About Performance Reports
Overview of GPS Operations Center Performance Reports
The GPS Operations Center is manned 24/7 and produces both prediction models and post-assessment
models of GPS accuracy using the program GIANT (GPS Interference And Navigation Tool).
These charts only represent the Signal In Space, or SIS, errors. There are additional sources
of position error that the user will encounter that we do not predict such as multipath, ionospheric
scintillation, or obscura to name a few. Posting is accomplished daily, and archived products are
also available through the website.
Prediction Products
The GPSOC creates prediction analysis for both, PDOP (Position Dilution of Precision) and 95th
Percentile Position Error. These analyses are based on a Best-4 Receiver’s logic. All predictions
are posted 3 days in advance, or 72 hours into the future. You can use the prediction maps to see
what the GPS performance will be in your area of interest. In the PDOP predictions, the color bands
indicate the maximum PDOP experienced at each grid point over the course of the day. In the
Position Error Predictions, the color bands indicate the 95% percentile position error seen
at each grid point over the course of the day. In brief, the 95th Percentile indicates, not
the maximum position error, but the most probable range of position errors. As an example,
if the chart indicates a color band of 4-8 meters for the 95th percentile (assume 8 meters for simplicity),
there is a 95% chance that any position solutions will have 8 meters or less of error.
Post Assessment Products
The Post Assessment products are based on the observed errors of the GPS constellation by the
Master Control Station. The GPSOC produces charts for both the Best-4 satellite receiver,
and the All-In-View (generally a 12-channel receiver). The All-In-View post assessments will
usually show better performance than a Best-4 post-assessment. The color bands indicate the
maximum actual position error (in meters) experienced at each grid point over the course of the day.
How to Use Reports
All predictions are posted 3 days in advance (24-hour period, 72 hours
into the future). The GPSOC is now open 24/7, so posting is accomplished daily. Use the
prediction maps to see what the GPS performance will be in your area of interest. In
the PDOP Predictions, the color bands indicate the maximum PDOP experienced at each grid
point over the course of the day. In the
Position Error Predictions, the color bands indicate the 95th Percentile Position Error
seen at each grid point over the course of the day. In brief, the 95th Percentile
indicates, not the maximum position error, but the most probable range of position
errors. As an example, if the chart indicates a color band of 4 - 8 meters for the
95th Percentile (assume 8 meters for simplicity), there is a 95% chance that any position
fixes will have 8 meters or less of error. Barring any kind of satellite
outage/anomaly, the remaining 5% rarely exceeds 20 meters of error. If you suspect
your area may be adversely impacted, SBMCS can be used to create a prediction for your
location of interest or you may contact
the GPS Operations Center for an analysis/prediction specific to your needs.
The GPS Post-Assessment, Daily Navigation Accuracy Maps are based on
observed GPS errors from the GPS Master Control Station. These maps can be used as a
quick-look post mission assessment to determine what ACTUAL, past GPS accuracy was
like in your area of interest. The color bands indicate the maximum actual position
error (in meters) experienced at each grid point over the course of the day. You
will need to contact the GPS Operations Center to
obtain detailed information on accuracies for a specific location.