Mandatory Requirement on Subordinate Level
The system shall monitor bus location with sufficient frequency to meet FPS requirements.
Comment - Some verbs are transinfinitive (want, require, need), which means they support both an object and an infinitive – they are at a higher level than the infinitive. Using "shall" on them captures the infinitive which they control.
Verbs such as "meet", which are not transinfinitive, do not capture the infinitive – they form the subject of the infinitive.
Monitor… to meet…
Here, the form is "meet" by means of "monitor", or "monitor" causes "meet".
A preferable form, where the "shall" directly controls the high level verb is
The system shall meet requirements by means of monitoring bus location with sufficient frequency.
This form indicates the circularity – a requirement requires that the system meet requirements.