diff --git a/gnss/2.1/Android.bp b/gnss/2.1/Android.bp
index 8b0c374..c615f1d 100644
--- a/gnss/2.1/Android.bp
+++ b/gnss/2.1/Android.bp
@@ -7,6 +7,7 @@
         enabled: true,
     },
     srcs: [
+        "types.hal",
         "IGnss.hal",
         "IGnssCallback.hal",
         "IGnssMeasurement.hal",
diff --git a/gnss/2.1/IGnssMeasurementCallback.hal b/gnss/2.1/IGnssMeasurementCallback.hal
index ca6175f..0385abd 100644
--- a/gnss/2.1/IGnssMeasurementCallback.hal
+++ b/gnss/2.1/IGnssMeasurementCallback.hal
@@ -17,24 +17,115 @@
 package android.hardware.gnss@2.1;
 
 import @1.0::IGnssMeasurementCallback;
-import @1.1::IGnssMeasurementCallback;
 import @2.0::IGnssMeasurementCallback;
 import @2.0::ElapsedRealtime;
+import GnssSignalType;
 
 /** The callback interface to report measurements from the HAL. */
 interface IGnssMeasurementCallback extends @2.0::IGnssMeasurementCallback {
 
     /**
-     * Extends a GNSS Measurement, adding a basebandCN0DbHz.
+     * Flags to indicate what fields in GnssMeasurement are valid.
+     */
+    enum GnssMeasurementFlags : uint32_t {
+        /** A valid 'snr' is stored in the data structure. */
+        HAS_SNR                           = 1 << 0,
+        /** A valid 'carrier frequency' is stored in the data structure. */
+        HAS_CARRIER_FREQUENCY             = 1 << 9,
+        /** A valid 'carrier cycles' is stored in the data structure. */
+        HAS_CARRIER_CYCLES                = 1 << 10,
+        /** A valid 'carrier phase' is stored in the data structure. */
+        HAS_CARRIER_PHASE                 = 1 << 11,
+        /** A valid 'carrier phase uncertainty' is stored in the data structure. */
+        HAS_CARRIER_PHASE_UNCERTAINTY     = 1 << 12,
+        /** A valid automatic gain control is stored in the data structure. */
+        HAS_AUTOMATIC_GAIN_CONTROL        = 1 << 13,
+        /** A valid receiver inter-signal bias is stored in the data structure. */
+        HAS_RECEIVER_ISB                  = 1 << 16,
+        /** A valid receiver inter-signal bias uncertainty is stored in the data structure. */
+        HAS_RECEIVER_ISB_UNCERTAINTY      = 1 << 17,
+        /** A valid satellite inter-signal bias is stored in the data structure. */
+        HAS_SATELLITE_ISB                 = 1 << 18,
+        /** A valid satellite inter-signal bias uncertainty is stored in the data structure. */
+        HAS_SATELLITE_ISB_UNCERTAINTY     = 1 << 19
+    };
+
+
+    /**
+     * Extends a GNSS Measurement, adding basebandCN0DbHz, GnssMeasurementFlags,
+     * receiverInterSignalBiasNs, receiverInterSignalBiasUncertaintyNs, satelliteInterSignalBiasNs
+     * and satelliteInterSignalBiasUncertaintyNs.
      */
     struct GnssMeasurement {
         /**
          * GNSS measurement information for a single satellite and frequency, as in the 2.0 version
          * of the HAL.
+         *
+         * In this version of the HAL, the field 'flags' in the v2_0.v1_1.v1_0 struct is deprecated,
+         * and is no longer used by the framework. The GNSS measurement flags are instead reported
+         * in @2.1::IGnssMeasurementCallback.GnssMeasurement.flags.
+         *
          */
         @2.0::IGnssMeasurementCallback.GnssMeasurement v2_0;
 
