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Historically, GPS receiver designers have relied on a combination of simulation and drive testing to characterize a receiver's performance in difficult environments. While GPS signal simulation provides a repeatable signal source, simulators cannot reproduce difficult multi-path signal anomalies often seen in the real world. Moreover, drive testing also contains inherent challenges. Not only is drive testing expensive, but it fundamentally introduces a receiver to signals that are difficult to repeat from one trial to the next.
As a result of these challenges, an increasingly common approach to receiver validation is to test receivers with recorded GPS waveforms. This approach uses an RF vector signal analyzer (such as the NI PXI-5661) to record live GPS signals as a continuous IQ data file. Then, an RF vector signal generator (such as the NI PXIe-5672) is used to play the signal back to the receiver. Testing GPS receivers with recorded signals has several benefits over traditional simulation or drive testing approaches. Not only does this approach introduce real-world impairments, but it does so in a manner that is repeatable. Thus, you can test many different receivers by observing how they will react to the same test stimulus.
Configuring an RF Recording Device
There are two main concerns to consider when configuring an RF record and playback system: using the full dynamic range of the RF vector signal analyzer and ensuring that the recording device does not add additional noise to the signal. Concerning the first issue, an easy way to capture the GPS signal's full dynamic range is to use a vector signal analyzer with significant dynamic range. With typical vector signal analyzers offering up to 80 dB of dynamic range, signals with a small signal-to-noise ratio (SNR) " usually less than 30 dB " such as GPS can easily be recorded without significantly affecting the SNR of the off-the-air signal. Thus, the only remaining task is to amplify the off-the-air signal while adding as little noise as possible.
In a typical environment, each GPS satellite will have an average power level (course acquisition (C/A) codes) ranging from -135 dBm to -125 dBm, depending on its position and environmental factors. A typical scenario will result in signals in the L1 (1.57542 GHz) band having a peak power that can range from -120 dBm to -110 dBm. In our testing, we observed a peak power of -116 dBm. As might be expected, recording such low-power signals requires careful attention to both antenna and amplifier selection. In fact, to use the full dynamic range of the RF vector signal analyzer, amplification is required. There are several ways to amplify an off-the-air GPS signal. However, you can achieve the best results by using an active GPS antenna in conjunction with an additional low noise amplifier. With two cascaded low-noise amplifiers (LNAs), each providing 30 dB of gain, the total gain applied is 60 dB. Thus, the resulting peak power observed by the vector signal analyzer is increased from -116 dBm to -56 dBm. The required power at an RF vector signal analyzer will vary from one instrument to the next, and this value is determined by the maximum gain applied by the vector signal analyzer.
Powering an Active Antenna
To capture GPS signals while adding the smallest amount of noise possible, use an active GPS antenna with a noise figure that is less than 2 dB. This achieves the best results. Active antennas provide the best gain performance versus noise figure, but they introduce the inherent challenge of providing a DC bias signal of anywhere from 2.5 to 5 V.
One common method that can be used to power an active antenna is with a DC bias "T." Using this component, a DC signal (3.3 V in this case) is applied to the DC port of the bias T, which applies the appropriate DC offset to the active antenna. Note that the precise DC voltage you should apply depends on the DC power requirements of the active antenna. Figure 1 shows a diagram illustrating the system setup.
1. You can use a DC bias "T" to power an active GPS antenna.
Observe in Figure 1 that you can use any off-the-shelf DC supply to supply the DC bias signal. While we used the NI PXI-4110 in our experiments, any generic power supply will work. Also, it is important that the DC bias T is rated for operation up to 1.57542 GHz, the frequency range of L1 GPS signals. A DC bias T like the one used in this experiment was purchased from minicircuits.com.
Once the RF front end of the recording device is configured, you can test the system simply by performing a basic RF spectrum measurement in the L1 band. To do this, configure the RF signal analyzer to a center frequency of 1.57542 GHz (the L1 band) and a span of 4 MHz. Note that the antenna should be placed in an open-air environment where it has a clear view of the sky. The GPS C/A code signal will occupy a bandwidth of about 1 MHz, so a slightly wider span is required to visualize this signal. In addition, because the power level is significantly low, a narrow resolution bandwidth (RBW) combined with a low RF reference level (-50 dBm) is also required. With a 10 Hz RBW configured with 20 averages, the GPS satellites should clearly be visible just above the noise floor. Figure 2 illustrates an example RF spectrum after 60 dB of gain.
2. GPS is visible only in the spectrum with a narrow resolution bandwidth.
Figure 2 shows a small "bump" right at 1.57542 GHz. This "bump" is the off-the-air GPS signals and it indicates that the RF front end is correctly configured. Now that the RF front end is configured, the next step is simply to perform a continuous IQ acquisition. Connected to a large storage volume, typical RF recording systems can capture up to 25 hours of continuous GPS waveform.
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