Atec HP-Symmetricom-58503B - Manual (Page 2)

2
FIG.2 SmartClock operation
SMARTCLOCK TECHNOLOGY
Integrated with the quartz oscillator,
SmartClock greatly enhances the performance
of the 58503B GPS time and frequency
reference receiver under both locked and
unlocked conditions. In the locked condition,
it has all the desirable short-term stability
attributes of a state-of-the-art quartz
oscillator, but it is drift free. In the event the
GPS reference is lost, the combination of the
quartz oscillator and SmartClock delivers
performance which approaches the
performance of rubidium oscillators.
Figure 2 gives typical data illustrating the
operation of SmartClock. All data is taken
with the unit locked to GPS. During the first
three days, SmartClock uses the GPS reference
to “learn” the aging behavior of the quartz
oscillator. This information, along with the
temperature compensation that is taught to
SmartClock when the unit is tested at the
factory, is used to steer the oscillator. The
light, shaded line shows the actual digital
steering commands sent to the oscillator to
keep it synchronized with GPS time.
The heavy, solid line shows a plot of the steering
commands computed using the SmartClock
algorithm starting with Day 4. The dominant
effects during the predicting period are changes
due to external temperature. If the GPS signal
had not been present, the oscillator would
have been steered in holdover by SmartClock
using the corrections that it had determined
from the learning period.
Holdover performance can be computed from
the difference between the actual steered
performance and the predicted performance.
The data for the first three days in holdover
are shown in Figure 2.
ENHANCED GPS
Enhanced GPS includes digital filtering
designed to remove most of the effects of
SA (Selective Availability) on the timing
performance of the 58503B.
Enhanced GPS is based on extensive study of
the spectrum of SA, a method superior to
simple averaging techniques. Enhanced GPS
couples SA data with clock instability models
to derive optimum digital filters which minimize
the effects of SA. The benefits of Enhanced
GPS apply only to stationary 58503B GPS
time and frequency reference receivers.
When a Symmetricom GPS receiver is initially
turned on and locked to the GPS satellite
system, it will achieve GPS lock within 30
minutes of operation. It has a 95% probability
of meeting unlocked (holdover) specifications
after 48 hours of GPS operation, followed by
24 hours of learning. The longer the GPS
receiver (and its quartz oscillator) operates,
the better its stability and unlocked
(holdover) performance becomes.
SATSTAT PROGRAM
The Symmetricom 58503B comes with a
Microsoft
® Windows 3.1 program called
“SatStat” which displays important internal
parameters. SatStat operates on any PC
which can run Windows 3.1 programs and
which has a serial interface available.
SatStat provides several useful functions. It
continuously polls the RS-232 interface and
displays receiver information most likely to
be of interest. This includes satellites being
tracked along with their elevation and
azimuth, receiver state (locked, holdover,
etc.), antenna coordinates, time and frequency
figures of merit and other data. In addition, a
clock window is provided to display time of
day in real time. Finally, SatStat allows you to
easily change many receiver parameters,
such as antenna delay, by simply picking the
item from a pop-up menu and entering a new
value. With SatStat and a PC, you can monitor
and control many aspects of the receiver status
without developing software.
714600
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714300
714200
714100
714000
713900
713800
0 1 2 3 4 5 6 7
3 DAY LEARNING PERIOD
EF
C
COMMAND
TIME (DAYS)
ACTUAL STEERING COMMAND PREDICTED STEERING COMMAND
START
OF
DAY
4
5
6
ACCUM.
TIME
ERROR
1.74∞S
4.03
7.50
AVG.
FREQ.
2.04e-11
2.63e-11
4.07e-11