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Author Topic:   GPS WAAS PRN Changes, July 2007
jimh posted 09-12-2008 08:03 AM ET (US)   Profile for jimh   Send Email to jimh  
(This topic has been mentioned in several other discussions, but it deserves its own discussion, as it has affected many devices.)

In July of 2007 there was a rather significant change in the Global Positioning System (GPS) and its Wide-Area Augmentation System (WAAS). To understand the change some background information is necessary. The GPS radio signals use a code division multiple access (CDMA) technique in which each satellite transmits its signal on the L1 frequency with a dither based on a predefined pseudo-random noise (PRN) pattern. Each of the high-earth orbiting GPS satellites transmits with a unique PRN. These PRN patterns or codes are identified with numbers that are usually in the range of 1-32.

The WAAS system uses leased transponders on geo-stationary satellites operating on the GPS L1 frequency to transmit its near real-time position correction data. These signals use PRN codes numbered much higher, typically above 120.

When the WAAS was first deployed, it used two geo-stationary satellites and these PRN numbers:

AOR W     122     54°W
POR        134     178°E

After July 2007 those satellites signals were removed from the WAAS and replaced by

ANIK F1R    138    107.3°W
GALAXY 15   135     133°W

To add further confusion to this discussion, there is a NMEA-based convention of giving GPS satellites an ID number. In the case of WAAS satellites their NMEA Satellite ID number is their PRN minus 87. Thus on a GPS receiver you often see the WAAS satellite signal represented as satellite ID numbers 35 (i.e., 122-87) or 47 (i.e., 134-87). The new WAAS satellite ID numbers are thus 51 and 48. You can typically see these satellite ID numbers or their PRN numbers shown on a GPS receiver's status display.

Graphic: Screen of GPS showing satellite status.
A GARMIN device displays the NMEA Satellite ID numbers of the old WAAS satellites.

Graphic: Screen of GPS showing satellite status
A LOWRANCE device shows the PRN numbers of the new WAAS satellites.

The effect of this change was significant in two realms. The footprint of the geo-stationary satellite coverage was slightly different, and the look angle to the satellites changed. In some cases this may have affected reception, typically for the better, particularly in the northeastern United States and Canada. More significantly, the stored instruction code in some GPS receivers was not prepared for processing the signals from sources with the new PRN numbers. These receivers were stuck searching for signals that were no longer available. The result was a loss of WAAS features on those receivers.

Many manufacturers were able to adjust to the change in WAAS PRN numbering by providing their customers with new firmware for their devices.

jimh posted 09-12-2008 08:35 AM ET (US)     Profile for jimh  Send Email to jimh     
In the Spring of 2006 there was also a change in the WAAS system when the operators of the AOR-W satellite moved its position further west to 142°W from 54°W. This reduced the footprint of the satellite in the northeastern United States, where the look angle to it became quite low. This may have affected users in the northeast. At very low look angles the path to the satellite increases in length and there is a corresponding loss in signal strength. Local terrain can also block the path.
jimh posted 09-14-2008 11:06 AM ET (US)     Profile for jimh  Send Email to jimh     
The WAAS system is actually intended for the aviation industry and its operation is provided by the Federal Aviation Administration (FAA). The real-time GPS correction signals are transmitted from both terrestrial stations and from satellites, that later known as signals-in-space or WAAS SIS. For maritime use it is the WAAS SIS that is being used. Its accuracy is generally excellent, with a horizontal error of about two meters. The vertical error, which is important to aviators, is not particularly significant for mariners.

There is a good article at WIKIPEDIA on WAAS:

jimh posted 09-25-2008 12:28 AM ET (US)     Profile for jimh  Send Email to jimh     
I was out with the boat last weekend and had my GPSr and chart plotter running for over an hour. I never obtained a WAAS-enhanced fix, and when investigating the GPS STATUS display, I noticed the device was looking for a satellite with a PRN of 122. That's the now discontinued PRN that was used by the AOR W satellite that is no longer in the WAAS program. I think this is prima facie evidence that the firmware in my GPS is now out of date and probably has been since July of 2007. I will have to check with the manufacturer to see if that is the case and if so, if they have a remedy.
jimh posted 09-25-2008 08:51 PM ET (US)     Profile for jimh  Send Email to jimh     
I received a very prompt reply from the manufacturer of my GPSr and chart plotter device. The GPS receiver uses a chip made by SiRF which cannot be updated and doesn't know about the new WAAS SIS satellites. So the WAAS function on my unit effectively ended in July of 2007. It is not terribly surprising, as I have had this receiver since c.2003. In modern electronics, a five-year-old device is quite old. Also, the WAAS function was quite new when I bought the device in c.2003, and having WAAS was considered leading-edge technology then. Here we are, five years later, and the WAAS system has evolved to a new deployment which my hardware cannot accommodate. So my device is back to being a non-enhanced GPS. It still gives me a position within about 10-meters, and that is close enough for the kind of boating I normally do.

