## *OBS Section: Observer Location |

**Type**- Secondary Section - depends on
`OBS`Flag. **Description**- Specify observer position in altitude [km] and, optionally,
horizontal angle [deg].
**Format**- Single or (
`GRA`Flag) two fields, identified by order.Field Type Description Range 1. `ALTOBS`R*4 Observer altitude [km] ≥ lowest point along ray path 2. `PSIOBS`R*4 Observer horizontal angle [deg] -90:90 (ignored unless `GRA`Flag enabled) **Notes**- Where the observer location is not specified (ie
`OBS`Flag not enabled) it is assumed that the observer is somewhere between the top of the atmosphere and infinity - the precise altitude makes no difference if the path is specified by tangent height (either geometric or refracted) and the observer is somewhere outside the atmosphere. With the`GRA`Flag, all tangent points are then taken to lie at`PSI=0`. - The actual range of allowed
`ALTOBS`is determined by the viewing geometry specified in the`*TAN`section:- Limb-viewing paths specified by tangent point
(
`*TAN`or`*GEO`):`ALTOBS`≥ highest tangent point - Limb-viewing paths specified by elevation angle (
`*ELE`):`ALTOBS`within atmosphere for positive angles, low enough to interset atmosphere for negative angles - Zenith paths (
`ZEN`Flag): within atmosphere - Nadir paths (
`NAD`Flag): no restriction

- Limb-viewing paths specified by tangent point
(
- When the
`GRA`Flag is specified (2D atmosphere), all viewing is assumed to be in the direction +ve to -ve horizontal angle, ie the observer location would normally be specified as a positive angle (subtended at the centre of the earth, so about 110km for 1 deg) relative to the atmospheric profile locations specified in the`*ATM`section. - When used in conjunction with the
`GRA`Flag, 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. - See
`LEV`Flag for method of generating outputs equivalent to multiple observer altitudes within a single run.

- Where the observer location is not specified (ie
**Examples***OBS 25.0 ! Observer at 25 km. (1-D atmosphere)

*OBS 25.0 4.0 ! @25 km. +4 deg from ref (2-D atmosphere).

**Bugs**- Bug#49 (Fixed v4.24)
- Bug#48 (Fixed v4.24)
- Bug#45 (Fixed v4.23)
- Bug#68 (Fixed v4.28)