Outer Marker

When doing an ILS approach, with the glideslope, is it a requirement to be
able to identify the outer marker or a substitute? FAR 91.175(k) lists the
outer marker as one of the "basic ground components" and gives the acceptable
substitutes, but doesn't explicitly say that it's required. The AIM 1-1-9(j)
on "Inoperative Components" mentions the localizer and glideslope, but says
nothing about the outer marker.

If you answer, please give a specific FAA reference that says whether or not
the OM or a substitute is required.

Thanks.

Barry