FOR EXPERT USE ONLY. The GRACE Level 1B data provide all necessary inputs to derive monthly time variations in the Earth's gravity field. Level 1B data are also used for GRACE orbit and mean gravity field determination. It contains K-Band Ranging Data Product (KBR1B), Star Camera Data Product (SCA1B), Accelerometer Data Product (ACC1B), GPS Data Product (GPS1B), Vector Products (VGN1B, VGO1B, VGB1B, VCM1B, VKB1B, VSL1B), Quaternion Products (QSA1B, QSB1B), and Housekeeping Products (AHK1B, IHK1B, THR1B, TNK1B, MAG1B, MAS1B, TIM1B)The GRACE Level-1B RL03 data consists only of updated spacecraft attitude (SCA1B) and K-band inter-satellite ranging (KBR1B) data. All other Level-1B were not changed and it is recommended to use the RL02 products with the updated RL03 KBR1B and SCA1B products. The RL03 SCA1B data were corrected for a stellar aberration error in the onboard star tracker software and incorrect data weighting in the star tracker combination software. For the RL03 SCA1B data a new software module was developed that uses Kalman filtering, field of view error modeling, relative alignment adjustment and the inclusion of angular spacecraft body acceleration measurements from the ACC instrument. This new processing resulted in a significant reduction in high frequency noise and the elimination of jumps during transitions between dual and single star tracker operation. The KBR1B product is updated as well because the KBR antenna phase center range correction, range rate correction and range acceleration are computed using the spacecraft attitude information (SCA1B). Only these three correction values were updated in the KBR1B product. All other entries in the KBR1B remained the same.