M e r i d i a n I I U s e r M a n u a l
131
"Smarter Timing Solutions"
Chapter
Eleven
Real-Time Ionospheric Corrections (RTIC) Option
This chapter describes the RTIC Option, its operation, and the time and frequency performance ben-
efits to Meridian II.
About RTIC
The Real-Time Ionospheric Corrections (RTIC) software option for the Meridian II Precision Time-
Base optimizes time and frequency stability and accuracy. The RTIC option uses proprietary algo-
rithms within EndRun’s L1 GPS timing receiver to directly measure and remove ionospheric delays
in real-time. This unprecedented real-time capability was previously only available with expensive
dual frequency L1/L2 GPS receivers.
Ionospheric Delay and Impact To GPS Time-Transfer
The largest contributor to GPS time-transfer error is the variable delay of the satellite signals as they
pass through the ionosphere, a layer of ionized particles a few hundred kilometers above the Earth’s
surface. The ionization is caused by solar radiation phenomena, and is maximum a little after local
noon and minimum a little after local midnight. The GPS signal delay through the ionosphere is
proportional to the ionization level, expressed as Total Electron Content (TEC). Data transmitted
from the satellites contain a model (Klobuchar) that receivers may use to partially compensate for this
delay. This model, however, provides only a coarse compensation for night-to-day variations in the
ionosphere. It is unable to compensate for the much shorter-term variations in the ionospheric delay
caused by various types of solar “storms”, and it cannot keep up with the day-to-day variations. As
such it was never intended to achieve more than about a 50% improvement over not compensating
for the ionospheric delay at all.
Our Proprietary Solution
The GPS code modulation and carrier phase delays are affected differently as they propagate through
the ionosphere. By recognizing a few other aspects of this behavior, EndRun developed the RTIC
algorithm to directly quantify the delay through the ionosphere, and resolve the code phase and car-
rier phase bias. The bias information enables real-time measured delay compensation in EndRun’s
proprietary, single-frequency, L1 GPS receiver.
To our knowledge, this capability is unique to EndRun Technologies
.
The algorithm is effective when the receiver is operating at a stationary location with continuous
tracking of at least one satellite at all times. After a 24-hour initialization period it begins producing
real-time ionospheric delay measurements, with full accuracy of those measurements achieved after
several days. When ionospheric conditions are quiet or relatively normal, this option improves the
accuracy and stability of the timing outputs by as much as a factor of three at observation intervals
between 3,000 and 100,000 seconds. During a major ionospheric storm event, the improvement
may be much greater. It is very difficult to achieve this stability over these observation intervals by
Summary of Contents for Meridian II
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