"I want to observe a signal that is between 990MHz and 1000MHz. What is the minimum sampling frequency that I need to use?".
Those answering 2GS/s faced an uphill struggle...."
You can use any sampling frequency. What is of interest here is how fast the sample is taken, not how often that sample is repeated.
You can take samples at a rate of 1000 samples per second... 10,000 samples per second... etcetera.
If a waveform is repetitive, you can take a sample every 1000 waves and still reconstitute the original waveform correctly.
Here the important thing is not the frequency but the duration of the pulse that takes the sample and being a frequency between 900 MHz and 1 GHz, whose period ranges from 1.11 ns to 1 ns, the duration of the sampling pulse should be of the order of 0.1 ns or less.