observer.

One who makes observations, monitors or takes notice.

Most impartial observers agreed that Sampras had not served well.

One who adheres or follows laws, guidelines, etc.

I shall be an observer of the local customs.

A person sent as a representative, to a meeting or other function to monitor but not to participate.

The UN sent many observers to the country's first elections.

A country or other entity which has limited participation rights within an organization.

The Vatican and Palestine have observer status at the UN.

A crew member on an aircraft who makes observations of enemy positions or aircraft.

The only crew-member to survive the crash was the Canadian observer.

A sentry etc. manning an observation post.

We waited till dusk when the observers' vision was poorest.

Synonym of spotter.