<p><pre><code>  The ShadowHawk uses military-grade encryption and changes
  GPS frequencies every half second, according to Buscher.
</code></pre>
That doesn&#x27;t sound true - the GPS satellites don&#x27;t do frequency hopping, you get 1575.42MHz, 1227.60MHz and that&#x27;s it. All the linked paper says is that you can use the precise time from a GPS receiver to start up a different radio system that needs precise timing.