DRAFT

3.3 Navigation, Timing, and Ancillary Data

Standards for this functional area are:

• Timing data synchronized to the United States Naval Observatory Coordinated Universal Time (USNO-UTC)
• A one-pulse per second (PPS) time tick provided over a MIL-STD-1553B bus
• IRIG-B timing standard
• SMPTE VITC Drop Frame Recommended Practices (SMPTE RP173: 1993 and SMPTE RP179: 1994) (video time base)

The navigation, timing, and ancillary data functions, color coded blue in the airborne reconnaissance FRM (Figure 3-1), are very important common support functions that directly affect the overall quality of the finished airborne reconnaissance product. All processing and exploitation functions use navigation, timing, and ancillary data in some way when processing the sensor data. In general, navigation data is information about the position and attitude (roll, pitch, and yaw) of the collection platform, timing data is an exact reference to absolute real-time, and ancillary data is information about the sensors and payload (e.g., sensor mode, settings, commands invoked, etc.).

Functionally, navigation, timing, and ancillary data are "captured" in the front-end subsystems and disseminated to all other functions. Navigation data is captured from the platform's inertial navigation subsystem and/or GPS; timing data is captured from the GPS and an on-board atomic clock frequency reference; and ancillary data is captured from the sensors themselves and the sensor control functions. The key term "data capture" includes data recorded either periodically (e.g., record the sensor mode every 15 seconds) or upon detection of specified triggering events (e.g., record the sensor mode whenever it is changed). "Recording" refers to real-time functions (on-board the platform) which logically attach time-tag data, navigation data, and ancillary data to the collected sensor data. The critical aspect is maintaining strict correlation of sensor data with the associated navigation, timing, and ancillary data.

Timing data synchronized to the United States Naval Observatory Coordinated Universal Time (USNO-UTC) is mandated. It will include a common precision frequency reference based on a rubidium or cesium atomic clock reference standard. Accuracy, precision, and latency will be set to meet the worst-case real-time processing requirements for SIGINT, IMINT, and MASINT payloads.

From the airborne platform inertial navigation system (INS) and Global Positioning System (GPS), navigational data and a one-pulse per second (PPS) time tick are provided over a MIL-STD-1553B bus. A fiber optic version of 1553B (i.e., MIL-STD-1773) is under investigation.

For video, the SMPTE VITC Drop Frame time base will be used for synchronization and timing. For all other data sources, the IRIG-B standard will be used in the case where a general purpose time reference is provided with a sensor data stream (e.g., on the audio track of a mission tape recorder or a time reference channel on the data link). IRIG-B calls for a 1 KHz signal with timing information encoded using pulse-width modulation. The IRIG-B message uses binary coded decimal (BCD) to encode the Julian date (three digits) and time of day (six digits) as follows: DDDHHMMSS. In some cases a high-precision time-tag will also be recorded with the data. Navigation data includes the following data elements (examples):

• Altitude (AGL and MSL)
• Latitude and longitude (or UTM)
• GPS delta
• Heading
• Ground speed and acceleration
• Drift angle
• Cross track
• Roll, pitch, and yaw

Ancillary data is particularly important for IMINT missions and payloads. Example data elements include:

• Sensor identification (model, type, serial number)
• Sensor configuration (installed options, lens type & identification)
• Film type (if applicable)
• Calibration data (such as identification of dead CCD elements and lens
  aberrations)
• Camera location
• Forward and side look angles
• Camera mode
• Frame number
• Focal length, field of view, and exposure

Currently, the specific data elements, format, and details for transmission of the data are usually specified in Interface Control Documents (or equivalent system-specific specifications) that are unique for given system-to-system interfaces. An "across-the-board" standard would greatly enhance interoperability among airborne reconnaissance systems; for example, it would provide a basis for allowing alternative ground/surface systems to receive and process information from platforms other than the one(s) they were specifically designed for. A common metadata standard for airborne imagery is being developed by CIO and DARO and will be incorporated in a future version of the ARTA. The requirement for precise geopositioning (e.g., for TDOA/FDOA calculations and IFSAR/SAR PHD processing) demands highly accurate and precise timing and navigation data. Detailed standards are under development and will be adopted across the INTs in a future version of the ARTA.

DRAFT