Report az/el for satellites excluded from the PVT fix, not just used ones

rescode() (rtklib_pntpos.cc) computes each satellite's azimuth/elevation via
satazel() before checking it against PVT.elevation_mask, and pntpos() copies
that azel into ssat[] unconditionally -- only ssat[].vs is gated by the mask
(the same applies to a satellite dropped by RAIM FDE). The used_satellites
loop in rtklib_solver.cc was keying its entire per-satellite entry on vs, so
a tracked satellite excluded from the fix (below the elevation mask, or by
RAIM) disappeared from MonitorPvt entirely -- indistinguishable from a
genuine tracking problem.

Key entries on whether azel was actually computed this epoch instead, and
add a separate `used` flag (default true) reflecting vs. A satellite that
is tracked but excluded from the fix now still reports its real az/el, with
used = false.

docs/protobuf/monitor_pvt.proto

@@ -53,11 +53,14 @@ message MonitorPvt {
   uint32 galhas_status = 35;  // Galileo HAS status: 1- HAS messages decoded and applied, 0 - HAS not available
   string geohash = 36;        // Encoded geographic location. See https://en.wikipedia.org/wiki/Geohash
 
-  // One satellite/signal used in this fix, with its azimuth/elevation and
-  // whether it was combined with another signal of the same satellite (e.g.
-  // Galileo E1+E5a iono-free combination). Signals are listed individually,
-  // not merged: a combined satellite appears as two entries, both with
-  // combined = true.
+  // One tracked satellite/signal with its azimuth/elevation, whether it was
+  // combined with another signal of the same satellite (e.g. Galileo E1+E5a
+  // iono-free combination), and whether it was actually used in this fix.
+  // Signals are listed individually, not merged: a combined satellite
+  // appears as two entries, both with combined = true. A satellite can have
+  // azimuth_deg/elevation_deg populated with used = false when it was
+  // tracked and had az/el computed but excluded from the fix itself (e.g.
+  // below PVT.elevation_mask, or by RAIM FDE).
   message UsedSatellite {
     uint32 prn = 1;
     string system = 2;  // "G" GPS, "E" Galileo, "R" GLONASS, "C" BeiDou, "S" SBAS, "J" QZSS
@@ -65,6 +68,7 @@ message MonitorPvt {
     double azimuth_deg = 4;
     double elevation_deg = 5;
     bool combined = 6;
+    bool used = 7;
   }
   repeated UsedSatellite used_satellites = 37;
 }

src/algorithms/PVT/libs/monitor_pvt.h

@@ -53,6 +53,12 @@ public:
         double azimuth_deg{};
         double elevation_deg{};
         bool combined{};
+        // false when this satellite/signal was tracked and had azimuth/elevation
+        // computed but was excluded from the fix itself (e.g. below
+        // PVT.elevation_mask, or by RAIM FDE) -- azimuth_deg/elevation_deg are
+        // still valid in that case, only the position solve ignored this
+        // observation.
+        bool used{true};
 
         template <class Archive>
         void serialize(Archive& ar, const unsigned int version)
@@ -66,6 +72,7 @@ public:
             ar& BOOST_SERIALIZATION_NVP(azimuth_deg);
             ar& BOOST_SERIALIZATION_NVP(elevation_deg);
             ar& BOOST_SERIALIZATION_NVP(combined);
+            ar& BOOST_SERIALIZATION_NVP(used);
         }
     };
 

src/algorithms/PVT/libs/rtklib_solver.cc

@@ -2837,13 +2837,27 @@ bool Rtklib_Solver::get_PVT(const std::map<int, Gnss_Synchro> &gnss_observables_
                     // iono-free combination -- see the "dual-frequency" branch of
                     // prange() in rtklib_pntpos.cc) gets one entry per signal, all
                     // flagged combined = true.
+                    //
+                    // rescode() (rtklib_pntpos.cc) computes azel via satazel()
+                    // *before* checking it against PVT.elevation_mask, and
+                    // pntpos() copies that azel into ssat[] unconditionally --
+                    // only ssat[].vs is gated by the mask. So a satellite that
+                    // is tracked and has a live observation this epoch, but
+                    // falls below PVT.elevation_mask (or was excluded by RAIM
+                    // FDE), still has a valid azel here; only vs is false.
+                    // Report it anyway with used = false instead of dropping
+                    // it, so an excluded satellite shows up in the monitor as
+                    // "not used" rather than as missing/no-az-el (which
+                    // otherwise looks identical to a tracking problem).
                     d_monitor_pvt.used_satellites.clear();
                     for (int sat_idx = 0; sat_idx < MAXSAT; sat_idx++)
                         {
-                            if (!pvt_ssat[sat_idx].vs)
+                            const bool has_azel = (pvt_ssat[sat_idx].azel[0] != 0.0) || (pvt_ssat[sat_idx].azel[1] != 0.0);
+                            if (!has_azel)
                                 {
                                     continue;
                                 }
+                            const bool used = pvt_ssat[sat_idx].vs != 0;
                             int prn = 0;
                             char sys_char = '?';
                             switch (satsys(sat_idx + 1, &prn))
@@ -2896,6 +2910,7 @@ bool Rtklib_Solver::get_PVT(const std::map<int, Gnss_Synchro> &gnss_observables_
                                     info.azimuth_deg = az_deg;
                                     info.elevation_deg = pvt_ssat[sat_idx].azel[1] * R2D;
                                     info.combined = combined;
+                                    info.used = used;
                                     d_monitor_pvt.used_satellites.push_back(info);
                                 }
                         }

src/algorithms/PVT/libs/serdes_monitor_pvt.h

@@ -129,6 +129,7 @@ public:
                 pb_sat->set_azimuth_deg(sat.azimuth_deg);
                 pb_sat->set_elevation_deg(sat.elevation_deg);
                 pb_sat->set_combined(sat.combined);
+                pb_sat->set_used(sat.used);
             }
 
         if (!monitor_.SerializeToString(&data))
@@ -188,6 +189,7 @@ public:
                 sat.azimuth_deg = pb_sat.azimuth_deg();
                 sat.elevation_deg = pb_sat.elevation_deg();
                 sat.combined = pb_sat.combined();
+                sat.used = pb_sat.used();
                 monitor.used_satellites.push_back(sat);
             }