There is, practically, no model in which GPS subscriptions make sense without enabling bidirectional communications with the satellite. GPS modules (ground side) do not need transmitters, only receivers and, potentially, a decryption module (soft or hardware).

A subscription GPS system with bidirectional communication (to authenticate users) would have grossly inefficient devices (transmitting to space is not cheap, plus the incurred latency on the communication). But that's the only way to enforce a subscription.

Subscription GPS systems with unidirectional communications (down only) would be cracked in about 5 minutes. Every authorized device would have to share the same key for it to be practical, which means even with key rotation schemes (sucks to be in the wilderness without internet connectivity for an extended period in this case) the keys would be on every device and trivially recovered.

The only way a unidirectional communication scheme works is if the GPS devices are already internet connected on the ground side, which largely defeats the purpose of GPS in many situations (at least historically, before widespread cellular data networks). You certainly couldn't use this effectively at sea or when hiking through the backcountry.

Little known fact, if you do have the GPS decryption module, you get way better sensitivity and spoofing protection. But you have to be the US military[1].

1: https://militaryembedded.com/comms/encryption/securing-milit...

Maybe, or maybe not (most likely not, in fact). Have you read the rest of my comment?