Q: Why does my mapped image have black bars across track?

A:
If you see black stripes across your mapped image, as in the shown figure, these are due to missing scans. The instruments occasionally "drop" scans, i.e fail to record the scan, due to instrument component failures or data recording rates. This is most notable in 2009/2010 data where there were issues with the frame grabber card. These stripes are essentially empty data (level 1 file has 0 values there) and are used as 'spacers' for the navigation synchronising.

When mapped with aplmap the values from the level 1 file (the 0s) are inserted into the map.