Equipment & Mission Planning

Overview#

The best understanding of GNSS theory is worthless if the equipment is inadequate, the mission is poorly planned, or the site is badly chosen. Equipment selection, mission planning, and site reconnaissance are where GNSS knowledge meets field reality. A surveyor who invests time in these steps before going to the field will consistently produce better results than one who arrives on site and simply turns on the receiver.

Equipment decisions determine what is possible. A single-frequency, code-only receiver cannot perform carrier phase surveying regardless of how long it observes. Mission planning determines what is practical -- whether enough satellites will be available with acceptable geometry during the planned observation window. Site selection determines what is achievable -- even the best receiver and perfect satellite geometry cannot overcome a site surrounded by reflective buildings and dense tree canopy.

"Planning a GPS survey is at least as important as conducting one. The time spent in the office examining satellite availability, predicting PDOP, and evaluating potential obstructions directly affects the quality and efficiency of the field work." -- Van Sickle, GPS for Land Surveyors (4th Ed.), Ch. 12, p. 271

GNSS Receiver Categories#

GNSS receivers span a wide range of capability and price, from sub-meter mapping units to millimeter-capable geodetic receivers. The primary distinctions are the number of frequencies tracked, the constellations supported, and whether the receiver processes carrier phase observations.

For professional land surveying -- boundary surveys, control establishment, construction stakeout, ALTA/NSPS surveys -- a survey-grade receiver is required. There is no workaround. Mapping-grade and GIS-grade receivers have their place in forestry, natural resource management, and asset inventory, but they do not meet the accuracy standards required for property boundary determination or engineering control.

Key Receiver Features

Modern survey-grade receivers offer a range of features that affect productivity and data quality:

Multi-constellation tracking. Receivers that track GPS, GLONASS, Galileo, and BeiDou simultaneously maximize satellite availability and improve geometry, particularly in challenging environments. A GPS-only receiver in an urban or forested environment may struggle to maintain a fixed RTK solution; a multi-constellation receiver tracking 20+ satellites will be far more reliable.

Multi-frequency capability. Dual-frequency (L1/L2) is the minimum for survey-grade work. Triple-frequency (L1/L2/L5) significantly accelerates RTK initialization and improves reliability under ionospheric stress. The extra-wide-lane combinations available with three frequencies can resolve ambiguities almost instantaneously.

Tilt compensation. Modern rovers incorporate an inertial measurement unit (IMU) that allows the surveyor to measure points without leveling the survey pole. The receiver computes the antenna phase center position even when the pole is tilted up to 15--30 degrees. This dramatically increases productivity in the field, particularly for topographic surveys and construction stakeout where precise pole leveling at every point is time-consuming.

"Tilt compensation has been one of the most significant productivity improvements in RTK surveying in the last decade. Eliminating the need to carefully level a range pole at each point can reduce occupation time per point by 50% or more." -- GEOG 862, GPS and GNSS for Geospatial Professionals, Penn State, Lesson 10

Internal radio and cellular modem. Many survey-grade receivers include built-in UHF radios and cellular modems for receiving RTK corrections. Integrated communications simplify the field setup and reduce the number of components that can fail.

Anti-jamming and anti-spoofing. As GNSS becomes critical infrastructure, protection against intentional interference is increasingly important. Some survey receivers incorporate adaptive antenna arrays or signal authentication capabilities.

Antenna Types#

The antenna is as important as the receiver for survey-grade work. The antenna determines how cleanly the satellite signals are received and how well multipath and interference are rejected.

Geodetic / Choke Ring Antennas

Choke ring antennas feature a series of concentric metal rings (resembling a choke) surrounding the antenna element. These rings attenuate signals arriving from below the horizon and at low elevation angles -- precisely the multipath-prone signals reflected off the ground and nearby surfaces.

Characteristics:

• Excellent multipath rejection
• Very stable phase center (minimal PCV)
• Large, heavy, requires a tripod or pillar
• Standard for permanent reference stations (CORS) and high-precision static surveys
• Not practical for rover (kinematic) use due to size and weight

Ground Plane Antennas

Ground plane antennas use a flat metal disk beneath the antenna element to reduce ground-reflected multipath. They are lighter and smaller than choke ring antennas while still providing good multipath mitigation.

