*OBS Section

RFM Driver Table Sections: Observer Location

01JUN24
Type
Secondary Section, requires OBS Flag.

Description
Specify observer altitude and, optionally, horizontal angle.

Format
PARAM=VALUE pairs, any order [2]
ParamTypeDescription Units
HGTOBS R Observer altitude [3] km
PREOBS R Observer pressure [3] hPa
PSIOBS R Observer horizontal angle [4] deg
Type: R=Real

The allowed range of HGTOBS or PREOBS depends on the viewing geometry specified in the *TAN section, to ensure that the resulting path always intersects the atmosphere at some point.
Circular Geometry
*TAN/*GEO Tangent Point Altitude HGTOBS ≥ highest tangent point
*ELE Elevation Angle ≥ 0 HGTOBS within atmosphere
Elevation Angle < 0 HGTOBS low enough so path intersects atmosphere
Plane Parallel Geometry
*SEC+ZEN Upward viewing HGTOBS within atmosphere
*SEC+NAD Downward viewing No restriction

Notes
  1. Where the observer location is not specified (ie OBS Flag not enabled) it is assumed that the observer is either somewhere between the top of the atmosphere and infinity — the precise altitude makes no difference since the path is specified by tangent height or zenith angle — or, with ZEN flag, at the surface. With the GRA Flag, all tangent points are then taken to lie at PSI=0.

  2. For compatibility with older versions of the RFM, the *OBS section can also contain just a single numerical value interpreted as HGTOBS and, if GRA Flag enabled, a second value interpreted as PSIOBS.

  3. Either HGTOBS or PREOBS can be specified, but not both. HGTOBS additionally requires that a height profile is specified in the *ATM section (a pressure profile is always required).

  4. PSI is the angle subtended at the centre of the earth, so about 110km for 1 deg. When the GRA Flag is specified (2D atmosphere), the observer location PSIOBS determines the horizontal angle of the tangent point(s) relative to the reference profile (PSI=0). (This information is written to the .pth diagnostics file). Viewing is assumed to be in the direction towards −ve horizontal angles, ie the observer location would normally be specified as a positive angle relative to the reference profile location.

  5. See LEV Flag for method of generating outputs equivalent to multiple observer altitudes within a single run.

Examples
1-D atmosphere
*OBS HGTOBS=25.0 ! Observer at 25 km.
2-D atmosphere
*OBS PREOBS=100.0 ! Observer at 100 hPa PSIOBS=4.0 ! +4 deg from ref (2-D atmosphere).

Bugs
Bug#48 (Fixed v5.21)
Bug#30 (Fixed v5.11)