interference measurement

I am trying to measure interference on GPS L5 (1176.42 Mhz). With RBW 300 KHZ and VBW 30 Khz I got interfering signal level -60 dbm whereas with RBW 300 Khz and VBW 3 KHZ, I get interfering signal level at - 40 dbm. can anyone explain what is actual signal level.

Recommended settings for VBW is 1/10 of RBW.
RBW is the right paramter to judge the signal level than VBW.

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