[aprssig] Re: D700A gps interface problem

Gerry Creager N5JXS gerry.creager at tamu.edu
Wed Jul 21 09:54:34 EDT 2004

KC2MMi wrote:
> By comparison here's a log snippet from my DeLuo:
> $GPGGA,090243.000,4041.0595,N,07332.4232,W,0,00,,,M,,M,,*57
Oops!  No lock

> $GPGSA,A,1,,,,,,,,,,,,,,,*1E

Oops! One satellite.  Acquiring more...

> $GPGSV,1,1,04,11,54,059,,28,70,296,,31,37,065,,15,34,030,*75
> $GPRMC,090243.000,V,4041.0595,N,07332.4232,W,,,270504,,*04
> $GPGGA,090244.000,4041.0595,N,07332.4232,W,0,00,,,M,,M,,*50
> Does your GPS have a clear sky view? Even with a 30-degree wide view up at
> the sky, that's not enough for a GPS lock most of the time. A good 90-degree
> view of the sky, all around, is about the minimum to expect a good lock
> with. And on first powr-up, or if the unit's internal memory has gone flat,
> the new ones can take 5+ minutes to get the first lock. Older models (like a
> 1987 Magellan) could take 45 minutes to get first lock!

A significant number of receivers (and the vendors who program them) 
recognize that going through a significant portion of the earth's 
atmosphere, where you encounter the maximum water vapor and traverse teh 
maximum amount of scintillating ionosphere... the region in elevation 
between 0 degrees (horizon) and at least 5 degrees, but more often as 
high as 15 degrees, are not going to contribute significantly to good 
navigation solutions.  Additions to the error budget outweigh the 
benefits of the additional sky-search parameters.

Generally, a receiver that knows where it is, in general terms, will 
seek a satellite high in the sky to gain its initial information and 
sync clocks.  By going to a maximum elevation SV for clocks, it 
minimizes the amount of delay caused by water vapor (tropospheric 
contribution) as well as scintillation (ionospheric contribution) which 
will result in delay and diminished signal at the antenna aperture.

After acquiring the first satellite, and getting its ephemeris, it'll 
have a better chance at local sky-search, because its time is now better 
regulated (sync'ing with the first satellite allows the cheap little 
quartz oscillator in the receiver to sync and lock with the cesium 
standard on the satellite at the level of 10e-06 or so, based on the 
clock of the C/A code.

Older Magellans were plagued with design problems based on Magellan's 
decision at some point to go for the consumer market and to bypass some 
elements of the GPS Interface Control Document and take shortcuts with 
the GPS Signal Specification, according to Magelllan design engineers I 
talked to repeatedly at the Institute of Navigation's annual GPS (now 
GNSS) meetings.

> If you have any GPS-interactive software running, most of it will have some
> screen that shows GPS performance, i.e. "poor-fair-good" or "no satellites"
> etc., in human-friendly form.

A quick perusal of any of several Web-available NMEA-0183 V2 or V3 
sentence explanations will allow you to use GGA and GSV to tell what's 
doing what.

Gerry Creager -- gerry.creager at tamu.edu
Network Engineering -- AATLT, Texas A&M University	
Cell: 979.229.5301 Office: 979.458.4020 FAX: 979.847.8578
Page: 979.228.0173
Office: 903A Eller Bldg, TAMU, College Station, TX 77843

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