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diff --git a/firmware/os/algos/calibration/sphere_fit/calibration_data.h b/firmware/os/algos/calibration/sphere_fit/calibration_data.h
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+/*
+ * Copyright (C) 2016 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/*
+ * This module contains a data structure and corresponding helper functions for
+ * a three-axis sensor calibration.  The calibration consists of a bias vector,
+ * bias, and a lower-diagonal scaling and skew matrix, scale_skew_mat.
+ *
+ * The calibration is applied to impaired sensor data as follows:
+ *
+ * corrected_data = scale_skew_mat * (impaired_data - bias).
+ */
+
+#ifndef LOCATION_LBS_CONTEXTHUB_NANOAPPS_CALIBRATION_SPHERE_FIT_CALIBRATION_DATA_H_
+#define LOCATION_LBS_CONTEXTHUB_NANOAPPS_CALIBRATION_SPHERE_FIT_CALIBRATION_DATA_H_
+
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#define THREE_AXIS_DIM (3)
+
+// Calibration data structure.
+struct ThreeAxisCalData {
+  // Scale factor and skew terms.  Used to construct the following lower
+  // diagonal scale_skew_mat:
+  // scale_skew_mat = [scale_factor_x    0         0
+  //                   skew_yx    scale_factor_y   0
+  //                   skew_zx       skew_zy   scale_factor_z].
+  float scale_factor_x;
+  float scale_factor_y;
+  float scale_factor_z;
+  float skew_yx;
+  float skew_zx;
+  float skew_zy;
+
+  // Sensor bias offset.
+  float bias[THREE_AXIS_DIM];
+
+  // Calibration time.
+  uint64_t calibration_time_nanos;
+};
+
+// Set calibration data to identity scale factors, zero skew and
+// zero bias.
+void calDataReset(struct ThreeAxisCalData* calstruct);
+
+// Apply a stored calibration to correct a single sample of impaired sensor
+// data.
+void calDataCorrectData(const struct ThreeAxisCalData* calstruct,
+                        const float x_impaired[THREE_AXIS_DIM],
+                        float* x_corrected);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif  // LOCATION_LBS_CONTEXTHUB_NANOAPPS_CALIBRATION_SPHERE_FIT_CALIBRATION_DATA_H_