02 / WHY IT WORKS
Keep settings consistent
Location, constellation selection and elevation mask determine which directions enter the geometry model. Compare times with those inputs held constant. If you change the mask, recompute both count and DOP rather than combining values from different configurations.
Why sampling matters
A 15-minute series only tells you about its sampled times. Threshold crossings occur somewhere between them. The highlighted bands join qualifying samples; they do not resolve the exact start and end of a usable interval. A finer production calculation can refine a candidate, within the tool’s limits.
03 / UNDER THE SURFACE
From intuition to a model
A transparent selection rule
For each sample, retain satellites at or above the fixed mask and compute the same unweighted ENU geometry as in the DOP lesson. A sample qualifies only when DOP is defined, at least four directions remain, and PDOP is at or below the chosen threshold. Four directions alone do not guarantee full rank.
N: number of retained directions; τ: chosen PDOP threshold; ℝ: finite real numbers. The threshold is a comparison setting, not a service standard.
A reproducible historical case
This lesson stores predictions for Beijing, 39.9075° N, 116.3913° E, height 50 m, from 00:00 UTC on 4 September 2024 to 00:00 UTC the next day, at 15-minute spacing. The mask is 15° and systems are GPS, BeiDou, GLONASS, Galileo, QZSS and NavIC as represented in the bundled data. The fixture records the TLE hash and epoch range so its provenance can be checked. It is loaded locally, not fetched from a live prediction endpoint.
04 / A WORKED EXAMPLE
Compare a strict threshold with a relaxed one
Start with PDOP ≤ 0.9. Drag the time slider and note count, PDOP and whether each sample qualifies. Lower the threshold to 0.8 and compare how highlighted windows change. If none qualify, that is a valid result—not a reason to replace missing values with zeros. Return to 0.9, inspect an interval boundary and remember its timing is only resolved to 15-minute samples.
Return to the experiment and try it ↑05 / PUT IT TO WORK
Try it in GNSS View
- Open the observatory and set your own position and current date.
- Choose systems, mask and time range, then wait for a successful calculation.
- Use count, DOP and skyplot together; inspect element age and local obstructions before making a field decision.
06 / COMMON QUESTIONS
Is the best DOP time always the best observation time?
Not necessarily. Task duration, obstructions, receiver conditions and data quality also matter.
Why does the sample use UTC?
UTC makes the saved timeline unambiguous across countries and browsers. Convert deliberately for field scheduling.
Will opening the tool copy the historical date?
No. The tool uses its current observation state. Set your own date and location explicitly.
07 / FURTHER READING
An original GNSS View explanation. These references document the models, terminology and system context.
ESA Navipedia · Positioning Error ↗CelesTrak · General Perturbations data ↗