UX Design · Study deck
GPS Accuracy: Enhancement Decisions
A basic receiver cannot meet the required accuracy on its own.
UX Uma is your guide for this deck.
After studying this chapter
Learning objectives
You will be able to:
- Explain: Hands-on exercises to understand GPS accuracy and enhancement:: Before collecting phone measurements, work through a signed-noise thought experiment that separates satellite geometry, receiver clock bias, and per-satellite measurement error.
- Explain: Figure: Stylized time-series reconstruction of GPS position error on shows a stylized reconstruction, so use it to understand the step change rather than to infer a particular receiver trace.
- Explain: We need BLUETOOTH BEACONS instead!" They put tiny beacons in each room, and now they could tell Whiskers was in the kitchen (probably near the food bowl).
- explain differential GPS corrections
Major section
Differential GPS (DGPS)
The useful question is which errors are sufficiently correlated across that baseline to be cancelled.
- Satellite clock, orbit, and broad atmospheric effects can be shared; rover-local multipath and receiver noise are not magically removed.
- The system therefore improves the error budget while still requiring correction age, link state, and local quality to remain visible.
Major section
Differential GPS (DGPS) (continued)
SA turned off in May 2000.
- The historical change in Figure: Stylized time-series reconstruction of GPS position error on separates intentional degradation from the natural errors that remain.
- That residual is why differential and later carrier-phase techniques still have a role.
- Figure: Stylized time-series reconstruction of GPS position error on shows a stylized reconstruction, so use it to understand the step change rather than to infer a particular receiver trace.
Major section
Advanced GPS Techniques
The three enhancements solve different positioning errors, so accuracy alone is not enough to select one.
- This comparison keeps speed, precision, and correction mechanism distinct.
- Problem: Downloading ephemeris (satellite orbital data) takes ~20s per satellite.
- Benefit: Faster Time To First Fix (TTFF), works indoors better.
Major section
RTK GPS in Precision Agriculture
RTK is the chapter's clearest example of accuracy as a business and safety requirement rather than a luxury specification.
- Understanding when augmentation (DGPS, RTK) is needed is critical for IoT system design.
- Problem: Modern farms use autonomous tractors for planting and harvesting.
- Crop rows are spaced 30-76 cm (12-30 inches) apart.
Major section
RTK GPS in Precision Agriculture (continued)
RTK GPS costs 10-50× more than standard GPS.
- Straight-line guidance: Maintain row centerline within ±2 cm over 1 km runs.
- The crop-row requirement now needs an end-to-end correction path, not just an RTK accuracy number.
- Problem: Driving under trees, bridges, or in narrow canyons causes "cycle slips".
Major section
RTK GPS in Precision Agriculture (continued)
That dependency chain explains why loss of satellite lock, base survey error, or a blocked correction link must trigger the fallback behavior discussed next.
- Base station: 5-15 watts continuous power (requires AC or large solar panel).
- Base station must be surveyed to ±1 cm accuracy (professional surveyor).
- Wrong: Track hospital wheelchairs indoors (GPS doesn't work indoors; use BLE beacons instead).
Major section
GPS Tech Cost Decisions
Key insight:: In this model, the delivery fleet would waste money on unnecessary RTK precision.
- The tractor deployment would carry large operational risk if standard GPS were used for row guidance.
- No → Use standard GPS ($0-200/unit).
- No → Use DGPS or SBAS ($0-500/unit for receiver).
Major section
GPS Tech Cost Decisions (continued)
Yes → Use RTK GPS ($8,000-15,000/unit + $10,000 base shared across all units).
- No → Use PPP ($2,000-5,000/unit + $50-200/month subscription).
- Yes → Must use RTK or real-time PPP (requires radio or cell connection).
- No → Can use post-processed PPP (acceptable for surveying, not for autonomous vehicles).
Major section
GPS Limits in Safety Navigation
Vine rows were spaced 2.5 meters apart.
- Assumed "GPS-guided" was sufficient without checking precision spec.
- Lesson:: Safety-critical navigation requires GPS PLUS other sensors, never GPS alone.
- No backup sensors:: GPS is only navigation input (no vision, lidar, lane detection).
- Clearance 5-20× GPS error → Need DGPS or sensor fusion (5m GPS + vision).
Major section
For Kids: Meet the Sensor Squad!
