The key's the key!
In order for your GPS to receive the secure signal, the receiver must be crypto-keyed. This is the job of your unit's COMSEC custodian. Your custodian will load the crypto variable (CV) key to your DAGR or PLGR to let it receive a secure signal.
Your receiver will tell you if it has the key and if it's the current one. Watch the screen when you turn on your receiver and if you don't have the current key, it will read "No CV Key for Today." If you don't have any key, it will read, "Warning No CV Keys Loaded." If either of these warnings show up, get to your COMSEC custodian before you use your GPS.
First, it's mandatory! Use of un-keyed GPS devices is only authorized for training and for research and development programs. DoD policy says combat and combat support operations must use crypto-keyed GPS receivers. If you want to read the official policy, go to the PM GPS website: https://gps.army.mil
Once there, go to "About GPS" and click on "PPS Policy."
Second, secure GPS devices have a selective availability anti-spoofing module (SAASM). Not only does SAASM limit spoofing, but it also provides greater position accuracy and situational awareness--two things that could save your life in combat.
Finally, most signal interference is not intentional. Crypto-keyed receivers can resist this unintentional jamming at 10 times the level of interference that un-keyed receivers can resist.
One last word: Put those commercial oft-the-shelf GPS devices back on the shelf. They may have a fancy doodad or two that the PLGR or DAGR doesn't have, but they are not secure! Also, they use the standard Positioning Service Signal, which is less accurate and more prone to interference than the military-only Precise Positioning Service (PPS) signal.