Android :: Method Undefined For Type SmsReciever - Context

I am developing an app that upon receiving an SMS message with a unique string of text, the GPS is enabled and starts tracking the location of the phone. The issue I'm having is with the getSystemService() method.
I receive the error "The method getSystemService(String) is undefined for the type SmsReceiver", I believe this is because it does not have a context. I have tried to add in a context with in my code with 'ctx', and that removes the error but my application crashes every time I run it on my phone. The code for recieving the SMS works fine, and the GPS location tracking code works fine if it's in my main class.
I still don't fully understand contexts.

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