Multipath Signals and Doppler Shift

Hi Skydel Team! I received an email from a customer and need help deciphering an answer for them. The question is below.

Hey so I have a question about how doppler is added to multipath signals. In the manual it states:
“The Doppler shift of an echo indicates the frequency offset the echo signal has compared to the Line Of Sight signal. The Doppler Shift will only affect the carrier’s phase and frequency. The echoed signal’s code is not affected by the Multipath’s Doppler shift.”

This seems to indicate that adding doppler to the multipath signals would not alter the time of arrival of the chips, however I am not observing what I would expect in that situation (or I am just not understanding it correctly). Hypothetically, if the doppler is only applied to the carrier and not the code portion of the signal, the time of arrival of the chips shouldn’t change and thus the time solution won’t change. What will change should be the clock drift rate calculation, which is ostensibly calculated from the carrier frequency. To test this, I added -100Hz doppler to multipath reflections of all the visible signals and blocked the LOS. This should increase the drift rate by about 63ns/s. This is observable with the ublox receiver I was testing with. If the code is unaffected then the bias calculation should not be affected by the frequency shift, and you would have a mismatch between the average rate of change of the bias and the calculated drift rate. This is not what I observed, however. The drift rate changed from 496ns/s to 564 ns/s and this matches almost exactly to the average rate of change of bias I calculated over the course of 60 seconds.

Is it possible the manual is incorrect on this point and doppler affects the chipping as well as the carrier?

I believe the customer might be thinking that the carrier works independently from the code. However, the code needs to be carried by the carrier in order to be retrieved. What is your take on this question?

Looking forward to hearing your thoughts!

1 Like

Hi Mallory,

Yes your findings are as expected. The multipath’s doppler shift should also affect the code. There is indeed a mistake in the User Manual. We will address this documentation in the next release.

Thank you for your investigation!