## *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]}Param Type Description Units HGTOBS R Observer altitude ^{[3]}km PREOBS R Observer pressure ^{[3]}hPa PSIOBS R Observer horizontal angle ^{[4]}deg **Type**: R=RealThe 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 +ZENUpward viewing HGTOBS within atmosphere *SEC +NADDownward viewing No restriction **Notes**- 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`. - 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. - 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). -
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.- See 2D Atmospheres

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

- Where the observer location is
**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#30 (Fixed v5.11)