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Author Topic:   WAAS Changes 2011
jimh posted 09-10-2011 02:52 PM ET (US)   Profile for jimh   Send Email to jimh  
There have been a few recent changes in the FAA's satellite-based augmentation system (SBAS) known as WAAS or wide-area augmentation system. Three satellites are now available for the WAAS signals-in-space corrections. Listing the satellites from East to West, they are:

Satellite Name / MNEA Number / PRN Number / Orbital Position


ANIK F1R NMEA-51 PRN-138 107.3°W

GALAXY-15 NMEA-48 PRN-135 133°W


I have been looking for a source of information on the date from which PRN-133 was operational. It seems like the onset of service from PRN-133 was earlier this year (2011), and the signal has been available for quite a while.

I have my HDS-8 running at the moment and will check to see if it can use PRN-133. It seems to prefer PRN-138.

Screen capture from HDS-8 showing satellites being received

Above is a screen capture showing the satellite status of my HDS-8 after it had been running for about a half-hour with a clear view of the Southern sky. Note that is has acquired a WAAS enhanced precision fix. From the display of satellite sources being tracked, it appears that the HDS-8 is using PRN-138, from ANIK F1R at 107.3°W. This is not the best choice for my location. The signal from INMARSAT 4F3 at 98°W would have a higher elevation and a shorter path. This means it should be stronger. The HDS-8 appears to not be aware of this signal (as it is not shown in the display of satellites in view), and thus I would infer it is not using this signal for the augmentation of precision.

[UPDATE: the above above remarks are not necessarily correct! In correspondence with Lowrance, I learned that the HDS display never really identifies which WAAS signal is being used for fix augmentation. Lowrance says the HDS receiver just picks the strongest signal and uses it. I have since observed that a WAAS augmented fix is being obtained and none of the WAAS PRN's are shown in the status display. There is also a slight difference among the three WAAS signals. PRN 135 and PRN 138 are sending ranging signals, so a GPS receiver can use them in a position solution. This means you could see PRN 135 or PRN 138 showing in the status display as part of the position fix. In contrast, PRN 133 is not sending a ranging signal; it only sends the augmentation information. This may account for why it is so seldom seen in the status page of Lowrance HDS GPS receivers.]

It may be interesting to get reports from readers who have various GPS receivers and can observe which satellite is being used. In the Eastern USA, PRN-133 should be preferred as it has a better look angle than the further West sources.

jimh posted 09-10-2011 02:54 PM ET (US)     Profile for jimh  Send Email to jimh     
For some previous changes to WAAS satellite deployment, see this earlier discussion:

David Pendleton posted 09-10-2011 09:28 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
No doubt Lowrance will update their firmware at some point to support the new WAAS satellites, and since you can update your own firmware, you don't even have to send it in.

Unfortunately, I do if I want to utilize the new birds. Since I've never had an issue using PRN-138, I'll probably just skip that step.

David Pendleton posted 09-10-2011 09:31 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
I'm also a little surprised to see an EPE of 55' after half and hour of reception. I'm assuming your boat is in the back yard (trees, houses, etc.), what do you see on the water?
David Pendleton posted 09-10-2011 09:33 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
I suspect that once your receiver picked up PRN-19, your EPE and HDOP would have dropped considerably...
swist posted 09-11-2011 08:16 AM ET (US)     Profile for swist  Send Email to swist     
Does any of this affect those of us with older GPS units (Garmin 492)? Do we have to do something to avoid inaccuracies or other errors?
jimh posted 09-11-2011 08:57 AM ET (US)     Profile for jimh  Send Email to jimh     
Dave--The GPSr was not on the boat for the test above. It did not have a very good view of the Northern sky, which is probably why it was not locked on PRN-19. The path to that satellite would have been through the garage.

It will be interesting to see if a firmware update from Lowrance will be able to help with this situation. Right now the software in the HDS does not offer much control over the GPS receiver and WAAS. There is just an ENABLE-DISABLE option for WAAS. I have used some other GPS receivers and their software offered some options for WAAS, such as setting a preference for what PRN to prefer.

