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Laboratory C: Signal Strength and Receiver Sensitivity
Lab Dates: Lab may be carried out any time from 27 Oct thru 16 Nov, 2014 Lab Goals:
• Observe and record C/No (carrier to noise-‐density ratio) in the open, and in a residence. • Compare to expected value from Module 4-‐4, and estimate signal losses in the phone/tablet. • See if your GPS receiver will work in your residence, and estimate the signal attenuation
caused by the building structure. • Compare data collected from all students.
Optional: Download a spreadsheet of all the data and do something with it (parse it, map it, analyze it, etc.)
To Prepare:
• Install and run the free GPS Test App (“AndroiTS GPS Test”). See installation instructions in Labs A and B.
To Submit Data: Please visit GPS-‐Lab.org to find links to submit.
1. Complete Lab Quiz C on the Coursera site (Due by Mon 17 Nov 2014 11:59 PM PST) 2. Record your results in the GPS Worldwide Lab C (Signal Strength) Form
Background
In Module 4-‐3 you learned about the expected signal strength outdoors. In summary: we expect a satellite signal to be -‐128.5 dBm at the output of the antenna, when outdoors with a clear view of a satellite.
In Module 4-‐4 you learned about C/No (carrier to noise-‐density ratio). And the expected C/No is 45 dB-‐Hz when the signal is -‐128.5 dBm.
14 Teff 296.4 K Friis's formula15 No -203.9 dBW/Hz =10*LOG10(k*Teff)
16 Signal Strength at antenna -128.5 dBm Minimum signal strength in IS-GPS 200, see Module 4-3
-158.5 dBW =dBm - 3017 C/No 45 dB-Hz SS (dBW)-No(dBW/Hz)
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The actual C/No will be lower if there are losses from the antenna and in the Front-‐End RF section of the receiver. When indoors, the C/No will be lower still from losses as the signal passes through the structure of the building. In this lab, you will estimate the values x and y shown in this diagram on the right.
(x) You will measure C/No values outdoors, and estimate the RF losses in the receiver beyond what we expected from our spreadsheet. These RF losses are caused by the combination of:
• The imperfect GPS antenna used in the cellphone/tablet. • Attenuation caused by the cellphone/tablet housing. • RF interference from within the device itself
(y) Then you will go into the building where you live, and see if you can still get a GPS fix. If you can, you will use the C/No to estimate the signal loss caused by the building
Figure 1. Block diagram of what we can measure.
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Data collection: Open sky Find a location with a clear view of the sky, start the GPS Test app, and observe the C/No values. You will notice that they jump around, this is because of the random nature of the noise, and the fading of the signal caused by multipath signals interfering with the direct signal. Record the strongest 3 signals, and calculate the average. Note that it’s okay to use the GLONASS satellites as well, their link budget works out close enough to GPS to work for this experiment. Why the strongest three? Because there will be many satellites partially blocked by trees, buildings, etc., and we don’t want them to corrupt the result. Also, we don’t want just the one strongest signal, because phone antennas often have non-‐uniform gain patterns (partly because of the effect of the phone housing itself), and so you may get one unusually strong signal. The strongest three signals give a good average number. Note that we have now estimated RF losses beyond the receiver nominal design from our spreadsheet. In this example we measured 38.3 dB-‐Hz, we expect 45 dB-‐Hz, based on our spreadsheet. So, for this phone the RF losses are about 7 dB. This is the value x in Figure 1.
45 38.3 7 dBx = − ≈
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Inside your residence With the receiver running, go inside your residence, and see if you can still track at least three satellites. If you can’t, that’s okay, that is the result of the experiment. If you can, repeat the above procedure of recording the strongest three C/No values. NOTE: Your receiver will continue to track a signal to weaker levels than it can acquire in the first place. That’s why, for this exercise, it’s good to be tracking signals before you go inside.
We know from the first part of the lab that this phone has an ‘x’ value of 7 dB attenuation beyond the nominal from our spreadsheet. So now we can compute the ‘y’ value depicted in Figure 1:
45 7 24 14 dBy = − − ≈
You have also made a measure of your receiver sensitivity. If it tracked these signals, it’s at least this sensitive. By repeating this kind of experiment you can find out for yourself the limits of your device.
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Results First, record your results in the Lab C Quiz on the Coursera site. This is necessary for you to get credit towards your Statement of Accomplishment with Distinction. Then go to this web page, and record your results in the Google Form http://goo.gl/forms/Npas10bKUm using this example as a guide: The most up-‐to-‐date links are available at GPS-‐lab.org.
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Analysis of completed lab results
As with the other labs, the data you submit on the Google Forms will be collected and make available to all of you as Excel and CSV (comma-‐separated values) files. You are then free to do with it whatever you like—analyze the statistics, plot histograms, plot a heat map, write a conference paper, etc.
Please Note: The data collected in the Google Form and in Excel and csv files is anonymous. Your identity is neither collected nor available to anyone.