A Must-Read for Beginners! The Complete Guide to RTK Antenna Height and Offset Settings [With On-Site Precautions]

Table of Contents

• Introduction

• What Is Antenna Height?

• What Is Antenna Offset?

• Why Antenna Height and Offset Settings Matter

• Correct Methods for Measuring Antenna Height and Offset

• On-Site Precautions

• Summary: Simple Surveying with LRTK

• FAQ

Introduction

RTK positioning (Real-Time Kinematic) is a widely used technique in surveying and on construction sites for obtaining high-precision, centimeter-level (half-inch accuracy) position information. Among these, vertical (elevation) measurements are particularly important for tasks such as managing drainage slopes in civil engineering and verifying design elevations, but ensuring accuracy in height is often more difficult than in the horizontal direction. An error of several tens of centimeters in elevation can cause drainage slopes to be wrong or structures to be set at incorrect heights, so maintaining height accuracy is critically important on site. However, due to satellite geometry and radio signal characteristics, RTK is said to be prone to larger positioning errors in the vertical direction. Against this background, correctly setting the antenna height (instrument height) and antenna offset is a fundamental and important point for achieving accurate elevations with RTK positioning.

This article provides a complete guide to antenna height and offset settings in RTK surveying for beginners. We explain what antenna height/offset are, why they are necessary, the correct measurement methods, and on-site precautions in detail. By mastering proper settings and operation tips, you can achieve high-precision RTK positioning.

What Is Antenna Height?

Antenna height (instrument height) is the vertical distance between the GNSS antenna’s radio receiving point, called the “phase center,” and the ground measurement point (the reference point). Simply put, when an antenna is mounted above a ground survey point, antenna height refers to how far it is from the ground to the antenna’s internal measurement reference point. In RTK positioning, this antenna height value must be entered into the positioning software or receiver and is basic data required to obtain correct coordinates.

If the antenna height is entered incorrectly, that error is reflected directly in the computed elevation. For example, if a rover GNSS antenna is installed at a position 2.00 m (6.56 ft) above the ground (on a pole, etc.), but the software is set to an antenna height of 0 m (0 ft), the calculated coordinate elevation will be 2.00 m (6.56 ft) higher than the true value. Similarly, care is required when handling antenna height for a base station. If a base station is set up on a known point, you must either set the base station’s height coordinate to the known point’s elevation plus the antenna height, or enter the known point’s coordinates and the antenna height separately into the positioning software. Neglecting such settings will cause the correction data transmitted from the reference station to be based on the wrong elevation, and all rover measurements will inherit that error.

In short, antenna height is the surveying “machine height,” and handling this value correctly is the first step to obtaining accurate elevations in RTK surveying.

What Is Antenna Offset?

Antenna offset is the internal correction value of the antenna that must be considered when measuring antenna height. Many GNSS antennas have a small difference between an external reference point (such as the antenna’s bottom surface or a designated measuring mark) and the actual radio receiving point (phase center). This differential is the antenna offset and is usually on the order of several centimeters to a dozen or so centimeters.

When measuring antenna height, if the antenna is mounted on a tripod you often measure the height from the ground marker to the antenna’s bottom surface, but that measured value does not include the antenna offset. If you do not add the offset value from the antenna bottom to the internal phase center (for example, ○○ cm), you will set an antenna height that is lower than the true value. For instance, if a model’s phase center is 15 cm (5.9 in) above the bottom surface and you input only the height from the ground to the antenna bottom, the resulting elevation will always be 15 cm (5.9 in) too low.

The antenna offset value is listed in the manufacturer’s specifications or catalog as the “phase center offset.” It is a model-specific value and must be taken into account for precise positioning. Some positioning software automatically applies the specified offset when you select the antenna model; in such cases, the user only needs to enter the height to the antenna bottom, and the software converts it to the phase center height. On the other hand, with simple GNSS receivers that do not offer antenna model selection or with generic NMEA-output devices, the user must calculate and set the height including the offset. In any case, the antenna offset is the unseen key to antenna height setting, and it is important not to forget to handle it properly.

Why Antenna Height and Offset Settings Matter

If antenna height or offset settings are wrong in RTK surveying, the results can be critically off. Vertical errors are less visually obvious than horizontal ones, but their impact on site work is large. For example, if an antenna height input mistake causes all elevations to be measured 5 cm (2.0 in) too low, you may need to add 5 cm (2.0 in) to all measured results later to correct them, leading to rework. In general, GNSS positioning’s vertical accuracy is said to be about 1.5 times worse than horizontal accuracy, and depending on satellite visibility and the surrounding environment, errors may grow larger (for example, in conditions where horizontal accuracy is about 3 cm (1.2 in), the vertical error may be around 5-6 cm (2.0-2.4 in)). Because vertical errors are inherently more likely to be larger, it is important to make the user-controllable errors (such as mistakes in measuring or setting antenna height) zero.

