Part 6: Field Testing

Learning Objectives

• Conduct outdoor GPS measurements
• Apply practical tips for best results
• Record and analyze waypoint data
• Evaluate GPS accuracy

🧪 Jump to Experiments

Equipment Checklist

• Arduino + GPS unit (assembled and tested)
• Laptop with Arduino IDE
• USB cable (long enough or power bank)
• Notebook for recording data
• Phone GPS for comparison (optional)

🛠️ GPS Options for This Lab

You can complete this lab using any device that provides GPS coordinates. Choose the option that works best for you:

Experiment 1: Device Validation

Before collecting detailed data, you must first verify that your GPS unit is working correctly. This initial test confirms your device can acquire satellites and provide accurate position data.

Step-by-Step Validation

1. Take your assembled GPS unit outside - Find a location with a clear view of the sky (avoid buildings, overhangs)
2. Power on and wait for satellite fix - Watch the Serial Monitor for satellite count. Wait until you see 4 or more satellites connected
3. Record the coordinates - Write down the Latitude and Longitude displayed (e.g., 48.584330, 7.750720)
4. Open Google Maps on your phone or laptop
5. Enter the coordinates in the search bar - Type: latitude, longitude (e.g., "48.584330, 7.750720")
6. Switch to Satellite View - Click on "Satellite" or "Layers" to see aerial imagery
7. Verify your position - Does the pin match where you are standing?

Validation Checklist

Practical Tips for GPS Users

These tips will help you get the best results from your GPS unit:

Cell Phone Interference

• Cell phones emit microwaves that can interfere with GPS signals
• Solution: Put your phone in airplane mode if it is near the GPS unit
• Keep phones at least 1 meter away from the GPS receiver

Body Positioning

• Your body can block GPS signals!
• Solution: Hold the GPS unit away from your body
• Position the antenna facing upward toward the sky
• Note: Trees and foliage usually do NOT block signals significantly

Best practice: Hold the receiver away from your body for the clearest signal.

Data Backup

• Do NOT rely solely on the device's storage
• Solution: Take a photo of the screen or write down the coordinates
• Consider logging data to a file for later analysis

Recommended Settings

Confirming a Good Fix

• Look for four black bars (or similar indicator) on the satellite page
• This confirms you have a solid fix
• Wait until you have at least 4 satellites before recording data

1. Satellite Status

Shows satellite signal strength and your current accuracy.

2. Map View

Visualizes your position and tracks on a background map.

3. Navigation

Uses a compass ring to guide you toward a waypoint.

Experiment 2: Distance Measurement

In this experiment, you will validate your GPS accuracy by comparing GPS-calculated distance to a known distance measured by pacing.

Procedure

1. Find an open field with a clear view of the sky (soccer field, park, open courtyard)
2. Take Position A measurement - Stand at one end and record your GPS coordinates:
   • Latitude A: _____________
   • Longitude A: _____________
3. Pace out the distance - Walk in a straight line, using approximately 1-meter strides
4. Count your steps - Walk at least 50 steps (50 meters), but 100 steps is best for more accurate results
5. Take Position B measurement - At your endpoint, record GPS coordinates:
   • Latitude B: _____________
   • Longitude B: _____________
6. Record your step count: _____________ steps (approximately _____ meters)

Data Recording Form

Step Count: _______ steps × 1 meter = _______ meters (paced distance)

Calculate GPS Distance

Use an online calculator to find the distance between your two GPS points:

1. Go to Sounny's Lat/Long Distance Calculator (recommended) or Movable Type Calculator
2. Enter Point A coordinates (Latitude, Longitude)
3. Enter Point B coordinates (Latitude, Longitude)
4. Read the distance in meters

GPS-Calculated Distance: _______ meters

Analysis

Experiment 3: Calculate Earth's Circumference

In this classic experiment, you will recreate the method used by Eratosthenes over 2,000 years ago to calculate Earth's circumference - but using modern GPS technology!

The Mathematical Principle

If you travel a certain distance along Earth's surface, you also travel through a certain angle of Earth's curvature. The ratio of your distance to that angle equals the ratio of Earth's full circumference to 360 degrees:

Procedure

Using your data from Experiment 2:

1. Calculate the angular difference between your two GPS points
2. Use the formula to calculate Earth's circumference
3. Compare to the actual value (approximately 40,075 km)

Step 1: Calculate Angular Difference

The simplest approach is to calculate the latitude difference (if you walked roughly north-south):

For more accurate results (if you walked at an angle), use Sounny's Lat/Long Distance Calculator or the Haversine formula calculator which accounts for both latitude and longitude changes.

Step 2: Calculate Circumference

Apply the formula:

Data Recording

Compare to Actual Value

Data Collection Form

Visit 3-5 waypoints around the ISU campus and record GPS data:

Testing Accuracy

Method 1: Compare to Known Point

1. Go to a surveyed benchmark or known location
2. Record your GPS coordinates
3. Calculate the difference from the known coordinates

Method 2: Repeat Measurements

1. Stand at one location for 5 minutes
2. Record coordinates every 30 seconds
3. Calculate the spread (standard deviation)

Method 3: Compare to Phone GPS

1. Open Google Maps or GPS app on your phone
2. Compare readings with your Arduino GPS
3. Note any differences

Environmental Effects

Test GPS performance in different environments:

Data Analysis Questions

After collecting field data, answer these questions:

1. How many satellites did you typically see? How did this vary by location?
2. What was your typical HDOP value? How did this affect accuracy?
3. How close were your readings to known locations or your phone GPS?
4. Did the environment affect your results? (buildings, trees, etc.)
5. How long did it take to get a first fix in different locations?

Workshop Assessment

Each group should complete:

1. Assembled GPS unit - Working hardware with successful satellite fix
2. Field data - Completed data collection form from 3-5 waypoints
3. Analysis summary - Answers to the analysis questions above
4. Accuracy report - Comparison of readings to reference points

Grading Criteria

Useful Resources

• Mapping Your Data: Google My Maps
• GPS Visualization: GPS Visualizer
• u-blox NEO-6M Datasheet: u-blox.com
• TinyGPS++ Library: GitHub / TinyGPSPlus