Image Frequencies
From The RadioReference Wiki
Images
An image is a false signal received on a frequency that does not actually exist due to poor receiver design.
For a detailed description of how and why this occurs see the article at Radio-Electronics.com linked at the bottom of this page.
Many people can receive control channels, repeater outputs, etc. in the upper end of the 700 MHz band around 790-800 Mhz. These are images. Scanners manufactured by GRE are particulary susceptible to 700 MHz images.
700 MHz
The 700 MHz band is split into two 30 MHz wide sections. Mobiles use a +30 MHz offset.
764-776 - Base (repeater outputs, mobile talk-around) 776-806 - Mobile (repeater inputs)
The Narrowband Public Safety allocations are:
769.003125-774.996875 - Base 799.003125-805.996875 - Mobile
If you are hearing control channel data or repeater outputs in the 776-806 MHz range they are images.