NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
GPS selective availiability 'switched off'
From: JC Sutherland
Date: 2000 May 04, 18:15 EDT
From: JC Sutherland
Date: 2000 May 04, 18:15 EDT
Hi all; Selective Availiability was so arranged that the precision of the a group of fixes taken over a period of time could not be improved by statistical methods. With SA discontinued this is no longer the case and much smaller circles of probable position can be obtained by averaging techniques even without P-code reception. Yesterday I set up my GPS at home and took fixes approximately every 10 mins throughout the afternoon, over a period of about 10 hrs. taking a total of 50 readings. This data gave a Probable Error of +/- 14.7 meters with a maximum diffference of 65 metres from the mean position. However this mean position was in error by 40 metres. But of course this could be an error in the map I was using. Later last night when the atmosphere was less turbulent I took 50 more readings as rapidly as I could, (about 6 secs between each). This second set gave a Probable Error of +/-1.5 metres and the mean fix was closer to the map position. I have drawn some interesting conclusions from this data which I would like to pass on for discussion. POINT 1 The more fixes you average the more precise you will think you will know your position. Also the more rapidly you take fixes the smaller will be the resultant scatter in the data. BUT THERE IS NO GUARANTEE THAT THE SMALLER GROUP WILL BE ANY MORE ACCURATE! In fact, because of the physics of propagation the smaller group could just as likely to be centered around a deflected position. POINT 2 The propagation of radio waves through the atmosphere is dependant on the ionization of the outer layers and this ionization is affected by Solar activity. We will need to be more consious of Solar flare predictions before we belive in this apparantly improved accuracy. POINT 3 What is the point of knowing your position in Lat and Long more precisely than the quality of the Chart or Map you are using. Sandbanks, mudflats and estuary bars as well as navigation buoys are known to move about. POINT 4 A lot of almanacs and pilot books are giving lists of Waypoints. Some of these are inevitably in strong tide areas and consequently many boats could be aiming for the same waypoint at the same time. Now that the autopilot position fixing could be accurate to less than a boat length it may not be long before the first GPS assisted collision occurs! After all if all GPS sets are receiving the same data at the same time they will all have the same position error! The moral is "Choose carefully your own waypoints" . Hope this is food for thought. Regards CLIVE