59 Location Awareness: Technology Tradeoffs

This decision tree helps you choose the right positioning technology based on environment, accuracy requirements, and infrastructure constraints. Read Figure 59.1 from its first environment question rather than jumping to a technology name: establish indoor or outdoor operation, narrow the accuracy class, and only then accept the infrastructure and power consequences of the suggested branch.

Figure 59.1 separates cheap presence or room evidence from precise ranging. Its outdoor branch moves from ordinary GNSS to correction services when metre-level error is insufficient; its indoor branch distinguishes reused Wi-Fi or BLE infrastructure from planned UWB anchors. The mixed-environment branch points to fusion, connecting technology choice back to the complete sensing-to-action loop rather than promising one universal method.

Different technologies for different needs:

Technology	How It Works	Accuracy	Best For
GPS	Satellites in space	3-5 meters	Outdoors
Wi-Fi	Known router locations	5-15 meters	Indoors/outdoors
Bluetooth Beacons	Small transmitters	1-3 meters	Indoor rooms
UWB	Ultra-precise timing	10-30 cm	Exact position
Cell Tower	Phone network	100+ meters	Rough area

Use Figure 59.2 to compare the same methods visually. Scan from the coarsest cell-based area estimate through Wi-Fi and BLE room or proximity evidence, then to outdoor GNSS and precise UWB ranging. Notice that accuracy improves for different physical reasons, so the methods also carry different coverage, infrastructure, and power assumptions.

In Figure 59.2, the accuracy ranges are design classes, not guarantees for every building or receiver. Cell identity supports broad area context, Wi-Fi and BLE often support indoor zones, GNSS supports open-sky outdoor positioning, and UWB supports tighter local ranging when anchors are maintained. This comparison supplies the evidence needed to use the preceding decision tree responsibly.

Different technologies achieve different accuracy levels through fundamental physics constraints:

GPS (outdoor):

Satellite signal travels at speed of light: $c = 3 \times 10^8$ m/s. Timing precision: ~10 ns (commercial receivers). Distance error: $\Delta d = c \times \Delta t = 3 \times 10^8 \times 10 \times 10^{-9} = 3$ m. With 4 satellites and geometric dilution of precision (GDOP ≈ 1.5), typical accuracy:. $\sigma_{\text{pos}} = \text{GDOP} \times \sigma_{\text{range}} \approx 1.5 \times 3 = 4.5 \text{ m}$

BLE beacons (indoor):

RSSI-based ranging: $\text{RSSI} = -10n \log_{10}(d) + A$ where $n \approx 2$ (path loss exponent), $A =$ signal at 1m. Multipath fading adds ±4 dB variance. Distance uncertainty: $\Delta d = d \times \ln(10) \times \frac{\Delta \text{RSSI}}{10n} \approx 0.46d$ (for 4 dB error). At 5 m distance, error ≈ 2.3 m; trilateration with 3 beacons yields ~3-5 m accuracy.

UWB (ultra-wideband):

500 MHz+ bandwidth enables <1 ns time resolution. Two-way time-of-flight: $d = \frac{c \times t_{\text{ToF}}}{2}$. With 0.5 ns precision: $\Delta d = \frac{3 \times 10^8 \times 0.5 \times 10^{-9}}{2} = 0.075$ m = 7.5 cm. Real-world accuracy 10-30 cm (multipath + clock drift).

Trade-off: GPS uses narrow bandwidth (20 MHz) → limited timing precision. UWB’s wide bandwidth = precise timing but limited range (~50 m vs GPS global).

Before deciding how Location shapes positioning accuracy tradeoffs, inspect Figure 59.4 beside (radius = anchor’s time-of-flight distance). Together, Location and (radius = anchor’s time-of-flight distance) frame the positioning accuracy tradeoffs claim: trilateration is the geometry behind the gps, ble, and uwb formulas above: each reference point a, b, or c measures a time-of-flight distance to the device, drawn as a circle of that radius, and the device’s location is the single point that lies on all three circles at once.

Read Location alongside (radius = anchor’s time-of-flight distance) in Figure 59.4; their named relationship makes trilateration is the geometry behind the gps, ble, and uwb formulas above: each reference point a, b, or c measures a time-of-flight distance to the device, drawn as a circle of that radius, and the device’s location is the single point that lies on all three circles at once concrete. For positioning accuracy tradeoffs, Location supplies visible evidence; (radius = anchor’s time-of-flight distance) constrains the decision. In Figure 59.4, retain Location beside (radius = anchor’s time-of-flight distance) so positioning accuracy tradeoffs remains explicit.