Relax p.676 Earth-space path condition

This makes p676_gas() usable down to eldeg = 5.

* src/prop.f90 (p676_gas): As stated. This relaxation is in ITU-R P676-11 over
  the condition in ITU-R P676-3, which was 10 deg.
* test/test-2.py: Exercise eldeg = 5.

Author: lloda

.github/workflows/c-cpp.yml

@@ -1,4 +1,4 @@
-name: C/C++ CI
+name: CI
 
 on:
   push:
@@ -15,7 +15,7 @@ jobs:
     - name: update
       run: |
          sudo apt update
-         sudo apt install gcc-11 gfortran-11 python3
+         sudo apt install gfortran-11 gcc-11 python3
     - name: configure
       run: cmake .
       env:

TODO

@@ -23,7 +23,7 @@ help: [C-uc .] insert timestamp [C-cc] flip checkbox [C-uucc] partial flip check
   - [X] scintillation loss (P.618-13 2.4)
   - [ ] total loss (P.618-13)
   - [ ] aeronautical mobile specifics (P682)
-  - [ ] lower elevation angle effects (<5°) P676, P619, P682
+  - [ ] lower elevation angle effects (<5°, <10°) P676, P619, P682
   - [ ] tropospheric effects (P834)
   - [ ] effects <10 GHz (ionospheric scintillation, Faraday rotation, etc.)
   - [ ] Earth to space interference (P619)

src/prop.f90

@@ -756,7 +756,7 @@ real(C_DOUBLE) function p676_gas(eldeg, freq, P, e, temp, Vt, hs) &
     real(C_DOUBLE), intent(in) :: P             ! dry air pressure (hPa)
     real(C_DOUBLE), intent(in) :: e             ! vapor part. pressure e(P) (hPa)
     real(C_DOUBLE), intent(in) :: temp          ! temperature (K)
-    real(C_DOUBLE), intent(in), optional :: Vt  ! total vaper vapor content (kg/m^2), cf p836_V()
+    real(C_DOUBLE), intent(in), optional :: Vt  ! total water vapor content (kg/m^2), cf p836_V()
     real(C_DOUBLE), intent(in), optional :: hs  ! station height above mean sea level (km)
 
     real :: go, gw, ho, hw, attw
@@ -823,7 +823,7 @@ real(C_DOUBLE) function p676_gas(eldeg, freq, P, e, temp, Vt, hs) &
        end block
     end if
 
-    if (eldeg>=10. .and. eldeg<=90.) then ! §2.2.1
+    if (eldeg>=5. .and. eldeg<=90.) then ! §2.2.1
 
        ! ! debug intermediate values
        ! write(*, *) 'ho', ho
@@ -836,7 +836,7 @@ real(C_DOUBLE) function p676_gas(eldeg, freq, P, e, temp, Vt, hs) &
        ! write(*, *) 'Aw/sinθ', hw*gw / sin(deg2rad(eldeg))
 
        att = (ho*go + attw) / sin(deg2rad(eldeg))  ! (29)
-    else
+    else ! FIXME §2.2.2
        stop 105
     end if
 
@@ -1059,7 +1059,7 @@ end function p836_height_scale
 
 
   ! Water vapor density, ρ in g/m³ exceeded for ppc of an average year.
-  ! After ITU-R P.836-6 Annex 2.
+  ! After ITU-R P.836-6 Annex 1.
 
   real(C_DOUBLE) function p836_rho(lat, lon, ppc, h) &
        bind(c) &
@@ -1075,7 +1075,8 @@ real(C_DOUBLE) function p836_rho(lat, lon, ppc, h) &
   end function p836_rho
 
 
-  ! Total vaper vapor content, after ITU-R P.836-6 Annex 2.
+  ! Total water vapor content.
+  ! After ITU-R P.836-6 Annex 2.
 
   real(C_DOUBLE) function p836_V(lat, lon, ppc, h) &
        bind(c, name='p836_V') &

test/test-2.py

@@ -35,4 +35,17 @@ def rel_error(a, b):
 print("lat {0:f} lon {1:f} hr {2:f} rerr {3:s}".format(lat, lon, hr, repr(rerr)));
 ne += (rerr>=1e-15)
 
+# not in validation tables, just exercise eldeg = 5 limit
+
+hs = 0.4
+temp = 290
+P = prop.p676_vapor_pressure(7.5, temp)
+Vt = prop.p836_V(45., 6., 1, hs)
+eldeg = 5
+fghz = 10
+A = prop.p676_gas(eldeg, fghz, 1013.25, P, temp, Vt, hs)
+rerr = rel_error(0.7464453612889095, A)
+print("eldeg {0:f} fghz {1:f} rerr {2:s}".format(eldeg, fghz, repr(rerr)));
+ne += (rerr>=1e-15)
+
 sys.exit(ne)