UAG-93: Chapters 1 to 5 - URSI

procedures employed with ordinary and with ordinary-p1us-extraordinary ray data. In these regions
a single defined solution is generally not possible, so physicall-y desirable features are combined
with the data in a least-square solution. Sections 4 to 9, and Appendices A to C, summarise much
unpublished work which provides the basis for the techniques used in P0LAN. The practical use of
P0LAN is described jn Section 10, while Section 11 describes a typical system for scaling, correction
and analysi s of jonosonde data.
Documentation of P0LAN and the associateo:,ubprograms is given in Appendices D to F; these
provide logic flow tables, varjable descriptions and computer listings. Appendix G gives standard
test data and the resulting output, so that correct operation of the main features of P0LAN can be
verified. The data also illustrate some of the refjnements avajlable in POLAN for obtaining maximum
.informatjon
and accuracy in different situations. All proqrams and test data are available from World
Data Center A.
For a given set of viitual-height data, real-height analysis using P0LAN takes roughly twice
as long as a simple lamination analysis. For a given overall accuracy, however, P0LAN requires
only about half as many data points. Thus there is little final difference.in computing time, and
there can be a worthwhile saving in the tjme required for scaling the ionograms. No problems have
been found in running P0LAN on a minicompuler. About 40kB of memory are required with a PDP-11.
The 24-bjt accuracy of such machines is sufficient for all modes of analysis, because of the stable
procedure used to solve the simultaneous equations. Comparison of results from a PDP-11 and from a
larger computer with 40-bit accuracy shows very few instances in which the calculated real heights
differ by more than 0.001 km.
In normal operation results are obtained using a polynomial representation of the real-height
profile, fitted to several points each side of the section being calculated. This provides an
accurate interpolation between scaled frequencies, which is necessary for an accurate analysis.
Vr'rtual height data define pr.imarily the real-height gradients at the scaled frequencies. Real
heights are therefore defined most accurately between the scaled frequencies (Titheridge, 1979).
Thus when an accurate analysis is used to obtain real heights at the scaled frequencies, it is dealing
directly with the most djfficult points. Tests have shown that direct second djfference interpolation
is then sufficient to reproduce the profile between scaled frequencies with little or no increase in
the mean error. Results obtained by P0LAN are therefore nornally stored as arrays giving the scaled
frequencies and corresponding real heights. Som extrapolated points are added above the layer
peaks, for simpler calculation of mean profiles and to give smooth plots with second or third order
parametric interpolation (which is necessary to cope with non-monotonic profiles). The frontispiece
shows some examples obtajned with fully automatic processing and plotting of digital ionosonde data.
For some purposes a mathematical representation of the calculated profiles is convenient.
This has recently been provided for in POLAN, as outljned in section 2.4 and appendix G.3. The basic
architecture of P0LAN is flexible and expandable. Further development depends on increased experience
in defining physical constra.ints, on formulatlng judgement criteria for difficult conditions and
specifying appropriate courses of action. Users of P0LAN can help in this development by informing
the author of difficult data types, how these might be identified, and ways in which they might best
be treated. Users are also urged to register wjth the author so that they will receive any further
information on new developments or suggested pnogram changes.