If you are using a GPS receiver in Europe, for example, you can't see these FAA SBAS signals. However, there are other SBAS signals available like the European geo-stationary navigation overlay service or EGNOS. I suspect that there must be some mechanism for a unit like an HDS to switch to using EGNOS for precision augmentation when the GPS receiver is in that region of the world. I think I mentioned in an earlier discussion that when rather far East a few summers ago I saw my GPS looking for PRN-120, an EGNOS source at 15.5°W.

swist--These changes in satellite position and PRN number affect all GPS users who are trying to use the FAA SBAS signals.

jimh posted 09-11-2011 11:18 AM ET (US)     Profile for jimh  Send Email to jimh     
According to an article on service_units/techops/navservices/gnss/waas/news/

the SBAS signal PRN-133 from INMARSAT 4F3 began transmission for testing in March 2010. I don't know the precise date when it became operational.

Here is a graphic representation of the satellite signal footprint on North America:

Graphic: Satellite signal footprint on North America service_units/techops/navservices/gnss/waas/news/view/GEO.cfm

K Albus posted 09-11-2011 12:09 PM ET (US)     Profile for K Albus  Send Email to K Albus     
Hey Jim - I'm glad you posted this. I don't know if you recall, but when we were in Tobermory I told you over the radio that my GPS unit had lost its WAAS fix. Earlier in the trip, before departing from Lion's Head, I went into the menu system of my GPS unit to erase all of my previous tracks and to reset my trip computer. My GPS isn't the most user friendly unit, and I mistakenly chose an option to reset or erase all of the settings in the unit's memory. This apparently included resetting the PRN number that the GPS unit would search for in order to find a WAAS signal. When we were in Tobermory, my GPS unit was trying to get a fix on PRN-122. My GPS unit eventually achieved a WAAS fix on its own. However, your post, and especially your link to the earlier thread, reminded me that if I go into the WAAS menu and manually set my GPS to look for PRN-138 (or now, possibly, PRN-133), the unit will acquire a WAAS fix much more quickly. Thanks again.
David Pendleton posted 09-11-2011 06:17 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
I was under the impression that the entire GPS constellation, including the WAAS satellites were dedicated.

After reading a bit more about this, I was surprised to learn that the WAAS satellites currently in use are commercial communications satellites and the FAA is only leasing "space" on them.

I also learned that if you subtract 87 from the PRN, you get the NMEA number, which is how older receivers will identify the WAAS satellites (if the receiver supports WAAS).

jimh posted 09-11-2011 08:27 PM ET (US)     Profile for jimh  Send Email to jimh     
Dave--You might find an interest in

David Pendleton posted 09-11-2011 08:30 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
And you'd be right, Jim. I hadn't seen that article before. Thanks.
jimh posted 09-11-2011 08:48 PM ET (US)     Profile for jimh  Send Email to jimh     
I agree that is does seem a bit odd that the FAA, a government agency, leases transponder space on commercial satellites. I suppose it is actually a good example of the government not building something itself at much greater cost. If the FAA had to build and launch its own geo-stationary satellites just to transmit the wide-area augmentation system signal, it probably would cost a lot more than the lease for some transponder space.

I have posted an inquiry on several popular boat electronic websites about the new WAAS signal, but so far no reports of anyone's GPS being able to detect or use it.

tmann45 posted 09-11-2011 10:13 PM ET (US)     Profile for tmann45  Send Email to tmann45     
Checked satellite reception thru out the day. My Garmin detected 46 most of the day but did not receive data from it. It did receive data from 48 and 51.

Screen captures at:

jimh posted 09-12-2011 08:23 AM ET (US)     Profile for jimh  Send Email to jimh     
Those screen captures from the GARMIN are interesting. In the upper right image we see that the new source, NMEA-46, has a stronger signal than the older source, NMEA-51, but the receiver is using the weaker WAAS signal.

