TOF Devices

Error Sources

Manufacture Specifications

Best Practices

Time of Flight (TOF) Devices

There are two approaches used in TOF devices, the time interval counter and the event timer approach. Time interval units measure the TOF of the optical pulse, while in event timers, the TOF is computed by differencing the laser fire epoch and the satellite echo epoch.

Raw satellite TOFs must be corrected (i.e. calibrated) for optical and electronic delays, inherent in SLR systems. This calibration value is commonly referred to as system delay. For a given satellite segment or pass, these calibrated satellite TOFs are compressed into normal points, using the Herstmonceux algorithm, and transmitted expeditiously to the ILRS global data centers.

TOF devices can be a major source of random (i.e. jitter) and systematic error in SLR. As the ILRS pushes toward 1 millimeter absolute accuracy, individual error sources must be reduced to the sub-mm level. Currently, there are 6 manufacturers of TOF devices used predominately in the ILRS network, each with its own specifications, but these different devices do have common error sources. In addition, there are some generally accepted BEST practices that can enhance the performance (jitter and accuracy) of any TOF unit.

Below are general PROS and CONS of each type of device:

Time Interval Units

PROS – inexpensive (<10K), can provide sub-cm accuracy with proper care and calibration versus a standard

CONS – maximum time-of-flight, laser repetition rate limited to <100Hz, requires special test equipment or multiple counters to measure non-linearity's, may be a limiting factor in 1 mm absolute accuracy

Event Timers

PROS – supports lunar, interplanetary, and KHz ranging, picosecond event timers are NOT a limiting factor in 1mm absolute accuracy

CONS – picosecond event timers are expensive

Responsible Government Official: Carey Noll
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