This file defines the columns of data in the poseandtwist.log files produced
by the GRID software receiver. Each poseandtwist.log file contains
time-stamped precise relative pose solutions. ORT time stamps indicate the
time at which the position solution applies.

Offset Receiver Time (ORT): This time stamp is equal to RRT plus an
offset that brings the result close (within a few ms) to true GPS time:

        ORT = RRT + tOffset

GRID automatically adjusts tOffset every so often to bring ORT within a
few ms of true GPS time.  When tOffset is adjusted, a small jump in ORT
is introduced and concomitant shifts occur in the pseudorange and
carrier phase data.  For maximum resolution, ORT is given in separate
columns for week, whole second, and fractional seconds. An explination of
tOffset can be found in navsol.txt



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Column        Quantity

1 ----------- ORT week number. 

2 ----------- ORT whole seconds of week.

3 ----------- ORT fractional second. 

4,5,6 ------- X,Y,Z position rW of the platform reference point (e.g., primary
              GNSS antenna's L1 phase center), in the W (ENU) frame in meters
              relative to reference antenna location.

7,8,9,10 ---- Quaternion qBW relating the body and local ENU frames.  Let qBW =
              [x,y,z,w] be the quaternion composed of these four elements in
              the order given.  Then vB = RBW*vW relates the expressions of the
              physical vector v in the world (W) and body (B) frames, where RBW
              is the direction cosine matrix corresponding to qBW.

11,12,13 ---- X,Y,Z velocity vW: the time rate of change of rW, in meters per
              second.
 
14,15,16 ---- X,Y,Z angular rate wB: angular rate of B relative to W, expressed
              in B in radians per second.

17,18,19 ---- X,Y,Z accelerometer bias baU expressed in the U (IMU) frame in
              meters/second^2.

20,21,22 ---- X,Y,Z rate gyro bias bgU expressed in U in rad/second.

23-40 ------- Diagonal elements of the error covariance matrix corresponding
              to the state vector [rW;e;vW,wB,baU,bgU], where e is the 3-by-1
              error state vector corresponding to the quaternion qBW.

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