Lesson Testing Cell Signal at Altitude

Learning Objectives

After this lesson, students should be able to:

• Analyze data based on a given question.
• Graph collected data in an appropriate form.
• Question and test the accuracy of collected data.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

NGSS: Next Generation Science Standards - Science

Common Core State Standards - Math

Worksheets and Attachments

Pre-Req Knowledge

Students should have a cursory knowledge of graphs and interpreting data. (Note: Interpreting the graph aims for Common Core Math Standards for 6th grade, including a measure of center. However, students do not need to calculate the center (average) in this lesson; rather they should understand the application of “center.”)

Introduction/Motivation

Many of us use cellphones every day. Do you know how a cellphone works? Cellphones rely on electromagnetic signals coming from cellphone towers. These signals allow us to use our phones to make calls and browse the internet. Sometimes these signals are very strong and other times they are weak. We can tell how strong the signal is by checking to see how many bars of service the phone is receiving. What kinds of things make a cell signal weak? There are many factors that can affect cellphone signal strength, such as terrain (like mountains), weather, and distance from the cellphone tower. For example, the farther your phone is from the nearest tower, the weaker the signal will be.

Now, most of us usually do not think about how cellphone towers transmit signal that reaches our cellphones. We just want our cellphones to have enough signal to work! But let’s think about this for a moment. We know cell towers transmit signal to our cellphones and we know that the farther your phone is from the nearest tower, the weaker the signal will be. So, we know that cellphones can pick up these signals close to the ground. But what about higher in the atmosphere? Can a cellphone pick up a signal on a high mountaintop? What about at 30,000 feet, where jet airplanes fly? What about in the ozone layer within the stratosphere, more than 10 miles above our heads? Are cell phone signals limited to the surface where people can use them? Let’s investigate this issue.

In this lesson, we will be analyzing cellphone signal data that were collected from a high-altitude weather balloon that ascended high into the atmosphere. This balloon was launched from Casper, Wyoming, in the fall of 2020 by a group of college students from the University of Wyoming. They used special electronics and sensors attached to the balloon to continuously measure the strength of cellphone signals coming only from cell towers on the ground. These sensors were powered by batteries and connected with wires to a small computer. The strength of the signal was measured in a unit called dBm; the higher the dBm, the stronger the signal. A good cellphone signal has a strength of around –70 dBm or larger. A very poor cellphone signal has a strength of less than –100 dBm. Below about –110 dBm, the signal is lost altogether. This is what happens when our cellphones tell us that there is no signal.

We are going to investigate is how the strength of a cell signal changes at different altitudes and what that might mean about how the signal is transmitted from towers. Think about what elements, either natural or man-made, can affect these changes. We can use the things learned here to discuss how cellphone towers work and how electromagnetic radiation, such as a cellular signal, moves through our atmosphere.

Lesson Background and Concepts for Teachers

There are three types of graphs students need to know for this lesson: bar graphs, line graphs, and pie charts.

• Bar graph: A graph in the form of boxes of different heights, with each box representing a different value or category of data, and the heights representing frequencies.
• Line graph: A graph in which a series of connected straight lines connect points, each of which represents the value of a particular variable.
• Pie Chart: A pictorial graph in the shape of a circle, with segments representing related proportions. Pie charts show the distribution of the components of one phenomenon.

The graph shown in slide 10 of the Analyzing Data PowerPoint (and shown in Figure 1 below) is a line graph. This type of graph best illustrates the changes in cellphone signal with altitude. Students will need to decide which variables go on each axis. Although the graph in Figure 1 has altitude on the x-axis, it is sometimes more helpful to put altitude on the y-axis to reflect the fact that altitude changes in the vertical.

Students are presented with the following questions:

• How do you think the cellular signal will change higher in the atmosphere, farther from the cellphone towers on the ground?
• Do you think the signal will change gradually with altitude or abruptly?
• What things could be causing these changes in the signal?

Students create an appropriate graph of the provided data that were collected during a high-altitude weather balloon flight over Casper, Wyoming, on a clear day in the fall of 2020. These data are provided as a CSV file that can be opened and graphed within Microsoft Excel. There are two columns of data that the students will use: altitude (ft) and RSSI (dBm). RSSI simply stands for Received Signal Strength Indicator and it is the measurement of cellphone signal strength obtained by the electronic sensors that were flying with the balloon. The altitude is in feet above sea level and starts at just over 5,200 feet, the ground elevation of Casper, Wyoming.

