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Author Topic:   GPS Receiver Performance: Time to WAAS Precision Fix
jimh posted 11-21-2008 01:11 AM ET (US)   Profile for jimh   Send Email to jimh  
I am interested in collecting some data about the time interval needed for various GPS receivers to obtain a WAAS precision fix. I have been testing a few devices. Most modern GPS receivers can obtain a 3D fix in a few seconds. I am interested in how long it takes the receiver to lock onto a WAAS signal-in-space, obtain the correction data, and provide a WAAS precision fix.

The receiver I am currently testing obtains a 3D fix a few seconds after power up, but it seems to take about ten minutes to obtain a WAAS fix. I am curious if this level of performance is typical of other marine units. If you have a marine GPS receiver with WAAS precision fix capability, I would appreciate it if you could mention the time interval typically needed to obtain a WAAS precision fix, measured from a cold start of the unit.

bluewaterpirate posted 11-21-2008 08:35 AM ET (US)     Profile for bluewaterpirate  Send Email to bluewaterpirate     
From a cold start about 1 - 2 minutes.
Jefecinco posted 11-21-2008 09:17 AM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     
Six to eight minutes with a Lowrance LGC-2000 in the Mobile Bay area. The MFD is a Lowrance LCX-111CHD.

I believe the new LGC-4000 would be faster based on the specs.

I turn on the 2000 when departing the house for the ramp about a half mile away. By the time the boat is on the water the WAAS fix is established.

It took up to a half hour when the birds first changed but after a couple of outings the 2000 seemed to have self taught where to look for the new birds.


Ridge Runner posted 11-21-2008 11:25 AM ET (US)     Profile for Ridge Runner  Send Email to Ridge Runner     
Eagle FishElite 642C iGPS (12 channel WAAS Receiver) - over 8 minutes to WAAS Fix
Garmin 498C (12 channel WAAS Receiver)- Aprx 5 minutes to WAAS Fix
Standard Horizon CP300 (16 Channel WAAS Receiver) - Aprx 60 seconds to WAAS Fix

Both the Eagle and Garmin have built-in antenna, the CP300 external.

Chuck Tribolet posted 11-21-2008 01:40 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Garmin 162, inside my garage. 2D and WAAS in 40 seconds, 3D
and WAAS 15 seconds later.


Bella con23 posted 11-21-2008 04:58 PM ET (US)     Profile for Bella con23  Send Email to Bella con23     
Garmin 172C - 1 to 2 minutes to WAAS
Bella con23 posted 11-21-2008 05:01 PM ET (US)     Profile for Bella con23  Send Email to Bella con23     
Sorry, make that the Garmin 178C.
bluewaterpirate posted 11-21-2008 07:36 PM ET (US)     Profile for bluewaterpirate  Send Email to bluewaterpirate     
Here you go .... 1 minute 37 seconds Garmin 2206. Video

glen e posted 11-21-2008 09:38 PM ET (US)     Profile for glen e  Send Email to glen e     
Garmin 4210 - 90 secs
Chuck Tribolet posted 11-22-2008 09:35 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
There are a couple of factors that come in to play:

How long has it been since the GPS was last run? My Garmin
eTrex Vista and the Pharos GPS that came with Microsoft
Streets and Trips 2007 are both really slow (a half hour)
to get a fix if they haven't been run in several months.
The newer generation Pharos (SirfStar III chipset) that came
with MS Streets and Trips 2009 locked right in, first time

How far away is the GPS from where it was last run. If you've
driven from, say, Michigan to Florida, it won't be looking for
all the right birds.


jimh posted 11-22-2008 02:34 PM ET (US)     Profile for jimh  Send Email to jimh     
I am more interested in the best (shortest) time rather than the worst (longest). So far, it looks like about a minute or two needed to acquire a WAAS precision fix is the range for the better units.

Again, I am interested in the time from a cold start to acquisition of a WAAS precision fix on a repeatable basis.

It is quite common these days that a GPS receiver can acquire a 3D fix within a few seconds of power up, as long as it has some notion of where it is located and the current time. Most receivers are able to maintain some data and a clock even when completely disconnected from a power source by using energy stored on a capacitor and by only consuming microwatts or femtowatts of power to sustain that data and clock function.

Chuck Tribolet posted 11-22-2008 08:11 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Jim, I'll bet GPS units remember things in flash, not a
capacitor-powered memory.


jimh posted 11-23-2008 11:17 AM ET (US)     Profile for jimh  Send Email to jimh     
It is hard to keep the clock going in flash memory.
Chuck Tribolet posted 11-23-2008 09:12 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
The clock will be a little CMOS counter. MEMORY will be flash.


fishgutz posted 11-24-2008 10:16 PM ET (US)     Profile for fishgutz  Send Email to fishgutz     
Garmin GPSMAP76: 34 seconds.
jimh posted 12-15-2008 12:01 AM ET (US)     Profile for jimh  Send Email to jimh     
Some of the times reported here as typical of the performance of a GPS receiver in obtaining a WAAS precision fix are awfully fast. I have noticed that there are really three steps to the WAAS precision fix solution.

First, a GPS receiver has to decide which WAAS source PRN it will look for. It must then see if it can receive and lock onto the signal. Once the WAAS signal-in-space source is acquired by the GPSr, the receiver can begin to acquire correction data from the source.

Next, the GPS receiver must receive the correction data being sent from the WAAS source, and use it to enhance the precision of its position fix. The WAAS correction consists of two types of data. The fast correction data provides for correction of orbiting satellite position and clock errors. These corrections can be applied immediately, and they apply to all receivers without regard to their position. (At least all the receivers in the WAAS coverage area of North America.)

The WAAS also provides slow correction data which corrects for errors caused by ionospheric propagation variations. These correction signals are not universal corrections but are dependent on the position of the receiver being corrected. Therefore it may take a moment for the data to be sent which is applicable for a particular location. This data is updated every minute or two. Once valid data is received the GPSr must apply it to the problem of enhancing the precision of its fix. These calculations can also take a moment to occur.

My suspicion is that some reports given here are the time to step one, acquisition and tracking of a the WAAS signal-in-space satellite source, and they are not the complete times to an initial precision fix solution.

Feejer posted 12-15-2008 11:14 AM ET (US)     Profile for Feejer  Send Email to Feejer     
My Garmin 478C will lock onto WAAS within a minute of turning on and hitting the OK button. Keep in mind this is also were the unit was turned off. If I take the boat to the Outer Banks 300 miles to the south it will take 2-3 minutes the 1st time I turn it on there.

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