GNSS in Drone and Aerial Survey Contexts (Part 107 and Photogrammetry GCPs)

GNSS in Drone and Aerial Survey Contexts

Global Navigation Satellite System (GNSS) technology is the backbone of modern drone-based mapping. Whether you are flying a photogrammetry mission to produce an orthomosaic or generating a surface model for a construction site, accurate GNSS positioning determines whether your final deliverable is usable. This lesson ties together three threads: how GNSS supports UAS operations, how ground control points (GCPs) are measured and used in photogrammetric workflows, and what FAA Part 107 requires UAS pilots to understand about altitude and airspace.

How GNSS Supports UAS Operations

Most commercial mapping drones carry an onboard GNSS receiver that logs the camera's position at the moment each photo is taken. These recorded positions are called geotagged image coordinates. By themselves, consumer-grade onboard GNSS positions are accurate to several meters — useful for orientation but not for survey-grade deliverables.

To achieve centimeter-level accuracy, surveyors add two techniques: Real-Time Kinematic (RTK) correction applied live during the flight, or Post-Processed Kinematic (PPK) correction applied after the flight using a base station log. In both cases, a known base station placed over a point of established position broadcasts or records corrections that dramatically tighten the rover (drone) position. Even with RTK/PPK, professional workflows include independently measured GCPs to validate and, when necessary, improve the georeferencing of the point cloud or orthomosaic.

Ground Control Points: Purpose and Measurement

A ground control point (GCP) is a clearly identifiable target on the ground whose horizontal and vertical coordinates have been measured with survey-grade accuracy. GCPs serve as anchors: photogrammetry software uses them to warp and scale the reconstructed model so it matches real-world coordinates.

GCP measurement procedure:

1. Set up a GNSS base station over a benchmark or known control point. Record occupation for the required duration to achieve the desired accuracy class.

2. Use an RTK rover to occupy each GCP target. Collect enough epochs (typically several seconds of averaged positions) per the project's accuracy standard.

3. Record coordinates in the project's required datum and coordinate reference system — commonly NAD 83 combined with a State Plane or UTM zone for horizontal, and NAVD 88 for vertical.

4. Use a geoid model (such as GEOID18) to convert ellipsoidal heights from the GNSS receiver to orthometric (elevation above mean sea level) heights referenced to NAVD 88. This step is critical and is frequently overlooked by beginners.

Coordinate formatting for photogrammetry software is typically decimal degrees (DD) for geographic coordinates or easting/northing in meters for projected systems, paired with an orthometric elevation in meters or feet. Always confirm what the software expects before importing.

GCP Layout Best Practices

Photogrammetry guidelines (drawn from established software documentation and academic research) consistently recommend:

• Minimum of five GCPs for a small rectangular mapping area, though more are better.
• Distribute GCPs around the perimeter of the project area, with at least one near each corner, and place one or more in the interior to control vertical doming — a systematic error where the center of a model curves upward or downward.
• Avoid clustering all GCPs along a single line or in the center only.
• Add check points (independent points not used in the adjustment) to validate accuracy after processing; a ratio of roughly one check point for every three to five GCPs is common practice.
• Target size should be proportional to the ground sampling distance (GSD) of the imagery — targets should be clearly visible and large enough to be unambiguously identified in multiple overlapping images.

FAA Part 107 and Altitude Referencing

FAA Part 107 governs commercial small UAS (unmanned aircraft systems) operations in the United States. Two altitude-related rules are directly relevant to GNSS-equipped operators:

1. Maximum altitude: Under Part 107, remote pilots must not fly above 400 feet above ground level (AGL) in uncontrolled airspace without a waiver, or above the altitude authorized by an airspace authorization in controlled airspace.

2. AGL vs. MSL awareness: Part 107 altitude limits are referenced to AGL, not to mean sea level (MSL). A drone's onboard GNSS receiver reports ellipsoidal height, which must be converted using a geoid model to get orthometric height (MSL). To find AGL, you subtract ground elevation from MSL elevation. Pilots operating over terrain with significant relief — hills, canyons, or tall structures — must track how terrain changes beneath the aircraft and ensure they remain compliant.

Airspace authorization: Class G airspace (uncontrolled) allows operations up to 400 ft AGL without prior authorization. Operations in Class B, C, D, or E airspace require authorization through the FAA's Low Altitude Authorization and Notification Capability (LAANC) system or a manual waiver. GNSS-based autopilot systems can enforce altitude ceilings programmatically, but the pilot in command remains legally responsible for staying within authorized limits.

Safety Considerations

• Pre-flight area survey: Before placing GCP targets, walk the site for hazards — overhead power lines, unstable terrain, traffic. Wear high-visibility PPE when working near roadways.
• GNSS base station stability: A base station that moves during occupation will corrupt all rover measurements. Use a tripod with a tribrach, level carefully, and never leave a base station unsecured.
• Airspace hazard awareness: GNSS outages (from multipath or jamming) can cause autopilot failures. Always maintain visual line of sight (VLOS) and be prepared to take manual control.
• Data backup: Always store raw GNSS observation files in at least two locations before processing. Corrupted data cannot be recreated once the equipment is demobilized.