GPS vs. DGPS

[jerry]

I have been looking at the Lowrance Global Nav 310. In the Cabela's catalog, it list the price for the GPS and a separate price for DGPS. What is the difference? I thought with Selective Availability turned off it didin't matter and that both would read with the same accuracy. Can someone please clear this up for me? Thanks.

[REW]

Yes and no.
Yes - SA has been turned off.
However, the accuracy of GPS - even with SA turned off - is affected to a certain affect by atmospheric conditions.

i.e.
If you compared a "normal" GPS with SA turned on -- the accuracy was guarenteed to be no better than 100 meters - or a circle the size of a foot ball field.

With SA turned on - the accuracy is typically 30 miters -- or the diameter of about 90 feet.

Many folks say - and I have also found this to be true -- that much of my accuracy is within 10 feet or 3 meters. i.e. a 10 foot circle. Again - there can be variations on a minute by minute basis - or hourly basis.

However, if DGPS is used -- the DGPS - ground based transmitter - knows exactly where it is. It is constantly receiving the same information from the GPS that you are. thus - assuming that you are able to receive a nearby DGPS signal - you will be experiencing about the same GPS reception as the DGPS station and will be receiving about the same level of inaccuracy - if any.
At any rate -- since the DGPS station knows exactly where it is - and it receives the signals from the GPS units in orbit -- it then computes the difference between GPS received information - from its know location. If there is any difference at all -- i.e. a difference as small as 1 inch - the DGPS station generates a correction signal. i.e. if the difference computed in location is small - the correction signal will be small; if the difference is larger - the correction signal will indicate a larger deviation.

Thus, if you are receiving both the GPS satellite signal - and the DGPS signal (an additional low frequency antenna is required) you will be able to insure your location - any where within range of this DGPS station of about 1 foot. Perhaps better, perhaps as far off as 2 feet.

Summary -
With the SA turned off - the typical 12 channel GPS receiver today - will routinely locate the GPS receiver within a circle that is about 10 feet in diameter. However, there may be a few times that it will grow to a circle as large as 30-100 feet.

If you want to know the exact location within about a 5 foot circle - be within range of a DGPS transmitter - not available everywhere - and be willing to purchase an additional DGPS receiver - and put up with a long low frequency antenna - to receive the DGPS signal.

Take care

REW

[jerry]

Thanks for the clarification, REW.

[Scott D]

Raytheon has gps units that use a new form of DGPS called WAAS. If I remember right it is called Wide Area Augmentation System. It utilizes stationary receivers that transmit corrections like REW said. The Raytheon units use only one receiver antenna compared to the other brands which use a separate DGPS antenna. Raytheon is the only units that utilize WAAS that I know of. I have been researching gps units and think that I have narrowed it down to the Raytheon 425 chartplotter. I am not sure if the other companies like Lowrance will be able to use WAAS in the future or if Raytheon has exclusive rights to it. The accuracy is supposed to be less than 3 meters 95% of the time.

[REW]

I see that Garmin is releasing a new unit called the 152.
This unit also contains the WAAS - and is specifically made for the small boat environment.

The link is as follows:

http://garmin.com/whatsNew/newProducts.html

[Scott D]

Thanks REW! I was led to believe that Raytheon had a monopoly on it. I guess they were just the first to utilize it. Maybe I will wait a little while longer on deciding on the best unit to get.

[SeaCat]

Standard Horizon has also released 3 WAAS units with CMap NT maps. The low end black and white unit is selling for about $400.

Ryan

[Dennis (MI)]

REW
I was wondering what you meant by "put up with a long low frequency antenna" my differential antenna is 36"

[Scott D]

SeaCat - I saw the Standard Horizon gps unit on the Boatersworld.com website for $400. Have you used it or know someone who has? It looks to be a good one for the price. I would be interested to see the features of the other two units. Do you know where I could find them?

[Captnric]

Standard Horizon has a GPS mapping unit with the WAAS antenna , that is just out