I-Lotus M12M Quick Start Manual - page 9
01 May 2008
M12M
™ Quick Start Guide (403-TTN-001)
Page 9
5
Time RAIM Algorithm (M12M Timing Receiver Only)
Time Receiver Autonomous Integrity Monitoring (T-RAIM) is an algorithm in Oncore timing
receivers (including the M12M T) that uses redundant satellite measurements to confirm the
integrity of the timing solution. The T-RAIM approach is borrowed from the aviation
community where integrity monitoring is safety critical.
In most surveying systems and instruments, there are more measurements taken than are
required to compute the solution. The excess measurements are redundant. A system can
use redundant measurements in an averaging scheme to compute a blended solution that is
more robust and accurate than using only the minimum number of measurements required.
Once a solution is computed, the measurements can be inspected for blunders. This is the
essence of T-RAIM.
In order to perform precise timing, the GPS receiver position is determined and then the
receiver is put into Position-Hold mode where the receiver no longer solves for position. With
the position known, time is the only remaining unknown. When in this mode, the GPS
receiver only requires one satellite to accurately determine time. If multiple satellites are
tracked, then the time solution is based on an average of the satellite measurements. When
the average solution is computed, it is compared to each individual satellite measurement to
screen for blunders. A residual is computed for each satellite by differencing the solution
average and the measurement. If there is a bad measurement in the set, then the average
will be skewed and one of the measurements will have a large residual. If the magnitude of
the residuals exceeds the expected limit, then an alarm condition exists and the individual
residuals are checked. The magnitude of each residual is compared with the size of the
expected measurement error. If the residual does not fall within a defined confidence level of
the measurement accuracy, then it is flagged as a blunder. Once a blunder is identified, then
it is removed from the solution and the solution is recomputed and checked again for
integrity.
A simple analogy can be used to demonstrate the concept of blunder detection and removal:
a table is measured eight times using a tape measure. The measurements are recorded in a
notebook, but one of the measurements is recorded incorrectly. The tape measure has 2
mm divisions, so the one-sigma (1
) reading error is about 1 mm. This implies that 95% of
the measurements should be within 2 mm of truth. The measurements and residuals are
recorded in the table on the following page. From the residual list, it is clear that trial six was
a blunder. With the blunder removed, the average and residuals are recomputed. This time,
the residuals fall within the expected measurement accuracy. This is shown in Table 3.4
below.