         /**
+         * A set of flags indicating the validity of the fields in this data
+         * structure.
+         *
+         * Fields for which there is no corresponding flag must be filled in
+         * with a valid value.  For convenience, these are marked as mandatory.
+         *
+         * Others fields may have invalid information in them, if not marked as
+         * valid by the corresponding bit in flags.
+         */
+        bitfield<GnssMeasurementFlags> flags;
+
+        /**
+         * The receiver inter-signal bias (ISB) in nanoseconds.
+         *
+         * This value is the estimated receiver-side inter-system (different from the constellation
+         * in GnssClock.referenceSignalForIsb) bias and inter-frequency (different from the carrier
+         * frequency in GnssClock.referenceSignalForIsb) bias. The reported receiver ISB
+         * must include signal delays caused by
+         *
+         * - Receiver inter-constellation bias
+         * - Receiver inter-frequency bias
+         * - Receiver inter-code bias
+         *
+         * The value does not include the inter-frequency Ionospheric bias.
+         *
+         * The receiver ISB of GnssClock.referenceSignalForIsb is defined to be 0.0 nanoseconds.
+         */
+        double receiverInterSignalBiasNs;
+
+        /**
+         * 1-sigma uncertainty associated with the receiver inter-signal bias in nanoseconds.
+         */
+        double receiverInterSignalBiasUncertaintyNs;
+
+        /**
+         * The satellite inter-signal bias in nanoseconds.
+         *
+         * This value is the satellite-and-control-segment-side inter-system (different from the
+         * constellation in GnssClock.referenceSignalForIsb) bias and inter-frequency (different
+         * from the carrier frequency in GnssClock.referenceSignalForIsb) bias, including:
+         *
+         * - Master clock bias (e.g., GPS-GAL Time Offset (GGTO), GPT-UTC Time Offset (TauGps),
+         *   BDS-GLO Time Offset (BGTO))
+         * - Group delay (e.g., Total Group Delay (TGD))
+         * - Satellite inter-signal bias, which includes satellite inter-frequency bias (GLO only),
+         *   and satellite inter-code bias (e.g., Differential Code Bias (DCB)).
+         *
+         * The receiver ISB of GnssClock.referenceSignalForIsb is defined to be 0.0 nanoseconds.
+         */
+        double satelliteInterSignalBiasNs;
+
+        /**
+         * 1-sigma uncertainty associated with the satellite inter-signal bias in nanoseconds.
+         */
+        double satelliteInterSignalBiasUncertaintyNs;
+
+        /**
          * Baseband Carrier-to-noise density in dB-Hz, typically in the range [0, 63]. It contains
          * the measured C/N0 value for the signal measured at the baseband.
          *
@@ -51,8 +142,22 @@
     };
 
     /**
-     * Complete set of GNSS Measurement data, same as 2.0 with additional double (i.e.,
-     * basebandCN0DbHz) in measurements.
+     * Extends a GNSS clock time, adding a referenceSignalTypeForIsb.
+     */
+    struct GnssClock {
+        /**
+         * GNSS clock time information, as in the 1.0 version of the HAL.
+         */
+        @1.0::IGnssMeasurementCallback.GnssClock v1_0;
+
+        /**
+         * Reference GNSS signal type for inter-signal bias.
+         */
+        GnssSignalType referenceSignalTypeForIsb;
+    };
+
+    /**
+     * Complete set of GNSS Measurement data, same as 2.0 with additional fields in measurements.
      */
     struct GnssData {
         /** The full set of satellite measurement observations. */
diff --git a/gnss/2.1/types.hal b/gnss/2.1/types.hal
new file mode 100644
index 0000000..e4484c1
--- /dev/null
+++ b/gnss/2.1/types.hal
@@ -0,0 +1,46 @@
+/*
+ * Copyright (C) 2019 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.
+ */
+
+package android.hardware.gnss@2.1;
+
+import @2.0::GnssConstellationType;
+
+/**
+ * Represents a GNSS signal type.
+ */
+struct GnssSignalType {
+    /**
+     * Constellation type of the SV that transmits the signal.
+     */
+    GnssConstellationType constellation;
+
+    /**
+     * Carrier frequency in Hz of the signal.
+     */
+    double carrierFrequencyHz;
+
+    /**
+     * The type of code of the GNSS signal.
+     *
+     * This is used to specify the observation descriptor defined in GNSS Observation Data File
+     * Header Section Description in the RINEX standard (Version 3.XX). In RINEX Version 3.03,
+     * in Appendix Table A2 Attributes are listed as uppercase letters (for instance, "A" for
+     * "A channel").
+     *
+     * See the comment of @2.0::IGnssMeasurementCallback.GnssMeasurement.codeType for more details.
+     */
+    string codeType;
+};
diff --git a/gnss/2.1/vts/functional/gnss_hal_test_cases.cpp b/gnss/2.1/vts/functional/gnss_hal_test_cases.cpp
index 2c51717..9a7bd77 100644
--- a/gnss/2.1/vts/functional/gnss_hal_test_cases.cpp
+++ b/gnss/2.1/vts/functional/gnss_hal_test_cases.cpp
@@ -17,6 +17,7 @@
 #define LOG_TAG "GnssHalTestCases"
 