However, we have our first evidence of a marine GPS appearing to be capable of receiving the new signal. I will say that GARMIN receivers seem to be quite good in this regard.

jimh posted 09-12-2011 08:46 AM ET (US)     Profile for jimh  Send Email to jimh     
Also, in the lower right pane of images, we see that sources 46 and 48 both appear. Source 46, the new satellite, is just about due South, and source 48, an older source, is in the West-Southwest. (I believe the GPSr was located in the New Orleans area when these images were taken.) The elevation to the due-South satellite should be higher than to the satellite in the West-Southwest. This normally means a stronger signal. In this case the due-South signal is not stronger.
tmann45 posted 09-12-2011 09:55 AM ET (US)     Profile for tmann45  Send Email to tmann45     
In the upper right image we see that the new source, NMEA-46, has a stronger signal than the older source, NMEA-51, but the receiver is using the weaker WAAS signal.

The bar being empty indicates there was a signal received from 46 but I assume that it is not transmitting useful data yet?

It appears that only one WAAS satellite can be used at a time, it this a correct assumption?

The elevation to the due-South satellite should be higher than to the satellite in the West-Southwest. This normally means a stronger signal.

Could be due to the view to the south was blocked by a wall but the west was clear.
jimh posted 09-12-2011 02:01 PM ET (US)     Profile for jimh  Send Email to jimh     
I suspect that a GPS receiver probably can only utilize one precision fix enhancement source at a time. This could explain why in some cases we see two WAAS sources shown, and only one marked with the filled-in green background, which appears to mean the receiver is utilizing the signal in some way.

Some GPS receivers have a control option to permit the operator to select the WAAS source that is preferred. I don't seem to have that control option on mine. Kevin mentioned his GPS receiver has it. I think Kevin has a Furuno.

David Pendleton posted 09-12-2011 03:10 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
Garmin uses the "empty" bar to indicate that a signal is being received, but the receiver does not have a "fix" on that particular satellite.

In other words, the signal is being received, but it is not being used to determine the current position.

jimh posted 09-13-2011 12:44 PM ET (US)     Profile for jimh  Send Email to jimh     
I made an inquiry with the FAA division that runs WAAS regarding the availability of PRN-133. I received an extremely thorough and quite technical reply--two replies actually--but I think the short and concise answer is this: PRN-133 is up there, available, and could be used by a GPS receiver if it wants WAAS data.

Since now there are three sources of WAAS data, exactly which one will be used by a particular receiver seems to be up to the details of the receiver.

I will give more of the technical details of the reply from FAA in a future posting.

David Pendleton posted 09-13-2011 06:23 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
I started up my Garmin GPS-V, which is easily 15 years-old, and I was able to find PRN-138 quite easily, but not the other two.
jimh posted 09-13-2011 09:50 PM ET (US)     Profile for jimh  Send Email to jimh     
Here is a bit more of the information I received from the FAA in regard to PRN-133:

--the PRN-133 signal has been available for about one year

--the PRN-133 is operational and can be used for WAAS precision fix augmentation, much the same as the other two signals, PRN-135 and PRN-138

--there is a slight difference in PRN-133: it does not appear to a WAAS receiver as another GPS navigational satellite ranging source. PRN-135 and PRN-138 do. I explain more below.

The WAAS geo-stationary satellite signals have dual and independent purposes. The first (and I infer the primary) purpose is to transmit a 250-bit correction message which WAAS-enable GPS receivers use to enhance the precision of their position fix. All three of the WAAS satellite signals in space provide this function. This correction signal helps reduce errors due to atmospheric propagation variations and other real-time errors in the sysetm.

It is also possible for the geo-stationary satellite to act as a ranging source, much like the many low-orbit GPS satellites. In this way the WAAS satellite can provide another ranging source in the solution of a position fix. Thus the WAAS signal can also enhance the accuracy of the position fix by reducing the dilution of position (DOP).

In the current configuration, PRN-133 has been set only to the first function, transmitting the error correction message, and it does not serve as a ranging source for position fix solutions.

