REVIEW 5 minor 1 cited by
Equivalence of Left- and Right-Invariant Extended Kalman Filters on Matrix Lie Groups
T0 review · 0 major / 5 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read With a full-order covariance reset, left- and right-invariant extended Kalman filters on matrix Lie groups give identical state estimates.
desk verdict The equivalence theorem is exact and the proof is clean; the paper's real weaknesses are in simulation reporting, not the math. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The machinery consists of the extended concentrated Gaussian distribution on a Lie group, written $g = g_o\exp(\xi)$ or $g = \exp(\bar\xi)g_o$ with the local error Gaussian on the Lie algebra, together with the full-order covariance reset $P^+ = J_\zeta(I-KC)PJ_\zeta^T$ and its right-invariant analogue. The group Jacobians $J_\zeta$ and $\bar J_\zeta$, defined as derivatives of the logarithm, carry the effect of moving the reference point after the Kalman update; Lemma 1 provides the Kullback-Leibler-minimizing formulas for that re-parameterization. The change of variables $\bar P = \mathrm{Ad}_{g_o}P\mathrm{Ad}_{g_o}^T$ is the transport that carries a left-filter trajectory onto a right-filter trajectory, and Proposition 1's adjoint identities are what make the update step commute with this transport. In short, the equivalence is a symmetry of the filter equations under conjugation by $g_o$.
What would settle it
Run the two full-order LEKFs on a Lie-group estimation problem with a strongly non-Gaussian or multimodal posterior, for example a very large initial attitude uncertainty, and check whether $\bar g_o(t) = g_o(t)$ and $\bar P(t) = \mathrm{Ad}_{g_o(t)}P(t)\mathrm{Ad}_{g_o(t)}^T$ hold to numerical precision throughout the run; a clear violation would show that Theorem 1 does not survive outside its second-order Gaussian regime.
Extended reading notes
Core claim
On the paper's own terms, the central claim is that the full-order covariance reset is the ingredient that makes the left- and right-invariant LEKFs equivalent. Theorem 1 states that Algorithm 1 and Algorithm 2 are related by the smooth change of variables $\bar g_o = g_o$ and $\bar P = \mathrm{Ad}_{g_o}P\mathrm{Ad}_{g_o}^T$, which is preserved by both the propagation and the measurement-update equations. Consequently the Lie-group-valued estimate $g_o$ is invariant to whether the estimation error is defined left-invariantly or right-invariantly. The covariance reset is not optional decoration; it is the exact re-parameterization that moves the posterior's reference point after an update, and the proof shows why truncating it to low order destroys the symmetry. The simulation then demonstrates the theorem numerically and shows that reduced-order resets produce different estimates for the two error conventions.
Load-bearing premise
The load-bearing premise is that the Gaussian re-parameterization step used for the covariance reset is accurate enough, since the paper's imported Lemma 1 (Section II-D) is proved only up to second-order linearization error and the left/right equivalence inherits that approximation.
Editorial extensions
If this is right
- Under the full-order covariance reset, the left- and right-invariant LEKFs produce identical state estimates, so a user can choose either error convention based on implementation ease.
- The two filters' covariance matrices are linked by $\bar P = \mathrm{Ad}_{g_o}P\mathrm{Ad}_{g_o}^T$, so results from one filter can be converted into the other's coordinates.
- Zero-order and first-order covariance resets break this equivalence; the Monte Carlo results show nonzero error between the left and right variants when the reset is truncated.
- In the simulated aided inertial navigation problem, the full-order reset filters had the lowest total mean absolute error, supporting the full-order reset as the appropriate re-parameterization step.
- For a group-affine system with the reset omitted, the LEKF reduces to an IEKF, which connects the equivalence result to the existing invariant-extended-Kalman-filter literature.
Reading between the lines
- An unstated consequence is that the equivalence is not tied to the particular $SE_2(3)\times\mathbb{R}^3\times\mathbb{R}^3$ navigation example; the proof uses only the group structure, so the same invariance should hold for any matrix Lie group and smooth measurement function covered by Lemma 1.
- The simulation shows the left and right reduced-order filters disagree by different amounts in different directions, suggesting that published performance rankings of left- versus right-invariant filters may actually be rankings of their truncated reset approximations.
- A practical extension is that the adjoint change of variables allows switching between left and right error conventions mid-run without re-initializing the filter, a possibility the paper does not discuss.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper derives a continuous-discrete extended Kalman filter for systems on matrix Lie groups, using the Stratonovich interpretation of the system SDE and discrete-time Euclidean measurements. The derivation yields a filter with a full-order covariance reset after each update. The main contribution is Theorem 1, which proves that the left-invariant and right-invariant variants of this filter (Algorithms 1 and 2) produce identical Lie-group-valued state estimates under the change of variables \bar g_o = g_o and \bar P = Ad_{g_o} P Ad_{g_o}^T. The proof is algebraic and uses standard identities for adjoint operators and group Jacobians. A Monte Carlo simulation of an aided inertial navigation problem confirms the equivalence for full-order resets and shows that reduced-order resets break it.
