Electronic Design

Bob’s Mailbox

Dear Mr. Pease:

Your Oct. 25 column hit close to home. I was one of the original members of the design team for the Trimble SCOUT (and all the handhelds from lineage, for that matter). Now, I don’t know where the picture of the SCOUT came from, but in either case I blame the marketing people who care more about how the product looks in the ad than how accurate the ad is! I’m sure this is a rare event and never happens where you work.

As far as the rest of the article:

1)      Storing that ‘info’ in EEPROM: EEPROM still is not that inexpensive to store all the data that GPS satellites generate. We had problems with total write-cycle numbers once, so we might still be a little gun shy, too. SRAM is cheap and easy to back up in a system that already has batteries. I’m sure that somewhere in their manual it states how long you should take to change out the batteries, and to leave them in until ready to do so. You wouldn’t do that with your HP calculator, would you?

2)      Have you heard about DPGS? The ‘D’ stands for Differential, where a GPS receiver that has been very accurately surveyed compares the GPS signal it receives to where it knows it is, and then broadcasts the correction factor over some RF channel. The Coast Guard is currently outfitting its stations with this capability with no charge to the end use. Of course, you need the appropriate receiver, and since, it is the Coast Guard, you need to be near WATER (within 100 miles or so, your mileage may vary). Some other services broadcast corrections over the FM subcarrier, but charge a monthly fee in addition to the receiver. But, DGPS can repeatedly get you within 5 to 10 meters of a spot!! And your Meridian, as are ours, is differential-ready.

3)      Yes, you were able to buy a GPS receiver at Fry’s, but it was not a very good one, with the worst user interface of any system I’ve seen in a long time!!! I guess that’s why you can’t buy it at Fry’s anymore. I’m glad you’re using GPS for hiking in the great outdoors. It’s a clever system with some amazing applications. If you ever want to take a look at a SCOUT, REI is selling them, or you can give me a call.




Trimble Navigation

Austin, Texas


Of course you are right. Marketing people don’t care about the factual details, just the image…No, my manual doesn’t give any clue how long it will store info with the batteries out. The Fry’s in Sunnyvale doesn’t sell GPS receivers—but the one in Palo Alto does. Yeah, after the differential GPS is working perfectly, the Air Force will probably decide to turn off the Selective Availability. –RAP


Dear Bob:

Regarding the GPS Selective Availability (S/A) issue mentioned in your Oct. 25 column: It boils down to the tax-paying public being denied the benefits of a government project for which there is no rational explanation. The DOD decided to intentionally degrade the accuracy of GPS for civilian use. But then the Department of Transportation spends millions more the defeat S/A by means of U.S. Coast Guard differential correction transmitters.

The “C” version of the 50-year-old loran system, which gave repeatable accuracies of ±10 meters in “good” reception areas is being phased out in favor of a global navigation system with an intentional ±100-meter error! Anyone who wishes to spend some extra money for GPS differential navigation equipment can improve their accuracy to under ±1 meter. So, what is the rationale for degrading the system accuracy to ±100 meters, when any user can defeat S/A by spending some extra money?

The National Academy of Public Administration is inviting comments from the public on a GPS project study. Those having comments on intentional inaccuracy of GPS, or any other aspect of use of the system, can send their comments to: NAPA, GPS Project, 955 L’Enfant Plaza North, Suite 4000, Washington D.C. 20024 USA; (202) 651-8014; fax: (202) 484-4899. Email: [email protected]



Navigation Technology Int’l

Reno, Nev.


Thanks for the info. I’ll write in. Readers should, too.—RAP


Dear Bob:

In addition to my bemusement at your unconventional mail stop designation, I found your comments on the “Taguchi effect” reminiscent of an old yarn which you may remember:

At an electric motor manufacturing plant, a newly graduated and recently hired production engineer was anxious to prove the superiority of his skills to those of an old engineer on the staff. He analyzed one of the high-production motors, and came up with a significant cost savings by a change in materials and modification in fabrication methods.

On the big day, he assembled the top management, and there on the table was the shining prototype of his modified design. He had charts and tables showing that the cost advantage of machining the brass frame with less-expensive tools and in less time more than offset the higher price of the material. The cost to produce the brass motor was reduced to 70% of the cost to produce it as presently designed with an iron frame.

The plant manager turned to the old engineer and asked, “Well, George, why didn’t you come up with these cost savings?”

The old engineer simply said, “Plug it in.”




A-Tech Engineering

Herndon, Va.


Lovely, Lovely! Thank you.—RAP

p.s. By any chance was the young engineer using Spice? 

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