If a marine GPS receiver is not utilizing error correction from PRN-133, that is due to some configuration or design of the particular GPS receiver. The signal is in service and available.

jimh posted 09-14-2011 12:22 AM ET (US)     Profile for jimh  Send Email to jimh     
To understand better how the use of a WAAS satellite signal as a ranging signal for position fixing can help with the dilution of precision (DOP), read this interesting article

that explains precisely what is meant by DOP error. The WAAS signal as a ranging signal may help decrease DOP by giving more favorable geometry to the position solution.

bluewaterpirate posted 09-14-2011 08:09 AM ET (US)     Profile for bluewaterpirate  Send Email to bluewaterpirate     
I was on the water yesterday and my Garmin 740 received data from 46 all day long.


jimh posted 09-14-2011 12:40 PM ET (US)     Profile for jimh  Send Email to jimh     
Tom--This is very good news and encouraging. I am glad to hear that your Garmin GPS receiver has this figured out and is using PRN-133. Since you were probably out in the Atlantic Ocean, PRN-133 is very certainly the WAAS satellite that has the highest elevation from your position, and that means the shortest path, and thus the strongest signal.

When Tom says his Garmin "received data from 46" he is referring to PRN-133 using the NMEA number. To understand what the relationship is between PRN number and NMEA number, see my article at

PRN Codes and NMEA Satellite ID

bluewaterpirate posted 09-14-2011 10:12 PM ET (US)     Profile for bluewaterpirate  Send Email to bluewaterpirate     
Just got in from a long day offshore we had to wait 35 miles SSE of Morehead City NC 3 hours to let a line of TS's dissipate.

While in the location I watched my Garmin 740's GPS display for the new WAAS birds. Interesting findings:

1. I observed PRN 130/43 (no data), PRN 133/46 (solid data), PRN 135/48 (no data), and PRN 138/51 (solid data after it was turned on).

2. It appears they're testing/calbrating the new birds selectively turning them on/off.

Here are some screen shots.

Note: PRN 130/43 was very low on WSW horizon so I wasn't able to get a screen shot very intermittent. There was 10 minute period where I had no WAAS birds displaying everyone was turned off.

PRN 133/46 Garmin%20740%20and%20SH%20GX2100%20Install/14SEP11_130_01. jpg?t=1316051024

PRN 135/48 Garmin%20740%20and%20SH%20GX2100%20Install/14SEP11_101_00. jpg?t=1316049984

PRN 138/51 coming back online Garmin%20740%20and%20SH%20GX2100%20Install/14SEP11_102_02. jpg?t=1316049994 Garmin%20740%20and%20SH%20GX2100%20Install/14SEP11_102_00. jpg?t=1316049982

PRN 138/51 fully functional Garmin%20740%20and%20SH%20GX2100%20Install/14SEP11_116_01. jpg?t=1316050013

Long day on the water ..... underway at 6AM tied up 8:50PM

My wife and I are dogged tired for sure.


jimh posted 09-15-2011 08:36 AM ET (US)     Profile for jimh  Send Email to jimh     
ASIDE: I can't find any information about PRN 130 that is current. It may have been associated with a satellite in the MSAS system. MSAS is similar to WAAS but located over Japan. The MSAS-1 satellite was proposed to use PRN-130, but it is not operational. MSAS-1R (where R may mean replacement) is on PRN-129. These satellites are very far away at 63-degrees-East longtide. The GARMIN GPS receivers may be programmed to be aware of them. In showing the satellite's position it may display it as being on the horizon when it is actually below the horizon. When on the Atlantic Ocean near the USA coast, a satellite at 63-degrees-East would be just about on the other side of the planet.
bluewaterpirate posted 09-15-2011 10:04 AM ET (US)     Profile for bluewaterpirate  Send Email to bluewaterpirate     
This was the approximate position I would see PRN 130/43 when it popped in on my display. It ould only be visible for maybe 3 - 5 seconds every 30 to 40 seconds. The anomaly lasted for about a 15 minute period. Garmin%20740%20and%20SH%20GX2100%20Install/14SEP11_114_00-1. jpg?t=1316095217


jimh posted 09-15-2011 01:08 PM ET (US)     Profile for jimh  Send Email to jimh     
Tom--Thanks for the follow-up information. Even if you could receive PRN-130, the data would not be useful.

