Narrative:

While being vectored onto final for runway 28 into ord; we were in solid IMC all the way. Our instruction given was to cross wavie at 5;000 ft; cleared for the ILS 28. When the approach was to be activated in the GPS unit; something gave an error and the GPS was not indicating correctly; causing a momentary loss of situational awareness. I knew we were established on the localizer; but for some reason thought we were past wavie at that point when in fact I was still about 3 miles away and had not crossed it yet. While the issue was solved in short order; during that time of getting the GPS to indicate properly; I allowed the airplane to descend down to 4;600 ft instead of maintaining 5;000 ft when I thought I was further ahead on the approach than I actually was. After getting a heads up from ATC to climb back to 5;000 ft; I initiated a climb and was back at 5;000 ft within 10 seconds. I never received any traffic alerts; TCAS advisories or any other collision warnings from the onboard equipment or ATC. Some of the factors that resulted in the deviation was losing situational awareness due to the GPS not sequencing properly; not engaging the autopilot while correcting the issue to hold altitude. For the future; I will be sure to make better use of the autopilot if something like this should happen as well as simply flying raw data in the event of a GPS malfunction on approach.

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Original NASA ASRS Text

Title: A C208 pilot reported descending early after being cleared for the ILS while attempting to correct a problem with the GPS. ATC notices the deviation and the reporter corrects back to 5;000 FT and continues with the ILS approach.

Narrative: While being vectored onto final for Runway 28 into ORD; we were in solid IMC all the way. Our instruction given was to cross WAVIE at 5;000 FT; cleared for the ILS 28. When the approach was to be activated in the GPS unit; something gave an error and the GPS was not indicating correctly; causing a momentary loss of situational awareness. I knew we were established on the localizer; but for some reason thought we were past WAVIE at that point when in fact I was still about 3 miles away and had not crossed it yet. While the issue was solved in short order; during that time of getting the GPS to indicate properly; I allowed the airplane to descend down to 4;600 FT instead of maintaining 5;000 FT when I thought I was further ahead on the approach than I actually was. After getting a heads up from ATC to climb back to 5;000 FT; I initiated a climb and was back at 5;000 FT within 10 seconds. I never received any traffic alerts; TCAS advisories or any other collision warnings from the onboard equipment or ATC. Some of the factors that resulted in the deviation was losing situational awareness due to the GPS not sequencing properly; not engaging the autopilot while correcting the issue to hold altitude. For the future; I will be sure to make better use of the autopilot if something like this should happen as well as simply flying raw data in the event of a GPS malfunction on approach.

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.