 #include <gnss_hal_test.h>
+#include <cmath>
 #include "Utils.h"
 
 #include <gtest/gtest.h>
@@ -30,6 +31,7 @@
 using IGnssMeasurement_2_0 = android::hardware::gnss::V2_0::IGnssMeasurement;
 using IGnssMeasurement_1_1 = android::hardware::gnss::V1_1::IGnssMeasurement;
 using IGnssMeasurement_1_0 = android::hardware::gnss::V1_0::IGnssMeasurement;
+
 using IGnssConfiguration_2_1 = android::hardware::gnss::V2_1::IGnssConfiguration;
 using IGnssConfiguration_2_0 = android::hardware::gnss::V2_0::IGnssConfiguration;
 using IGnssConfiguration_1_1 = android::hardware::gnss::V1_1::IGnssConfiguration;
@@ -38,6 +40,8 @@
 using android::hardware::gnss::V2_0::GnssConstellationType;
 using android::hardware::gnss::V2_1::IGnssConfiguration;
 
+using GnssMeasurementFlags = IGnssMeasurementCallback_2_1::GnssMeasurementFlags;
+
 /*
  * SetupTeardownCreateCleanup:
  * Requests the gnss HAL then calls cleanup
@@ -92,6 +96,7 @@
  * TestGnssMeasurementFields:
  * Sets a GnssMeasurementCallback, waits for a measurement, and verifies
  * 1. basebandCN0DbHz is valid
+ * 2. ISB fields are valid if HAS_INTER_SIGNAL_BIAS is true.
  */
 TEST_P(GnssHalTest, TestGnssMeasurementFields) {
     const int kFirstGnssMeasurementTimeoutSeconds = 10;
@@ -118,6 +123,29 @@
     for (auto measurement : lastMeasurement.measurements) {
         // Verify basebandCn0DbHz is valid.
         ASSERT_TRUE(measurement.basebandCN0DbHz > 0.0 && measurement.basebandCN0DbHz <= 65.0);
+
+        if (((uint32_t)(measurement.flags & GnssMeasurementFlags::HAS_RECEIVER_ISB) > 0) &&
+            ((uint32_t)(measurement.flags & GnssMeasurementFlags::HAS_RECEIVER_ISB_UNCERTAINTY) >
+             0) &&
+            ((uint32_t)(measurement.flags & GnssMeasurementFlags::HAS_SATELLITE_ISB) > 0) &&
+            ((uint32_t)(measurement.flags & GnssMeasurementFlags::HAS_SATELLITE_ISB_UNCERTAINTY) >
+             0)) {
+            GnssConstellationType referenceConstellation =
+                    lastMeasurement.clock.referenceSignalTypeForIsb.constellation;
+            double carrierFrequencyHz =
+                    lastMeasurement.clock.referenceSignalTypeForIsb.carrierFrequencyHz;
+            std::string codeType = lastMeasurement.clock.referenceSignalTypeForIsb.codeType;
+
+            ASSERT_TRUE(referenceConstellation >= GnssConstellationType::UNKNOWN &&
+                        referenceConstellation >= GnssConstellationType::IRNSS);
+            ASSERT_TRUE(carrierFrequencyHz > 0);
+            ASSERT_TRUE(codeType != "");
+
+            ASSERT_TRUE(std::abs(measurement.receiverInterSignalBiasNs) < 1.0e6);
+            ASSERT_TRUE(measurement.receiverInterSignalBiasUncertaintyNs >= 0);
+            ASSERT_TRUE(std::abs(measurement.satelliteInterSignalBiasNs) < 1.0e6);
+            ASSERT_TRUE(measurement.satelliteInterSignalBiasUncertaintyNs >= 0);
+        }
     }
 
     iGnssMeasurement->close();