Any Unknown or Unlisted Aircraft Manufacturer
Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor
Flight Crew Last 90 Days 20; Flight Crew Total 11000; Flight Crew Type 850
Confusion
Ground Excursion Taxiway
General Maintenance Action
Airport; Weather; Human Factors
After landing on Runway 28L we exited left onto Taxiway C for passenger pick-up; located on the 'TRANSIENT RAMP' on the airport diagram. Surface markings were partially obscured by a developing layer of frost early in the clear night. During flight planning I expected the ramp entrance to be a confusing entrance so we taxied slowly past Taxiway B and began a left turn near Taxiway C1 where the alert controller corrected us that we were to turn in at the next intersection. We turned left as instructed. Upon entering this area the taxiway lighting abruptly ended and marking and definition of the pavement edge was insufficient. We both assumed we had entered the ramp area and focused our attention on the Marshaller with lighted wands in the distance. The aircraft departed the short pavement stub/entrance to the west and all three wheels decelerated as they depressed into the soft sod. Days earlier the mostly frozen ground was subjected to rain and in some areas several inches of top soil had thawed. After shutdown and exiting the aircraft we contacted airfield operations to inform them of our position. Noteworthy items: After exiting the aircraft we noticed our right main landing gear had just missed a blue-topped plastic pavement edge marker; and another marker was in front of the aircraft. Although lighting from the hangar illuminated the ramp near the hangar; the lights had detrimental effects at our distance. The airfield diagram depicts continuous pavement between the terminal and FBO but this is not necessarily available for aircraft passage. Separate ramp entry points are necessary under current layout. Linemen and airfield maintenance personnel promptly returned our aircraft to the pavement undamaged.
Pilot reported when exiting taxiway C in the dark; the lighting ends and there are only reflective posts on the west side of the taxiway leading to the transient ramp.  Being there was ground frost; ground markings were obscured and the variance between concrete and grass was not discernable. Pilot suggested that lighting be extended to the ramp to aid navigating.
Captain reported a taxiway excursion while taxiing in the dark with limited airport lighting and ground frost conditions.