What must an alternate airport be served by according to the applicable rules?

The alternate airport must be served by an approach based on other than GPS navigation to ensure that there are multiple options for safe landing in the event that the primary airport is not accessible. This requirement helps mitigate the risks associated with reliance on a single type of navigation system, which could fail or become unreliable.

Having an approach based on other than GPS provides redundancy and ensures that pilots have alternative means to land the aircraft safely. This is particularly important in conditions of poor visibility or when GPS signals may be compromised. Therefore, the regulations emphasize this necessity to enhance operational safety and reliability during flight planning.

Other options do not adequately meet the standard requirements for alternate airports. For instance, GPS navigation alone may not be as reliable in all scenarios, and visual flight rules (VFR) are not suitable for all weather conditions, so relying solely on those could pose significant risks. Similarly, suggesting that any available navigational aid is sufficient does not guarantee the necessary level of safety and reliability for alternate airport procedures.