Iranian missiles are using different instruments for guidance system, for example LPS instead of GPS when it comes to nearby enemies such as Israel. So no need to decode GPS at war times.
Other possibility, is a mixed use of GPS, GLONASS and Chinese Beidou.
Also CDS technology is widely used in Iranian cruise missiles.
We have a long history of producing cruise missiles from different kinds. Anti ship, ground based, etc
Saeqeh cruise missile, 500 Kg warhead, 306 m/s speed, with 300 km range
Nasr cruise missile, 150 Kg warhead, 35 km range
Advanced version of Nasr, improved range and capabilities. Zafar
Qader cruise missile, 306 m/s speed, 200 Kg warhead, 200 km range
Noor cruise missile, 175 Kg warhead, 120 km range
Ghadir cruise missile, 330 km range, 200 Kg warhead
Ya Ali cruise missile, 700 km range
Ya Ali is similar to the Russian Kalibr
And finally the Soumar cruise missile, due to its technological requirements, it was unveiled with delay.
@TheImmortal