Just how High are you? Cellular Altitude – Part 2

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In my blog last week, I reported on the FCC initiative requiring carriers to provide what is called the Z-axis on cellular calls to 911. For those of you who don’t know, the z-axis reports altitude. Knowing this elevation, calculating the floor that a person is on in a multi-story building is possible and delivered to first responders. This blog sparked many questions, with several people wondering why this has been such a problem? 

To answer some of those inquiries, I decided to dive deeper into the topic to expand everyone’s knowledge. The first part of the question is: 

“How do I calculate altitude?”

In most cases, calculating altitude is accomplished through barometric pressure measurement. But this measurement on its own is worthless without knowing the barometric pressure at the ground level of a location. If you think about it, the ALTITUDE is not the information that a first responder needs; it is the ELEVATION or distance above ground level so they can determine the floor number.  

For example, measuring altitude is typically measured as the distance above average sea level. So if a device is 120 feet above sea level, and the ground level is 100 feet above sea level, my elevation is 20 feet, or on the second floor, allowing 10 feet per floor and another variable that needs to be understood and agreed upon by the measurement device and the receiving device. 

Even if your new smartphone can measure the barometric pressure and determine that you are at a particular altitude, calculating the elevation is impossible without knowing the base ground level. Another simple way to collect altitude information is through GPS. If you have a smart device with a GPS receiver and good visibility to three or four satellites, the altitude calculation can be reasonably accurate. However, a problem with GPS radio signals is that they are challenging to receive while indoors. These signals are already weak when they reach the ground from satellites in space. They are easily affected by the concrete and steel in buildings where they are blocked entirely or reflected, creating an inaccurate measurement. When a GPS receiver is outdoors, it can achieve accuracy within a few meters; however, indoors requires additional assistance.

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Cell phones utilize broadcasts from wireless access points and their BSS IDs to identify their X/Y location in the two-dimensional world. Thanks to Apple, Google, and Skyhook, this makes for a very accurate database by collecting vast amounts of crowd-sourced data provided by the billions of cell phones sharing location data every day. How can they get away with that? Read through the terms of service agreement you agreed to by clicking “Accept.” Unfortunately, only two-dimensional information was collected, and never altitude. 

Can these be used in the future? Absolutely, yes. But to do that, the databases would have to be created with the altitude information and then maintained with accurate information as the access points moved or replaced; not something very likely to be maintained and even less likely to be shared.

Once a device has altitude and elevation can be calculated, how do I get it to the 911 call taker at the ECC/PSAP? 

Today, in the heritage 911 network that exists, the location information is determined by looking up in a database based on the caller’s telephone number. For landlines, it’s your billing address, and for wireless calls, location gets provided through a query to the wireless carrier at the time of the call. The response that an ECC obtains may seem a bit crude; however, keep in mind this capability was a Band-Aid fix to a system built for stationary endpoints. Here you can see a standard wireless ALI screen and the information presented to the call taker. 

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Displayed are the latitude and longitude, along with certainty and accuracy figures; however, a field may not exist for altitude or elevation. Even if the data exists with the carrier, there needs to be a way to convey that information to emergency service call-takers at the ECC. That’s another change needed extending beyond carriers, and in all fairness, before Z-axis can become a reality. 

Once again, I’ll use my TV analogy. Despite a TV program broadcast in color, if you only have a black-and-white TV, that’s what you’ll see. Likewise, if you have a color TV, but you are watching a 1950s “I Love Lucy” re-run, all you’re going to see in black and white. Remember, 911 information is part of an end-to-end solution.


If you missed the point earlier, altitude reporting must be ratified through standards and its relationship to the ground elevation at the device’s location. Questions such as:

Buildings with different elevations in the front and the rear; which one is measured? 

What data source is used for the altitude of the ground outside of the building, needed to calculate the elevation of the caller. 

Many times when large buildings a re constructed, the grade or elevation of the property is dramatically changed from when it may have been measured previously. 

For an AUDIO VERSION of this Blog [CLICK HERE]

A change of 20 feet could seriously affect the accuracy of the elevation calculation. With a call for emergency services, wrong location information can be as significant as no information.

In closing, while determining location is not difficult, even today, determining altitude can be a bit of a challenge, and just wanting to get the information isn’t enough. It reminds me of an old joke from one of my favorite comedians, Steve Martin, on becoming a millionaire. He starts with the advice, “First, get a million dollars . . .”

If it were only that easy.


Please follow me on Twitter @Fletch911
Be sure to check out my Blogs and Podcasts on: Fletch.TV

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