum triggering level and 50 db above that level and upon receipt of properly spaced interrogations when the received amplitude of P2 is equal to or in excess of the received amplitude of P1 and spaced 2.0 ±0.15 microsecond from P3.

This is 100% totally wrong.  In addition to P2 isn't anywhere within 2.0  ±0.15 microseconds of P3, there is a requirement to not respond to critical interrogations.  What it should say:

c.  Mode S and TCAS Non-Suppression.  The transponder must not be suppressed for a period of 35 ±10 microseconds following receipt of a pulse pair of the following spacing.  The transponder must not be suppressed to more than a 1 percent efficiency over a received signal amplitude range between 3 db above minimum triggering level and 50 db above that level and upon receipt of properly spaced interrogations when the received amplitude of any pulse is equal to or in excess of the received amplitude of P1 and spaced 2.0 ±0.15 microsecond from P3.

The pulse P4 is the only pulse 2.0 ±0.15 microsecond from P3, and is always there (even if you call it by the improper name of P2), on the newer interrogations from FAA ground radar, and the airborne TCAS, if they follow their TSO specifications.  The transponder should reply ideally all of the time, never suppress.  Nobody in the FAA seems to know this.

But (yet another error) that leaves us with the problem that critical transponder side lobe suppression is not accomplished.  This additional problem results in all FAA ground radar seeing phantoms of the true target aircraft.  It's there, all right, but there is more than one of him, and you don't know which is real and which is not really there.  To prevent that problem, a new section must be added to the specification (old section from pre current C74c restored):

d.  Side-lobe Suppression.  The transponder must be suppressed for a period of 35 ±10 microseconds following receipt of a pulse pair of proper spacing and suppression action must be capable of being reinitiated for the full duration within 2 microseconds after the end of any suppression period.  The transponder must be suppressed with a 99 percent efficiency over a received signal amplitude range between 3 db above minimum triggering level and 50 db above that level and upon receipt of properly spaced interrogations when the received amplitude of P2 is equal to or in excess of the received amplitude of P1 and spaced 2.0 ±0.15 microsecond from P1.

The pulse P2, the only pulse 2.0 ±0.15 microsecond from P1, is there only when the transponder is not supposed to reply to any FAA ground radar.  The transponder should suppress, not reply, to a pulse near P1 (and should not to a pulse near P3).  Nobody in the FAA seems to remember this.

These are NOT THE ONLY technical errors in the current (since 1973) published TSO C74c.  The FAA doesn't seem to know about those, either.  User beware.

The person who boards an aircraft is admonished to obtain from the FAA what GUARANTEES for separation of what (VFR, IFR) aircraft potential collision permutations is provided (only IFR from IFR, which is about 1%).

The person who boards an aircraft is admonished to obtain from the FAA what the DEFINITION is for coast mode (we can't see the airplane on radar, we put a radar target on the scope presentation anyway), and DISCLOSURE of the presently used number for the amount of time that an aircraft is in coast mode before a C is put on the lower right hand corner of the data block for that target on the scope presentation (the radar controller is first advised, if he is paying attention to every target block, that his traffic has been in coast mode).

With the chip, these two, and also additionally all other, problems get fixed; which is why the transponder works so well after the repair, in comparison to what it can do before the repair.  Plus, with the repair, you get a second collision avoidance system that is the ONLY system that works anywhere all of the time for all aircraft both in flight and on the ground for runway incursion avoidance.  The military are allowed this, only the FAA prevents it for use in protecting your life.