Delay compensation issue

Between April 14-th 2014 (LOFAR Release 2.2) and September 13-th 2016 (LOFAR Release 2.16.7) there was an issue with the LOFAR array, specifically with the antenna cable delay compensation not being properly set which has potentially affected some observations.

 

The issue was caused by RCUs keeping the previous cable delay settings (and ignoring new ones), if the RCUs were still powered on after the end of a previous observation. This happens whenever there is a gap smaller than about 3 minutes between observations.

 

The issue occurs if and only if RCUs switch modes while the intra-observation time gap is not larger than 3 minutes. It also occurs if the switch is from HBA to any LBA mode, or between any two LBA modes. If a chain of observations all have less than 3 minute gaps, the cable delay setting used for the entire sequence is that of the first observation. Within this scenario, if the splitter in the core stations switches state (a change between HBA and HBA_DUAL mode) the RCUs belonging to ring 1 will be reset and receive proper delays at the time of the ring splitter switching state, meaning that for HBA1 the issue ceases to exist, but is still present for HBA0.

 

The issue manifests itself as a multi-MHz-scale ripple over the pass-band of the affected stations causing a "ripple effect" in sensitivity with an amplitude of up to 50% of the peak gain (as shown in the figure below); some SBs having higher sensitivity than others, with lower frequencies being more affected than higher ones. Unfortunately, there is no way to recover the loss of sensitivity and distorted beam shapes in affected data. A list of project / observations which may be affected by this issue can be downloaded here.

An example of bad amplitude solutions (reflecting the station sensitivity). with a clear modulation in frequency, here indicated as SB number.

Figure 1: An example of bad amplitude solutions (reflecting the station sensitivity). with a clear modulation in frequency, here indicated as SB number.

Design: Kuenst.    Development: Dripl.    © 2020 ASTRON