Accurate antenna height and offset settings are prerequisites for RTK to deliver its full precision. Even if you use the latest high-precision RTK equipment, if you estimate antenna height loosely or ignore offsets, the resulting data will be less accurate than the equipment’s capability. Conversely, by firmly handling the basics of measuring and entering antenna height, you can eliminate unnecessary error sources and significantly improve RTK vertical accuracy. Beginners in surveying tend to think “antenna height can be approximate,” but not neglecting the basics leads to accurate data collection.

Correct Methods for Measuring Antenna Height and Offset

Now, let’s go through the actual on-site procedure for measuring antenna height and setting it correctly.

• Install the antenna: Place the GNSS antenna securely directly above the point you want to measure using a tripod or pole. For tripods, adjust the setup so the center is directly above the survey point; for poles, place the tip on the survey point and stand the pole vertically (check with a bubble level).

• Measure the height from the reference point to the antenna: Measure the vertical distance from the ground reference point (a bolt or marker) to the designated antenna measurement reference point using a tape or dedicated scale. Many antennas have a protrusion or mark on the bottom or side for hooking a tape—this is the measurement reference. For tripod setups, the antenna bottom is usually the reference point, so measure from the ground marker to the antenna bottom. For pole setups, read the pole length (to the antenna mounting point) from the scale or measure directly with a tape.

• Confirm the antenna is vertical: Ensure the measured height is a true vertical distance. For poles, check with the bubble level that the pole is plumb; if it is tilted, the true vertical height will be shorter than the pole length. For tripods, reconfirm that the reference point is directly under the antenna.

• Determine the antenna offset: Check the GNSS antenna’s specifications for the offset to the phase center. For example, “antenna height offset 0.134 m (0.440 ft).” Commercial antennas always have this value specified.

• Calculate the height to the phase center: Add the offset from step 4 to the height measured to the antenna bottom (or reference) in step 2. This yields the accurate height from the ground to the antenna phase center. For example, if the height to the antenna bottom is measured as 1.800 m (5.905 ft) and the antenna offset is 0.134 m (0.440 ft), the phase center height is 1.934 m (6.345 ft).

• Enter the value into the positioning device: Enter the antenna height into the GNSS receiver or surveying app settings. If you can select the antenna model, choose the appropriate model and input the measured “height to the antenna bottom” (in the example, 1.800 m (5.905 ft)); the software will automatically account for the offset. If your device does not allow antenna model selection, enter the calculated phase center height directly (in the example, 1.934 m (6.345 ft)). When setting up a base station, either enter the known point’s elevation and the antenna height separately or compute and enter the combined elevation according to the device’s requirements.

• Double-check the entered value: Verify that the antenna height has been entered correctly and that there are no unit or digit errors (m vs. cm, etc.). Once a value is stored in the device, it may persist and be applied to subsequent measurement points. When you reposition the antenna or change the pole length, make sure the value is updated accordingly each time.

The above steps cover the correct measurement and setting of antenna height and offset. For precise surveying, measure and record antenna height carefully to millimeter-level accuracy each time. Neglecting small errors with the thought “a little difference won’t matter” can lead to significant elevation discrepancies, so take care.

On-Site Precautions

Once antenna height and offset are correctly set, proceed with positioning work on site, but observe the following points to maximize accuracy.

• Record measurements only after obtaining a Fix solution: In RTK, centimeter-level accuracy is only achieved after receiving correction data and resolving integer biases to a “Fix” solution. Float or Single solutions are prone to large and unstable vertical errors. Always confirm the solution is Fix before logging observations.

• Choose locations with good sky visibility: The more open the sky above the measurement point, the better the satellite geometry and the better the vertical accuracy. Locations surrounded by buildings or trees tend to impair satellite reception and increase vertical uncertainty. Where possible, choose spots with fewer obstructions or temporarily raise the antenna to a higher position.

• Be aware of multipath: Reflective objects such as metal sheets, fences, or vehicles around the antenna can cause multipath (reflected signals), introducing vertical errors. If reflectors are present, set the antenna further away from them or mount it higher; avoid placing the antenna close to reflection sources.

• Keep the antenna stable: When using a pole, keep it vertical and steady during measurement; do not sway it. On windy days, poles tend to move and change height, so be cautious. For tripods, ensure legs are tightened and will not shift; on soft ground, tripod legs may sink, so check before and after each measurement that antenna height has not changed.

• Unify coordinate systems and vertical reference: Check the positioning software’s coordinate system settings and whether a geoid model (geoid height conversion) is being applied. Even if antenna height is correct, GNSS yields ellipsoidal heights unless geoid correction is applied. In Japan, the difference between ellipsoid height and orthometric height can be around 30-40 m, so for surveys that require orthometric height, always apply the latest geoid model. If on-site heights are off by tens of meters, suspect a geoid correction setting error before antenna height issues.

• Verify base station settings: When using your own base station, be careful not to make coordinate entry mistakes on the base side. If you forget to account for the antenna height when entering the base station’s known point elevation, the transmitted corrections will be based on the wrong height and all rover heights will be offset. As noted earlier, enter known point elevation + antenna height correctly or set the antenna height field in the software as instructed.

• Always apply the latest values: If antenna height changes between points (for example, extending or retracting the pole or changing tripod height), update the height value before measuring. In busy field conditions, it’s common to forget to change the antenna height from the previous point; frequent checks prevent such errors.

