From d54fe7c1f704a63824c5bfa0ece65245572e9b27 Mon Sep 17 00:00:00 2001 From: Joseph Hunkeler Date: Wed, 4 Mar 2015 21:21:30 -0500 Subject: Initial commit --- src/slalib/sun67.htx/node167.html | 232 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 232 insertions(+) create mode 100644 src/slalib/sun67.htx/node167.html (limited to 'src/slalib/sun67.htx/node167.html') diff --git a/src/slalib/sun67.htx/node167.html b/src/slalib/sun67.htx/node167.html new file mode 100644 index 0000000..f422ec8 --- /dev/null +++ b/src/slalib/sun67.htx/node167.html @@ -0,0 +1,232 @@ + + + + +SLA_REFRO - Refraction + + + + + + + + + + + + +
+ +next + +up + +previous +
+ Next: SLA_REFV - Apply Refraction to Vector +
+Up: SUBPROGRAM SPECIFICATIONS +
+ Previous: SLA_REFCOQ - Refraction Constants (fast) +

+

+

SLA_REFRO - Refraction +   +

+
+
ACTION: +
Atmospheric refraction, for radio or optical/IR wavelengths. +
CALL: +
CALL sla_REFRO (ZOBS, HM, TDK, PMB, RH, WL, PHI, TLR, EPS, REF) +

+

+
GIVEN: +
+
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
ZOBSDobserved zenith distance of the source (radians)
HMDheight of the observer above sea level (metre)
TDKDambient temperature at the observer (degrees K)
PMBDpressure at the observer (mB)
RHDrelative humidity at the observer (range 0-1)
WLDeffective wavelength of the source ($\mu{\rm m}$)
PHIDlatitude of the observer (radian, astronomical)
TLRDtemperature lapse rate in the troposphere +(degrees K per metre)
EPSDprecision required to terminate iteration (radian)
+

+
RETURNED: +
+
+ + + + + +
REFDrefraction: in vacuo ZD minus observed ZD (radians)
+

+
NOTES: +
+
1. +
A suggested value for the TLR argument is 0.0065D0. The +refraction is significantly affected by TLR, and if studies +of the local atmosphere have been carried out a better TLR + value may be available. +
2. +
A suggested value for the EPS argument is 1D-8. The result is + usually at least two orders of magnitude more computationally + precise than the supplied EPS value. +
3. +
The routine computes the refraction for zenith distances up + to and a little beyond $90^{\circ}$ using the method of Hohenkerk + & Sinclair (NAO Technical Notes 59 and 63, subsequently adopted + in the Explanatory Supplement to the Astronomical Almanac, + 1992 - see section 3.281). +
4. +
The code is based on the AREF optical/IR refraction subroutine + of C.Hohenkerk (HMNAO, September 1984), with extensions to + support the radio case. The modifications to the original HMNAO + optical/IR refraction code which affect the results are: +
    +
  • Murray's values for the gas constants have been used + (Vectorial Astrometry, Adam Hilger, 1983). +
  • A better model for Ps(T) has been adopted (taken from + Gill, Atmosphere-Ocean Dynamics, Academic Press, 1982). +
  • More accurate expressions for Pwo have been adopted + (again from Gill 1982). +
  • Provision for radio wavelengths has been added using + expressions devised by A.T.Sinclair, RGO (private + communication 1989), based on the Essen & Froome + refractivity formula adopted in Resolution 1 of the + 12th International Geodesy Association General Assembly + (Bulletin Géodésique 70 p390, 1963). +
+ None of the changes significantly affects the optical/IR results + with respect to the algorithm given in the 1992 Explanatory + Supplement. For example, at $70^\circ$ zenith distance the present + routine agrees with the ES algorithm to better than + $0\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.05$

+for any reasonable combination of parameters. However, the + improved water-vapour expressions do make a significant difference + in the radio band, at $70^\circ$ zenith distance reaching almost + $4\hspace{-0.05em}^{'\hspace{-0.1em}'}$ for a hot, humid, low-altitude site during a period of + low pressure. +

5. +
The radio refraction is chosen by specifying WL >100 $\mu{\rm m}$. Because the algorithm takes no account of the ionosphere, the + accuracy deteriorates at low frequencies, below about 30MHz. +
6. +
Before use, the value of ZOBS is expressed in the range $\pm \pi$. If this ranged ZOBS is negative, the result REF is computed from its + absolute value before being made negative to match. In addition, if + it has an absolute value greater than $93^\circ$, a fixed REF value + equal to the result for ZOBS $=93^\circ$ is returned, appropriately + signed. +
7. +
As in the original Hohenkerk and Sinclair algorithm, fixed values + of the water vapour polytrope exponent, the height of the + tropopause, and the height at which refraction is negligible are + used. +
8. +
The radio refraction has been tested against work done by + Iain Coulson, JACH, (private communication 1995) for the + James Clerk Maxwell Telescope, Mauna Kea. For typical conditions, + agreement at the + $0\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.1$ level is achieved for moderate ZD, + worsening to perhaps + $0\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.5$ - + $1\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.0$ at ZD $80^\circ$. At hot and humid sea-level sites the accuracy will not be as good. +
9. +
It should be noted that the relative humidity RH is formally + defined in terms of ``mixing ratio'' rather than pressures or + densities as is often stated. It is the mass of water per unit + mass of dry air divided by that for saturated air at the same + temperature and pressure (see Gill 1982). The familiar + $\nu=p_w/p_s$ or $\nu=\rho_w/\rho_s$ expressions can differ from + the formal definition by several percent, significant in the + radio case. +
+
+ +next + +up + +previous +
+ Next: SLA_REFV - Apply Refraction to Vector +
+Up: SUBPROGRAM SPECIFICATIONS +
+ Previous: SLA_REFCOQ - Refraction Constants (fast) +

+

+

+SLALIB --- Positional Astronomy Library
Starlink User Note 67
P. T. Wallace
12 October 1999
E-mail:ptw@star.rl.ac.uk +
+ + -- cgit