For the error correction and enhancement data to be useful for a GPS receiver, the data must be provided for the region or area where the receiver is located. For example, you might be able to detect one of the FAA WAAS PRN sources from way out in the South Pacific Ocean. If you enable your GPS to use that source there is no basis to think you will get a more accurate position fix. The correction data is for corrections to receivers in the WAAS coverage area, which is nominally the USA and Canada. That is where the ground stations are located which provide the information to create the correction signals. There would be no basis to think that applying a correction for a receiver that was assumed to be in Ohio would make a receiver aactually in the South Pacific be able to provide a more accurate position fix.

The only useful WAAS PRN sources are the three I mentioned at the beginning of the discussion, 133, 135, and 138. Even if you receive another PRN, it won't make your position fix more accurate in North America.

jimh posted 09-15-2011 01:16 PM ET (US)     Profile for jimh  Send Email to jimh     
Here is an interesting article which talks about the FAA, WAAS, leased transponders, and coverage areas:

The article is from April 2010 and pre-dates the deployment of PRN-133 from the INMARSAT-4F3 satellite at 98°W.

There is a good graphic in the article which shows the elevation or look-angle to PRN-135 (dashed line) and PRN-138 (solid line), the two older sources.

Look Angle to PRN 135 (dashed line) and PRN 138 (solid line)

As you can see, for users in the North and East of the USA, the elevation to PRN-138, the best choice, is as low as 25-degrees. Boating in Lake Superior the look angle would be around 33-degrees. The addition of PRN-133, while not shown in the graphic, can be inferred by moving the solid lines of PRN-138 to the right.

bluewaterpirate posted 09-15-2011 01:16 PM ET (US)     Profile for bluewaterpirate  Send Email to bluewaterpirate     
Here's why we were late getting in ....... my 1st and only mate and I had some puckered (new word) moments.


jimh posted 09-15-2011 01:32 PM ET (US)     Profile for jimh  Send Email to jimh     
According to a report from the FAA, the PRN-133 source became operational on 11/11/2010. I found that information in .

tmann45 posted 09-15-2011 05:53 PM ET (US)     Profile for tmann45  Send Email to tmann45     
[Changed TOPIC. Please start a new discussion to engage this topic. Thanks.--jimh]
jimh posted 09-15-2011 10:09 PM ET (US)     Profile for jimh  Send Email to jimh     
Re Tom's noting of PRN-130: here is the definitive source for PRN code number assignments for systems that employ the GPS spectrum spreading code and can interoperate with GPS receivers on their L1 coarse acquisition carrier:

This document shows PRN-130 is aboard a satellite at 142.5-degrees-East longitude. This signal would be impossible to receive from a location in the Eastern USA coastal area.

jimh posted 09-16-2011 08:12 AM ET (US)     Profile for jimh  Send Email to jimh     
In another forum discussion where I initiated a discussion similar to this, a participant posted this screen capture from his HDS receiver: d=1316093572

This is an interesting display. The status indicates that a precision fix using WAAS has been obtained, but the azimuth-elevation (az-el) display does not show any WAAS satellite source.

One possible explanation for this contradiction may be in the nature of the az-el display. If the az-el display only shows satellites which are presenting information for a ranging solution, the display would never show PRN-133. This is because PRN-133 is sending only the correction data. The two other WAAS satellites, PRN-135 and PRN-138, send correction data, but they also send a signal that could be used in a ranging solution. PRN-133 does not (at this moment) transmit that second signal; it sends only the correction signal. If the az-el display only shows signals with a ranging solution, it would not show PRN-133.

This means my inference that the source of the WAAS correction must always appear in the az-el display may be mistaken.

jimh posted 09-16-2011 08:32 AM ET (US)     Profile for jimh  Send Email to jimh     
Here is the text from the Lowrance manual that explains the satellite status display and the meaning of the blue indicators:



Monitors the location of satellites in view and the quality of the unit’s satellite lock-on. The Satellite page has two display options.

The Satellite screen displays a circular graphic that shows where satellites are located and a bar graph that monitors the strength of satellites within range of your unit. Your unit is locked-on to satellites shown with blue bars.

