Hello, I'm currently building an application that can be used to process orders with mobile devices on events. The application allows self-service for guests with their own mobile device.<p>To get to the order process and to determine if the person is really at the event the application uses unique URLs and QR codes for each table.<p>The problem is that orders can still be done even if the guests are no longer on the event (even if they are logged out after a certain time), because they know the URL or have taken a picture of the QR code. Also URLs/codes can be sent to other persons.<p>I think about using GPS as additional check, because it's little harder to bypass, but risky if the event is indoor (?).<p>Are there any other possibilities to ensure that a person is at a specific location?<p>Thank you, btw. the application: http://www.orderlink.io