The GPS Operations Center (GPSOC) uses an assessment tool called
GIANT to produce the many types of GPS performance reports
posted on this website. GIANT uses data obtained directly from the GPS Master Control
Station (MCS). The MCS data includes Precise Positioning
Service (PPS) Estimated Range Deviations (ERDs), Observed Range Deviations (ORDs)
and the satellite
Almanacs
.
GIANT has been certified for use within the GPSOC by USSTRATCOM (formerly USSPACECOM).
The GPSOC routinely generates a number of performance reports including Dilution of
Precision (DOP) reports, Navigation Solution (NAVSOL) reports, and User Range Error (URE)
reports. These reports are produced on both a global basis and for specific Areas of
Operation (AOOs) as defined by USSTRATCOM/J3.
The AOOs in our reports are currently defined as follows:
Area of Operations (AOO) |
Geographic Location |
Continental United States (CONUS) |
21.0 Degrees North |
235.0 Degrees East |
53.0 Degrees North |
295.0 Degrees East |
Navigation Performance Reports
Historical navigation performance reports are generated using GIANT.
GIANT accepts ERD data from the MCS, satellite almanac data,
and Notice Advisory to NAVSTAR Users (NANU) satellite outage
information, issued by the 2nd Space Operations Squadron (2SOPS). GIANT
produces a wide array of performance statistics including the 50%, Root Mean Squared (RMS) and 95% navigation solution performance statistics for
Horizontal, Vertical, and Position (3-D) Error. The 50% Horizontal Error is the Circular
Error Probable (CEP) metric and the 50% Position Error is the
Spherical Error Probable (SEP) metric. The statistics can be
computed and graphically displayed either for a specific site or for an entire area. All
error measurements are in meters. See the
Data
Descriptions page for more detail.
Report Generation
Each performance report is generated with GIANT using the following criteria:
Parameter |
Description |
Default |
Mask Angle |
The angle above the receiver's local horizon below which
satellites are not used to perform the calculations. |
5.0 degrees |
PDOP
Threshold |
GIANT reports additional statistics for
positions within a region that exceed this threshold value. |
6.0
PDOP values under 6 are considered good, above 6 are bad.
Reference the SPS Signal Spec |
Satellite Selection |
The GIANT satellite selection algorithm can use the Best-4
SVs or All-In-View criteria. All-In-View solutions tend to be more precise than
Best-4 solutions, however, Best-4 solutions are more common in military receivers. A
Best-4 solution is generated by selecting the 4 currently visible satellites that will
minimize the Dilution of Precision for a given site. An All-In-View solution uses all
available satellites in view to minimize the Dilution of Precision for a given site.
All satellites used, regardless of algorithm, must be above the receiver's mask angle. |
Best-4
(All-In-View is available upon request) |
Time Increment |
For each grid point in the regional calculation, the
performance data is calculated at specific time steps defined by this parameter. For
example, if the time increment is 60 seconds, for each grid point, there will be 86400/60
= 1440 calculations made. Each standard performance run is done for an entire 24
hour period (24 hours = 86400 seconds) |
60 seconds |
Degree Increment |
The grid spacing used for the regional or global
calculation. For example, using the CONUS AOO above, and a grid spacing of 0.5
degree, performance data will be calculated from 21.0 N to 53.0 north latitude at one
degree steps. The number of longitude increments for a given latitude decreases as
the latitude increases. For example, at the equator with a 1 degree latitude increment,
360 longitude steps are made, but at 80 degrees latitude, only 62 longitude steps are
taken. |
Global: 2.0 degrees
AOOs: Optimized for each Region |
PRN Removed |
Based on active NANUs, specific satellites are removed
from the calculations. Satellites are specified by PRN and the time the NANU
specifies for the outage. For navigation accuracy predictions, the
NANU forecast outage times are used. For post-processing navigation
accuracy, the NANU summary outage times are used. |
Varies by Day, but will always reflect
the current state of NANUs when the data runs are done. |
Dilution of Precision (DOP) Reports
DOP assessments provide valuable information regarding the effect each satellite's
position in the sky has on the accuracy of a GPS solution. All DOP reports are
generated using GIANT. GIANT accepts
satellite almanac data (SEM format) and satellite outage information (NANU), both of which
are provided by the 2nd Space Operations Squadron (2SOPS). GIANT
produces a wide array of performance statistics including the 50%, RMS and 95% value for
each of the following DOP metrics: Horizontal DOP (HDOP), Vertical DOP (VDOP) and Position
DOP (PDOP). DOP values are unitless. See the
Data
Descriptions page for more detail.
Report Generation
Each DOP report is generated with GIANT using the following criteria:
Parameter |
Description |
Default |
Mask Angle |
Any satellite below the mask angle is not counted in the analysis.
Daily products are set at 5º, due to the fact that satellites below 5º would have a signal too weak
to provide a solution to the receiver. |
5.0 degrees |
PDOP
Threshold |
GIANT reports additional statistics for
positions within a region that exceed this threshold value. |
6.0
PDOP values under 6 are considered good, above 6 are bad.
Reference the SPS Signal Spec |
Satellite Selection |
: Satellites are selected either with All-in-view or Best 4 solutions
for the various analyses. Prediction analyses use Best 4 solutions while Post Assessment analyses use
Best 4 and All-in-view solutions. Best 4 analyses are calculated using the four satellites with the
best geometry, ideally one straight overhead and 120º of separation for the other three. This provides
the greatest angle of satellite separation from the perspective of the receiver. |
Best-4
(All-In-View is available upon request) |
Time Increment |
Analysis is normally performed using a 24 hour time period starting at
midnight Zulu, and calculated with 60 second increments, providing 1440 time increments in a day. |
60 seconds |
Degree Increment |
This can range from a single point on the map as in a visibility chart
or the entire World. The regions are calculated in various increments depending on the degree of
precision required for the region.. The world is calculated in 2º increments
(providing a grid of 16,380 points) while CONUS is calculated in 0.5º increments,
(providing a grid of 7865 points)
In summary when GIANT calculates GPS constellation accuracy,it’s calculating the 95% percentile of
the (grid) x (time) = 23,587,200 data points for Global accuracy and 11,325,600 data points for
CONUS accuracy. |
Global: 2.0 degrees
AOOs: Optimized for each Region |
PRN Removed |
Based on active NANUs, specific satellites are removed
from the calculations. Satellites are specified by PRN and the time the NANU
specifies for the outage. For DOP calculations with a future date, the NANU forecast
outage times are used. For DOP calculations in the past, NANU summary outage times
are used. |
Varies by Day, but will always reflect
the current state of NANUs when the data runs are done. |