CenterClick NTP200 and NTP250 Documentation - Antenna
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o Antenna Issues
o PPS Output
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Troubleshooting Antenna Issues
GPS reception issues can be one of the most difficult problems to resolve. The guide below has some steps you can take to help determine the root cause.
Tools available in the NTP2xx appliances
We've added information in our appliances to help debug.
Changes to antenna position/reception are generally observable within 5-10 seconds, however some items such as initial aquisition or trends, can only be determeined with longer-term testing (1-6+ hours). In some cases it may take several tests to try to improve reception
Lets look at this example, an appliance that has reception but it comes and goes. Symptoms include occasional 'no-gps' errors and stratum >1 on occasion, however most of the time there is gps lock and satellites used is >6.
Let's first look at the easiest indicator for reception quality, the immediate overall SNR:
In this example, a SNR of 20.5 is marginal, however if I refresh the page a minute later:
we can see it's now 17.8 which is poor.
The SNR displayed here is the real-time average of the 6 satellites with the highest SNR and is a good reading of the signal strength. A reading of less than 20 is poor, 20-25 is fair, 25-30 is good, and greater than 30 is great.
For toubleshooting purposes, you should ignore the 'seen' satellite count as only the 'used' count is generally useful. And having said that there isn't much gained by a larger number of 'used' satellites as long as it is more than 6-8. The SNR is much more important than the number of 'used' satellites.
The overall SNR is a good readout of the real-time reception, however it can be quite noisy and doesn't give a good picture of the longer-term quality. For that we must look at the graphs for the same device. The NTP/GPS Graphs page provides a graph of the same SNR value, but over time:
We can see that most of the time, it's centered around 18 +-3, but occasionally drops out for a few hours as well. Also on this page we can see the counts over time:
We can see that while the used count on our two stapshots above were 10 and 12, in actuality, most of the time it stays within the 5-10 range and there are lots of dropouts.
In conclusion, if were were to encounter the appliance above we'd conclude that it is unlikely there is a hardware issue, and the cause is simply poor reception.
Here's another example, this appliance has a slightly better reception than example 1, and doesn't have issues.
The SNR is averaging around 21 over time vs the 18 in example 1 and those extra 3dB make a difference. There are zero dropouts in the 'used' count and we would conclude that while reception is near the limit of usability it is stable and the appliance is fully functional.
In this example, this appliance has worse symptoms than example 1 or 2. GPS shows no-fix and zero 'used' satellites frequently, but not all the time.
There is a period of several days were no signal was good, but it occasionally comes into operation for a few hours before dropping off every so often. Also, it is important to note that when there are used satellites the SNR is still low. In this example, it is unclear if the antenna is bad or just very poor reception.
Also note that the 'seen' count is quite useless as it never goes down during the outages.
Is my hardware bad?
Antenna SMA connector short circuit
The most common antenna defect is a poorly made connection joint between the cable and SMA connector. This will result in a near or full short circuit between the pin and sheild of the SMA connector but may only do so under some physical stress (i.e. only when screwed down hard, bent in one direction, etc...).
We test all antennas we ship against NTP2xx appliances and can usually elimiante these types of defects, however, on occasion this type of fault can be seen if the fault got worse due to flex/bending of the cable/connector junction.
You can usually test for this fault using a multimeter:
WARNING: Do not intentionally push on the the SMA pin in the direction towards the cable. This can result in the pin actually sliding into the connector making it slightly too short (see below).
In some cases a short circuit defect only occurs when the SMA is screwed down and the pin has pressure on it. Here is an example of a short circuit that only occured once the SMA connector was screwed down. We are using a bare SMA edge connector to attach the multimeter.
There are some symptoms of this type of fault while under operation:
SMA connector pin alignment or length
Sometime mechanical damage just happens. We've seen a few occurances of the SMA connector being damaged, usually on the antenna end but occasionally the NTP2xx as well.
The pin can get bent resulting it not aligning with the slot on the connector. This usually results in it being very difficult to screw down the sheild and in the process it deforms the appliance-side SMA connector preventing another pin from ever connecting properly. This type of damage is easially observable after the fact.
Alignment issues can also cause the SMA pin on the antenna side being pushed back into the connector towards the cable. This makes it too short to later contact with the appliance in future attempts. The pin of the SMA connector should protude about 2.5mm out of the base.
GPS module or power supply damage
The GPS module on the appliance can be damaged, you can check the basics with a multimeter:
We have found that in most cases, people do not have issues with using the included antenna indoors. however it is best to:Avoid:
The Antenna's effectiveness will be improved if attached to the top of a metal sheet or plate that is parallel to the ground of at least about 5" in size. The antenna puck includes a magnetic base.
Antenna Cables and Extensions
The included antenna has a 3m/10ft RG174 cable. RG174 is small an flexible, but not excellent for long distances as it has relatively high loss per foot compared to other options. While it can work for GPS at longer than 10ft lengths, it is not recommended that you extend RG174 beyond 10ft unless the expected signal gain by doing so would counteract the added loss due to cable length.
For longer runs, it is recommended to use RG58 cable instead of RG174. Both are 50 Ohm Impedance and commonly found with SMA connectors for GPS and cellular applications. RG58 and similar types of coax cables marketed as 'low loss' or 'ultra low loss' can be used.
NOTE: Avoid cables designed for WiFi as they likely have RP-SMA instead of SMA connectors.
Regardless of what cable type you use, a shorter antenna cable is highly desirable as losses are cumulative per added foot. The NTP250 features PoE for powering itself over its Ethernet connection so that the NTP250 itself can be located in otherwise difficult to install locations. Ethernet does not suffer significant losses even out to 200+ feet and it is preferable to relocate BOTH the NTP250 and its antenna to a better location than to extend the antenna cable. The NTP200 can also be relocated but a local power source would be needed wherever it is located.
There are a variety of antennas available for purchase, the following hard requirements are needed when picking an antenna:
The following products have been selected after review of their data-sheet or specs, they appear to meet the criteria above, however unless specified below they have not been verified to work with the NTP200/NTP250. If you obtain one, we'd like to hear about your results.