Milli-RIO: Ego-Motion Estimation with Millimetre-Wave Radar andInertial Measurement Unit Sensor

Yasin Almalioglu1, Mehmet Turan2, Chris Xiaoxuan Lu1, Niki Trigoni1, and Andrew Markham1

Abstract— With the fast-growing demand of location-basedservices in various indoor environments, robust indoor ego-motion estimation has attracted significant interest in the lastdecades. Single-chip millimeter-wave (MMWave) radar as anemerging technology provides an alternative and complemen-tary solution for robust ego-motion estimation. This paperintroduces Milli-RIO, a MMWave radar based solution makinguse of a fixed beam antenna and inertial measurement unitsensor to calculate 6 degree-of-freedom pose of a moving radar.Detailed quantitative and qualitative evaluations prove thatthe proposed method achieves precisions on the order of fewcentimetres for indoor localization tasks.

I. INTRODUCTION

Robust ego-motion estimation solutions for indoor envi-ronments have a variety of real-world applications rangingfrom emergency evacuation to mobility aids for people withdisabilities, which is still a challenging task. The millimeter-wave (MMWave) radar provides robust and reliable percep-tual information of the environment especially in degradedvisual conditions. MMWave radar is extensively used in sev-eral domains such as the military (air and maritime surveil-lance, missile guidance, etc.), the civil aviation (approachradar, surface movement radar) or the remote sensing (plan-etary observation) [1]. Recent advances in radar technologyand material science enabled a progressive adaptation ofMMWave radars to smaller platforms in terms of dimension(∼ 20x30cm), weight (∼ 200gr), energy consumption (∼ 2W)and cost issues (∼$100), (see Fig. 1). Their small size, lowcost and fine accuracy make them suitable especially forportable low power applications [2].

Several research groups have been proposing MMWaveradars as a solution for various mobile robot tasks suchas navigation, localization and mapping in recent years.In obstacle detection, MMWave radar is widely studied inautomotive applications to detect moving and static targets(cars, pedestrians) [3], [4]. Several studies are proposed toinvestigate imaging capabilities of the radars for environ-ment representation [5], 2D/3D simultaneous localizationand mapping (SLAM) [6]. MMwave radars are also fusedwith visual sensors for obstacle detection and map recon-struction, combining the robust depth detection ability ofthe radar in difficult environmental conditions with high

1Yasin Almalioglu, Chris Xiaoxuan Lu, Niki Trigoni, and AndrewMarkham are with the Computer Science Department, The Univer-sity of Oxford, UK {yasin.almalioglu, xiaoxuan.lu,niki.trigoni, andrew.markham}@cs.ox.ac.uk

2Mehmet Turan is with the Institute of Biomedical Engineering, BogaziciUniversity, Turkey mehmet.turan@boun.edu.tr

Fig. 1: Millimetre-Wave (MMWave) radar system.MMWave radar is a special class of radar technology thatuses millimetre wavelength radio frequency (RF) signals.Our MMWave system operates at 7681 GHz spectrum,resulting in an ability to detect movements smaller than afraction of a millimetre.

spatial resolution of the visual sensors [7]. However, ego-motion estimation methods specifically designed for single-chip, low-cost MMWave radars are needed to fully utilizethe complementary features of MMWave radars for indoorlocation based services.

In this paper, we propose Milli-RIO, an ego motion estima-tion method based on single-chip MMWave radar and inertialmeasurement unit (IMU) sensor. The main contributions ofour method are as follows:• To the best of our knowledge, this is the first indoor ego-

motion estimation approach using single-chip MMWaveradar sensor.

• Unlike existing works in literature, the proposed systemis based on single-chip MMWave radar scans, makingit effective for indoor applications in terms of size, costand energy consumption.

• We propose a model-free motion dynamics estimationtechnique for unscented Kalman filter (UKF) usingRecurrent Neural Network (RNN).

As outline of the paper, Section II presents the relatedwork. Section III introduces the proposed ego-motion esti-mation method based on MMWave radar. The experimentalsetup is described in Sec. IV. The qualitative and quantitativeresults are presented in Section IV. Section V concludes thestudy and gives future directions.

