John Deere OMPC20964 User Manual

Page 25

background image

GS2 Display—StarFire iTC



GSA - GPS DOP and active satellites. This sentence
provides details on the nature of the satellite constellation
fix. It includes the numbers of the satellites being used in
the current solution and the DOP. DOP (dilution of
precision) is an indication of the effect of satellite
geometry on the accuracy of the fix. It is a unitless
number where smaller is better. For 3D fixes using 4
satellites a 1.0 would be considered to be a perfect
number, however for overdetermined solutions it is
possible to see numbers below 1.0.

There are differences in the way the PRN’s are presented
which can effect the ability of some programs to display
this data. For example, in the example shown below there
are 5 satellites in the solution and the null fields are
scattered indicating that the almanac would show
satellites in the null positions that are not being used as
part of this solution. Other receivers might output all of the
satellites used at the beginning of the sentence with the
null field all stacked up at the end. This difference
accounts for some satellite display programs not always
being able to display the satellites being tracked. Some
units may show all satellites that have ephemeris data
without regard to their use as part of the solution but this
is non-standard.

GSA String Example




Satellite status


Auto selection of 2D or 3D fix (M = manual)


3D fix - values include::
1 = no fix
2 = 2D fix
3 = 3D fix


PRNs of satellites used for fix (space for 12)


PDOP (dilution of precision)


Horizontal dilution of precision (HDOP)


Vertical dilution of precision (VDOP)


the checksum data, always begins with *




Continued on next page