I am not familiar with SAASM, but I have read a little bit about the older scheme PPS-SM and can answer that.
The SV (space vehicle - aka the satellite) transmits an encrypted
P(Y) code. The encryption key, called Cryptovariable Daily Key (
CVd) changes every day.
If the receiver wants to acquire the signal, it needs to produce a copy of the
P(Y) code and therefore it needs the
So how does the
CVd key is obtained? There are two ways:
By loading the receiver a weekly key
CVW. This key can generate daily keys for a specific week.
The daily key is actually transmitted by the SV in the navigation message. But it is encrypted by a yearly key called
GUV. This key needs to be loaded into the receiver every year.
Both keys are stored inside the
PPS-SM module, which is supposed to be secure and prevent tampering or leakage. It is enough for the military to state that the receiver is unclassified.
Back to your question:
The major reason for this scheme is to prevent spoofing.
- My guess is that obtaining a PPS-SM module and extracting the keys require tremendous effort.
- If someone does obtain any of the keys, it is an achievement that is limited by time (day, week or year).
- Keys can be changed and re-distributed before major conflicts or once spoofing effort is detected.
- The obviously weak point of this scheme is the insane logistics it requires. An attacker would try to insert itself into the distribution chain to obtain continuous updates of key changes.