#24116 - 02/06/04 02:11 PM
TerraFix 406 GPS review
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Old Hand
Registered: 05/10/01
Posts: 780
Loc: NE Illinois, USA (42:19:08N 08...
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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?
_________________________
Willie Vannerson McHenry, IL
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#24118 - 02/06/04 07:20 PM
Re: TerraFix 406 GPS review
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new member
Registered: 11/18/02
Posts: 34
Loc: SF Bay Area, California
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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
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#24119 - 02/06/04 08:16 PM
Re: TerraFix 406 GPS review
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Pooh-Bah
Registered: 01/28/01
Posts: 2198
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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.
Edited by Doug_Ritter (02/07/04 01:20 PM)
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