Motorola M12 Oncore User's Guide download pdf (Page 44)

Chapter 3 – Receiver Description

Motorola GPS Products - Oncore User’s Guide

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

k-1

. 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 T

k-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

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 Oncore 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 about one second later than the previous measurement

epoch, where any difference from 1.000000000 seconds is the result of the receiver

local oscillator intentional offset (about +60 us/s) and the oscillator's inherent

instability (+/-30 ppm over specified temperature range).

When the Oncore processor computes receiver local time, this time corresponds to

the time of the last receiver measurement epoch. This time is precisely known by the

Oncore process to an accuracy of approximately 20 to 300 ns depending on satellite

geometry and the effects of selective availability.

Revision 5.0 08/30/02

3.44

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Motorola M12 Oncore User's Guide Page 44

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