@wyq200704 First, if you doubt the accuracy of L1 only, then you have clearly not tried it or compared it to your Novatel.
Second, are using it on a moving drone, in the air? If so, then the only advantage L1/L2 has over L1 only is when your baseline starts to get longer than 10km.
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I am also a land surveyor in the U.S. It has been several years since I read a book detailing GNSS positioning for land surveyors (author Van Sickle). However, with respect to the use of L1 only vs. L1/L2 receivers, my memory is that using a receiver with multiple frequencies will allow for better resolution of atmospheric ambiguities. The GPS signal travels at the speed of light from the satellite (20km above surface of the earth) to your receiver. The condition of the air through which that signal travels influences its speed. In other words, the signal will travel at different speeds depending upon temperature, pressure and humidity of the air through which it passes, particularly through the layers of the atmosphere called the troposphere and the ionosphere. I believe the L1/L2 receivers will collect two streams of data, which can be post processed to resolve some of the ambiguities. In the air, with clear sky, in real time (RTK), I doubt it will make much difference whether L1 vs. L1/L2. I have used both types of receivers and have obtained similar quality results from each.