UAV - Unpiloted Aerial Vehicle
Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Flight Crew Last 90 Days 22; Flight Crew Total 3000; Flight Crew Type 1500
Human-Machine Interface; Situational Awareness
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance
Aircraft; Human Factors; Procedure
I put the aircraft on a pre-programmed mission with altitude set to FL270 and airspeed set to 83 KIAS.  The altitude was not properly propagated to one of the points on the mission and I missed that.  Turning to point 3; the aircraft started descending from FL270 and went as low as FL254.  Center called as I was doing a manual take over and climbed back to the assigned altitude fixed the mission and continued over the area of interest until a crew swap out 2 hours later.The location was approximately 20 miles north of Sioux Falls; SD.
Reporter mentioned that there was an error in the program. FL200 was entered when it should have been FL270.
UAV Operator reported misprograming the UAV causing it to descend unintentionally.