AP_GPS: sanity check update rate

Also minor spelling and parameter documentation fix

libraries/AP_GPS/AP_GPS.cpp

@@ -157,6 +157,7 @@ const AP_Param::GroupInfo AP_GPS::var_info[] = {
     // @Description: Controls how often the GPS should provide a position update. Lowering below 5Hz is not allowed
     // @Units: milliseconds
     // @Values: 100:10Hz,125:8Hz,200:5Hz
+    // @Range: 100 200
     // @User: Advanced
     AP_GROUPINFO("RATE_MS", 14, AP_GPS, _rate_ms[0], 200),
 
@@ -165,6 +166,7 @@ const AP_Param::GroupInfo AP_GPS::var_info[] = {
     // @Description: Controls how often the GPS should provide a position update. Lowering below 5Hz is not allowed
     // @Units: milliseconds
     // @Values: 100:10Hz,125:8Hz,200:5Hz
+    // @Range: 100 200
     // @User: Advanced
     AP_GROUPINFO("RATE_MS2", 15, AP_GPS, _rate_ms[1], 200),
 
@@ -254,6 +256,12 @@ void AP_GPS::init(DataFlash_Class *dataflash, const AP_SerialManager& serial_man
     // Initialise class variables used to do GPS blending
     _omega_lpf = 1.0f / constrain_float(_blend_tc, 5.0f, 30.0f);
 
+    // sanity check update rate
+    for (uint8_t i=0; i<GPS_MAX_RECEIVERS; i++) {
+        if (_rate_ms[i] <= 0 || _rate_ms[i] > GPS_MAX_RATE_MS) {
+            _rate_ms[i] = GPS_MAX_RATE_MS;
+        }
+    }
 }
 
 // baudrates to try to detect GPSes with
@@ -978,13 +986,25 @@ float AP_GPS::get_lag(uint8_t instance) const
     } else if (drivers[instance] == nullptr || state[instance].status == NO_GPS) {
         // no GPS was detected in this instance
         // so return a default delay of 1 measurement interval
-        return 0.001f * (float)_rate_ms[instance];
+        return 0.001f * (float)get_rate_ms(instance);
     } else {
         // the user has not specified a delay so we determine it from the GPS type
         return drivers[instance]->get_lag();
     }
 }
 
+/*
+  return gps update rate in milliseconds
+*/
+uint16_t AP_GPS::get_rate_ms(uint8_t instance) const
+{
+    // sanity check
+    if (instance >= num_instances || _rate_ms[instance] <= 0) {
+        return GPS_MAX_RATE_MS;
+    }
+    return MIN(_rate_ms[instance], GPS_MAX_RATE_MS);
+}
+
 /*
  calculate the weightings used to blend GPs location and velocity data
 */
@@ -1010,8 +1030,8 @@ bool AP_GPS::calc_blend_weights(void)
         if ((state[i].last_gps_time_ms < min_ms) && (state[i].last_gps_time_ms > 0)) {
             min_ms = state[i].last_gps_time_ms;
         }
-        if (_rate_ms[i] > max_rate_ms) {
-            max_rate_ms = _rate_ms[i];
+        if (get_rate_ms(i) > max_rate_ms) {
+            max_rate_ms = get_rate_ms(i);
         }
     }
     if ((int32_t)(max_ms - min_ms) < (int32_t)(2 * max_rate_ms)) {

libraries/AP_GPS/AP_GPS.h

@@ -31,6 +31,7 @@
 #define GPS_MAX_RECEIVERS 2 // maximum number of physical GPS sensors allowed - does not include virtual GPS created by blending receiver data
 #define GPS_MAX_INSTANCES  (GPS_MAX_RECEIVERS + 1) // maximumum number of GPs instances including the 'virtual' GPS created by blending receiver data
 #define GPS_RTK_INJECT_TO_ALL 127
+#define GPS_MAX_RATE_MS 200 // maximum value of rate_ms (i.e. slowest update rate) is 5hz or 200ms
 
 // the number of GPS leap seconds
 #define GPS_LEAPSECONDS_MILLIS 18000ULL
@@ -142,7 +143,7 @@ public:
 
     // return number of active GPS sensors. Note that if the first GPS
     // is not present but the 2nd is then we return 2. Note that a blended
-    // GPS solution is treated as an addditional sensor.
+    // GPS solution is treated as an additional sensor.
     uint8_t num_sensors(void) const {
         if (!_output_is_blended) {
             return num_instances;
@@ -152,7 +153,7 @@ public:
     }
 
     // Return the index of the primary sensor which is the index of the sensor contributing to
-    // the output. A blended solution is avalable as an additional instance
+    // the output. A blended solution is available as an additional instance
     uint8_t primary_sensor(void) const {
         return primary_instance;
     }
@@ -327,6 +328,9 @@ public:
     float get_lag(uint8_t instance) const;
     float get_lag(void) const { return get_lag(primary_instance); }
 
+    // return gps update rate in milliseconds
+    uint16_t get_rate_ms(uint8_t instance) const;
+
     // return a 3D vector defining the offset of the GPS antenna in meters relative to the body frame origin
     const Vector3f &get_antenna_offset(uint8_t instance) const {
         if (instance == GPS_MAX_RECEIVERS) {

libraries/AP_GPS/AP_GPS_UBLOX.cpp

@@ -1269,7 +1269,7 @@ AP_GPS_UBLOX::_configure_rate(void)
 {
     struct ubx_cfg_nav_rate msg;
     // require a minimum measurement rate of 5Hz
-    msg.measure_rate_ms = MIN(gps._rate_ms[state.instance], MINIMUM_MEASURE_RATE_MS);
+    msg.measure_rate_ms = MIN(gps._rate_ms[state.instance], GPS_MAX_RATE_MS);
     msg.nav_rate        = 1;
     msg.timeref         = 0;     // UTC time
     _send_message(CLASS_CFG, MSG_CFG_RATE, &msg, sizeof(msg));

libraries/AP_GPS/AP_GPS_UBLOX.h

@@ -48,7 +48,6 @@
 #define UBX_MSG_TYPES 2
 
 #define UBLOX_MAX_PORTS 6
-#define MINIMUM_MEASURE_RATE_MS 200
 
 #define RATE_POSLLH 1
 #define RATE_STATUS 1