Characteristics:

• Good multipath rejection (less than choke ring)
• Moderate phase center stability
• Suitable for static, rapid-static, and base station applications
• Lighter than choke ring; usable on tripods and tribrachs

Rover / Pole-Mounted Antennas

Rover antennas are compact, lightweight designs optimized for mounting on a survey pole (range pole). They prioritize portability and durability over maximum multipath rejection.

Characteristics:

• Compact, lightweight, weather-resistant
• Acceptable phase center stability when calibrated
• Integrated into many modern RTK receivers
• Always use with proper antenna height measurement (to ARP)

Antenna Phase Center Calibration

Every antenna has a calibrated offset from the Antenna Reference Point (ARP) to the mean electrical phase center, plus phase center variation (PCV) tables that describe how the phase center shifts with satellite elevation and azimuth. These calibrations are published in ANTEX (Antenna Exchange) format by NGS and other agencies.

"Mixing antenna types on a baseline without applying proper phase center corrections can introduce systematic vertical errors of 5--10 centimeters or more. This is one of the most common sources of preventable error in GPS surveys." -- Ghilani & Wolf, Elementary Surveying (13th Ed.), Ch. 14, p. 408

For highest accuracy (static geodetic surveys), all receivers in a session should use the same antenna model. When different antennas must be used, absolute PCV corrections must be applied in post-processing.

Mission Planning#

Satellite Availability and PDOP Prediction

Before conducting any GNSS survey, the surveyor should predict satellite availability and geometry for the planned observation period. Mission planning software (provided by receiver manufacturers, or available from NGS and online tools) uses current almanac data to compute:

• Number of visible satellites at any given time
• PDOP (and HDOP, VDOP) time series
• Satellite sky plots showing tracks and elevations
• Rise and set times for each satellite

The key inputs to mission planning are:

Planning Criteria

For survey-grade GNSS observations, the following planning criteria should be met:

If the predicted conditions during the planned observation window are marginal, the surveyor should consider rescheduling to a time with better geometry. For static surveys, the observation session should be centered on the period of lowest PDOP rather than scheduled for operational convenience.

Obstruction Analysis

An obstruction diagram (also called a station visibility diagram) maps the horizon as seen from the observation point, showing the elevation angle of obstructions at each azimuth. When overlaid on the predicted satellite tracks, it reveals which satellites will be blocked and how severely the geometry is affected.

Obstructions can be documented by:

• Field reconnaissance with a compass and clinometer (measuring azimuth and elevation angle to each obstruction)
• Handheld inclinometer apps (approximate but useful for quick assessment)
• Digital terrain models (for natural terrain obstructions)
• Photographs (panoramic site photos annotated with azimuth and elevation)

Site Selection#

Principles of Good Site Selection

The ideal GNSS observation site has a clear, unobstructed view of the sky from the elevation mask (10--15°) to zenith in all directions. In practice, perfectly open sites are rare, and the surveyor must evaluate trade-offs.

Primary site selection criteria:

1. Sky visibility. Maximize the visible sky. Avoid sites adjacent to tall buildings, dense tree canopy, steep terrain slopes, and overhead power lines.

2. Multipath environment. Avoid proximity to large reflective surfaces -- metal buildings, chain-link fences, vehicles, bodies of water, concrete walls. A minimum distance of 5--10 meters from any large reflective surface is recommended.

3. Electromagnetic interference. Avoid sites near radar installations, microwave towers, high-voltage transmission lines, and other sources of radio-frequency interference (RFI). These can degrade signal tracking or cause complete signal loss.

4. Physical stability. The ground must be stable (no soft soil, no frost-heave-prone areas for long-term monitoring). The monument or tripod must be securable against disturbance.

5. Accessibility. For repeated observations (monitoring programs), the site must be safely accessible in all weather conditions. For RTK base stations, the site must accommodate the base station equipment and have adequate power.

