GPS World, January 2013

Integration with Other Technologies GNSS deSiGN FiGUrE 9 Cellular antennas atop Westview Condominium Building in downtown Nashville median site heights are 57 and 52 meters respectively RALR atop the Archdale Building in Raleigh North Carolina was the tallest identified site at 641 meters This site however was decommissioned in January 2012 although it was identified as operational in a February 2012 NGS listing of sites The second tallest site identified is WVHU in Huntington West Virginia at 396 meters which is still operational atop of a Marshall University building 223 of the 1543 CORS sites within CONUS have AGL heights greater than 10 meters and furthermore the taller sites tend to be in urban areas where cellular networks tend to have the greatest base station density High Precision and Networks End Users Many high precision end users employ GPS receivers at considerable heights above ground For instance high precision receivers are relied upon within modern construction methods The adjacENt PHotos show GPS receivers used for the construction of a 58 story skyscraper called The Bow in Calgary Canada For this project a rooftop control network was established on top of neighboring buildings using both GPS receivers and other surveying equipment for example 360 degree prisms for total stations and GPS receivers were moved up with each successive stage of the building to keep structural components plumb and properly aligned Similar techniques are being used for the Freedom Tower the new World Trade Center in New York City and many other current construction projects Other terrestrial applications that rely on high precision GPS receivers at high altitudes include structural monitoring and control of mechanical equipment such as gantry cranes At times even ground based survey receivers can be substantially elevated Although a conventional surveying pole or tripod typically places the GPS antenna 15 2 meters above the ground much longer poles are available and occasionally used in areas where obstructions are present 4 meter GPS poles are often utilized and poles of up to 40 ft 122 meters are available from survey supply companies General Location Navigation Although controlling received power from a cellular network at 2 meters AGL may be suitable to protect many general navigation location users it is not adequate by itself For example GPS receivers are used for tracking trucks and for positive train control the latter mandated in the United States per the Rail Safety Improvement Act of 2008 GPS antennas for trucks and trains are often situated on top of these vehicles Large trucks in the United States for use on public roads can be up to 13 ft 6 in 41 meters and a typical U S locomotive height is 15 ft 5 in 47 meters Especially in a mature network that is using high downtilts received power at these AGL heights can be substantially higher than at 2 meters Within the TWG and NPEF studies the general location navigation GPS receiver category is defined to include non certified aviation receivers One notable application is the use of GPS to navigate unmanned aerial vehicles UAVs are increasingly being used for law enforcement border control and many other applications where the UAV can be expected to occasionally pass within the boresight of cellular antennas at short ranges cellular The majority of Americans own cell phones and a growing number are using cell phones as a replacement for landlines within their home Already 70 percent of 911 calls are made on mobile phones Although pedestrians and car passengers are often within 2 meters of the ground this is not always the case FiGUrE 9 shows three cellular sector antennas situated atop a building lled with residential condominiums The rooftop is accessible and frequently used by the building inhabitants According to an online real estate advertisement The Garden Roof was voted the Best Green Roof in Town and provides amazing 360 degree views of downtown Nashville as well as four separate sitting areas and fabulous landscaping One of the sector antennas is pointing towards the opposite corner of the building If the downtilt is in the vicinity of 2 6 degrees then it is quite likely that a person making a 911 call from the rooftop could see a received power level of 10 dBm to 0 dBm high enough to disrupt GPS within most cellular devices if the antennas were transmitting in the 1525 1559 MHz band This situation is not unusual Many cellular base stations are situated on rooftops in urban areas and many illuminate living areas in adjacent buildings In recent years New York City even considered legislation to protect citizens from potential harmful effects of the more than 2600 cell sites in the city since many sites are in very close proximity to residential areas Propagation Models Within the LightSquared proceedings there was a tremendous amount of debate regarding propagation www gpsworld com January 2013 GPS World 65