I have not made public before that the original manuscript for the UNM was just over 160,000 words and the published version contains 88,000 words.
Harper Collins felt that the detail, particularly in the Satnav Sections (
including Emergency Management Planning) were far too technical.
It was difficult for me taking the words out; nevertheless I understand HC’s reasoning that the book needed to be a manual rather than a text book.
My original thinking had been to publish this additional information on this website, but, frankly, it is so large that it needs a lot of work to illustrate and assemble it in a logical order. Instead, it is forming the basis of my next book - of which two are in the offing.
However, an anomaly has arisen: I created this Forum because I really want this space to be a place where best practice and current thinking is shared, with open access for everyone, who has an interest in land navigation, plus I do state in the UNM Satnav section that further reading will be available on this site!
So here goes with the first snippet and I need your feedback here please: If it is too technical and innapprioriate for this forum tell me; I won’t be offended
Magnetic Declination & Satnav This is a subject where I had only ever applied the data i.e. corrected for magnetic declination, and never really thought about it until I visited the US 50th Space Wing.
Based east of Colorado, on the edge of the Great Plains, this big base is the key part of the Air Force Space command and controls and operates all of the US GPS satellites (
referred to as the space segment in my book and elsewhere).
Satnav has become ubiquitous tool for all military operations, for the reason that it provides reliable and secure situational analysis:
navigation and position.
Translating this data into operational field tactics, where everyone from foot soldiers to gunnery units’ use both compasses and more frequently satnav in the field, they need to accurately corrected for magnetic declination.
So mathematical models have been created to account for the isogonic lines that traverse the earth but, and this is a big BUT, as soon as these models are created, just like maps they:
A. Are immediately out of date
B. Contain anomalies – glitches as one engineer more appropriately told me!
The two main models used are:
1. The World Magnetic Model (WMM), that Stu referred to, which was developed by the National Geophysical Data Center (NGDC) and the British Geological Survey
2. The International Geomagnetic Reference Field (IGRF), which was introduced by the International Association of Geomagnetism and Aeronomy (IAGA), a collaborative effort between magnetic field modellers and the institutes involved in collecting and disseminating magnetic field data from satellites and from observatories and surveys around the world.
When referring to either you should correctly state the specific model, as both are updated every 5 years, so currently the versions are IGRF2010 and WMM2010.
(There is an apparent third model called the Definitive Geomagnetic reference (DGRF) which replaced some IGRF models but is generally not used as values cannot be definitive -
exact -).
So let’s looks at why they are:
A. Immediately out of dateThe Earth’s magnetic field is in reality a combination of 5 different principal magnetic fields:
I. The Earth's core, specifically the outer fluid part of it
II. Electrical currents that flow through the Earth's crust
III. Electric fields created in the ionosphere and magnetosphere
IV. Magnetised rocks and minerals in the Earth's crust
V. Ocean currents
With the exception of magnetised rocks and minerals in the Earth's crust, the other four components all are part of chaotic systems; in other words they vary unpredictably and all at different times.
It is estimated that around 92% of the Earth’s geomagnetic field is generated by the outer fluid part of its core and this is what is used to determine both the WMM and the IGRF.
B. Contain anomaliesThe clue is in the title ‘
mathematical models‘ because these are theoretical representations that use mathematical language to describe the behaviour of a system, so they are predictions only.
As I have explained, with the exception of item IV, all of the other components are unpredictable to a greater or lesser degree.
Generally, the WMM & IGRF are stated to be accurate to within 30 minutes of the arc of D. Yet it is very important to be aware that local anomalies local anomalies of ~3° are not that uncommon and in some instances anomalies <10° exist, albeit in relatively few areas. Two such locations in Minnesota, USA,are separated by only 7 km yet the models predict one with a magnetic declination of 16° east and the other 12° west!
The 50th Space Wing continually update the known land areas with known anomalies, yet here again terminologyis important where Istate known areas, implying there are unknown areas. Plus they are working with a group that is empirically defining the influence created by item IV (
in addition to some other classified factors to apply correction) and may transmit this data via satellites to encrypted military satnav units - I put 'may' because I do not know. Where troops are going in Theatre they are provided with a map of the area which highlights any anomolies, for the field commanders and controllers to employ.
The WMM, and on some manufacturers models the IGRF, is
flashed onto individual satnav units when the user updates their firmware - another important reason to check for updates from the manufacturer!
The irony is that the incredibly sophisticated technology of satnav redefines for me again the value of the simple map and compass, because the best way of determining what your correction should be for magnetic declination for the precise area you are in is with a map & compass (P.131 UNM).
