A GNSS position applies to a documented reference point, not automatically to the vehicle axle, camera centre or tool that uses it. To obtain another point on a rigid platform, add the measured offset after rotating it into the position coordinate frame, using attitude and position for the same epoch. A correct antenna position can therefore coexist with an incorrect work-point position if the offset, rotation or timing is wrong.
This is a geometry and synchronization problem. Improving the GNSS solution alone does not identify the required point, measure its displacement or align a camera exposure with the navigation record.
Identify the point that the output already represents
Start with the message definition and active configuration. A GNSS-only output may represent an antenna-related reference; a fused GNSS/INS output may instead refer to its inertial navigation centre or a configured external point. Do not apply an antenna-to-tool offset to a solution that has already been translated to that tool.
For a manufacturer example, SBG Systems' IMU configuration guide distinguishes the internal measurement centre, enclosure targets and custom reference points. It also permits alternative monitoring locations. A mark visible on a housing is therefore not necessarily the origin used by an output or installation parameter. These are SBG configuration choices, not ResiNav output capabilities.
Keep the mechanical reference separate from electrical antenna calibration. An antenna reference point (ARP), phase-centre offset and phase-centre variation describe a different part of the measurement chain from the displacement to an axle or camera. The surveying and mobile-mapping guide explains that calibration boundary. Here, assume the position has already been expressed at the explicitly identified point A.
Rotate the lever arm before adding it
A lever arm is a directed displacement, not just a distance. For a fixed point T on the same rigid body, define lATb as the vector from A to T, expressed in body axes b. Let pAn(t) be A's position in Cartesian navigation frame n, and Rbn(t) the rotation mapping body-frame components into n. Then:
pTn(t) = pAn(t) + Rbn(t) lATb.
All position and lever-arm components are in metres. The rotation is dimensionless. Use a compatible Cartesian frame, such as Earth-centred Earth-fixed coordinates or a consistently defined local frame; do not add metre offsets directly to latitude and longitude in degrees. This equation assumes a rigid, constant body-frame offset and matched epochs. It does not estimate attitude or correct an erroneous position at A.
NovAtel's SETINSTRANSLATION documentation illustrates why origin, direction and axes matter: its offsets run from the IMU to the specified point, and supported input frames require the corresponding rotation. A vector measured from antenna to IMU cannot be entered unchanged where IMU to antenna is required. Follow the exact product convention rather than copying the notation above into a command.
Hypothetical geometry example, not a product test: use a local east–north–up navigation frame. Define right-handed body axes x forward, y left and z up. The vehicle is level, and T is 2.00 m behind and 1.00 m below A, so its body-frame lever arm is (−2.00, 0, −1.00) m. Hold A at (100.00, 200.00, 5.00) m to isolate rotation:
- Facing east, body x points east. The rotated offset is (−2.00, 0, −1.00) m, giving T = (98.00, 200.00, 4.00) m.
- Facing north, body x points north and body y points west. The rotated offset is (0, −2.00, −1.00) m, giving T = (100.00, 198.00, 4.00) m.
The same installed lever arm has different navigation-frame components. Subtracting a constant 2 m easting would be correct for the first orientation and wrong for the second. Holding A fixed is a mathematical comparison, not a claim about how a real vehicle turns.
Attitude error becomes position error away from the origin
The rotation uses estimated attitude. An error in that attitude rotates the lever arm incorrectly even when its measured length is exact. For a lever-arm component of length L perpendicular to a single erroneous rotation axis, the displacement between the correctly and incorrectly rotated endpoints is:
e = 2L sin(|δθ| / 2) ≈ L |δθ|, with δθ in radians for the approximation.
For a hypothetical 2.00 m horizontal offset and 1.00° yaw error, the exact endpoint separation is about 0.03491 m, or 34.9 mm. This is the isolated geometric contribution, not total position error or a receiver accuracy prediction. A component parallel to the rotation axis contributes no displacement from that rotation. Roll and pitch also matter when the installation has horizontal or vertical separation.
NovAtel's translation documentation explicitly accounts for attitude and offset uncertainty in supported user-point outputs. Its implementation is one example of why an uncertainty reported at the navigation origin should not automatically be relabelled as uncertainty at a distant point. The general geometry does not supply a particular receiver's covariance model or a universal mounting-distance limit.
