The GPS signal is different than standard radio signals (FM signal/AM signal, cellular, TV, two-way radio, etc.). It is different mainly by the amount of power it has when received by your roof/external antenna.
If you were standing next to the satelllite, you would be able to see a pattern, which is is represented (or known) as the normalized sinc function, similar to the picture below:
If that is the shape of the signal, why cant you see it? The short answer is you are not standing next to the satellite. The long answer is the GPS satellites are 10,898 nautical miles in space and they dont transmit with enough power for GPS signals to be as powerful as the terrestrial radio signals that you see with your spectrum analyzer.
Also, standard radio signals have a positive SNRs (signal-to-noise ratio). This means they are stronger than the noise in their band. The GPS signals have a negative SNR, i.e., they are weaker than the noise in their band. If GPS had a positive SNR, you would probably not need an antenna on your roof. It is possible you could receive it indoors without an outside antenna, just like your cell or AM/FM radio. But, the GPS signal has a negative SNR. By the time it makes it to the earths surface, it is about 26dB below the noise floor of the earth. That is why an external/roof antenna is needed. The GPS signal is available, but it is hidden under all the noise.
The GPS signal can be extracted from the noise. A typical GPS receiver has approximately 40dB of processing gain. When it looks at the noise it can see/detect something. It does this by performing what is called a correlation of the noise against the same codes that were used to create the GPS (CDMA) signal. Aclose analogy would be that of a comb with some of the teeth broken out that represents the pattern. If you comb the noise with a comb that is missing the exact same teeth as the comb that created the patternat exactly the right instant, you will get a match.
So what is the bump that you are seeing on your spectrum analyzer? What you are seeing is the response/shape of the antenna element (it acts like a filter) and the filter response of the antennas preamplifier. Their frequency response has a bell (Gaussian) curve shape, so that you see a bell-shaped bump on your analyzer. The noise is shaped by the filter, because noise is random. It has equal power across the band, so you wind up with noise I the shape of the filter response.
The GPS signal is there, but it is 26dB below the peak of the bump down in the noise or roughly 1,024 times weaker than the power of the noise. If the GPS signal is 26dB weaker than the signal when it arrives, is the GPS signal 26dB below the noise being measured? No, not necessarily. It is an okay rule of thumb, but keep in mind the exact signal level depends on the bandwidth of your system, the exact gains of all devices, their noise figures, temperature (remember noise figure changes with temperature), etc. Essentially, this rule of thumb will usually get you in the ballpark provided none of the gain stages are saturated.
The GPS signal has a negative SNRso what?
Signal-to-noise ratio (or SNR) is a measure used to quantify how much a signal has been corrupted by noise. It is defined as the ratio of signal power to the noise power corrupting the signal. A ratio higher than 1:1 indicates more signal than noise. The GPS signal has a negative SNR, and is weaker than the noise in the band.
It is important when designing a GPS network with cascading gain stages that you keep the noise level in mind, because the noise is so much stronger than the GPS signal. It can drive cascaded gain stages into saturation. An experienced GPS network designer will strategically place gains and losses to avoid saturation issues. The other side of this coin is that you need to maintain link margins so that the signal does not dissipate before you place a following gain stage.
The way to think about it is like this, "if I have a GPS network that needs 60dB of gain, I shouldnt place a 60dB amplifier at the output of the antenna and try to drive to the end of the network, because it will saturate and distort the signal. Likewise, I cant place all the gain at the end of the network, because the signal will end before it reaches the amplifier at the end of the network". In other words, you will not have enough link margin. One needs to distribute the gains and losses so that you avoid saturation and maintain link margin across the GPS network. Call (719) 561-) if you need help with calculating link margins and designing your GPS network.
It is difficult enough looking for the right GNSS antenna or module. There are many options to choose from, and getting the most popular GNSS option may not be the best choice for your application. [Read our article on "GPS vs GNSS"] Yet as the number of GNSS systems increases and navigation technology improves, keeping track of GNSS choices can be challenging.