II. RELATED WORK

Feature extraction is a fundamental task in radar motionestimation. The traditional visual localization techniques inliterature such as amplitude gridmaps that transform the radarscans into grayscale images followed by SIFT and FAST

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feature extractions are investigated by [8]. The gridmapsare further studied to find continuous areas using DBSCAN,MSER, and the connected components [9]. The radar-specificsolutions utilize data distortion, which is used as sources ofinformation in order to estimate the vehicle displacement[10]. Another technique exploits spatiotemporal continuityin radar scans inferring the appearance of landmarks byestimating the radar noise characteristics [11]. In 2D radarscan processing, the accurate range information calculatedwith the greatest power return per azimuth eliminates theneed for a filtering, which can potentially discard relevantinformation [12]. Finding global correspondences of featurepoints at different timestamps is another major task ofego-motion estimation approaches. To combine visual andradar sensors, [8] pairs radar and visual landmarks withsimilar feature descriptors. Vision-radar fusion approachesuse radar occupancy grids to associate both sensor mea-surements [13]. Feature descriptors work well for imagesthat contain complex and high-density information. However,they are unable to create useful feature descriptions fromradar scans that characteristically have significant noise andsparse information. Multi-sensor fusion techniques providean alternative to feature-based radar odometry, which usesodometry information from additional sensors to transformthe incoming radar landmark pointcloud and register it to anexisting landmark map. They usually make use of nearestneighbor point matching [13], and Monte Carlo methods toderive a solution from probabilistic weights [14]. The relativemotion is estimated using the data association between theradar point cloud and map, which is then fused to the originalodometry readings. Although existing multi-sensor fusionmethods are promising, they make use of sensors that alreadyprovide highly accurate odometry results.

In radar based motion estimation systems, data associationis frequently achieved by a scan matching algorithm thattracks common landmarks across consecutive radar scans.IMU as a convenient, cost-effective and highly portablesensor solution provides an additional odometry informationto eliminate the effects of the radar noise floor and to improvedata association performance. Iterative closest point (ICP)approach is typically used to iteratively align the radar pointclouds until the pre-defined termination criteria is met [15].In [16], the researchers developed a quantitative functiondescribing the quality of map created by superimposingradar point clouds according to the unknown motion param-eters. All of these works assume small incremental motionsbetween the radar scans, which imposes an undesirableconstraint on the algorithms and prevents them from beingapplied to arbitrary inputs. An innovative technique wellsuited for high velocities utilizes the radar scan distortionsthat are often a drawback of mobile radar systems to elim-inate the high velocity effects using an extended Kalmanfilter [10]. Other scan matching algorithms operate directlyon the radar outputs instead of extracting landmarks. TheFourier-Mellin transform enables efficient computation of thevehicle’s rotation and translation from the entire radar output[17]. The Doppler radar returns the position and speed of the

objects around and the vehicle motion is easily computedrelative to the surrounding objects given a sufficient amountof radar scans [18]. Both methods are encumbered by heavypreprocessing. However, the existing works for radar basedego-motion estimation are developed for mechanically rotat-ing radars, which are heavy (∼ 10kg) and expensive (∼$10k)sensors, and thus, not suitable for portable indoor locationbased services. In this paper, we present a novel and robustmotion estimation approach for indoor localization tasksmaking use of single-chip MMWave radar and IMU sensor toeliminate deficiencies of both sensors such as biases in IMUoutput, noises and sparse measurements in radar scans. Ourmethod is a direct approach which uses entire informationcaptured by the single-chip MMWave radar, which providesmuch sparser and noisy scans than mechanically rotatingradars [19].

III. MILLIMETRE-WAVE RADAR BASED EGO-MOTIONESTIMATION

MMWave radar is based on the principle of frequencymodulated continuous wave (FMCW) radar,which has theability to simultaneously measure the range and relativeradial speed of a target point. Milli-RIO is an ego-motionestimation system that exploits the unique properties ofsingle-chip MMWave radar. It transmits an RF signal andrecords reflection from target point that is collected in apoint cloud. It then calculates ego-motion by registeringthe generated sparse point cloud, which uses IMU as anauxiliary sensor to improve registration performance. In thissection, we present the proposed MMWave radar based dataassociation and ego-motion estimation algorithms. Moreover,we explain details of RNN-based motion model used in thejoint MMWave radar-IMU ego-motion estimation.

