I’ve read that that frequency is a ‘window’ in weather, impacted less by raindrops for example. I otherwise would’ve thought that simply increasing the wavelength would decrease impact. I don’t understand why 1575 MHz is a ‘window’. Also, what factors other than weather impacted the decision to use 1575 MHz?
The technical decision was to use L-Band which extends from 1-2 GHz. The specific frequencies to use within L-Band depended on political decisions including existing allocations and users of those frequencies and possible interference. Unprotected GPS signals are very sensitive to interference rather than GPS interfering with other users.
L-band is attractive for GPS for these reasons:
L-band has good penetration of foliage, rain, snow and clouds and signal propagation is less influenced by weather.
Frequencies below 1 GHz and significantly above 2 GHz are much more affected by the ionosphere (resulting in variable delays).
The GPS waveform itself requires a relatively high bandwidth (C/A code has a main lobe that is approximately 2 MHz wide), where higher carrier frequencies are more likely to support wider bandwidth allocations.
The antenna pattern in L-band can easily realize a wide pattern for whole earth coverage and reception; higher frequencies tend to be much more directional in practical realization.
In addition to the civil L1 signal at 1575.42 MHz, two additional frequency carriers are emerging as part of GPS modernization for civil use in L-band, one at 1227.60 MHz (L1C) and another at 1176.45 MHz (L5 for aviation safety services). There will also be an improved civil signal at 1575.42 MHz (L1C) that will work in conjunction with the existing civil L1 signal and military signals there. A dominant feature of the new carrier for normal civil use will be the ability to do ionospheric correction due to the differences in the receiver of the signals at each carrier.