To access the Satellite screen select Satellites from the System menu and press enter.

The term "WAAS" is only mentioned twice in the 133-page manual. One mention is in regard to enabling or disabling WAAS, and the second is in the listing of device specifications.

jimh posted 09-17-2011 05:51 PM ET (US)     Profile for jimh  Send Email to jimh     
Winner--Winner--Chicken Dinner!

Another HDS-8 user posted this screen capture of the satellite status display showing PRN-133 making an appearance:

Graphic: HDS GPS receiver display of satellite position and signal strength

This tends to confirm a few things:

--the GPS receiver in the HDS can receive PRN-133; and,

--the status display will show satellites that it is receiving, even if it is not using their signal in the ranging solution for position fix.

David Pendleton posted 09-17-2011 10:57 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
Sure, but did you ask him what his firmware version was?
jimh posted 09-18-2011 09:52 AM ET (US)     Profile for jimh  Send Email to jimh     
We've come to the end of a long and winding road. Yesterday I moved my HDS to several different places in the house and backyard, hoping to see the GPS receiver find PRN-133. However, I could only see PRN-138 most of the time, and once or twice found PRN-135.

The look angle to PRN-135 is quite low, about 20-degrees from my location in SE Michigan. In contrast, the look angle to PRN-133 will be 39-degrees, almost twice as high in the sky.

This morning, I powered-on the HDS at its usual spot on the work bench. About 15 minutes later I checked the satellite status page. It was a EUREKA moment. There was PRN-133 on my HDS-8 satellite display page!

Graphic display of satellite status from Lowrance HDS GPS receiver

It will be interesting to see if the GPS receiver becomes more likely to use PRN-133, now that it has finally discovered it. Since the look angle to PRN-133 is 39-degrees, from its position in the graphic display (above) it seems to imply that the inner ring of the display may be denoting an elevation angle of 45-degrees.

By the way, my HDS is running on version 4.0 of the operating system.

jimh posted 09-18-2011 12:24 PM ET (US)     Profile for jimh  Send Email to jimh     
It seems reception of PRN-133 by my HDS-8 GPS receiver is ephemeral, to use perhaps precisely the right adjective. I walked by the receiver an hour later and PRN-133 was gone, replaced by much lower PRN-135.
K Albus posted 09-18-2011 06:31 PM ET (US)     Profile for K Albus  Send Email to K Albus     
I rolled my boat out of the garage today for a little bit of cleaning. I fired up my Furuno GP-1850 and I was able to get a good strong signal from PRN 133:

On my display, the WAAS satellite appears as a "W". In the bottom left corner you will see that I have a "GPS W3D" fix. The W indicates a WAAS fix.

I also tried PRN 135 and PRN 138, but couldn't get a fix with either of them, although the Furuno was clearly "seeing" them. PRN 135:

PRN 138:

You'll notice on these last two, I was only getting a "GPS 3D" fix, with no "W". You'll also notice the alarm icon is lit, warning me that the WAAS fix has been lost.

jimh posted 09-18-2011 09:20 PM ET (US)     Profile for jimh  Send Email to jimh     
Kevin's data is most interesting because it shows the signal strength of the three WAAS sources, and clearly PRN-133 is the strongest. That is just what I suspected would be obtained. This information makes it harder for me to understand why the Lowrance HDS internal GPS receiver is so reluctant to use PRN-133 in my various tests.
jimh posted 11-05-2011 10:18 AM ET (US)     Profile for jimh  Send Email to jimh     
Regarding the WAAS correction signals and their various satellite sources, there are a couple of additional points that should be noted.

As far as I can tell, and from all that I have read, the correction signals being transmitted from the three WAAS sources in space are the same signals. It does not matter which source is used to obtain the WAAS correction signal, as they are all transmitting the same correction data. The correction data is derived from analysis of ground station refernce sites and the real-time errors observed at those locations. The correction signal is sent up to the satellites to be transmitted from space. The signal is not derived from the WAAS satellites themselves. Therefore it is reasonable that the same signal would be transmitted by all three.