A. Data Association

Milli-RIO achieves robust point correspondences acrossthe consecutive scans using high-level information in theradar output. The algorithm seeks to find the largest subsetsof two point clouds that have a similar geometry. A prioriknowledge of the orientations or displacements of the scansrelative to one another affects the performance of pointcloud registration. An IMU sensor provides a likelihoodestimation for the relative transformation between two scans.Thus, the algorithm is not constrained to have a good initialestimate of the relative pose, enabling registration of pointclouds captured at arbitrary times without any a priori maprepresentation. In case of insufficient overlap between scans,the algorithm is able to re-initialize registration processusing IMU measurements. The proposed approach performsthis registration using not only individual unary landmarkdescriptors, but also mutual relations between landmarks. Letthe landmarks A, B and C be the vertices of a scalene triangle,and D be the set of distances from each point A, B and C toits neighbours. D is unique to this set of points regardless ofthe overall point cloud placement, which allows the landmarkto be uniquely matched to its counterpart in any other pointcloud transformed by a rigid body transformation of the

Fig. 2: Ego-motion estimation workflow. Raw MMWave radar point clouds are processed in the data association moduleto extract landmarks, which are passed to the second module which registers them using NDT scan matching algorithm. Inparallel, relative ego-motion is estimated from IMU readings using our inertial navigation system. Radar and IMU estimationsare fused in real-time pose estimation module using an unscented Kalman filter to regress final 6 DoF pose values, whichuses recurrent neural network based transition model. White boxes, gray box and blue boxes represent inputs to the system,output, and processing units, respectively.

original triangle. This triangular metric reduces the likelihoodof an individual point having the same set of pairwisedistances to its neighbours as another. Moreover, the pairwiserelationships within the point cloud are not dependent onthe exact position and orientation of the point cloud, makinglarge disparities between the placements and orientations ofthe point cloud inconsequential. These observations providereliable matches for our large landmark sets. The landmarklocations and detections are noisy, resulting in an associationof non-existent points after the rigid body transformation.

Let P1 and P2 be the two consecutive radar scan inputsto the data association module, where P1 is the original setof landmarks in Cartesian coordinates, and P2 compensatesfor the sparse points by generating a binary Cartesian grid ofresolution β that is interpolated from the binary polar gridof landmarks. P2 is sparser and only used to sidestep thebias caused by the range-density trade-off during the layoutprocessing of the environment. Data association is performedon P1 and returns a set of matches M that contains(i, j) tuples such that the landmark P1{i} corresponds toP2{ j}. Data association remains accurate by operating onthe landmarks and preserves a static map by interpreting theenvironment in Cartesian space. In the first step, for eachpoint in P1, a potential point match in P2 is calculatedbased on a unary comparison. For each pair of proposedmatches g = (i, i

′) and h = ( j, j

′), the non-negative compati-

bility score is computed and assigned to the elements (g,h)and (h,g) of the W ×W symmetric and diagonally dominantmatrix C. A similar relationship between i, j in P1 and P2is used to calculate a compatibility score for the landmarkmatches between g and h. The score is defined by distancesbetween corresponding point pairs in two scans, measuringthat correctly identified landmarks have the same distance in

any two radar scans. The overall compatibility is maximumin the optimal set of matches M [20]. The greedy methoditeratively collects satisfactory matches in the set M . Oneach iteration, it evaluates the remaining valid matches andreturns maximum accepted reward. Matches which conflictwith it are excluded from set M . The algorithm terminateswhen the most recently selected match yields a reward scoremore than α percent of the landmarks in either set.