Match the position and attitude to the event epoch
Use the pose corresponding to the event being located. A navigation packet received immediately after a camera trigger can still describe an earlier epoch. Likewise, position at the correct epoch combined with attitude from another epoch applies the wrong rotation during motion. Raising output rate alone does not establish their alignment.
SBG's synchronization and event documentation separates event timestamps from the later message-output window. NovAtel's receiver-time documentation similarly describes electrical event arrival and associated logs. Those functions time defined electrical events; the relationship between that event and actual exposure must still be established for the selected camera.
Consider two separate hypothetical timing sensitivities:
- At a constant 5.00 m/s with no rotation, using a position 20.0 ms too early creates a 0.100 m along-track displacement: 5.00 × 0.0200. This assumes the same velocity throughout the interval.
- With position otherwise aligned, a constant 20.0°/s yaw rate and attitude 20.0 ms too early create a 0.400° rotation mismatch. For a 2.00 m horizontal lever arm, the endpoint separation is about 0.01396 m, or 14.0 mm.
These examples isolate different causes. They are not independent random standard deviations to combine by default, and their magnitudes should not simply be added as a predicted system error. In a moving installation, error directions and correlations matter.
Establish the time scale and clock relationship, exposure-event meaning, any documented trigger delay, and how the processing obtains position and attitude at that epoch. A suitable interpolation or propagation method can use surrounding navigation records; copying the nearest packet without assessing motion is not equivalent. The freshness and integrity guide covers data age and clock relationships, while the receiver interface guide covers message and transport definitions.
Distinguish four corrections that solve different problems
| Item | What it establishes or changes | What it does not replace |
|---|---|---|
| Lever-arm measurement | Directed displacement between named reference points, expressed in declared axes | The attitude needed to rotate that displacement |
| Body-to-sensor alignment or boresight | Angular relationship between the platform and sensor axes | Translation to the sensor centre or the sensor's own calibration |
| Event and navigation time alignment | Which position and orientation belong to a measurement event | Mechanical offsets or a correct coordinate reference |
| Antenna phase-centre calibration | The applicable electrical antenna-reference correction | The measured displacement from the resulting reference point to the work point |
A camera-centre position is not yet a mapped ground point. Locating an imaged feature also needs the relevant camera model, viewing direction and scene geometry. Boresight affects that direction; a translation alone cannot provide it. GeoCue's antenna lever-arm guidance is a concrete example of software-specific camera reference and sign conventions, not a universal measurement recipe for every camera or gimbal.
A constant lever arm also stops being sufficient when the target moves relative to the body. NovAtel's variable-lever-arm documentation describes a supported gimballed installation that uses reported gimbal angles to update the geometry. A moving boom, flexing mount or gimbal needs the appropriate additional geometry and motion information; the fixed-offset equation must not silently assume those motions away.
Evaluate the delivered point rather than only the antenna trace
Name the consumer's point and epoch in the output specification. A useful delivery record identifies the original output point, target point, offset direction and units, input axes, attitude convention, event-time relationship and whether translation is applied in the receiver, host or post-processing. This prevents both missing a correction and applying it twice.
When validating that delivery, compare against evidence for the same point at matched times. Include relevant orientation and motion changes: straight, level travel alone can conceal a wrongly fixed navigation-frame offset or a roll/pitch omission. A disagreement that varies with attitude makes geometry worth investigating, but is not by itself a unique diagnosis of its cause. Keep reference uncertainty and synchronization in the comparison.
After a remount or sensor relocation, update the affected geometry. NovAtel's lever-arm calibration routine explicitly calls for redefining its installation offset after relevant moves. That manufacturer procedure is not a ResiNav calibration service or an automatic guarantee that an estimated offset is correct.
For a later trajectory, specify this output location in the real-time versus post-mission processing scope. A file at the antenna and a file at the camera can contain different coordinates without either being an erroneous GNSS solution. The useful result is the point the job needs, in the agreed frame and at the agreed epoch. GNSS reception protection, if separately required, does not supply this geometric transformation or synchronization function.
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