Are our car-mount­ed GPS units always reli­able? Every dri­ver of a car equipped with this pre­cious instru­ment has won­dered if the direc­tions indi­cat­ed are not sim­ply improbable. 

“In the field of avi­a­tion, the key word is sys­tem integri­ty”, explains Vin­cent Drev­elle. “For air traf­fic con­trol pur­pos­es, the aim is to locate any air­craft and guar­an­tee the max­i­mum rate of error”. Dur­ing approach for land­ing phase, the data from satel­lites give a posi­tion with a max­i­mum error of 10m. “If the nav­i­ga­tion sys­tem detects an error exceed­ing 10m, it issues an alert inform­ing the pilots that the GPS can no longer be used”. 

Vin­cent Drev­elle — who grad­u­at­ed from UTC as an engi­neer, major­ing in Com­put­er sci­ences and appli­ca­tions, fol­lowed by a Master’s degree in Infor­ma­tion and sys­tems tech­nolo­gies — want­ed to trans­pose the idea of integri­ty from air­craft to cars and oth­er road vehi­cles in general. 

For air­craft, cat­a­stroph­ic fail­ures in the chain of mea­sure­ments and/or the satel­lites them­selves are rather rare events. In con­tradis­tinc­tion, GPS sig­nals in an urban envi­ron­ment can be blocked or reflect­ed by obsta­cles such as build­ings or even trees and can induce large-scale errors”. To cor­rect­ly locate an air­craft you need 4 satel­lite read­ings. In town, the num­ber avail­able is 2 or 3 giv­en the con­straints. “If you are in the Defence sec­tor in West Paris or in some out­back vil­lage in South France the fig­ure might even be as low as 1 satellite”. 

To be even more accu­rate, we must add data on alti­tude and the ter­rain. “We have done some work with the Insti­tut Géo­graphique Nation­al (IGN) to map part of the 12^th arrondisse­ment in Paris, notably includ­ing the width of the car­riage­ways”. In par­al­lel, the test vehi­cles were fit­ted with sen­sors to mea­sure wheel speed, rota­tion rate and steer­ing wheel angle. An on-board PC was used to cal­cu­late the exact posi­tion while the vehi­cles were in motion. 

In a field, the max­i­mum error is between 5 and 10 meters. In a dif­fi­cult urban area the max­i­mum error is 20 meters. How­ev­er, “if we exclude the maps, alti­tude and oth­er sen­sors, the max­i­mum error can rise occa­sion­al­ly to 100 meters, or even (worst case) the GPS can­not locate the vehi­cle at all!”

The objec­tive we assigned our­selves was not in fact to achieve accu­rate loca­tion but to have the pos­si­bil­i­ty to assess max­i­mum error”, insists Vin­cent Drev­elle. “If a call is made to report an emer­gency of a road acci­dent, we must be able to ascer­tain the error mar­gin in which to locate the vehicle/persons”.

The work in Vin­cent Drevelle’s the­sis can also prove use­ful for appli­ca­tions such as pay­ing for a rent­ed vehi­cle as a func­tion of its real uti­liza­tion (miles) or for truck tolls.