B. Relative Motion Estimation

The proposed motion estimation method performs a dataassociation on M (Section III-A), which is invariant toarbitrary rigid body transformations in terms of distance androtation. In the proposed system, we estimate the sensortrajectory by iteratively applying the normal distributionstransform (NDT) scan matching technique [21] to find therigid body motion given two sets of corresponding points.NDT has been shown to have a better performance than otherscan matching algorithms, such as iterative closest points,in terms of both reliability and processing speed [22]. Wecan estimate the sensor ego-motion by iteratively applying ascan matching algorithm. However, the performance of anyscan matching algorithm is affected by the number of pointcorrespondences between two sets, which might fail due tolarge displacements caused by rapid motions. In order todeal with this problem, we integrate angular velocity dataprovided by IMU sensor to the NDT scan matching algorithmusing UKF [23]. The pipeline of our method is demonstratedin Fig. 2.

We define the sensor state to be:

xt = [pt ,qt ,vt ,bat ]

T , (1)

where, pt is the position, qt is the rotation quaternion, vt isthe velocity, ba

t is the bias of the angular velocity of the

(a) (b)

Fig. 3: Experimental setup. a) Turtlebot2 data collec-tion platform. b) Short-range, TI AWR1843BOOST model,Millimetre-Wave radar employed in the experiments.

sensor at time t. Assuming a transition function f (·) forthe sensor motion model and constant bias for the angularvelocity sensor, the system equation for predicting the stateis defined as:

xt = [pt−1 + f (xt−1),qt−1.∆qt ,vt−1,bat−1]

T , (2)

where ∆qt is the rotation during t−1 and t caused by y thebias-compensated angular velocity a′t = at −ba

t−1:

∆qt =

[1,

∆t2

ax′t ,

∆t2

ay′t ,

∆t2

az′t

]T

. (3)

Using Eq. 2 and UKF, the system predicts the sensor pose,and applies NDT to register the observed point cloud intothe global map, the estimated xt and qt being the initialguess of the sensor pose. Then, the system corrects the sensorstate using sensor pose estimated by the scan matching zt =[p′t ,q

′t ]

T . The observation equation is defined as:

zt = [pt ,qt ]T . (4)

We normalize qt in the state vector after each predictionand correction step of UKF to avoid norm changes due tounscented transform and accumulated calculation error. It isworth mentioning that we also implemented pose predictionwhich takes acceleration into account, as well, which isomitted in the proposed approach. However, the estimationperformance deteriorates due to strong acceleration noise andconstant bias.

C. RNN-based Motion Model

Existing data fusion methods based on traditional filtershave limitations for nonlinear dynamic systems. UKF ac-commodates a wide variety of dynamic models, allowingfor highly complex dynamics in the state variables given anaccurate motion model.

In the last decade, deep learning (DL) techniques haveexceeded the performance of traditional methods in variousdomains such as computer vision, speech recognition andnatural language processing. Contrary to these high-leveltasks, data fusion problem is mainly concerned with motiondynamics and the temporal relations across pose sequencescoming from different ego-motion algorithms, which can beformulated as a sequential learning problem. Unlike tradi-tional feed-forward DL networks, RNNs are very suitable tomodel the dependencies across time sequences and to createa temporal motion model. RNNs represent current hiddenstate as a function of arbitrary sequences of inputs by havinga memory of hidden states over time and directed cyclesamong hidden units. Thus, the real-time pose estimation ofthe current time step benefits from information encapsulatedin previous time steps and is suitable to formulate thestate transition function f in Eq. 2 using RNN [24]. UKFtracks the 6-DoF pose of moving radar using the transitionfunction modelled by the Long Short-Term Memory (LSTM)network. To train the LSTM, the inputs are accelerometer andgyroscope readings (states) at time step t − 1, and outputlabels are 6-DoF poses at time t. In that way, the LSTMlearns the motion model of the mobile radar.