The correction signal does not necessarily have to be a real time correction, that is, it is not mandatory that the correction signal be received at the same instant that the GPS signals to be corrected are received. The correction signal represents a correction for the ionospheric conditions that prevail at the moment, but those conditions do not change particularly radically from second to second. Actually, the ionospheric correction signal is only updated every two minutes. A six-minute old correction is still considered to be a valid correction signal. Some modern GPS receivers will hang on to the WAAS correction signal and continue to use it, even if they are currently not receiving that signal, as long as there is a basis to provide an enhanced precision in the position fix by using the correction. A GPS receiver might be momentarily shadowed and the signal from a WAAS source blocked. An advanced receiver will retain the correction signal and continue to use it for some time, while waiting for the WAAS source signal to reappear. After a correction signal becomes too stale, the advanced receiver will stop using it.

I don't know if any of the current marine GPS receivers are configured to retain WAAS correction signals for use after loss of signal. This feature may be limited to some advanced GPS receivers used primarily for land surveying.

jimh posted 01-12-2013 09:17 AM ET (US)     Profile for jimh  Send Email to jimh     
Since I initially wrote several of the comments in this thread, I have learned from Lowrance technical support the HDS Satellite status display does not show the PRN of the WAAS satellite that is in use. Lowrance says the GPS receiver just picks the best (and by that I think they mean strongest) signal from the WAAS signals available, and it uses that signal for the WAAS correction data. The satellite status display does not clearly indicate which PRN is in use for WAAS.

The WAAS satellites seem to occasionally show up in the HDS's overhead Az-El display. Because they are in geo-stationary orbit, they should always show up at about the same elevation and azimuth for a particular terrestrial location; they don't move through the sky like the low-orbit satellites.

It is also possible that PRN 135 and PRN 138 could be used in a position solution, and you might see them appear in the signal strength bars and be marked in blue if they are being used in the position fix.

I don't have any good explanation why PRN 133 would so seldom appear, particularly since it seems like it is the strongest and has the best path from my location.

jimh posted 01-12-2013 10:02 AM ET (US)     Profile for jimh  Send Email to jimh     
[Located missing graphic showing look angles and added it as in-line image, located much earlier in discussion. The original link had gone dead.]
jimh posted 01-12-2013 11:01 PM ET (US)     Profile for jimh  Send Email to jimh     
I have never seen a plot of the look angles to the WAAS satellite at 98W, PRN133, so I made one:

Plot of look angles from eastern North American to WAAS satellite at 98W, PRN133

jimh posted 01-12-2013 11:10 PM ET (US)     Profile for jimh  Send Email to jimh     
Comparing the look angles to PRN133 (at 98W) and PRN138 (at 107.3W), the difference is not as much as I expected. For Great Lakes boaters in Lake Superior, PRN133 gains about three or four degrees elevation compared to PRN138. (The plot for PRN138 is shown much earlier in the discussion; the plot for PRN133 is just above.) I was anticipating a better outcome.

For GPS users in the West, you can just take the plot above and flip it horizontally.

A sidebar on how I made the plot

To create the look angle plot, I first needed some data on the look angles. I used an on-line look angle calculator to do the heavy lifting. I found one that was easy to use and allowed me to enter all the data manually, without having to use a click and point map or other clumsy interface.

To make the plot I used four sets of longitudes: The satellite position is geo-stationary at 98-degrees-West. That position would be my first set of data. I wanted to find the latitudes where the look angle would be 70, 60, 50, 40, 30, and 20 degrees. The calculator solves for look angle from an argument of latitude, so I had to enter my guesses for latitude until I got the look angle I wanted. In a few minutes of entering latitudes and checking the calculated look angle, I soon had a table of data for 98-degrees-West. Next I moved East ten degrees, and recalculated for 88-degrees-W. By using the results for 98W I could rather quickly discover the latitudes for this position. And in a similar manner I found results for 83W, only five degrees away but of special interest to me because it is the location of most of my boating, and then other five degrees to 78W.