I suspect that there are a lot of c.2003 devices whose WAAS function has become impaired as a result of the July 2007 change.

WAAS celebrated its fifth birthday on July 10, 2008.

gss036 posted 09-25-2008 09:56 PM ET (US)     Profile for gss036  Send Email to gss036     
Jimh, was you unit a Garmin by chance?
bluewaterpirate posted 09-25-2008 09:58 PM ET (US)     Profile for bluewaterpirate  Send Email to bluewaterpirate     
That's a Lowrance display we're looking at ......
jimh posted 09-25-2008 11:05 PM ET (US)     Profile for jimh  Send Email to jimh     
My GPSr is in a Standard-Horizon CP-150. The unit has been obsolete for several years. I think they stopped making them in c.2005.

I suspect there are quite a few more models of GPSr that have been affected by the July 2007 WAAS change.

gss036 posted 09-26-2008 02:15 PM ET (US)     Profile for gss036  Send Email to gss036     
I have a Garmin 176C, bought in 2001, and it had corrupt software, sent it in a Garmin fixed. I e-mailed them after checking for up dates and could not access thier site with my computer connection. We don't get very good WAAS signal up here in the SanJuans anyway. MAybe I will get a reply even though my unit is so old, they don't off rebuild prices any longer, but do for the B&W 176, charges $150 flat rate.
gss036 posted 09-26-2008 02:41 PM ET (US)     Profile for gss036  Send Email to gss036     
WOW!, cannot believe such quick response from Garmin. The tech support dept says I do not need upgrades to detect the new bird. (There is not a software update required on the unit to find a new WAAS satellite)
jimh posted 09-26-2008 08:37 PM ET (US)     Profile for jimh  Send Email to jimh     
I'd say that is quite amazing that a unit from 2001, which pre-dates the whole WAAS launch, is capable of WAAS enhancement and even capable of knowing about the PRN's of the current satellites. You seldom get that sort of longevity out of a modern electronic device. I wonder how Garmin knew in advance about WAAS in 2001? It wasn't deployed until 2003!
gss036 posted 09-26-2008 11:15 PM ET (US)     Profile for gss036  Send Email to gss036     
I have no idea how they knew, must have been incorporated into the software that was installed. [Changed topic to talk about digital chartcartograpy. Please use a new thread for that topic. Here we are discussing changes in the FAA's WAAS Signals In Space program and how it affects different units.--jimh]
bluewaterpirate posted 09-27-2008 10:08 AM ET (US)     Profile for bluewaterpirate  Send Email to bluewaterpirate     
Garmin has made software changes to unit over the years. There were improvements to the existing functionalities. You can see the functionalities that have been added and improved from this Garmin site. product=010-00214-00&cID=168&pID=144 4892322_2160052_48157729_WebSmall_3/ Image-4892322-48157729-2-WebSmall_0_a6c5fb88ed1e902056200aa6f63bf121_1

gss .... When you returned your unit to GArmin they made all those changes to your unit.


jimh posted 09-27-2008 11:28 PM ET (US)     Profile for jimh  Send Email to jimh     
But you guys are absolutely certain that GARMIN made a GPS receiver in 2001 that was ready to receive and use WAAS signals that weren't operational for two years later. And now seven years later they update the unit and it can use PRN codes that only became active a year ago. Is that right?

I don't care about other changes they made--I am sure they added features and other stuff. But how did they know so far in advance about WAAS and changes coming? What chip set do they use?

gss036 posted 09-27-2008 11:42 PM ET (US)     Profile for gss036  Send Email to gss036     
Well I am about as certian that as I am about sitting here looking at the receipt where I purchased from on November 8th, 2001. It is a Garmin 176 Color unit that operates hard wired or portable with 4 NMH AAA batteries.
swist posted 09-28-2008 07:38 AM ET (US)     Profile for swist  Send Email to swist     
I don't know about 2001, but I have a Garmin 492 made in (I'm guessing) the 2003-2004 timeframe and it sure doesn't seem to be confused by changed in WAAS. More accurate than ever!

