Abstract: An instrument housing for a drill string, comprising: a cylindrical housing having a cavity for receiving an instrument assembly such as a transmitter sonde; an elongated side load opening disposed parallel with and toward one end of the cavity and formed through a side of the cylindrical housing into the cavity. The side load opening is substantially shorter than the length of the instrument assembly; and an elongated side load door assembly is configured to fit within the side load opening, to enclose and secure the instrument assembly within the cylindrical housing such that the instrument is protected from loss or damage due to loss or damage to the side load door during operation.