This starts to be a perfect 1000fpm descent until FL270 !??
Thereafter again from FL300 (after 5 minutes) until FL220.
Are we talking about TCAS or something else ?
Check and see what alt the flight filed for. I think if the postion report does not have and alt (perhaps the secondary radar return was not received by ATC) they (I assume ATC) subs the filed alt in the data. Notice very often the first one or two postion reports after departure are for the files alt, then “descent” to an actual. It also occurs very often during approach descent, possibly because during approach, the aircraft is transiting between high alt long distance radar coverage to local systems.
Below are the first several reports for COM153, 4/12/06 JAX-BOS. Filed for alt FL330. Notice the 7:25 reports Alt 33000 from 5500. Pretty good performance for a CRJ.
The secondary radar data is based on an interogation method. When the primary radar signal hits the aircraft, the transponder “squawks”- it transmits back a quick stream of data which includes the squawk code (so ATC systems can positivily ID the target and alt as read from the aircraft’s navagation systems. This alt is much more reliable than can be gathered from the primary radar return (radio echo). The primary return is good at getting range and bearing, giving a position. Lots of factors could result in momentary loss of the secondary signal. In addition to switching radar facilities, a large number of secondary returns (high traffic) can cause some of the returns to be lost.
I work the sector in the Washington ARTCC that does the CLT arrivals over LYH for the MAJIC9 arrival. One of the radar site for our mosaic is over ROA, during a decent, it is not uncommon for the computer to miss the MODE C readouts.