• Stop and verify abnormal values: If clearly erroneous elevation data appear during measurement, stop and investigate rather than continuing hastily. Check for antenna height input errors, correct geoid settings, base station or NTRIP reference frames, and satellite reception conditions (move or reinstall the antenna if necessary). Sometimes restarting the device and reacquiring a Fix resolves the issue.

By observing these points, you can maximize the benefit of correctly setting antenna height and offset and make RTK positioning on site more reliable and secure.

Summary: Simple Surveying with LRTK

So far, we have explained the importance of antenna height and offset in RTK surveying, how to set them correctly, and the on-site points to watch. Applying these practices will enable you to obtain much more accurate elevation data than before, but managing everything perfectly in the field is not always easy. Beginners especially may find advanced device settings and real-time decisions during positioning to be burdensome.

This is where simple surveying with LRTK becomes attractive. LRTK is a solution designed to make high-precision GNSS positioning easier to perform. By combining a dedicated high-precision GNSS receiver with a smartphone app, LRTK is designed so that centimeter-level (half-inch accuracy) positioning is possible in real time without having to be conscious of complicated settings. For example, LRTK apps let you set the coordinate system and geoid model once and then automatically obtain corrected orthometric heights, and antenna height input is guided so a single operator can complete measurements without mistakes. Procedures that once required specialized knowledge for RTK settings can be completed intuitively with LRTK, reducing on-site workload significantly.

LRTK is a strong ally not only for surveyors seeking high vertical accuracy but also for construction supervisors and infrastructure inspection staff who are not surveying specialists. Consider introducing the simplicity of LRTK-based surveying on your site to obtain accurate elevation information without being bogged down by complicated steps.

FAQ

Q: Which has better vertical accuracy, RTK surveying or leveling? A: Generally, leveling (spirit leveling) outperforms RTK in vertical accuracy. Precision leveling performed by experienced surveyors can measure elevation differences to millimeter accuracy over short distances. However, leveling requires more manpower and time and is unsuitable for measuring elevation differences over wide areas. RTK surveying, on the other hand, can obtain elevation information over large areas in a short time. Its accuracy is on the order of a few centimeters, which is adequate for many applications such as construction control and topographic surveys. A common practice is to combine the two methods—measure benchmarks with leveling and use RTK for broader surveys—to achieve both efficiency and accuracy. For extremely high-precision tasks (sub-millimeter), leveling is indispensable, but for most general elevation surveys, RTK offers a good balance of speed and sufficient accuracy.

Q: How accurate is RTK vertical positioning? A: It depends on equipment and environment, but as a rule of thumb, when RTK-GNSS equipment achieves horizontal accuracy of a few centimeters, vertical accuracy is about 1.5 times worse—that is, on the order of a few centimeters to a dozen centimeters. With good satellite geometry, correct geoid correction, and accurate antenna height settings, vertical errors under 5 cm (2.0 in) can be expected. There are reported cases using the latest high-precision GNSS receivers and services (such as LRTK) where vertical errors were contained to about 3 cm (1.2 in).

Q: Is using a geoid model always necessary? A: If you need accurate orthometric heights (elevation above sea level), geoid correction is almost essential. Without geoid correction, RTK heights remain ellipsoidal and can differ from actual orthometric heights by about 30-40 m depending on the region. If you plan to post-process and apply correction later, or if you only need relative height differences within a small area, you may not need to apply geoid correction on site. However, if you will compare results with official benchmarks or share data with other sites, it is safer to apply the geoid model on site from the start.

Q: Any tips for measuring antenna height? A: When measuring antenna height, measure vertically from the antenna’s designated reference point (many GNSS antennas have marks or protrusions for hooking a tape) down to the ground survey point. For tripods, measure from the ground marker to the antenna bottom and add the antenna’s phase center offset. For poles, accurately determine the pole length to the antenna mounting point and check that the pole is plumb using a bubble level. Even a millimeter-level measurement error can affect vertical accuracy, so measure carefully.

Q: What should I do if RTK vertical accuracy is poor (large errors)? A: First, confirm that the solution is a proper Fix. If it remains Float, vertical errors will be large and unstable. If Fix is hard to attain, check satellite reception and whether correction data (base station/NTRIP) are being received correctly. If the area has many obstructions, move slightly or raise the antenna. Also recheck base station settings for coordinate system or vertical reference mistakes. Try changing the time of day to improve satellite geometry, suspect antenna cable or connector faults, or restart the receiver and reacquire corrections. If none of these help, reboot the device and reattempt to acquire a Fix.

Q: What is LRTK? A: LRTK is a solution for performing simple yet high-precision GNSS surveying. It combines a dedicated high-precision GNSS receiver with a smartphone app to enable real-time centimeter-level (half-inch accuracy) positioning. LRTK simplifies RTK positioning—which traditionally required specialized knowledge and complex settings—so that non-surveying professionals can use it easily. With LRTK, small teams (sometimes a single operator) can efficiently carry out field surveys and obtain the required elevation data with high accuracy, making it a reassuring tool for less experienced users.