150 ft above ground level is the height which is below the radar horizon and hence, the best height to fly for penetration
This may result in a radar gap at some higher height. In this case, the radar gap is at 500 ft AGL (above ground level). This is the better height to penetrate than 150 ft AGL:::Under the radar.
Flights by aircraft as low as possible to avoid being picked up by hostile radar. Normally, this would mean flying below the radar horizon. However, anomalous propagation can provide gaps in radar coverage. These gaps, if known, are the best areas for penetration into hostile territory.