APOLLO Electronics System
The APOLLO system is in essence a precision stopwatch using individual photons to trigger the measurement interval. The APD is the photon-sensitive device, sending START signals to the time-to-digital converter (TDC). The TDC measures intervals less than 100 ns, to 15 ps precision. The STOP signals fed to the TDC are derived from a high-stability GPS-disciplined quartz clock, whose (multiplied) 50 MHz signal allows measurement of the 2.5 second round-trip to the moon to be made to about 7 ps precision (one millimeter of one-way travel distance).
The APOLLO Command Module (ACM) performs the digital logic functions associated with:
The TrueTime XL-DC GPS-disciplined high-stability oscillator provides our primary time and frequency reference. Combining the superior short-term stability of quartz with the exceptional long-term stability of the global atomic clock ensemble represented by GPS, we can measure the time-of-flight to the moon and back with confidence in both the short-term (2.5 second) and long-term (months) sense.
Millimeter LLR in itself only requires microsecond absolute time accuracy (relative to UTC)--this is about how long it takes the earth to rotate one millimeter. The GPS clock is typically within 40 ns of GPS, and still within 110 ns under selective availability. More critical is the accuracy of its 10 MHz output, which we deem is good to about 5 ps RMS over 2.5 second intervals.
Because of lightning problems at Apache Point, we will use a fiber link between the antenna and the XL-DC clock.
The main computer, houston, acts as mission control:
The fast photodiode (labled PIN in diagram) detects the outgoing laser pulse as it leaves the laser enclosure, alerting the ACM that the process has started. In addition, a START pulse is sent to the TDC when the photodiode is triggered, so that the laser fire time can be determined to a precision better than 20 ps.
Various small motors actuate key optical elements in such a way as to allow dynamic focus, collimation, and alignment control. The New Focus 8732 multi-axis controller provides the (GPIB) interface to the optical motors.
A CCD camera running in video mode catches light rejected from the narrow-band filter at the entrance aperture of the receiver. In this way, the instrument operator can see where the telescope is pointing, and perform laser beam diagnostics such as transmitter/receiver co-alignment, outgoing beam collimation, etc.
APOLLO Apparatus Page
APOLLO Main Page