GSG-5/6 has the capability to simulate noise on the GPS L1 band. Noise simulation can be a powerful tool for receiver testing, since it allows for a strong signal to be submitted, without jamming the receiver.
To access the Transmit Power view, which—among other things—allows to adjust the noise settings, navigate to Options > Transmit Power:
Adjusting noise settings in the Transmit Power view
The noise generated by GSG-5, GSG-56 and GSG-6 is similar to the noise of GSG-55, but differs in so that the noise bandwidth is constant and set to cover both the GPS L1 as well as the GLONASS L1 band. The noise central frequency is not configurable.
International regulations keep the L1 band practically clean from disturbing signals, so the only noise source is the natural background noise, as expressed in the following equation:
- PN = kTBN
Where k is the Bolzmann’s constant, T is the ambient temperature (in Kelvin), and BN is the bandwidth (in Hertz).
For example, an ideal GPS L1 C/A code filter would have a passband of 2MHz, and noise power passed by the filter at a temperature of 290 K would be equal to -141 dBW.
The ambient noise power spectral density is given by the equation:
- NT = kT = 4.00 x 10-21 W/Hz = -204 dBW/Hz = -174 dBm/Hz
By definition, carrier-to-noise density is the carrier power divided by the noise power spectral density. The GPS ICD specifies that the received signal level at the surface level is -130 dBm or better. Carrier-to-noise density is then:
- C/N0 = -130 dBm/(-174 dBm/Hz) = 44 dB Hz
C/N0 (not SNR) is the figure that the receivers typically display as an indication of quality for the received, digitally modulated signal. If the receiver has bandwidth of 6 MHz, SNR would be:
- SNR = 44 dB Hz/(6 x 106 Hz) = 44 – (10 x log10(6 x 106)) dB = -23.8 dB.
If a stronger input signal for the receiver is required, while maintaining the same C/N0, additional noise needs to be introduced into the transmitted signal. One may think of this as having an active antenna at the receiver input. The signal level is higher, but so is the noise level.
When you change the values in the Transmit Power dialog, you may notice that other settings may change as a consequence of the changes made. For example, if you have Transmit Power set to -70 dBm, and External Attenuation set to 5.0 dB, the unit actually transmits signals at -65 dBm to compensate for the external losses.
Note, however, that manually adjusting the attenuation to 10 dB in such a situation will cause the Transmit Power to drop to -75 dBm as a consequence. This is a result of the hardware configuration, as the unit cannot deliver more than a total of -65 dBm. The Transmit Power setting gives the power level at the end of your antenna cable.
In general, when changing the Transmit Power setting, it is recommended to follow this order:
- Set the External Attenuation
- Set the Transmit Power
- Set the Noise Bandwidth
- Set the Carrier-to-Noise Density
- Set the Noise Offset (this can be done at any time without affecting the other settings)
If you use the SCPI protocol to change the power/noise settings, use the order above to do modifications, and check the SCPI error after each command. If there is a Parameter Conflict error, it would indicate that the unit accepted your command, but due to a conflict with a different parameter, your parameter value was modified.
The conditions under which a Parameter Conflict may occur include the following:
- A Transmit Power value has been requested that is too high. The requested Transmit Power is within the specified limits, but the External Attenuation setting limits the maximum power to below the requested setting. Transmit Power is set to the maximum available, rather than the value requested by the user. Increasing the Transmit Power may lead to an increase of C/N0, as described under bullet #3 below. To prevent this from happening, especially when using the SCPI protocol for making adjustments, always use the command order described above, and check the SCPI errors after each command.
- An Unachievable Carrier-to-Noise ratio has been requested. The requested value is within specifications, but the Transmit Power setting is too low to achieve the required setting. In this case, the ambient noise power spectral density limits the achievable carrier-to-noise ratio. The Carrier-to-Noise density will be set to its maximum value, not to the value requested by the user. The noise generator does not generate any additive noise in this situation. Increase the Transmit Power, then set C/N0 again.
- A Carrier-to-Noise ratio has been requested that is too low. The requested value is within specifications, but the Transmit Power setting is too high to achieve the required setting. The signal/noise generator does not have the capability to generate a noise signal this strong (remember that noise power is more than the signal power – SNR is negative). The Carrier-to-Noise density will be set to its minimum value, not to the value requested by the user. Decrease the Transmit Power to decrease the required noise power.
- A Noise Bandwidth value has been requested that is too wide. (SCPI command only) In effect, this leads to the same situation described under bullet # 3 above. GSG accepts the noise bandwidth setting, but increases the C/N0 to its minimum value. The noise bandwidth required depends on the filters of the receiver. You have to search for the value that is wide enough for your receiver. Set up a relatively strong signal (for example: -100 dBm, C/N0 44 dB-Hz), and narrow noise bandwidth. Then increase the noise bandwidth until the C/N0 value shown by your receiver stabilizes. It is a good idea to use the narrowest bandwidth needed.
Note: The receivers use different methods to calculate C/N0 (or SNR), so the value given by the receiver may be different from the C/N0 setting of the GSG unit.