With the Sierra Sector we have our first border in the RL Sector Map series of maps. This is the Romulan border, the Romulan Neutral Zone, and shows the positions of known outposts and starbases from all time periods. Oddly enough there is a fairly empty band of space where the ST:Star Charts representation of the Romulan Neutral Zone is shown. Also shown with dotted lines is the alteration of the Neutral Zone due to the conditions of the Treaty of Algeron, as interpreted from an Okudagraphic shown in the TNG episode, “The Defector.”
The Tomed system, focus of a Vulcan expedition in the early 22nd Century (ENT episode “Kir’Shara“) and possibly the location of the Tomed Incident of 2311 which lead to the Treaty of Algeron. This location would make sense since the only known alteration to the Romulan Neutral Zone due to the Treaty stipulations was here. Since the exact circumstances of the Tomed Incident are unknown (asides from non-canonical information,) it could be safer to assume a more general confrontation testing the Federation, similar to but far more bloody then the incident that took place in the TOS episode “Balance of Terror.” The only thing we know for sure is that the Federation was banned from cloaking and the Romulans surrendered some more buffer space from their side of the zone in this area. This to me would indicate the incident occurred in this region, the Federation must have come out in the superior position from the incident because giving up cloaks was likely not as big a deal as forcing the Romulans to give ground (it could also have been possible that some form of Federation cloaking device experimentation was occurring in or near the Tomed system and the Romulans were going to war to maintain their best technological monopoly, were successful in destroying the prototype but were delivered such a bloody nose in doing so they decided to withdraw from dealing with the Federation for almost 60 years.) The Treaty also removed the relatively more lenient conditions of violating the Neutral Zone found in the TAS episode “The Survivor.” It is also very possible that Starbase 39 itself (and the Sector name change from Gamma Hydra Sector to Sierra Sector) occurred due to the change of posture by the Federation towards the Neutral Zone after the Tomed Incident.
The infamous “Gamma Hydra, Section 10” location of Kobyashi Maru fame would be located somewhere in the section of Neutral Zone just below Nelvana.
Central to this sector is Starbase 39-Sierra, located in star system LHS 293, called the Sierra system. Starbase 39-Sierra is mentioned in passing during the ST:TNG Episode “The Neutral Zone.” The STO summary for Starbase 39-Sierra states that Romulans had mined the Sierra system in the 22nd century.
Kaleb, or Caleb, system is where Kang and Kor (for some reason apparently operating from Romulan space, perhaps during their alliance days) attacked a Federation outpost (mentioned in DS9 episode “Once More Unto the Breach.”) This system I’ve located at 61 UMa and was apparently important enough that at one time it served as the alternate name for the Sector (Kaleb Sector.)
And finally (for non-STO systems) we have Tagus towards the bottom of the sector. This was the home of the Taguans and a site of civilization for a reported 2 billion years. It was featured in the TNG episode, “Qpid.”
The Federation Neutral Zone outposts from bottom to top: Quebec IV, Romeo I, II, & III, Sierra I, II, III, IV, V, and VI, and Science Station Tango Sierra. It is unclear from canon if the outposts employ a 360 degree monitoring sensor array, as the US military PAVE PAWS at RAF Flyingdale, or a more concentrated 270 degree array like most of the US military PAVE PAWS network. The spacing of the outposts suggests a minimum of 2 LY for outpost long range sensors, and I would guess the maximum effective range of 4 to 5 LY.
Terrh, Sienae, Agrama, Rhi, and Chiron are all STO locations. Agrama shares the same approximate location as a star labeled “Epsilion Legato” on an Okudagraphic shown in the TNG episode, “The Defector.” Moore’s Star, Theta Curry, Epsilion Legato, Gamma Hydra (289 G. Hyrda) and Beta Hutzel are shown in the same Okudagraphic, oriented to Outposts on the Federation side with the same numbering as shown here, Gamma Hydra IV hosted a short-lived Federation outpost which suffered from exposure to an exotic radiation which caused rapid aging in humans, as seen in the TOS episode “The Deadly Years.”
It should be noted that it is also possible that Sierra, Gamma Hydra, and Kaleb sector references in canon are not mutually exclusive in reference to all or part of this sector. There is mention of “sub sectors” and “sections” in canon, so it is entirely possible that while the overall sector is known as “Sierra Sector”, Kaleb could refer to the “Alpha” sections of the Sierra Sector (Sections 1,2,3,4 in upper left of Sector) and Gamma Hydra would refer to the “Gamma” sections of the Sector (Sections 9,10,11,12)
If there were true then the “Delta” sections of Sierra Sector (13,14,15,16) would likely carry a colloquial title of Tagus or Barradas…etc.