Global Navigation Satellite System signals have been used for years to study high-frequency fluctuations (f>0.1Hz)intheionosphere. The customary procedure uses the geometry-free (GF) combination ofL1andL2carriers, for which it is necessaryto acquire theL2GPS signal. Initially,L2had to be acquired from a codeless signal, L2P(Y), using several techniques, someof them requiring the aid ofL1. New GPS satellites transmit the newC2civil code, which can be used to acquire directlyL2,i.e. L2C. Several publications have reported differences in the GF combination when it is computed from L2P(Y) or L2C.Using two ionospheric scintillation monitoring receivers (ISMRs), these differences were shown to be related to how theyacquireL2, i.e. if the receiver acquiresL2with theL1aid. However, ISMRs are scarce, so the extension of such a study is notstraightforward. The present work uses the geodetic detrending technique to identify whether a conventional geodetic-gradereceiver acquiresL2with the aid ofL1. The study employs six different receiver types with measurements stored in RINEXformats version 2 and 3. In both formats, we are able to identify ifL2signal is acquired withL1aid. In this way, we showthat some receiver types heavily underestimate high-frequency ionospheric fluctuations when using the GF combination.Our results show that the ionosphere-free combination of these carrier phases is not free from high-frequency ionosphericfluctuations, but in some receivers, almost 90% of the high-frequency effects inL1remain in such combination.