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NI PXI-6683 Series User Manual
The embedded GPS receiver in the NI PXI-6683 Series device requires signals from several
satellites to compute accurate timing and location. The more satellites available to the receiver,
the more accurately it can determine time and location. Therefore, the antenna location should
be such that it receives signals from the greatest number of satellites possible. As the number of
satellites visible to the antenna decreases, the synchronization performance may also decrease.
Choose the antenna location so that the antenna has a clear view of the sky. There is no strict
definition for a clear view of the sky, but a suitable guideline is that the GPS antenna should have
a straight line of sight to the sky in all directions (360°) down to an imaginary line making a 30°
angle with the ground. Locations far from trees and tall buildings that could block or reflect GPS
satellite signals are best.
Maximum Cable Length
Maximum cable length depends on the GPS antenna gain and the cable’s loss per unit of
distance. National Instruments recommends a GPS signal strength of between -135 dBm and
-120 dBm at the NI PXI-6683 Series device SMB input. GPS signal strength on the Earth’s
surface is typically -130 dBm. Targeting a signal strength of -125 dBm at the SMB input, you
can compute the maximum cable length as:
Max_cable_loss = -130 dBm + antenna_gain - (-125 dBm)
Max_cable_length = Max_cable_loss / (loss_per_unit_of_distance)
For example, if you use an active antenna with gain of 28 dB and RG-58 cable, which has a rated
loss at 1.5 GHz of about 0.8 dB/m (24.5 dB/100 ft), the maximum cable length you could use is:
Max_cable_loss = -130 dBm + 28 dB - (-125 dBm) = 23 dB
Max_cable_length = 23 dB / (0.8 dB/m) ≈ 29 m
Note
The GPS antenna kit offered by National Instruments comes with a 30 m
cable which has a loss of 15 dB/100 ft, making the total loss in the cable
approximately 14.8 dB.