Motorola M12 Oncore Guide de l'utilisateur télécharger pdf (Page 49)

Chapter 3 - Receiver Descriptions

To compensate for the one second computational pipeline delay, a one second

propagated position is computed that corresponds to T

based on the position and velocity data

computed from measurements taken at time T

k -1

. In this way, the position data output on epoch

will most closely correspond with the receiver true position when the data is output on the

serial port. Of course, there can be a position error due to the propagation process if the receiver

is undergoing acceleration. The error can be as large as 4.5 m for every G of acceleration. There

is no significant error under stationary or constant velocity conditions.

Position Data Latency

The position data output in the current data packet (i.e., at time T

) is the result of a Least

Squares Estimation (LSE) algorithm using satellite pseudorange measurements taken at time T

. The resulting LSE position corresponding to time T

k-1

is then propagated one second forward by

the velocity vector (the result of an LSE fit using satellite pseudorange rate measurements taken

at T

k-1

). The resulting propagated position is output at the T

epoch.

Velocity Data Latency

The velocity data output in the current data packet (i.e., at time T

) is the result of an LSE fit using

satellite pseudorange rate measurements taken at time T

k-1

. The pseudorange rate

measurements are derived from the difference in integrated carrier frequency data sampled at

measurement epochs T

k-1

and (Tk

-1

-200 ms). In effect, the resulting velocity data represents the

average velocity of the receiver halfway between T

k-1

and (T

k-1

-200 ms).

Time Data Latency

The time data output in the current data packet (i.e., at time T

) is the result of an LSE fit using

satellite pseudorange measurements taken at time T

k-1

. The time estimate at T

k-1

is then

propagated by one second plus the computed receiver clock bias rate at time T

k-1,

before being

output at time T

. The resulting time data is the best estimate of local time corresponding to the T

measurement epoch based on data available at T

k-1

ONE PULSE PER SECOND (1PPS) TIMING

Measurement Epoch Timing

The M12+ receiver timing is established relative to an internal, asynchronous, 1 kHz clock

derived from the local oscillator. The receiver counts the 1 kHz clock cycles, and uses each

successive 1000 clock cycles to define the time when the measurement epoch is to take place.

The measurement epoch is the point at which the receiver captures the pseudorange and

pseudorange rate measurements for computing position, velocity, and time.

When the receiver starts, it defines the first clock cycle as the measurement epoch. Every 1000

clock cycles from that point define the next measurement epoch. Each measurement epoch is

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