AP_HAL_SITL: Spam u-blox NAV-SVINFO at 0.1Hz

This resolves EKF3 being hung on launch

libraries/AP_HAL_SITL/sitl_gps.cpp

@@ -271,12 +271,34 @@ void SITL_State::_update_gps_ubx(const struct gps_data *d, uint8_t instance)
         uint32_t headVeh;
         uint8_t reserved2[4]; 
     } pvt;
+    const uint8_t SV_COUNT = 10;
+    struct PACKED ubx_nav_svinfo {
+        uint32_t itow;
+        uint8_t numCh;
+        uint8_t globalFlags;
+        uint8_t reserved1[2];
+        // repeated block
+        struct PACKED svinfo_sv {
+            uint8_t chn;
+            uint8_t svid;
+            uint8_t flags;
+            uint8_t quality;
+            uint8_t cno;
+            int8_t elev;
+            int16_t azim;
+            int32_t prRes;
+        } sv[SV_COUNT];
+    } svinfo;
     const uint8_t MSG_POSLLH = 0x2;
     const uint8_t MSG_STATUS = 0x3;
     const uint8_t MSG_DOP = 0x4;
     const uint8_t MSG_VELNED = 0x12;
     const uint8_t MSG_SOL = 0x6;
     const uint8_t MSG_PVT = 0x7;
+    const uint8_t MSG_SVINFO = 0x30;
+
+    static uint32_t _next_nav_sv_info_time = 0;
+
     uint16_t time_week;
     uint32_t time_week_ms;
 
@@ -365,6 +387,26 @@ void SITL_State::_update_gps_ubx(const struct gps_data *d, uint8_t instance)
     _gps_send_ubx(MSG_SOL,    (uint8_t*)&sol, sizeof(sol), instance);
     _gps_send_ubx(MSG_DOP,    (uint8_t*)&dop, sizeof(dop), instance);
     _gps_send_ubx(MSG_PVT,    (uint8_t*)&pvt, sizeof(pvt), instance);
+
+    if (time_week_ms > _next_nav_sv_info_time) {
+        svinfo.itow = time_week_ms;
+        svinfo.numCh = 32;
+        svinfo.globalFlags = 4; // u-blox 8/M8
+        // fill in the SV's with some data even though firmware does not currently use it
+        // note that this is not using num_sats as we aren't dynamically creating this to match
+        for (uint8_t i = 0; i < SV_COUNT; i++) {
+            svinfo.sv[i].chn = i;
+            svinfo.sv[i].svid = i;
+            svinfo.sv[i].flags = (i < _sitl->gps_numsats) ? 0x7 : 0x6; // sv used, diff correction data, orbit information
+            svinfo.sv[i].quality = 7; // code and carrier lock and time synchronized
+            svinfo.sv[i].cno = MAX(20, 30 - i);
+            svinfo.sv[i].elev = MAX(30, 90 - i);
+            svinfo.sv[i].azim = i;
+            // not bothering to fill in prRes
+        }
+        _gps_send_ubx(MSG_SVINFO, (uint8_t*)&svinfo, sizeof(svinfo), instance);
+        _next_nav_sv_info_time = time_week_ms + 10000; // 10 second delay
+    }
 }
 
 static void swap_uint32(uint32_t *v, uint8_t n)