A smartphone, comprising: a radar system, implemented at least partially in hardware, configured to: provide a radar field; sense reflections from an object in the radar field; analyze the reflections from the object in the radar field; and provide, based on the analysis of the reflections, radar data; one or more computer processors; and one or more computer-readable media having instructions stored thereon that, responsive to execution by the one or more computer processors, implement a persistent radar-based interaction manager configured to: maintain the radar system in an idle mode, the idle mode requiring no more than approximately 30 milliwatts of power; determine, based on a first subset of the radar data, a presence of the object within an awareness zone of the smartphone; responsive to determining the presence of the object within the awareness zone, cause the radar system to enter an attention mode, the attention mode requiring no more than approximately 60 milliwatts of power; responsive to entering the attention mode, and based on a second subset of the radar data, determine an intention level associated with the object; based on the intention level, determine whether the object is a user intending to interact with the smartphone; responsive to determining that the intention level indicates the object is not a user intending to interact with the smartphone, cause the radar system to exit the attention mode and enter the idle mode; and responsive to determining that the intention level indicates the user intends to interact with the smartphone, cause the radar system to exit the attention mode and enter an interaction mode, the interaction mode requiring no more than approximately 90 milliwatts of power. a radar system, implemented at least partially in hardware, configured to: provide a radar field; sense reflections from an object in the radar field; analyze the reflections from the object in the radar field; and provide, based on the analysis of the reflections, radar data; provide a radar field; sense reflections from an object in the radar field; analyze the reflections from the object in the radar field; and provide, based on the analysis of the reflections, radar data; one or more computer processors; and one or more computer-readable media having instructions stored thereon that, responsive to execution by the one or more computer processors, implement a persistent radar-based interaction manager configured to: maintain the radar system in an idle mode, the idle mode requiring no more than approximately 30 milliwatts of power; determine, based on a first subset of the radar data, a presence of the object within an awareness zone of the smartphone; responsive to determining the presence of the object within the awareness zone, cause the radar system to enter an attention mode, the attention mode requiring no more than approximately 60 milliwatts of power; responsive to entering the attention mode, and based on a second subset of the radar data, determine an intention level associated with the object; based on the intention level, determine whether the object is a user intending to interact with the smartphone; responsive to determining that the intention level indicates the object is not a user intending to interact with the smartphone, cause the radar system to exit the attention mode and enter the idle mode; and responsive to determining that the intention level indicates the user intends to interact with the smartphone, cause the radar system to exit the attention mode and enter an interaction mode, the interaction mode requiring no more than approximately 90 milliwatts of power. maintain the radar system in an idle mode, the idle mode requiring no more than approximately 30 milliwatts of power; determine, based on a first subset of the radar data, a presence of the object within an awareness zone of the smartphone; responsive to determining the presence of the object within the awareness zone, cause the radar system to enter an attention mode, the attention mode requiring no more than approximately 60 milliwatts of power; responsive to entering the attention mode, and based on a second subset of the radar data, determine an intention level associated with the object; based on the intention level, determine whether the object is a user intending to interact with the smartphone; responsive to determining that the intention level indicates the object is not a user intending to interact with the smartphone, cause the radar system to exit the attention mode and enter the idle mode; and responsive to determining that the intention level indicates the user intends to interact with the smartphone, cause the radar system to exit the attention mode and enter an interaction mode, the interaction mode requiring no more than approximately 90 milliwatts of power.