An interesting answer from EzSurv software editor.
I was trying this GNSS calculation software with L1/L2 Trimble receiver data and I was surprised that only L1 baselines solutions were given.
For baselines less than 20km, that is because L1 only is more accurate.
The answer :
"For short vectors L1 fixed solution is the most precise (because less noisy).
So by default EZSurv calculates, if possible, a L1 fixed solution for vectors of 20km or less.
Beyond 20 km, an L3 or L4 solution (mixed L1L2) will be calculated.
In GPS L1 fixed is the best and most accurate solution for short vectors (L1 is less noisy and over short distances the ionospheric correction is negligible)
L3 fixed is the most accurate solution for longer vectors (signal more noisy than L1, but here the ionospheric correction is applied)
L4 fixed is used for long vectors, it is the least accurate fixed solution"