Of course this unit was WAAS-enabled when new. Clearly their software was good enough to adapt to satellite configuration changes in 2007.

deepwater posted 09-28-2008 07:54 AM ET (US)     Profile for deepwater  Send Email to deepwater     
I wonder how much input Garmin had in providing hardware or software to a company that help build or provide hardware or software for the satellites (Wass program)
bluewaterpirate posted 09-28-2008 11:02 AM ET (US)     Profile for bluewaterpirate  Send Email to bluewaterpirate     
Jim if you'd read thru the softeware changes you'd see the word WAAS numbers of time. My best guess is Garmin has adult leadership ..... what a novel idea.
jimh posted 09-28-2008 02:56 PM ET (US)     Profile for jimh  Send Email to jimh     
I did read the mentions regarding WAAS and EGNOS in the GARMIN literature, but I found they do not explicitly mention anything about the expansion of the PRN numbers.

A simple confirmation from a user of one of these c.2001 GARMIN devices that they are tracking the current WAAS satellites using PRN codes 138 and 135 would be excellent proof. Reports that an unspecified device "works good" are not quite specific as to the satellite PRN being tracked, the device, or when it was made.

Given GARMIN's position in aviation electronics, it would not surprise me that they would be on top of the WAAS program, in as much as WAAS is primarily an aviation-driven development.

It is somewhat ironic that the marine oriented differential GPS system (which operates only in coastal areas using land based transmitters at low radio frequencies) is not at all affected by these changes, and old GPS receivers using differential GPS still work just fine.

gss036 posted 09-30-2008 06:49 PM ET (US)     Profile for gss036  Send Email to gss036     
For what it is worth, I was out all day today and ran my Garmin 176C unit. I was looking at a total of birds. The last time I looked I was using 03-06-07-08-13-16-20-23-25-27-48-51. So, if 48 and 51 are the new numbers being used, then I guess I am reading them on my 2001 unit. Accuracy was as low as 9 ft and as high as 19 ft. So that is close enough. I even fired up my old-old Impulse Loran unit and it looked in 4 stations.
Even managed to hook onto a nice 10-11# hook nose hatchery COHO.
jimh posted 09-30-2008 08:34 PM ET (US)     Profile for jimh  Send Email to jimh     
The NMEA satellite ID numbers of the new WAAS satellites are 48 and 51. If your GPS receiver shows that you are tracking those satellites, then you are tracking the new WAAS satellites using PRN codes 135 and 138. This is excellent confirmation that your GARMIN 176C device has been successfully updated to accommodate the July 2007 changes. Thank you for the data.
K Albus posted 10-13-2008 05:35 PM ET (US)     Profile for K Albus  Send Email to K Albus     
I have a Furuno GP 1850 WF combination Fishfinder/GPS unit, which I purchased in March 2004. After reading this thread, I checked and sure enough, my GPS unit was searching in vain to locate the PRN 122 satellite.

After a quick internet search, I found Service Advisory #10-071 from Furuno which explained the fix. Among other things, the Service Advisory provided as follows:

We are pleased to report that all Furuno WAAS Enabled GPS Products have been designed to adapt automatically to these new WAAS Satellites, which are scheduled to be in continuous service for the next decade. NOTE THAT NO FURUNO GPS PRODUCT SOFTWARE UPDATES ARE PLANNED OR REQUIRED!
. . .

RECOMMENDED SETTING CHANGE: If an East Coast Furuno GPS Customer complains that WAAS Mode is now more intermittent, we simply recommend changing the WAAS SEARCH MODE from "AUTO" to "MANUAL" and then manually setting the WAAS PRN Value to "138" in the product software. Vessels Operating on the North American West Coast will not need to make any setting changes as their systems will correctly acquire PRN 135 and continue to function normally.

Being in the Midwest, I fall into the "East Coast Furuno GPS Customer" category. I made the recommended change, which was quite easy, and my GPS unit is now receiving the PRN 138 signal strong and clear. See:

jimh posted 10-15-2008 10:28 PM ET (US)     Profile for jimh  Send Email to jimh     
I am in the process of bench testing a GPS receiver. It defaults to an automatic mode for WAAS PRN selection. The receiver begins with PRN 122 and hunts upward. This device also has an option to "hot wire" the WAAS PRN to a specific code sequence. Changing the WAAS selection mode to specific from automatic generally results in a big reduction in the time needed to acquire a WAAS fix, and particularly now that PRN 122 is no longer available.
bluewaterpirate posted 11-03-2008 10:48 AM ET (US)     Profile for bluewaterpirate  Send Email to bluewaterpirate     
Had not seen PRN 135 (48) before but got a solid lock yeaterday offshore. Both of my Garmins tracked 32/48/51. Image-4892322-51741714-2-Web_0_f287e2667b1a42e77dc9b67d8af9f01a_1

jimh posted 09-10-2011 10:03 AM ET (US)     Profile for jimh  Send Email to jimh     
[I revived this older discussion, but later I decided to begin a new thread on the more recent changes. Please see the newer thread. I have closed this thread.]

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