During this lesson, students will discover the limitations of the data (i.e., it is often messy and noisy) and may realize that their questions need to be refined or restated. The primary discovery will be that the signal strength decreases gradually from the ground (5,240 feet) up to about 10,000 feet. Above that altitude, the strength of the signal remains quite poor (below about –90 dBm) all the way up to almost 100,000 feet, which is into the ozone layer within the stratosphere.

Astute students may notice that even at high altitudes the strength still fluctuates quite a bit between –90 dBm (poor signal) and –115 dBm (no signal). In other words, the signal never completely disappears for good. One possible reason for this is sensor error which could produce “ghost” signals, especially when the real signal is very weak or nonexistent. Another possible explanation is that the antenna used to capture the incoming signal was constantly changing direction as the balloon moved through the atmosphere. When the antenna was angled downward, it could pick up a weak signal. When it was angled upward, it measured no signal at all. Regardless of the possible reasons for these fluctuations, students should still be able to see that as the sensors move higher in the atmosphere, the signals they measured from the towers on the ground weakened.

Following this discovery, it may be worth having the students search online for more information about how cellphone towers work, specifically the direction in which cell towers transmit their signals and how far their signals will travel (i.e., the range) before they become too attenuated by the atmosphere to be useable. Since almost all cellphone users are situated on the ground, cell towers tend to transmit their signals horizontally, although it is possible that some towers transmit in all directions (i.e., isotropically). Thus, it would not be all that surprising that cellphone signals decrease with altitude. Many signals can travel up to 100 km (60 miles) or more before they become completely attenuated by the atmosphere, so range does not seem to be the primary factor for why the signal decreased as much as it did up to 100,000 feet (only ~18 miles above the ground). Other elements such as weather (especially precipitation), hills and mountains, and even buildings can decrease their range. Since none of these were present between the balloon and cell towers on the day of this flight, it is likely that the signal decreased primarily because most cell towers do not transmit their signals very far in the upward vertical direction. 

Associated Activities

• Electromagnetic Waves Are Everywhere! - Students discover the different of uses of electromagnetic radiation by learning about devices that use electromagnetic waves such as laser pointers, remote controls, and X-ray machines. Students conduct basic research into each of these devices to determine which use higher and lower frequencies of the electromagnetic spectrum. Students become researchers, using problem-solving and creative thinking to determine how these devices use electromagnetic radiation to operate. Students practice writing about science, engineering, and technology. Students then become experts on one electromagnetic radiation device and practice their research skills to create a poster presentation.

Lesson Closure

Today, we practiced our data analysis skills. We graphed the weather balloon data and got to practice thinking like scientists and engineers. We questioned our results and asked how they could be better.

Today, we learned what elements, natural or human-made, can impede cellphone signal and how this could possibly affect our data. We learned that a group of college students designed and built electronics to attach to a high-altitude balloon and collect data about the strength of cellphone signals. We examined their data to draw inferences and better understand how cellphone towers work and how they transmit their signals.

Vocabulary/Definitions

bar graph: A graph in the form of boxes of different heights, with each box representing a different value or category of data, and the heights representing frequencies.

data: Information, especially in a scientific or computational context, or with the implication that it is organized.

graph: A data chart (graphical representation of data) intended to illustrate the relationship between a set (or sets) of numbers (quantities, measurements or indicative numbers) and a reference set.

line graph: A graph in which a series of connected straight lines connect points, each of which represents the value of a particular variable.

pie chart: A pictorial graph in the shape of a circle, with segments representing related proportions.

x-axis: The axis on a graph that is usually drawn left to right and usually shows the range of values of an independent variable.

y-axis: The axis on a graph that is usually drawn from bottom to top and usually shows the range of values of variables dependent on one other variable, or the second of two independent variables.

Assessment

Pre-Lesson Assessment

Brain Dump: Ask students to list what they know about graphs and collecting data.

Lesson Embedded (Formative) Assessment

Excel graphs of data: While students are creating their data sets and graphs, ask what categories they are making and why. Additionally, ask if there is anything that their graphs make them question or wonder about the data.  

Post-Lesson (Summative) Assessment

Class discussion of results: After completing their graphs and discussing what the data showed, ask the students what might explain the changes in cellphone signal at higher altitudes. Also ask the students how they might apply this knowledge to their everyday lives using cellphones.  

References

Copyright

Contributors

Supporting Program

Acknowledgements

This material was developed based upon work supported by the National Science Foundation under grant no. 1821566—the LIFT (Learning to Integrate Fundamentals through Teaching) Project, Wyoming NASA Space Grant Consortium, University of Wyoming. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.