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25 Jun 2022
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TECHNICAL NOTES

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 GIANTGIANT 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 GIANTGIANT 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.