Mounted search and rescue

["IRVIN LICHTENSTEIN" <ilichten1@verizon.net>]

The low bridge and high crown crossing information is extremely helpful to goose neck rigs.  Many are too high for some bridges around here, particularly in park areas where the 9-2 and 10-2 bridges have not been replaced deliberately to keep trucks off the short cuts.  Also, US Rider reports many long rigs are bellying out on railroad crossings and other high road crown situations.  Getting stuck on a railroad track is just a disaster waiting to happen.  All the clearances under the 309 Freeway were just lowered by 8-10 inches as new steel was put in the bridges to upgrade them and the roads underneath were repaved at the same time.  Bridges that were over 15 feet and unposted are now marked as 14-2, at least a 10 inch drop with no obvious change to the bridge.

 

I have worked with GPS since it was called Navstar and the receivers were so large they had to be on aircraft carriers.  The radio frequency used is very close to PCS cell phones, L-band aircraft and boat radars and even speed radars.  It is a digital signal but is very complex because it includes a time reference and the ephemeris of the satellites. The basic principal of GPS is Time Difference of Arrival, the same technique used for LORAN and some cell phone locators.  By very precisely measuring the time difference between the arrival of an identical time mark you can calculate the distances to the several transmitters.  The accuracy of the clock used by the system and the knowledge of the path traveled by the radio signal are the governing accuracy factors.  Your handheld unit most likely does not have a system clock but relies on the time signal in the radio broadcast to correct its time measurement hardware.  You need to receive 3 different satellites to produce a valid fix. A fourth will add altitude above the reference point. Many handhelds use all the signals they can receive and work the fix in combinations to get the most accurate solution.  As the system progresses “good enough” software solutions get replaced with slightly faster systems and perhaps with fewer bugs. 

 

Realize that this is a radio based system.  At RADAR frequencies the signal bounces off buildings, is absorbed by trees, and destroyed by vehicles acting as Faraday cages (shields). It can also be interfered with by PCS phones, other radars, and deliberate jamming. 

Unless a map system is matched to the GPS receiver you must translate the co-ordinates derived to the map.  This means that you must check the coordinate references used by the GPS and the map, set the GPS to present them in a compatible form (UTM, NGS, LAT/LONG, seconds or decimal) and read them correctly on both forms.

There are also ground truth stations at the ends of many airport runways that broadcast corrections to the satellite system so that airplanes aiming for the threshold of a 150 wide runway get the right data. 

 

Irv Lichtenstein

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From: msar-riders-bounces@lists.ibiblio.org [mailto:msar-riders-bounces@lists.ibiblio.org] On Behalf Of Karen Nesbitt
Sent: Wednesday, August 27, 2008 10:54 AM
To: MSAR
Subject: [MSAR] GPS