GPS accuracy is like giving directions -- sometimes "near the school" is good enough, but sometimes you need "third desk from the left!".
- Regular GPS said "Main Street, near the park." That was close enough!
- "We need RTK GPS!" said the battery.
- The rows were perfectly straight!
Major section
For Kids: Meet the Sensor Squad! (continued)
The pizza was only a few minutes away. "5-10 meter accuracy is PLENTY for tracking delivery trucks," Sammy announced.
- Challenge 2: Guide the Robot Farmer "The robot tractor needs to plant seeds in rows that are only 50 centimeters apart!" said the microcontroller.
- "GPS doesn't work inside buildings!" explained Sammy. "The satellite signals are too weak to go through walls.
- We need BLUETOOTH BEACONS instead!" They put tiny beacons in each room, and now they could tell Whiskers was in the kitchen (probably near the food bowl).
Major section
Try It Yourself
Hands-on exercises to understand GPS accuracy and enhancement:: Before collecting phone measurements, work through a signed-noise thought experiment that separates satellite geometry, receiver clock bias, and per-satellite measurement error.
- The field exercise should still record the actual sky view, fix status, reported accuracy, and repeat scatter.
- Indoors, you may get NO fix or GDOP > 10.
- This demonstrates how DGPS removes common-mode errors.
- Crop rows are 50cm apart (standard corn/soybean).
Major section
Try It Yourself (continued)
After differential correction, scatter reduces to 1-3m.
- Tractor width is 3 meters, so it must stay within 25cm of row centerline.
- Standard GPS accuracy is 5-10 meters (100× too large!).
- Tractor must not exceed 25cm deviation.
- DGPS (0.5-1m) would still damage crops.
Deck summary
Key takeaways
The useful question is which errors are sufficiently correlated across that baseline to be cancelled.
- SA turned off in May 2000.
- The three enhancements solve different positioning errors, so accuracy alone is not enough to select one.
- RTK is the chapter's clearest example of accuracy as a business and safety requirement rather than a luxury specification.
- RTK GPS costs 10-50× more than standard GPS.
Retrieval practice
Recall check 1 of 4
UX Uma says: answer from memory, then check your reasoning.
Q1What is the typical horizontal accuracy of standard civilian GPS under open-sky conditions?
Show answer
Answer: C Standard civilian GPS typically achieves 5-10 meter horizontal accuracy under good conditions (open sky, no multipath).
Retrieval practice
Recall check 2 of 4
UX Uma says: answer from memory, then check your reasoning.
Q2What is the main advantage of Differential GPS (DGPS) over standard GPS?
Show answer
Answer: B DGPS uses a base station at a known location to calculate correction factors for errors that are common to the local area (ionospheric delay, satellite clock errors).
Retrieval practice
Recall check 3 of 4
UX Uma says: answer from memory, then check your reasoning.
Q3An autonomous tractor needs to plant crop rows that are 50cm apart. Which positioning technology is appropriate?
Show answer
Answer: A To navigate 50cm rows without damaging crops, the tractor needs accuracy well under 50cm—ideally ±2cm to stay centered.
Retrieval practice
Recall check 4 of 4
UX Uma says: answer from memory, then check your reasoning.
Q4Place each GPS accuracy responsibility where it lives so you can trace a product promise from measured error sources through the combined budget and geometry to an honest enhancement choice.
Show answer
Answer: A GPS accuracy is an evidence chain: bound the error sources, combine them with geometry, then make a product promise and enhancement choice that the field results can support.
Q5Complete the real-time sensor update handler:
Show answer
Answer: A getElementById() finds elements by ID.
Print reference
Answers 1 of 2
Answer key.
- C · Standard civilian GPS typically achieves 5-10 meter horizontal accuracy under good conditions (open sky, no multipath).
- B · DGPS uses a base station at a known location to calculate correction factors for errors that are common to the local area (ionospheric delay, satellite clock errors).
- A · To navigate 50cm rows without damaging crops, the tractor needs accuracy well under 50cm—ideally ±2cm to stay centered.
Print reference
Answers 2 of 2
Answer key.
- A · GPS accuracy is an evidence chain: bound the error sources, combine them with geometry, then make a product promise and enhancement choice that the field results can support.
- A · getElementById() finds elements by ID.