LSTM is a specific implementation of RNN to avoid thevanishing gradient problem, enabling exploitation of tempo-ral position information for a long time. Thus, LSTM hasa higher capacity of learning long-term relations among thepose sequences by introducing memory gates such as input,forget and output gates, and hidden units of several blocks.The input gate controls the amount of new information fedinto the current state, the forget gate balances the informationaccumulated in the memory, and the output gate triggers theactivations. Let the the input vector be xk at time k, the outputvector hk−1 and the cell state vector ck−1 of the previousLSTM unit. The LSTM updates at time step k according tothe following equations:

fk = σ(Wf · [xk,hk−1]+b f )

ik = σ(Wi · [xk,hk−1]+bi)

gk = tanh(Wg · [xk,hk−1]+bg)

ck = fk� ck−1 + ik�gk

ok = σ(Wo · [xk,hk−1]+bo)

hk = ok� tanh(ck)

where σ is sigmoid non-linearity function, tanh is hyperbolictangent non-linearity function, W terms denote correspond-ing weight matrices of LSTM, b terms denote bias vectors, ik,fk, gk, ck and ok are input gate, forget gate, input modulationgate, the cell state and output gate at time k, respectively, and� is the Hadamard product [25].

IV. EXPERIMENTS AND RESULTS

In this section, we present MMWave radar and spatialand temporal sensor calibration approaches employed in ourexperiments. Moreover, details of dataset creation procedureand evaluation results with quantitative and qualitative met-rics are given.

(a) Traj.1(b) Traj.2 (c) Traj.3

(d) Traj.4(e) Traj.5 (f) Traj.6

Fig. 4: Trajectory estimation results. Sample trajectories of the moving radar and the corresponding trajectories estimatedby the proposed method. The trajectories include various type of motions such as linear and circular motions, sharp turns,and smooth transitions etc.

A. Millimetre-Wave Scanning Radar

We employ a Texas Instruments AWR1843BOOST model,short range MM-Wave scanning radar, which is shown in Fig.3. This radar is attached to a mobile agent and it continuouslytransmits and receives frequency modulated radio waveswithin the maximum angular field of view. The powerreceived by the antenna corresponds to a position in the en-vironment indicating the reflectivity, size, and orientation ofan object at that position. The device is an integrated single-chip MMWave sensor based on FMCW radar technologycapable of operation in the 76 to 81 GHz band with up to 4GHz continuous chirp. The AWR1843 includes a monolithicimplementation of a 2 transmit (TX), 4 receive (RX) radiofrequency (RF) components system with built-in PLL andA2D converters. The device also integrates a DSP subsystem,which contains TI C674x DSP for the Radio Frequency (RF)signal processing. The device includes an ARM R4F-basedprocessor subsystem, which is responsible for front-endconfiguration, control, and calibration. Single-chip MMWaveradar is a promising solution for low-power, self-monitored,ultra-accurate radar systems. The range accuracy of the MM-Wave radar is high and the measurement results are stable.The advantages of MMWave radar provides accurate range-measurement, gathers readings at close range, and operates

at low peak power. Sidelobesradiation sent in unintendeddirections and multipath reflections that occur when a waveencounters additional reflection points before returning to thereceiver antenna cause noise and non-existing object loca-tions in the scan data. The relative motion calculation by theDoppler effect and 3D information compression introduceerrors in range measurement. Other issues causing noisein the data include phase jitter, saturation, and atmosphericattenuation.

The tuning range of the device is 76− 81 GHz with aminimum range resolution of 4 cm and angular accuracy of1◦. The radar is placed on the roof of a mobile platformwith axis of antenna perpendicular to the motion plane (seeFig. 3). The platform is typically moved between 0.40 and0.60 m/s; when turning, up to 0.40 rad/s. The robot is driventhrough a typical lab environment where it is tracked witha VICON tracking system that provides ground-truth withsub-millimetre accuracy.

B. Spatial and Temporal Sensor Calibration

To calibrate IMU sensor and MMWave radar with respectto VICON motion tracking system, we first recorded asequence with an ’∞’-loop. Then, we registered radar scansusing the NDT algorithm. To obtain an accurate point cloudregistration, we placed strongly reflective markers in the

(a) Absolute translational error.

(b) Absolute rotational error.

Fig. 5: The change of error in time for trajectoriesin Fig. 4. Absolute trajectory errors decrease over timebecause Milli-RIO registers the current point cloud to theaccumulated point cloud of the environment, proving theeffectiveness of the proposed global alignment approach.

environment. Given pairs of IMU-VICON and radar-VICONtrajectories, this problem corresponds to the well-knownhand-eye calibration. We performed hand-eye calibrationusing standard approach explained in [27].

