Phase of matched filter for chirp signals I am using the following system: Ultrasonic chirp signals with central frequency 100kHz are being transmitted using ultrasonic sensor. After the transmission through the medium(air/solid/liquid/combination) they are being received with hydrophone. The system is being controled by DSP and signal generation and acquisition is obtained via DAC and ADC. After reception we are performing demodulation and then complex matched filtering. Our aim is to detect the time of arrival between transmission and reception. So far I have been only observing the peak of absolute value of matched filter output as the moment that corresponds to time of arrival, but if I am correct the phase of the matched filter output can also be used.
My question is, should the phase of matched filter output be zero at the moment of the peak in the absolute value of matched filter output?
For a constant frequency signal, with no frequency offset between transmitter and receiver, the phase of a complex correlation, at the correlation peak, is the phase difference between the received signal and the reference signal used for the matched filter.  It need not be 0.  And that is for the actual correlation peak, which may have to be obtained by interpolation between the two samples that surround the peak.
I haven't worked out the math for a chirp.  I would not rely on the phase of the complex correlation being 0 at the correlation peak.