Timing and timestamping
Timestamping of received data is relatively basic and not yet suitable for high-precision applications. This page describes the basics as currently implemented.
There is significant room to improve on ORCA here. USRP devices are capable of multiple means of synchronization between devices that could be used to provide better timestamping.
Relative timing of samples
Transmit and receive events are scheduled as
timed commands.
Generally, bursts of samples are recorded at intervals specified by pulse_rep_int.
When an error occurs, extra time is added to allow the system to catch up. The
amount of time added is configured by the error_recovery_time parameter. When
time is added, a message is printed to stdout with the chirp number and the
amount of time added, such as the following:
[1691434273.020] [TX] (Chirp 975541) time_offset increased by 0.0016
Note that due to the enqueueing of commands, multiple errors may occur before the
next set of commands is enqueued with a time offset. If more than one error occurs,
the time will be offset by number of errors * error_recovery_time.
These output messages are read during the Zarr file creation process in order to provide a corrected slow time record.
Older versions
Older versions of ORCA did not print out the time offset messages and had hard-coded error recovery time. When this situation is detected (by the combination of a non-zero number of logged errors but no time offset messages), the time offset is estimated based on the starting and ending system clock, rounded to the nearest multiple ofpulse_rep_int.Timestamping samples
ORCA currently assumes that the SDR clock is useful to relative timing only and
relies on the host computer clock to provide a starting timestamp. When using
the run.py interface,
stdout output is logged with a Unix timestamp from the host computer. The
timestamp on the starting message (plus any specified time_offset in the config
file) is used to provide the absolute time of the first transmission.
This relies upon the system clock being set accurately and should never be assumed to provide sub-second accuracy.
Accessing timestamps in Xarray
The default coordiantes of the radar DataArray are sample_idx and pulse_idx.
These correspond to the fast time and slow time indices of the as-recorded data.
Considering pulse_idx this means, for example, that if num_presums is 10,
then there are 10 chirps transmitted and received for each index of pulse_idx.
Similarly, any errors are ignored in counting pulse_idx.
The additional data variables chirp_sequence_idx, slow_time, and timestamp
may be used if other information is needed. They are as follows:
chirp_sequence_idxrefers to the index as tracked by the SDR of the first chirp included in the associated data recording. With no errors,chirp_sequence_idx = num_presums * pulse_idx.slow_timerefers to the relative time (in seconds) of the start of the first chirp included in the assocaited data recording.timestampis annp.datetime64series equal toslow_time + start timestamp