911 and the Trouble with the Z-Axis


Today, 80% of the 240 million 911 calls made annually are from mobile phones. Unfortunately, vertical positioning (Z-axis) data to help locate callers in multi-level structures has tended to be missing or inaccurate. Learn how testing and measurement suites are used to validate data sent to 911 including Z-axis accuracy.

In the UK, it’s 999. The EU has 112. Australia calls it Triple Zero. But when you need help, most countries have a short number you can call and somebody, hopefully, will come to your aid.

An estimated 240 million calls are made to 911 in the US each year. Each call can potentially be a life and death situation, for both the caller and first responders, and as such the US has invested heavily in building infrastructure that aims to make response fast and effective.

As part of this goal, in 1996, the Federal Communication Commission (FCC), the US communications regulator, initially mandated that telephone network operators needed to relay a caller's telephone number to 911 operators to allow for “call back” if a call is disconnected. It then required that carriers must be capable of relaying the location of the base station or cell site receiving a 911 call to help with the routing of 911 calls to an appropriate Public Safety Answering Point (PSAP). As most mobile calls at the time were made from outdoors, this initial foundation was adequate for 911 dispatch.

However, the FCC foresaw that mobile adoption, and the underlying technology, was progressing quickly and stated that within five years, the location of the mobile station must be provided to the PSAP in two dimensions (X and Y), with an accuracy within a radius of 125 meters in roughly two thirds of all cases.

Over the years, more communication systems were added to this mandate that now includes interconnected Voice over Internet Protocol (VoIP) services, Internet-based Telecommunications Relay Services (TRS), and mobile text service.

The 50-meter mark

But the biggest change came in 2015, when the FCC started requiring the accuracy of the data provided by operators from 911 calls to hit 50 meters - the approximate accuracy of civilian GPS. But crucially, the requirement also included vertical location data, and the commission gave operators three years to comply.

In 1996, when the idea was first conceived, the vast majority of 911 calls were from landlines, while mobile calls made only a small percentage and were largely outdoor. Today, in many areas, 80% or more 911 calls are from wireless devices, according to data from NENA, the only non-profit organization solely focused on improving 911 services.

Yet this requirement to provide vertical location data, or Z-axis, is more difficult than it was first realized and the lack of it can be dangerous. Imagine the situation: a call comes in from a tower block. There is a fire, and the panicking caller is not sure what floor they are on. The fire crew is on its way but does not know what floor to go to because the Z-axis data is missing or inaccurate. These situations unfortunately do occur. The FCC places the onus on the carriers to provide this positional data, and carriers in turn have relied on smartphone manufacturers to help with this process.

A smarter phone

If you’re using a smartphone to dial 911 in the US, the phone will automatically switch on the GPS receiver and transmit your coordinates to the carrier. This is a mandated requirement for phones sold in the US market, and even most international phones have this feature hardwired into the firmware. Unfortunately, GPS receivers are not all the same quality. GPS receivers and servers (where carriers must calculate GPS location) are typically optimized only for 2D positioning. Altitude, i.e., Z-axis, can easily have errors that span hundreds of meters, which is larger than the height of most tall buildings in major cities.

However, there are other options than just GPS. Newer phones, such as models from Apple, Samsung, and others, have barometric sensors that can measure altitude. If configured correctly, this information can be added to the GPS information transmitted to emergency services when a 911 call is initiated for higher accuracy. However, until recently, the calibration of these devices has been rather hit and miss. This is now changing.

Errors can occur with the use of GNSS alone by transmitting inaccurate vertical information resulting in delayed response time. The combination of GNSS with other technologies increases vertical accuracy.

Industry united

Spirent has worked with several vendors and operators that are taking this issue seriously with test and measurement suites to validate data sent to 911 including Z-axis accuracy to improve the situation. In some cases, particularly with older models, the design of the smartphone chassis and positioning of the electronic components means that there is no easy way to correct Z-axis distortion. In some instances, test data can help with the calibration process and a simple over-the-air update can dramatically improve the accuracy of a smartphone’s location services. In addition, the data gained from this process can be fed back into the design and manufacturing stages so that newer models are built from the ground up to meet FCC 911 rules more easily.

This is important for the industry because operators are potentially liable if they fail to pass accurate information on to 911 services. Fines are just one penalty. A more serious concern is that inaccurate information might lead to unnecessary delays to first responders, meaning a life-threatening situation needlessly becomes a fatality.

If operators are made aware that a particular brand or popular handset is not able to deliver against FCC 911 requirements, operators have the right, and possibly a moral and legal obligation, to ban a phone from connecting to its network. This is of course a grey area, but it does highlight the need for both operators and device manufacturers to work more closely to build FCC 911 regulations into ongoing test and assurance processes.

Smartphones are getting lighter, more powerful, and increasingly flexible. However, without proper testing, the Z-axis problem is something that won’t go away by itself.

Learn more about Spirent’s 8100 location technology and how Z-axis testing can be addressed.




William Chan


William Chan是思博伦通信的高级产品经理,主要负责自动化设备测试的蜂窝辅助定位解决方案。他积极参与3GPP、CTIA、PTCRB及GCF组织,并在其中作为定位测试的测试规范、测试计划及验证流程的主要参与者,范围涵盖OTDOA及ECID等GNSS和蜂窝定位技术。从GSM时代的EOTD到目前5G NSA/SA系统的定位测试,他在这一领域已经工作超过18年。他经常与美国各大运营商的专家们一起工作,确定测试的规范与计划,以满足FCC E911不断发展的要求。