Augustine Nguyen, Applications Engineer at Symmetry Electronics, provides a few considerations for those adding GNSS to their design.
It may be easy to account for GPS, but how about BeiDou or Galileo? Different regions use different GNSS systems, and if your product has a regional focus, its best to know the corresponding GNSS system to build around.
This is an important factor to consider when designing a product because adding support for multiple GNSS systems requires additional testing during the production process, says Augustine Nguyen. Each GNSS system serves a specific set of regions with their own regulations, and certification is required before the product can be introduced to a region. This can be a lengthy and expensive process which any project manager would need to consider.
GNSS systems available or planned to be available in the future are GPS (USA), Galileo (European Union), BeiDou (China), GLONASS (Russia), NAVIC (India), and QZSS (Japan).
The answer depends on the global reach of the application. Is the product a domestic product, with a strong focus on a specific region, or will the product extend across other parts of the world?
Do you want your product to provide GNSS capabilities for multiple regions around the world? You are essentially dictating the demographic you are selling to. It comes down to how flexible and versatile you want your product to be. For example, your cell should have location tracking no matter where you are in the world, therefore supporting multiple GNSS systems. An application focusing primarily on domestic farmers, on the other hand, would only need to support the GNSS system that pertains to that specific region. Supporting only the regions you need keeps the product cheaper and easier to produce.
Take into consideration the dimensions of the product, as well as the size of the PCB. Augustine explains further:
Knowing the size of the product is important because the dimension of the product and its PCB determines where you can place the antenna. Poor placement of the antenna would affect the antennas performance. The size of the antenna also changes how the antenna behaves. The antennas output power, efficiency, and directivity are the major parameters that can be affected by poor design.
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Internal, or external? Soldered or connected via cable? Location determines the optimal performance of the system and the user experience.
"The location of the antenna on your products PCB affects the radiation pattern and efficiency of it. Depending on the type of antenna, the size of the PCBs ground plane should always be considered in comparison to the antenna manufacturers reference plane and calculated values. The manufacturers data sheet of that particular antenna only gives you the general idea of how it performs and will often bear no resemblance to the antenna performance on your final product. This is because antenna manufacturer use reference boards (that are of a specific size) to ONLY provide a baseline of their products performance.
The location of the antenna also has to do with how you want your design to look like. You dont find GNSS antennas external to the enclosure of the because it would be unattractive. There is a balancing act between the products functionality and its aesthetics.
Not all antennas are the same, and good manufacturers go above and beyond to make sure buyers are satisfied. Choose reputable parts for quality and service. A specialty distributor can help you find the right antenna and avoid costly mistakes along the design cycle.
Finding the right solution for your specific needs is key to successful design. The size of the product, location of the antenna, and geographic range are important factors to consider before adding GNSS to your design.
Ethertronics offers antennas of all sizes to accommodate your product needs.
For smaller antennas mounted on the surface of a PCB, Ethertronics offers internal antennas that are placed inside the enclosure of the product as a surface-mounted device, or SMD.
Ethertronics: (SMD) - Prestta Standard GPS Embedded Ceramic Chip Antenna
Ethertronics: M (SMD) - GPS Savvi Ceramic Chip Antenna
Ethertronics: (SMD) - Embedded GPS ISM Stamped metal SMT Antenna
Ethertronics EtherHelix
Ethertronics also offers bigger, external or off-board antennas connected to the PCB via a coaxial cable with RF connectors at both ends (SMA, u.FL). These external antennas are either placed outside the enclosure of the product or off the products PCB to provide more design flexibility.
Ethertronics: - EtherHelix Mission Critical GPS Antenna
Ethertronics: - GPS Active Patch Antenna
In addition to industry-leading wireless antennas, Ethertronics has an in-house team of engineers to help analyze your product or system and fine-tune your RF design around your application for optimal performance. Click here to see the selection of Ethertronics antennas available.
Are you interested in learning more about GPS smart timing antenna? Contact us today to secure an expert consultation!