"No amount of processing sophistication can compensate for a poorly selected observation site. Multipath, signal obstruction, and interference are best addressed by choosing a better location, not by trying to fix the data after the fact." -- Van Sickle, GPS for Land Surveyors (4th Ed.), Ch. 12, p. 280

Site Documentation

For every GNSS observation site (particularly control points), document:

• Point description (monument type, stamping, reference ties)
• Obstruction diagram (azimuth and elevation angles of obstructions)
• Photographs (panoramic and close-up of monument)
• Known sources of multipath or interference
• Access directions and any access restrictions

Observation Planning#

Session Design

Antenna Height Measurement

One of the most common and preventable errors in GNSS surveying is an incorrect antenna height. The antenna height is the vertical distance from the survey mark to the antenna reference point (ARP). It must be measured carefully and recorded accurately.

Best practices:

• Measure the slant height to the measurement mark on the antenna, then convert to vertical ARP height using the manufacturer's specifications
• Measure at the beginning and end of the session -- if they differ by more than 2 mm, investigate
• Record both the raw slant measurement and the computed vertical height
• Note the measurement method (slant to bottom of antenna mount, vertical to ARP, etc.)
• Use a steel tape or calibrated height rod, not a cloth tape

The vertical height to ARP is computed from the slant height as:

$h_{\text{ARP}} = \sqrt{h_{\text{slant}}^2 - r^2}$

where $r$ is the horizontal radius from the monument center to the measurement point on the antenna.

Power and Communication Logistics

For RTK operations:

Pre-Field Checklist#

Before leaving for the field, verify:

• Receivers and data collectors are fully charged
• Firmware is current (or at least consistent across all receivers)
• Almanac data is current (< 1 week old)
• Mission planning confirms acceptable satellite availability and PDOP for the planned observation window
• Antenna calibration files are loaded in processing software
• Communication links tested (radio and/or cellular)
• Coordinate system, datum, geoid model, and projection are correctly configured
• Known control point coordinates are loaded for initialization checks
• Sufficient memory and spare batteries are available
• Tripods, tribrachs, height rods, and measuring tapes are in good condition

Key Takeaways#

• Survey-grade receivers (multi-frequency, multi-constellation, carrier phase) are required for professional land surveying. Mapping and GIS-grade receivers do not meet boundary or control accuracy standards.
• Multi-constellation tracking (GPS + GLONASS + Galileo + BeiDou) significantly improves satellite availability and geometry, especially in obstructed environments.
• Antenna selection matters as much as receiver selection. Choke ring antennas provide the best multipath rejection and phase center stability for static work; rover antennas trade some performance for portability.
• Antenna phase center calibrations (absolute PCV in ANTEX format) must be applied in processing, especially when mixing antenna types.
• Mission planning (satellite availability, PDOP prediction, obstruction analysis) should be performed before every survey. Schedule observations during periods of optimal geometry.
• Site selection is the first line of defense against multipath and signal obstruction. No processing technique can fully compensate for a bad site.
• Antenna height measurement is a critical field procedure. Measure twice, record the method, and verify at session end. Incorrect antenna heights are one of the most common GNSS survey errors.
• A thorough pre-field checklist prevents the most common logistical failures that waste field time.

References#

1. Van Sickle, J. GPS for Land Surveyors (4th Ed.). CRC Press, 2015. Chapters 12--13.
2. Ghilani, C.D. & Wolf, P.R. Elementary Surveying: An Introduction to Geomatics (13th Ed.). Pearson, 2012. Chapter 14.
3. GEOG 862: GPS and GNSS for Geospatial Professionals. Penn State College of Earth and Mineral Sciences. Lessons 9--10.
4. National Geodetic Survey. "Antenna Calibrations." NOAA/NGS. https://geodesy.noaa.gov/ANTCAL/
5. Trimble Inc. Trimble Access Field Software User Guide. Trimble Navigation, 2023.
6. Leica Geosystems. Leica Captivate Technical Reference Manual. Leica Geosystems AG, 2023.