In the new review Doug posted, he states,

"The TerraFix 406 GPS I/O (Interface/Onboard) incorporates an integral GPS receiver while still retaining the external GPS hook-up (protoype photos included here). That provides a potentially useful additional capability over having only an integral GPS"

I've never used a GPS (or PLB), so it's not clear to me why having an internal GPS in the PLB and an external GPS plugged into it would be "useful." Would the 2 units symnchronize their data or does it improve their ability to acquire the satalites?

First, there are circumstances where you may want to load a location into the beacon before you set out or before you enter a particular area, just as back-up or because you are going someplace you know does not allow for reliable, or perhaps any, GPS reception.

Also, the GPS "engine" and antenna in these units is generally not as effective as those of better quality stand-alone GPS units in my experience. Thus, you may not be able to gain a GPS location using the PLB alone in some circumstances where the high-end handheld you are using for navigation may be able to lock on.

In this PLB, the integral GPS is primary and will override any external GPS derived location if it acquires one.

Note that the 406 MHz distress signal's ability to reach the COSPAS-SARSAT satellites is totally unrelated to the ability of the integral GPS to acquire a location. One is transmitting at approximately 5 watts, the other is receiving a very weak and easily blocked signal from space.

Hope this helps.

The GPS signals are very weak compared to, e.g., a cell-site transmitter. In spite of a "strong" signal, cellular coverage problems are well known. And on top of that, the GPS satellites are about 11,000 nautical miles away. Needless to say, GPS radio receivers must be very sensitive. Antenna design is also a very important aspect of GPS receiver function.

GPS-equipped PLBs contain other radio transmitters, e.g., the 403 MHz beacon and a 121.5 MHz homing signal, as well as the highly sensitive GPS receiver. In addition to these "intentional radiators," other electronics in the PLBs generate "unintentional (radio) radiation," or radio-frequency noise.

This noise and the transmitted signals can interfere with the operation of the internal GPS receiver. Similar behavior can be observed in broadcast radio receivers placed next to computers (especially AM radios).

Consequently, in a variety of circumstances, it can be advantageous to utilize an external GPS unit. Such circumstances include, for example, the relative quaility (gain and pattern) of the GPS antenna between the two devices, and an ability to physically separate a GPS receiver from the electrical noise of the PLB.

Such potential benefits of an external GPS must be balanced with the additional complexity and failure modes of such a system, e.g., breaking the cable.

/Neal

Neal,

As you note. And, just to tidy up loose ends, it should be noted that the GPS L1 (1575.42 MHz) is very nearly the 13th harmonic of the 121.5 MHz continuous analog homing signal, adding to the potential for interference and the difficulty in designing a GPS receiver that will adequately filter out this relatively very powerful signal immediately adjacent to the GPS antenna so that the GPS signal can be received and will not be overpowered. The manufacturers use various means to accommodate this issue, including shutting down the 121.5 for brief periods, but simply moving the GPS receiver away a small distance can make a big difference. The bottom line, in this instance, is that the user has a choice, whereas prior to this product, it was external or integral, one or the other.