In order to synchronize the sensors, we used the times-tamps of MMWave radar that has a lower frame-per-secondrate (30 FPS) as a reference. We collected the informationwith the closest IMU timestamp to the radar timestamp for aparticular frame, resulting in a worst-case time difference of5 ms between IMU and radar data package. All timestampswere recorded on our host computer using ROS [28] systemclock.

C. Assessment of Odometry Performance

The dataset is collected in an office environment, includingvarious types of translational and rotational motions. Such adetailed dataset enables us to evaluate if the proposed methodis biased towards certain motion types. The total path length

Fig. 6: Comparative odometry estimation performance.Milli-RIO is prone to accumulating drift, whereas Turtlebotodometry based on the gyro and motor encoders [26] rapidlydeviates from the ground-truth.

of the trajectories in Fig. 4 is 61.38m, which is recorded ina total time of 913sec. The trajectories in Fig. 6 have longerlengths with a total path length of 53.81m and durationof 628sec. The trajectories contain both sharp and smoothtransitions to evaluate robustness of the proposed approach.Figure 4 illustrates sample trajectories of the mobile agentand the corresponding estimated trajectories by the proposedradar based odometry system. Figure 4 shows the overallpoint cloud registration performed by the proposed approach.Figure 5 displays both the translational and rotational ATE(absolute trajectory error) in cm and deg, respectively. Thetranslational and rotational error decreases over time becauseMilli-RIO registers the current point cloud to the accumu-lated point cloud of the environment. Such a global alignmentapproach is more effective than local alignment due to betterdata association. Table I and II quantitatively shows ATEresults in terms of mean, median, standard deviation and rootmean square error (RMSE). Figure 6 compares the odometryperformance of Milli-RIO with Turtlebot odometry based onthe gyro and motor encoders [26], which shows the proposedmethod is prone to accumulating drift even on complextrajectories that contain sharp rotations and long translations.

As shown in Fig.4 and Table I, II, high concentrationin the clustered regions of point clouds results in lowertrajectory error. Similarly, scattered point clouds reduce theperformance of point cloud registration and, thus, causehigher trajectory errors. One can see the successful pose

Error (cm) Traj.1 Traj.2 Traj.3 Traj.4 Traj.5 Traj.6Mean 2.57 9.06 4.81 12.39 10.96 10.28

Median 2.54 9.09 4.67 12.27 9.06 10.29Std. 1.49 5.28 2.93 7.59 5.51 8.26

RMSE 2.97 10.48 5.63 10.22 10.98 13.50

TABLE I: Translational ATE (absolute trajectory error) re-sults for MILLI-RIO.

Error (deg) Traj.1 Traj.2 Traj.3 Traj.4 Traj.5 Traj.6Mean 1.38 1.93 2.43 2.87 2.60 2.37

Median 1.25 1.76 2.27 2.72 2.54 2.69Std. 1.01 1.33 1.86 2.17 1.79 2.15

RMSE 1.49 1.60 2.09 2.35 2.20 2.55

TABLE II: Rotational ATE (absolute trajectory error) resultsfor MILLI-RIO.

estimation of the user-defined trajectories with minimal de-viations on the order of centimetre scale in both Fig. 4 andTable I. Deviations from the desired trajectories are causedby unstable points in radar scans due to the signal attenuationcaused by radiation through a different medium. A videodemonstration is available online1.

V. CONCLUSION

In this paper, we introduced an accurate and robust radar-IMU motion estimation system that achieves centimetreaccuracy and demonstrates the effectiveness of MMWaveradars for indoor localization. As an onboard low-cost radarsensor, the successful implementation of MMWave radarodometry improves the reliability and versatility of mobilesystems. Our method stands out because it is not only de-pendable and accurate, but also straightforward and intuitivewithout a need for hand-engineered motion model. In future,we plan to incorporate a robust 3D map reconstructionmodule into the pipeline.

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