GRAPPLE – GRoundwAve Propagation Predictions for the Littoral Environment – is applicable between 10 kHz and 30 MHz. It uses the ITU-R groundwave model (GRWAVE) for the underlying calculations, and is able to account for mixed land/sea paths by employing Millington's method.

The groundwave model of propagation tends to be extremely stable. However, in the littoral environment the prevailing sea conditions can impact (in some cases quite severely) upon the propagation loss.

The propagation loss is sensitive to surface conductivity. The sea conductivity is a function of sea surface temperature and salinity, and as such varies with time. On smaller time-scales, the propagation loss also varies with sea-state. Barrick, in the 1970s, proposed a theory that accounted for the additional loss experienced in rough seas. These additional losses, depending upon the prevailing conditions and operational frequency, can exceed 10 dB.

The GRAPPLE tool is able to predict these temporal variations.

Click on images to enlarge

At HF frequencies and below, the field strength of the groundwave undergoes an abrupt change as ground types with differing conductivities are encountered (e.g. see UK conductivity map above). These changes are most significant across land/sea boundaries. Somewhat surprisingly, the field strength increases as the propagation path crosses from land to sea. This was first predicted by Millington in the 1950s and it is his method that has been implemented within the GRAPPLE tool.

Model required for:

• Scenario modelling
• Link budget definitions
• Vulnerability assessment
• Tactical decision aid

The SNR coverage predictions (out to 400km) for an 100W transmitter situated on the Isle of Man. A dramatic decrease in SNR can be observed at sea/land boundaries around the Irish Sea. Enhancements in the signal level occur when a land/sea boundary is encountered (e.g. the East Coast of England).

The SNR coverage predictions for an 100W transmitter situated on Gotland. The SNR decrease over the sea is much greater than for the UK, due to the lower salinity and temperatures of the Baltic Sea.