Significance. The result is significant because it eliminates a design choice (left- vs right-invariant error) when a full-order covariance reset is used. The proof is rigorous and general for matrix Lie groups, and the paper also provides a Stratonovich-based derivation of the continuous-discrete Lie-group EKF that simplifies existing Ito-based derivations. The simulation supports the theoretical claim and demonstrates that the common reduced-order resets do not share the invariance. The paper's claims are falsifiable and clearly stated.
minor comments (5)
- [Section III.B, Eq. (35)] Equation (35) is written incorrectly: the right-hand side should be multiplied on the right by \bar g_e, so that the time derivative is an element of the tangent space at \bar g_e. As written, the right-hand side is an element of the Lie algebra. The subsequent derivation in Eq. (36) uses the correct form, so this is likely a typo, but it should be fixed.
- [Algorithm 1 and text after Eq. (23)] The notation 'ad_G(a_u(g_o))' appears to be a typo; it should be 'ad_{a_u(g_o)}' throughout. The same ambiguity appears in the definition of A(g_n,u) in Section III.B.
- [Lemma 1] The phrase 'up second-order linearization error' should read 'up to second-order linearization error'; also, a proof sketch would improve self-containedness, as the lemma is central to the derivation of the right-invariant reset formula.
- [Theorem 1] The statement of Theorem 1 should explicitly state the initial condition assumption \bar g_o(0) = g_o(0) and \bar P(0) = Ad_{g_o(0)} P(0) Ad_{g_o(0)}^T, since the proof implicitly relies on this initialization.
- [Section IV] The sentence 'Theorem 1 does not hold when using a zero-order or a first-order covariance reset approximation' is imprecise; it would be clearer to say that the equivalence property is lost when the assumptions of Theorem 1 (full-order reset) are not satisfied.
Circularity Check
No significant circularity: the equivalence theorem is an exact algebraic identity between two filter algorithms, with no fitted input renamed as a prediction and no load-bearing self-citation.
full rationale
Theorem 1 is proven by exhibiting the smooth change of variables \bar g_o = g_o and \bar P = Ad_{g_o} P Ad_{g_o}^T, then checking propagation (Eqs. 46-47) and update (Eqs. 48-50) using only Proposition 1 identities; the proof does not invoke the theorem's conclusion. The filter derivation uses a classical EKF linearization and a KL-minimizing covariance reset imported from external references [27], [28]; even though Lemma 1 is stated to hold only 'up to second-order linearization error', that caveat bears on whether the reset is the optimal Bayesian reparameterization, not on whether the left and right algorithms are algebraically equivalent under the same approximate reset rule. The cited reset formulas are external prior results (Ge, Van Goor, Mahony), not self-citations by the present authors, and they are load-bearing only insofar as they determine the explicit P^+ formulas, which the equivalence proof treats as given operations. The Monte Carlo study sets the right filter's initial covariance by (45b), a consistency requirement rather than a fitted parameter, and the agreement of L-FO and R-FO is a confirmation of the algebra, not a construction of it. No equation was found in which a defined quantity is implicitly defined by the predicted quantity, and no 'prediction' reduces to a fit.
Assumptions & free parameters
free parameters (1)
- Measurement noise inflation factor =
3
assumptions (4)
- standard math Lemma 1 (spatial counterpart of [28, Lemma 1]): the KL-minimizing re-parameterization of an extended concentrated Gaussian is given by equations (14)-(15), up to second-order linearization error
- domain assumption The system noise is modeled on the Lie algebra and the SDE (16a) is interpreted in the Stratonovich sense
- domain assumption The error state xi is assumed small enough that higher-order terms O(|(xi,mu)|^2) can be neglected in equations (23)-(25)
- standard math Smoothness of the mappings a, B, c, D and the availability of right/left derivatives on the matrix Lie group
Cite this review
Pith. "Pith review of Equivalence of Left- and Right-Invariant Extended Kalman Filters on Matrix Lie Groups." pith.science (2026). https://pith.science/paper/7CF7AXTB
@misc{pith2026250601514,
author = {Pith},
title = {Pith review of: Equivalence of Left- and Right-Invariant Extended Kalman Filters on Matrix Lie Groups},
year = {2026},
howpublished = {\url{https://pith.science/paper/7CF7AXTB}},
note = {Machine review of arXiv:2506.01514}
}
read the original abstract
This paper derives the extended Kalman filter (EKF) for continuous-time systems on matrix Lie groups observed through discrete-time measurements. By modeling the system noise on the Lie algebra and adopting a Stratonovich interpretation for the stochastic differential equation (SDE), we ensure that solutions remain on the manifold. The derivation of the filter follows classical EKF principles, naturally integrating a necessary full-order covariance reset post-measurement update. A key contribution is proving that this full-order covariance reset guarantees that the Lie-group-valued state estimate is invariant to whether a left- or right-invariant error definition is used in the EKF. Monte Carlo simulations of the aided inertial navigation problem validate the invariance property and confirm its absence when employing reduced-order covariance resets.
Figures
Forward citations
Cited by 1 Pith paper
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Derivations of Error-State Kalman Filter Kinematics for Globally Applicable Aided Inertial Navigation Systems
Four classical and invariant error-state Kalman filters for global aided INS are derived in full, with system matrices, Jacobians and reset rules presented for direct comparison.
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Reviewed August 7, 2026 · model on record in the stance chip above.
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