"Sinai Field Mission," at 9 tonight on PBS finds Frederick Wiseman still preoccupied with garrison life among fellow Americans stationed in unglamorous foreign climes. At a shade over two hours, this new documentary impression of institutional Middle Americana in exile avoids the limbo of tedium attained by the three-hour "Canal Zone." However, "Sinai Field Mission" scarcely qualifies as electrifying stuff. Despite curious and fascinating observationa here and there, the film's content seems as dry as the setting.

As part of the Sinai II Agreements signed by Israel and Egypt in 1975, a surveillance base was constructed in the buffer zone of the Sinai Desert over-looking the central highlands in general and the Mitla and Giddi Passes in particular. The United States consented to build and man three watch stations. American personnel monitor electronic sensors placed along roads and guarantee that neighboring Israeli and Egyptian surveillance stations maintain specified force levels and keep informed about each other's movements.

About two dozen American government employes supervise the operation of the Dinai Field Mission, of SFM. The remainder of the work force - about 140 people - consists of technicians and clerical staff hired by a Texas engineering company called E Systems contracted by the government to build and maintain the facilities, a collection of prefabricated boxes that suggests a lunar outpost.

The purpose of the mission is defined by an American official seen briefing a group of Israeli newcomers to see that neither side uses the location as a military staging point. He compares the job to a referee's and adds that making "quick, hard calls" when disputes arise will ultimately establish credibility as mediators. There's never any reason to doubt his perception or tha later assertion by another official that the mission has been a success, creating a good precedent for similar American peacekeeping outposts in former battle zones of the Middle East.

As far as one can tll the SFM performs a necessary diplomatic function. On the other hand, it seems unlikely that the work lends itself to exciting motion picture documentation. There may be a background of volatility about this setting, as there was about "Canal Zone," where one could imagine all those homesick Americans becoming hostages to a political failure but the routine and banality in the foreground tend to smother hints of conflict.

Faced with a cycle of sensor checks, vehicle checks orientation sessions and placating discussions (an Israeli officer who seems a bit turned on by the camera protests the failure of a U.N. escort to arrive promptly), one overreacts to apparent changes of pace and fleeting ironies. Anything to break the coming and going monotoney. For example, It seems a great relief when Wiseman inserts a scene of cooking preparations in the base kitchen. Shifting to the lounge, he observes the employes amusing themselves by reading, drinking, playing pool and darts, dozing off. It's the lighter side of boredom.

In a similar respect the deferential role of the Americans and the British military style of Ghanaran troops assigned to escort duty on behalf of the U.N. give off odd post-imperial vibrations. Instead of overt American of British power there are these subdued reminders of its heritage. While no one at the Sinai Field Mission suggests lawrence of Arabia who's complaining?

When Wiseman introduces life at the base with the following public address transmission - "Your attention please: Mrs. Roberts, please call two-zero-five" - he appears to be recalling the theatrical Mr. Roberts, and the sedentary nature of the duty aboard U.S. Navy Cargo ship AK 601. unfortunately, he doesn't turn up such a colorful crew.

The tone of working and social life at SFM ultimately recalls spaceship duty as envisioned by Kubrick in "2001." Again, it's tempting to jump to concinsions when any random connection can be made from fundamentally tedious material. Perhaps Wiseman has seen the technological future and also fount it boring. Perhaps Wiseman has seen the technological future and also found it boring. Perhaps Wiseman has seen the technologica future and also found it boring. Perhaps he's just stumbled upon the boredom inherent in most custodial jobs. After "Sinai Field Mission" one may feel a renewed respect for the traditional mission of the movies to relieve the boredom of work-a-day life.