Phoebe guidesA field team faces an unresolved physical question: When is a browser location accurate enough? They must answer it before changing reported location accuracy radius on the real device. Predict the direction first.
See the relationship before changing it
The figure reads from left to right. The blue card is reported location accuracy radius. The middle card applies this page's relationship. The green card is chapter's 40 m / 10 m. Walk the arrows once: set the input, apply the rule, then read the result with its unit.
The retained audit below checks several chapter fixtures. This added model holds every other chapter fixture fixed, so the numeric fixture does not switch without explanation.
Derive the baseline in four named moves
- 1
Name the input. The chapter baseline for reported location accuracy radius is 25.
- 2
Name the relationship. task multiple=rreported/rrequired; FSPL=20log10(4πd/λ); λ=c/f; tchip=1/Rchip
- 3
Substitute the chapter fixture. Set reported location accuracy radius to 25. The page ledger gives chapter's 40 m / 10 m as 4.00 times.
- 4
Read the result. Keep times beside the value. Use it only inside the technical boundary on this page.
Predict, then change reported location accuracy radius
Try Predict the direction of chapter's 40 m / 10 m. Move one control, calculate, then check your prediction.
Observe The physical radio numbers explain why positioning is demanding, but the decision needs only the provider's uncertainty radius compared with the required spatial scale. Reset the control to 25 and compare chapter's 40 m / 10 m.
Explain Only reported location accuracy radius moves here. The other chapter fixtures remain fixed.
Check yourself
What should you do before trusting a moved-control result?
What does this small model leave out?
1. Accuracy is an uncertainty radius
coords.accuracy is the provider's 95% uncertainty estimate in metres. The browser does not promise which radios, cached values, inertial inputs, or databases produced it.
2. See why radio scale is difficult
Spread power over areaS=Pt/(4πd²).
Express the ratio as lossFSPL=(4πd/λ)², or 20log10(4πd/λ) dB.
3. Bound timing without overclaiming
Invert the chip rate1/(1.023 Mchip/s)=0.978 µs per chip.
Multiply by light speed0.978 µs × 3.00×10⁸ m/s ≈ 293 m.
Receivers estimate sub-chip timing and combine several corrected pseudoranges. One chip's travel distance is a physical scale, not a final position-accuracy guarantee.
4. Try the reported radius
TryMove the reported accuracy radius while the first chapter task still needs a 5 m distinction.
ObserveA 25 m radius is five times the first task's 5 m scale. The chapter's separate 40 m radius is four times its 10 m distinction.
ExplainThe physical radio numbers explain why positioning is demanding, but the decision needs only the provider's uncertainty radius compared with the required spatial scale.
FSPL is a free-space power model, not a location solver.
- geometry
- Needs separate evidence
- clocks
- Needs separate evidence
- atmosphere
- Needs separate evidence
- multipath
- Needs separate evidence
- blockage
- Needs separate evidence
- radio databases
- Needs separate evidence
- inertial data
- Needs separate evidence
- caches
- Needs separate evidence
- filtering
- Needs separate evidence
- permissions
- Needs separate evidence
- platform policy
- Needs separate evidence
- A 95% radius is not a hard boundary for every fix
- Needs separate evidence
Use field evidence or a deeper model before release.
5. Reproduce the chapter decisions
Both fixes are much coarser than the stated distinction. Request a better fix or defer; do not make the location decision anyway.
6. Preserve what the API actually says
Record coordinates, timestamp, radius, permission state, request settings, and whether the value was fresh enough for the task. enableHighAccuracy is a hint, not a promise of GNSS or a particular precision.
7. Check yourself
What does a 25 m accuracy value mean here?
Can a 40 m radius answer a 10 m nearest-point question?
Does 293 m per C/A chip set GPS accuracy?
These are the chapter inputs, worked results, and named teaching assumptions.
- 1575.42 MHz carrier
- Frequency, sample rate, or event rate
- about 20,200 km path
- Distance, wavelength, or size
- 182.5 dB free-space loss
- Gain, loss, margin, or level ratio
- 1.023 Mchip/s timing scale
- Time, interval, or service-life value
- 293 m chip travel
- Distance, wavelength, or size
- 25/5 plus 40/10 task ratios reproduce the chapter
- Percentage, ratio, or gain
They do not identify the browser's hidden provider or predict its position error; Under the Hood keeps those system limits.