Basically the cell phone bands can interfere with the GPS bands. GPS frequency is a narrow band at about 1575MHz. Some cell bands are very close by, other utilize frequencies in which harmonics can occur in the GPS frequency. There are critical specifications for the amount of signal level which can be produced in the GPS band by any cell phone, cell site, or other system. All products must be certified for this emission requirement.
As the cell band now require higher data rates (smart phones streaming video, accessing the internet, etc.) new modulation schemes are being utilized to make efficient use of the allotted cell frequencies. These new modulations generate a considerable amount of noise, each user adding to the noise level, some of it bleeding into the GPS band. Now approaches a noise level where a GPS user might have trouble acquiring the GPS signal. This is the concern.
A`ohe `ulu e loa`a i ka pokole o ka lou.
No breadfruit can be reached when the picking stick is too short.