Why so many different Galileo signals ?

In the end, a good engineering compromise has been found, but since those compromises may not satisfy all type of users (an indoor, static user may like long codes, while outdoor, fast-moving users would prefer short codes), the solution has been to provide alternative codes with different characteristics on the various Galileo signals. This is one of the reasons why there are so many signals.
 
In GPS only one signal is available, which does not allow the kind of optimisation performed for Galileo. This will be overcome with the GPS modernisation programme, where more GPS signals will be made available.
 
Another reason for having so many signals in Galileo is to allow the receiver to estimate the ionospheric delay error. This error is due to the delay that the navigation signals suffer when they travel through the ionosphere. This delay makes the distance from the satellite to the user, as measured by the receiver, appear longer than it actually is and if not corrected would lead to large positioning errors. Fortunately, this delay is proportional to the frequency of the signal, with lower frequency signals experiencing a longer delay than higher frequency signals. Therefore, by combining measurements to the same satellite at two different frequencies it is possible to produce another measurement where the ionospheric delay error has been cancelled out. This cancellation becomes more effective as the separation between the two frequencies increases. This is the reason why Galileo services are generally realised using pairs of signals.

Galileo signal frequencies

The open services are realized by using the signals at L1, E5a and E5b, whether data or pilot. Several combinations are also possible, such as a dual frequency service based on using L1 and E5a (for best ionospheric error cancellation) or single frequency services (at L1, E5a, E5b or E5a and E5b together) in which case the ionospheric error is removed using a model, and even triple frequency services using all the signal together (L1, E5a and E5b), which can be exploited for very precise, centimetric applications.
 
The safety-of-life services are based on the measurements obtained from the open signal and use the integrity data carried in special messages designated for this purposed within the open signals. The safety-of-life service is like a data channel within the open signals.
 
The commercial service is realized with two additional signals in the 1278.75 MHz band plus also the capability to include commercial data within the open signals.
 
The Public Regulated Service is realized by two signals, one in the 1575.42 MHz band and the other in the 1278.75 MHz band. These signals are encrypted, allowing the implementation of an access control scheme.
 
Finally, the distinctive shape of the spectrum of the signals is due to the special modulation adopted for Galileo. This modulation has been chosen to avoid interference with other satellite navigation systems within the same band, which is indeed the case of GPS at L1. The modulation adopted is called BOC(1,1), meaning Binary Offset Carrier of rate (1,1). This kind of modulation allows GPS and Galileo signals to occupy the same frequency while avoiding mutual interference. This makes building receivers that use both GPS and Galileo simpler, because GPS and Galileo use the same frequency.