@adammoffett wrote:
1) I've seen a wimax product that requires 20+ minutes to acquire GPS lock on a cold start. I'm told in the carrier Wimax/LTE/cellular space that's actually fast, and some products require up to an hour, and I heard of one older product that supposedly needed 24 hours.
Canopy seems to only need 30-90 seconds to have GPS sync up and running, and the PMP320 might have been 5 minutes at the most. I've been wondering what the difference is. I suppose it's a long shot that you (Forrest) would know, but I thought I'd ask.
2) Forrest, do you foresee Packetflux making GPS sync products for any platform besides Canopy?
3) The biggest downside I see to GPS sync is that you have a problem when it stops working. What options do we have to "harden" our GPS sync setups? Would it be worth having surge protectors on sync cables for example?
Answering your question by the numbers:
1) A lot of it comes down to two factors: a) the quality of the GPS receiver and b) the amount (and type) of timing accuracy required by the application.
Older GPS receivers were not that sensitive and only listened on a few channels at a time (12 channels used to be a good receiver). This resulted in very long lock times. With more modern receivers, coupled with more advanced algorithms and processors to go with, the base acquisition goes down to 35 seconds (cold). As an aside, the current chipset we use receives on 66 channels and can receive signals as low as -165dBm.
With that in mind, I suspect the main difference is the quality or type of the timing needed. Most off the shelf receivers such as is used by cambium GPS sync sources have 1PPS (one pulse per second) sources accurate to somewhere around 10 nanoseconds (1/10,000,000th of a second) per second once they have a lock. The cambium radios don't even need that level of accuracy, mainly since they only use it for transmit/receive switching which happens over tens or hundreds of microseconds (1/1,000,000th of a second) instead of nanoseconds.
On the other hand, some other technologies need a much more accurate clock (perhaps 1nS or better), or more likely - a high precision 10Mhz (or higher) frequency source. The most cost effective way to attain a very high accuracy source is through a process called disciplining where the frequency of a already high-accuracy oscillator is adjusted using the GPS signal as a reference. Over a fairly long period a disciplined oscillator compares the signal being generated by the oscillator with the 1PPS signal from the GPS. If there are less than 10 million counts received in a 1PPS interval, it turns up the frequency. If there are more than 10 million counts, it turns it down. It has to do this over a fairly wide time period since the best accuracy is obtained over a long period. For instance, if you only measured every 100th 1PPS count, you now have an accuracy of 10ns over that 100 seconds, which is, of course 100 times more accurate than once per second. If you need that level of accuracy, you have to wait 100 seconds for the first measurement, and may need more measurements than that to trim the frequency source to within tolerance. This is what is refered to as "warm-up time", and it can definitely take a lot of time.
As an aside: GPS antennas are now showing up at broadcast sites - they're used for the frequency source for the transmitter and/or a subcarrier and as a result, are definitely of the "needs warm up time" type.
2) It really depends. A lot of the reason why I am able to provide synchronization for cambium products is that they rely on an external synchronization source. Many of the other radios which need a synchronization (or timing) source rely on an internal receiver which isn't really able to be bypassed. At most I could sell a external antenna but those are widely available and I prefer a bit less competititon in my markets.
If there was a radio (or device) which needed a external precision timing and/or frequency source and the available sources were priced significantly higher than I could produce a unit for, I sure would consider it.
3) The easiest way is to use multiple GPS sources. Modern cambium gear handles multiple inputs from multiple sources. For instance on the 450, you can use the internal GPS, and two external GPS sources (sync over power and.or timing port). With the ePMP you have both internal and sync over power. With my products being relatively inexpensive, it doesn't make a lot of sense not to provide multiple sources, especially on those radios which have a built in GPS aleady there.
Beyond that, there isn't much you can do to improve other than standard precautions you'd take with any radio receiver (away from other RF sources, pay attention to mounting location, etc). You could add a surge supressor, but generally that's not necessary (I have integral surge supression on both ends of the sync cable in our products).
