Up until around 1980, high frequency (HF) radio links were the principal means of communicating over medium and long distances. In practice, there was a major subdivision within the HF band - long-haul communications that passed through the ionosphere (often called Skywave), and medium-to-short-haul communications that relied on the ground wave.
All this changed with the advent of satellite communications (satcoms) and, from the late 1970s onwards, HF was superseded for both long and short haul communications. This transition was evident in all walks of life, from military systems and diplomatic communications to maritime communications, and even to domestic radio communications in hitherto poorly-served areas of Africa and elsewhere.
As with so many innovations, the pendulum has swung back again and HF communications are once more in vogue, due largely to the overcrowding of satcom channels and, as far as the military are concerned, the vulnerability of satellites to attack.

Changing times
Equally, HF communications has has moved on, with the frequency band extended from 2-30 MHZ up to 40 MHZ and the technical implementation of HF systems changing almost out of recognition.
No longer does a military HF manpack radio contain kilograms of quartz in the form of side-band filters and a multitude of oscillators for stable channel selection. Instead, advanced digital processing techniques extract signals from the increasingly crowded radio spectrum. Again, today’s HF systems counter eavesdropping, as well as natural and man-made interference, with sophisticated frequency hopping and encryption.
In the face of all these changes, radio waves are still subject to the same laws defined by the 19th Century mathematical physicist, James Clerk Maxwell. But, in parallel with other developments, our knowledge of environmental and propagation parameters has increased markedly.

Rapid propagation
This knowledge is embodied in a variety of radio propagation models used by network planners whether for a trunk microwave system, GSM radio, TETRA, PMR, tactical military communications or conventional TV/radio broadcasting.
Perversely, the lower down the frequency spectrum one goes, the more difficult it is to calculate the propagation behaviour of a radio transmission.
Microwave calculations can be based on standard optical principles (line of site), taking into account prevailing meteorological conditions and the influence of built structures. 
VHF and UHF propagation must correct for Fresnel diffraction and land cover - often called ‘clutter’ in the lexicography of geographic information systems - as well as earth curvature.
MF (medium frequencies) and low frequency sky wave propagation is variously governed by ionospheric effects, earth curvature, and ground conductivity.
The accuracy of these models has increased over the years, as has our knowledge of environmental parameters.

The relevance of mapping
Geo Strategies is one of a number of companies specialising in the creation of digital terrain models and clutter (land use) datasets for the radio planning community. Having supplied data for over 60 countries, it came as no surprise to be contacted by a leading manufacturer of military radios. The requirement, however, was somewhat unusual.
In brief: it had been involved in planning a major radio upgrade for Britain’s Maritime and Coastguard Agency (formerly HM Coastguard). The requirement was for a spectrum planning tool that accounted for all relevant physical phenomena and allowed the selection of frequencies on an hour-by-hour basis. However, the range of propagation had to be limited to avoid interfere with other land-based services. This is not dissimilar to naval requirements for frequencies that facilitate ship-to-shore communications but do not propagate into enemy territory).
As the coastguard relies on HF ground wave for such communications, calculations hinge on a detailed knowledge of local ground conductivity.
Ground conductivity maps have been around for many years, but the presence of sea water and dry ground in the transmission path poses unique problems.
The conductivity of sea water is typically 4,000 or 5,000 mS/m (27,000 for the Dead Sea!), and dry ground is in the range 1-to-10 mS/m, i.e., a ratio of ~1,000:1
This huge difference in conductivity means that the relative lengths of the transmission path over sea water and over dry land need to be known with considerable precision for predictions to hold any validity.

Solution
In response, Geo Strategies produced a highly unusual spatial database embodying the latest information on ground conductivity and featuring highly accurate coastlines.
The final database, which covers the whole world, accommodates coastline data accurate to approximately 50 metres, but with 10 metre accuracy in selected areas.
As a result, it is now possible to offer a real-time spectrum planning tool that is ideal for deploying both tactical HF radio and mission critical communications systems where security is paramount.
Needless to say, the exercise proved an excellent example of joint working between organisations that rely on each other’s expertise.

For further information, please contact Geo Strategies at:
	Geo Strategies Ltd			Geo Strategies SA
	St John's Innovation Centre			Str G-ral V. Milea, 10A
	Cowley Road			Sibiu 550331
	Cambridge  CB4 0WS			Romania
	United Kingdom
	Tel:	+44 (0)1223 205080		Tel:	+40 (0)269 210832
	Fax:	+44 (0)1223 205081		Fax:	+40 (0)269 211165
	Email:	maps@geo-strategies.com		Email:	maps@geo.strategies.ro