diff --git a/libraries/AP_GPS/RTCM3_Parser.cpp b/libraries/AP_GPS/RTCM3_Parser.cpp
new file mode 100644
index 0000000000..64587fb063
--- /dev/null
+++ b/libraries/AP_GPS/RTCM3_Parser.cpp
@@ -0,0 +1,191 @@
+/*
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+/*
+  RTCMv3 parser, used to support moving baseline RTK mode between two
+  GPS modules
+*/
+
+#include <string.h>
+#include "RTCM3_Parser.h"
+
+// reset state
+void RTCM3_Parser::reset(void)
+{
+    pkt_len = 0;
+    pkt_bytes = 0;
+    found_len = 0;
+}
+
+// clear previous packet
+void RTCM3_Parser::clear_packet(void)
+{
+    if (found_len == 0) {
+        return;
+    }
+    // clear previous packet
+    if (pkt_bytes > found_len) {
+        memmove(&pkt[0], &pkt[found_len], pkt_bytes-found_len);
+        pkt_bytes -= found_len;
+    } else {
+        pkt_bytes = 0;
+    }
+    found_len = 0;
+    pkt_len = 0;
+}
+
+// return length of found packet
+uint16_t RTCM3_Parser::get_len(const uint8_t *&bytes) const
+{
+    if (found_len > 0) {
+        bytes = &pkt[0];
+    }
+    return found_len;
+}
+
+// return ID of found packet
+uint16_t RTCM3_Parser::get_id(void) const
+{
+    if (found_len == 0) {
+        return 0;
+    }
+    return (pkt[3]<<8 | pkt[4]) >> 4;
+}
+
+// look for preamble to try to resync
+void RTCM3_Parser::resync(void)
+{
+    const uint8_t *p = (const uint8_t *)memchr(&pkt[1], RTCMv3_PREAMBLE, pkt_bytes-1);
+    if (p != nullptr) {
+        const uint16_t idx = p - &pkt[0];
+        memmove(&pkt[0], p, pkt_bytes - idx);
+        pkt_bytes -= idx;
+        if (pkt_bytes >= 3) {
+            pkt_len = (pkt[1]<<8 | pkt[2]) & 0x3ff;
+        } else {
+            pkt_len = 0;
+        }
+    } else {
+        reset();
+    }
+}
+
+// parse packet
+bool RTCM3_Parser::parse(void)
+{
+    const uint8_t *parity = &pkt[pkt_len+3];
+    uint32_t crc1 = (parity[0] << 16) | (parity[1] << 8) | parity[2];
+    uint32_t crc2 = crc24(pkt, pkt_len+3);
+    if (crc1 != crc2) {
+        resync();
+        return false;
+    }
+
+    // we got a good packet
+    found_len = pkt_len+6;
+    return true;
+}
+
+// read in one byte, return true if a full packet is available
+bool RTCM3_Parser::read(uint8_t byte)
+{
+    clear_packet();
+
+    if (pkt_bytes > 0 && pkt[0] != RTCMv3_PREAMBLE) {
+        resync();
+    }
+
+    if (pkt_bytes == 0 && byte != RTCMv3_PREAMBLE) {
+        // discard
+        return false;
+    }
+
+    if (pkt_bytes >= sizeof(pkt)) {
+        // too long, resync
+        resync();
+    }
+
+    pkt[pkt_bytes++] = byte;
+
+    if (pkt_len == 0 && pkt_bytes >= 3) {
+        pkt_len = (pkt[1]<<8 | pkt[2]) & 0x3ff;
+        if (pkt_len == 0) {
+            resync();
+        }
+    }
+
+    if (pkt_len != 0 && pkt_bytes >= pkt_len + 6) {
+        // got header, packet body and parity
+        return parse();
+    }
+
+    // need more bytes
+    return false;
+}
+
+/*
+  calculate 24 bit RTCMv3 crc. We take an approach that saves memory
+  and flash at the cost of higher CPU load. This makes it appropriate
+  for use in the f103 AP_Periph nodes
+  On a F765 this costs us approx 2ms of CPU per second of 5Hz all
+  constellation RTCM data
+*/
+uint32_t RTCM3_Parser::crc24(const uint8_t *bytes, uint16_t len)
+{
+    uint32_t crc = 0;
+    while (len--) {
+        uint8_t b = *bytes++;
+        const uint8_t idx = (crc>>16) ^ b;
+        uint32_t crct = idx<<16;
+        for (uint8_t j=0; j<8; j++) {
+            crct <<= 1;
+            if (crct & 0x1000000) {
+                crct ^= POLYCRC24;
+            }
+        }
+        crc = ((crc<<8)&0xFFFFFF) ^ crct;
+    }
+    return crc;
+}
+
+#ifdef RTCM_MAIN_TEST
+/*
+  parsing test, taking a raw file captured from UART to u-blox F9
+ */
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <stdio.h>
+
+int main(int argc, const char *argv[])
+{
+    const char *fname = argv[1];
+    int fd = ::open(fname, O_RDONLY);
+    if (fd == -1) {
+        perror(fname);
+        ::exit(1);
+    }
+    RTCM3_Parser parser {};
+    uint8_t b;
+    while (::read(fd, &b, 1) == 1) {
+        if (parser.read(b)) {
+            const uint8_t *bytes;
+            printf("packet len %u ID %u\n", parser.get_len(bytes), parser.get_id());
+        }
+    }
+    return 0;
+}
+#endif
diff --git a/libraries/AP_GPS/RTCM3_Parser.h b/libraries/AP_GPS/RTCM3_Parser.h
new file mode 100644
index 0000000000..f1e606870a
--- /dev/null
+++ b/libraries/AP_GPS/RTCM3_Parser.h
@@ -0,0 +1,59 @@
+/*
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+/*
+ RTCMv3 parser, used to support moving baseline RTK mode between two
+ GPS modules
+*/
+
+#include <stdint.h>
+
+class RTCM3_Parser {
+public:
+    // process one byte, return true if packet found
+    bool read(uint8_t b);
+
+    // reset internal state
+    void reset(void);
+
+    // clear previous packet
+    void clear_packet(void);
+    
+    // return length of found packet
+    uint16_t get_len(const uint8_t *&bytes) const;
+
+    // return ID of found packet
+    uint16_t get_id(void) const;
+    
+private:
+    const uint8_t RTCMv3_PREAMBLE = 0xD3;
+    const uint32_t POLYCRC24 = 0x1864CFB;
+
+    // raw packet, we shouldn't need over 300 bytes for the MB configs we use
+    uint8_t pkt[300];
+
+    // number of bytes in pkt[]
+    uint16_t pkt_bytes;
+
+    // length from header
+    uint16_t pkt_len;
+
+    // length of found packet
+    uint16_t found_len;
+    
+    bool parse(void);
+    void resync(void);
+    uint32_t crc24(const uint8_t *bytes, uint16_t len);
+};
+
diff --git a/modules/mavlink b/modules/mavlink
index d2cc7dbff6..6e08c0757a 160000
--- a/modules/mavlink
+++ b/modules/mavlink
@@ -1 +1 @@
-Subproject commit d2cc7dbff67f8c318a40e1ef57a99488b4737fab
+Subproject commit 6e08c0757a1e90c461a8caf1a8d5b724abf3ca57