GNSS Field Protocol: Base Station and Checkpoints

Read this before the Fall 2026 Lake Wheeler field flight.

Note

Distance education students: read this protocol, then watch How to Place Ground Control Points (GCPs) for Precise Drone Mapping for the GCP placement and survey in sections 3 and 5.

Equipment List

• Emlid Reach RS4 rover on a survey pole
• Emlid Reach RS4 base with a fixed-height tripod and tribrach
• Tablet running Emlid Flow, paired to both receivers, with the NCRTN login entered
• Four GCP targets
• DJI Mavic Air 2 with two charged batteries and the flight plan loaded
• Field sheets
• LoRa radio link between base and rover, as the fallback if cell coverage drops

Warning

Bring closed-toe shoes, clothes for an hour outdoors, water, and a phone for point photos. There is no bathroom or water at the site.

Base coordinate methods

The base station sits on a practice mark. We will establish the coordinate of that mark three ways and compare them:

• Method A: the base receiver’s own averaged position, with no corrections.
• Method B: a real-time network (RTK) fix on the mark, collected with the rover using corrections from the North Carolina Real Time Network (NCRTN).
• Method C: the base receiver’s raw log, post-processed after the trip against NCRTN station NCRD. This is the most accurate, especially in height.

Methods A and B are collected in the field. Method C needs only the base log.

Plan

1. Arrive and gather at the command area.
2. Overview: what is being flown, why the GCPs and checkpoints exist, and where not to stand during the flight.
3. Method B on the mark with the rover (section 1).
4. Set up the base on the mark and start the log (section 2).
5. Lay the four GCPs and survey them, everyone together, rotating on the rover (section 3).
6. Preflight and launch. One battery over the GCP field (section 4).
7. While the aircraft flies, groups rotate on the rover for 8 to 10 checkpoints, off the flight line (section 5). Then a second group re-occupies 3 of them.
8. Land and check the photo count and geotags. Stop the rover and export the project. Stop the base log last. Pack up (section 6).

1. Method B: fix the mark with the rover

Do this before the base is set up.

1. In Emlid Flow, connect the rover to the NCRTN caster: host rtn.nc.gov, port 2101, mountpoint VRS_RTCM34_MSM5, course login. Corrections come through the tablet’s cell data.
2. Set the antenna height to the pole length. Do not change the pole length for the rest of the day.
3. Hold the pole plumb on the mark. Wait for FIX. Record the satellite count and correction age. Collect a 2 minute averaged occupation.
4. Record the coordinate on the field sheet as Method B.

2. Base station log

1. Set the tripod over the mark and level it.
2. Measure the antenna height from the mark to the bottom of the receiver. Record the height and note that it was measured to the bottom of the receiver. Emlid Flow adds the phase center offset. An antenna height error becomes a height error in Method C.
3. Confirm open sky. Record the satellite count and PDOP.
4. Start RINEX logging at 1 Hz. Record the start time. The log runs until the last rover point is collected.
5. Method A: with the base in SINGLE, average its position for 1 minute and record the coordinate. Skip this if short on time; it can be computed from the log later.
6. Do not reconfigure, restart, or bump the base until the end of the session.

3. GCPs

Set the rover project to NAD83(2011) / North Carolina in meters (EPSG:6542) with NAVD88 (GEOID18) height. Write the coordinate system on the field sheet. The processing labs use EPSG:3358 (NAD83(HARN)), which differs from NAD83(2011) by about 2 cm at this site.

• Lay the four GCPs on the pre-chosen spots, one toward each corner of the flight area, none on the very edge. Weight them down. Do not move a GCP after it has been surveyed.
• Measure the center of each GCP with a 30 second averaged FIX occupation. Rotate on the rover so everyone runs an occupation. These four points control the photogrammetric processing.
• Photograph each GCP with the pole tip visible and record its number on the field sheet.

4. The flight

Lake Wheeler is Class G airspace, so no authorization is needed, and the site is inside the North Carolina State University Lake Wheeler West FRIA, so Remote ID is not required. Daylight is 6:59 am to 7:22 pm. The flight stays at or below 400 ft AGL. Fly one battery, one block over the GCP field; the second battery is the spare.

Preflight checklist

Before leaving:

• Two aircraft batteries and the controller charged; spare battery packed
• Aircraft and controller firmware current; SD card in, formatted, with space for the flight
• Flight plan loaded and checked: area covers all four GCPs, altitude, overlap, one battery
• Weather checked: wind, precipitation, visibility
• Airspace and NOTAMs checked, including the GPS interference advisory above

At the site:

• Walk the area: obstacles near the takeoff and landing spot and along the flight lines, people and vehicles nearby
• Wind checked at the site
• Takeoff and landing spot chosen, flat and clear; everyone briefed on where to stand
• Visual observer assigned
• Airframe inspected: propellers, arms, motors, gimbal cover off, battery seated and latched
• Power on; compass and IMU status normal; satellite count adequate; home point recorded
• Return-to-home altitude set above any obstacle
• Camera set: photo mode, geotagging on, exposure checked, lens clean
• Flight plan uploaded to the aircraft; mission start confirmed

During the flight:

• Aircraft in visual line of sight at all times
• Battery level and satellite count watched; land at the app’s battery warning
• Anyone entering the flight area is called out and the aircraft is paused

After landing:

• Motors stopped, aircraft powered down, battery removed
• Photo count matches the plan; photos carry geotags
• Flight logged: time, battery, notes

WarningGPS interference test on the flight day

The FAA advisory FTBRNC GPS 26-76 schedules GPS interference testing centered on Fort Bragg on September 14 to 16, daily 1500Z to 2300Z, which is 11:00 am to 7:00 pm EDT. The test starts in the middle of the field window. Lake Wheeler is 85 km (46 NM) from the center, and the advisory’s area covers the whole state from the surface up.

5. Checkpoints

Checkpoints are withheld from processing and used to measure the accuracy of the products.

• Pick points you can find in the imagery: pavement corners, sidewalk joints, manhole edges, paint marks. Avoid grass and gravel.
• Hold the pole plumb, confirm FIX, and collect a 30 second averaged occupation.
• Photograph the point with the pole tip visible and record the point number on the field sheet.
• Spread the points across the area and its elevation range. Put several on level ground for the vertical check.

6. What goes to the shared folder

1. The base RINEX log
2. The Emlid Flow CSV export (all points, both height types)
3. The Method A and B coordinates, with antenna heights and times
4. The point photos, named by point number
5. A photo of the field sheet

NCRTN station NCRD

Method B uses live corrections from the NCRTN. Method C post-processes the base log against one of its stations. The nearest station to Lake Wheeler is NCRD (Raleigh DOT), a Trimble Alloy at 35.763752, -78.578998, ellipsoid height 51.802 m, 11.5 km from the mark. The next nearest stations are 23 to 52 km away.

The NCRTN Sensor Map with NCRD selected, 11.5 km from Lake Wheeler.

The network monitors each station’s position against its published coordinate. Over one hour NCRD varies by 1 mm at 3 sigma.

One hour of NCRD position scatter: 3 sigma is 1 mm.

Both pages are public at rtn.nc.gov under Sensor Map and Status Messages.