Narrative:

On departure roll we got a nav error; 'unable required nav perf.' I first saw this message as we approached V1 and the takeoff was continued. Weather was VMC to our cruise altitude of 5;000 ft. From our vantage point we could see down the channel and we decided that we could remain VFR. Remaining on the rnp departure/rnp arrival would have taken us into IMC conditions and terrain; so we decided that it was better to go off our track and remain in VMC. Weather at destination was reported at takeoff as 5;000 ft overcast and 10 miles visibility. With the rnp unavailable; we selected the localizer approach for situational awareness. I programmed the FMC; tuned and identified the lda-C approach. As we approached the lda we were still VMC; but noticed a big rainsquall north of the airport. While maneuvering to land; we entered rain and visibility went to zero. Now what do we do! At that point we were about 1;200 ft MSL and beyond the published missed point for the lda. Captain elected to go around. Within a few seconds we were out of the rain and back into VMC. We climbed to 5;000 ft; which is the missed approach altitude on the RNAV approach. In order to get out of the terrain I built the missed approach to predicate upon the missed approach RNAV. At this point we still did not have the rnp for the missed approach; but we were running out of options. Well actually we did not have any other options. We reported our missed with center. [At the missed approach holding point]; we rebuilt the lda approach and were able to get in on the second attempt. Did we make an error? Yes; we deviated from our route on an IFR flight plan. But I am not sure that proceeding into terrain and IMC conditions with unreliable nav is a good idea. The assumption that we would be able to maintain VMC was just that; an assumption! In the process of re-living this event in my head; it would have been better to return; remain in VMC to the north or go to [the VOR] to hold and regroup and reassess the situation. The first cause of the event was the degraded GPS signal.

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Original NASA ASRS Text

Title: B737 First Officer reports a 'Unable Required Nav Perf' FMC message just prior to V1 and the takeoff is continued. The crew elects to deviate from the cleared route to remain VMC and avoid high terrain. The visual approach at the nearby destination turns IMC and a missed approach is executed to the published hold at the VOR. Two attempts are required to land using the LDA approach.

Narrative: On departure roll we got a Nav error; 'Unable Required Nav Perf.' I first saw this message as we approached V1 and the takeoff was continued. Weather was VMC to our cruise altitude of 5;000 FT. From our vantage point we could see down the channel and we decided that we could remain VFR. Remaining on the RNP departure/RNP arrival would have taken us into IMC conditions and terrain; so we decided that it was better to go off our track and remain in VMC. Weather at destination was reported at takeoff as 5;000 FT overcast and 10 miles visibility. With the RNP unavailable; we selected the localizer approach for situational awareness. I programmed the FMC; tuned and identified the LDA-C approach. As we approached the LDA we were still VMC; but noticed a big rainsquall north of the airport. While maneuvering to land; we entered rain and visibility went to ZERO. NOW what do we do! At that point we were about 1;200 FT MSL and beyond the published missed point for the LDA. Captain elected to go around. Within a few seconds we were out of the rain and back into VMC. We climbed to 5;000 FT; which is the missed approach altitude on the RNAV approach. In order to get out of the terrain I built the missed approach to predicate upon the missed approach RNAV. At this point we still did not have the RNP for the missed approach; but we were running out of options. Well actually we did not have any other options. We reported our missed with Center. [At the missed approach holding point]; we rebuilt the LDA approach and were able to get in on the second attempt. Did we make an error? Yes; we deviated from our route on an IFR flight plan. BUT I am not sure that proceeding into terrain and IMC conditions with unreliable Nav is a good idea. The assumption that we would be able to maintain VMC was just that; an assumption! In the process of re-living this event in my head; it would have been better to return; remain in VMC to the north or go to [the VOR] to hold and regroup and reassess the situation. The first cause of the event was the degraded GPS signal.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.