I then had a table of data of look angles for four longitudes and in ten degree increments. The next problem was how to create a plot of this data. I needed a map on which I could drop some points, then draw a line between the points, and to do this all electronically. I started using GOOGLE EARTH, but it was not working very well for me. Then I realized that plotting a series of points and connecting them with a line is the same as making a ROUTE on a chart plotter. I switched to POLAR VIEW and began making a series of routes. The route connecting the points, for example, for the positions where the look angle is 40-degrees was saved as the LookAngle40 route. PolarView is quite flexible about displaying routes, so I was able to have it show five routes at one time. I did not plot the data for LookAngle20 because it was too far North to be of interest

PolarView also has a function to export the screen, and I used that to get a PNG graphic. I opened the PNG in PhotoShop to add the legends.

jimh posted 01-13-2013 12:16 PM ET (US)     Profile for jimh  Send Email to jimh     
Here is a newer plot, made with more accuracy;

Plot of look angles to WAAS satellite PRN133 from eastern USA and Canada by Jim Hebert
Look angles to WAAS Satellite PRN133 from Eastern USA and Canada

jimh posted 01-13-2013 12:55 PM ET (US)     Profile for jimh  Send Email to jimh     
Investigating the look angle calculation for an algorithm I might use, I was reminded of the children's tale of Goldilocks and the Three Bears. One of the initial searches found this method:

Determination of Look Angles to Geostationary Communication Satellites

This method, like Papa Bear's porridge, chair, and bed, was too hot, too large, and too complicated. It includes variations in the Earth's shape into the calculation. Too much precision for me.

Next I found an on-line classroom which had some homework exercises:

Satellite Earth Station Problem

This method, like Mama Bear's porridge, chair, and bed, was too cold, too small, and contained some unexplained elements. I didn't want to follow the algorithm when I did not understand some of the terms being used.

Finally I found

Computing Azimuth and Elevation Angles with JavaScript

Like Baby Bear's porridge, chair, and bed, this method was just right. It provided a formula for calculating the look angle, and there were no unexplained elements in the formula, just geometrical relationships. As a bonus, there was a really fast calculator built with JavaScript using the formula on that same web page. That saved me from having to implement some way to calculate the look angles, other than by evaluating them by hand and entering a lot of numbers into a simple calculator. (For a moment I though I was going to get back to my other project, learning to program in Python. I started that about three months ago but I have not gotten very far into it.)

I also refined the method for creating the routes. I just took one of the existing routes I had made, exported it in GPX format, and then edited the route point positions using a text editor. This gave more precision and more speed than trying to edit the routes by moving the route points with a mouse to the specified locations. I could the save the route, and import it back to PolarView. I could also use one route as a template and create the other quite easily and quickly.

That is the story behind the latest plot. It took a hour or so of a Sunday morning and a pot of coffee. The result is a very accurate plot of the look angles for PRN133.

jimh posted 01-13-2013 04:15 PM ET (US)     Profile for jimh  Send Email to jimh     
One element of the data regarding look angles is subject to some controversy. Exactly where is the position of INMARSAT 4-F3, the satellite carrying the FAA's transponder for WAAS using PRN 133? It seems that the most general reference to its location is at 98-degrees-West longitude. However, there are also references to the position as 97.6-degrees-West and 97.65-degrees-West. I don't know which sources are the most authoritative. In my calculation I used 97.6-degrees-West as the longitude of the satellite.
jimh posted 07-28-2014 03:16 PM ET (US)     Profile for jimh  Send Email to jimh     
Regarding the three sources of augmentation signals from WAAS, the two older sources, PRN135 and PRN 138, are different from the newest source, PRN133. The two older sources send psuedo-range signals in addition to the WAAS signal. This means that these PRNs can be used in a position fix solution. They are just like the orbiting satellites; you can include them in a position fix. The newest WAAS source, PRN133, sends only the WAAS augmentation signal. It does not send a psuedo-range signal, so it cannot be part of a position fix solution. This explains why it never shows up in the listings of which satellites are "locked" or being used as part of the position solution; PRN133 will never be in that category. It does show up from time to time in the polar plot of azimuth and elevation.

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