We need to change this line: https://github.com/sparkfun/SparkFun_u-blox_GNSS_v3/blob/939754f0c4f12123a47f3aeea60aa851be2eec46/src/u-blox_config_keys.h#L1022 The type should be UBX_CFG_I1 (8-bit signed), not UBX_CFG_U1 (8-bit unsigned). You would probably never notice, unless you try to set a negative elevation limit.