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ContinuousWave: Small Boat Electrical
GPS Receiver Performance: Time to WAAS Precision Fix
|Author||Topic: GPS Receiver Performance: Time to WAAS Precision Fix|
posted 11-21-2008 01:11 AM ET (US)
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.
posted 11-21-2008 08:35 AM ET (US)
From a cold start about 1 - 2 minutes.
posted 11-21-2008 09:17 AM ET (US)
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.
posted 11-21-2008 11:25 AM ET (US)
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.
posted 11-21-2008 01:40 PM ET (US)
Garmin 162, inside my garage. 2D and WAAS in 40 seconds, 3D
and WAAS 15 seconds later.
posted 11-21-2008 04:58 PM ET (US)
Garmin 172C - 1 to 2 minutes to WAAS
posted 11-21-2008 05:01 PM ET (US)
Sorry, make that the Garmin 178C.
posted 11-21-2008 07:36 PM ET (US)
Here you go .... 1 minute 37 seconds Garmin 2206. Video
posted 11-21-2008 09:38 PM ET (US)
Garmin 4210 - 90 secs
posted 11-22-2008 09:35 AM ET (US)
There are a couple of factors that come in to play:
How long has it been since the GPS was last run? My Garmin
How far away is the GPS from where it was last run. If you've
posted 11-22-2008 02:34 PM ET (US)
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.
posted 11-22-2008 08:11 PM ET (US)
Jim, I'll bet GPS units remember things in flash, not a
posted 11-23-2008 11:17 AM ET (US)
It is hard to keep the clock going in flash memory.
posted 11-23-2008 09:12 PM ET (US)
The clock will be a little CMOS counter. MEMORY will be flash.
posted 11-24-2008 10:16 PM ET (US)
Garmin GPSMAP76: 34 seconds.
posted 12-15-2008 12:01 AM ET (US)
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.
posted 12-15-2008 11:14 AM ET (US)
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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