- EUTELSAT 117 WEST B, 117° West, PRN 131, NMEA 44
GALAXY 30, 125° West, PRN 135, NMEA 48
SES-15, 129° West, PRN 133, NMEA 46
A "look angle" is the apparent elevation in the sky of a WAAS satellite from your particular location (and also the azimuth angle or heading to the satellite, but in this discussion, the azimuth won't be considered because the antenna used in marine GNSS receivers are omni-directional, that is, they do not need to be pointed at a particular heading in order to receive a satellite signal, as distinct from satellite high-gain dish antennas). All geostationary-orbit satellites are located on the equator. If the satellite longitude and your longitude are the same, the look angle is simply calculated by subtracting your latitude from 90-degrees. As can easily been inferred, the farther your location is north (or south) of the equator, the lower the look angle becomes. For example, if your location were at latitude 50-North, then the highest possible look angle would be 90 - 50 = 40-degrees for a satellite located on your longitude. If your location is not on the same longitude as the satellite, the look angle will be even lower. Calculating look angles involves some spherical trigonometry, but there are several on-line calculators that can solve for the look angle, given a particular ground location and a particular satellite.
Using a typical ground position for my boating, near 45-degree-North, 85-degrees-West, and using the easternmost WAAS satellite (EUTELSAT 117 WEST B at 117-degrees W), the look angle is rather low, 29.3-degrees. If you were a boater in Maine (around 44.8-North, 68.7-West, the look angle will be even lower: 20-degrees.
The lower the look angle, the longer the signal path. Also the signal path through the atmosphere will be corresponding longer. The longer the path length for the signal, the more attenuation of the signal, resulting in a lower signal level at the receiver. For this reason, boaters in the eastern USA should prefer to use PRN code 131, as it is being transmitted from the easternmost satellite and should be the strongest signal. Most GNSS receiver today can automatically choose the WAAS PRN source, but older receivers may need to be manually set to a specific satellite.
BIAS TO THE WEST
Earlier configurations of the WAAS satellites position tended to have more favorable locations for the eastern USA. For example, from November 2010 to November 2017, the INMARSAT4 F3 satellite at 97.6-degree-West longitude was part of WAAS, transmitting as PRN 133. This was the most favorable location as it was the most eastern satellite in the constellation. With this satellite location, the look angle from my boating area increased to 36.7-degrees.
Because the FAA system was designed for enhancement of position information from GPS for aircraft, these relatively low look angles in the eastern USA are more understandable: an aircraft in flight at 30,000-feet altitude does not really have to worry about any ground obstructions blocking a signal.
Because geo-stationary satellites over the eastern USA may have more customers for leasing a transponder, the cost of a leased transponder may be higher in those locations than for geostationary satellites in farther west longitudes. I don't have any data on this, and it is just my speculation.
PRN CODE and NMEA NUMBER
To understand why there is a "NMEA" number for these SBAS satellite PRN codes, I include an earlier article I wrote on this subject 15-years ago, in 2008:
GPS: PRN Codes and NMEA Satellite ID
The GPS system uses a form of spread-spectrum or code-division multiple access (CDMA) communications in which each signal source is modulated by a PRN code. PRN means "pseudo-random noise." (It does not mean "pseudo-random number" as is sometimes mistakenly reported.) The PRN code (along with other encoded sub codes) dithers the L1 carrier (or the coarse-acquisition carrier) of the signal. All receivers listen on the same L1 frequency, but they sort out the satellite signals by demodulating them according to their PRN encoding.
There is an allocation plan for assignment of PRN codes. In as much as the GPS is controlled by the military, not surprisingly the Air Force (now Space Force) is the source of authority. The allocation of PRN codes is:
- 1 to 63: Reserved for GPS satellites;
64 to 119: Reserved for ground-based augmentation systems (GBAS) and other sources;
120 to 158: Reserved for satellite-based augmentation systems (SBAS) such as the FAA's WAAS;
159 to 210: Reserved for future use
Source: http://www.losangeles.af.mil/shared/med ... 30-036.pdf
Also see: http://www.losangeles.af.mil/library/fa ... sp?id=8618
In the current GPS implementation, all the satellites are using a PRN code in the range 1 to 31. For the WAAS system all PRNs will be in the 120 to 158 range. [When this article was originally published there were ]only two active satellites, and they use PRN 135 and 138. Previously PRN 122 and 134 were active.
In the NMEA 0183 specification, which is proprietary and not available except by purchase, there is a field designator SATELLITE ID. As far as I can tell, again this is based without access to the actual specifications, the SATELLITE ID and the PRN are the same for values of 1 to 32.
Identification of WAAS SIS ("signals in space") sources in NMEA apparently uses a different technique. The SATELLITE ID is set to the PRN minus 87. We can build a table thus:
- PRN -- NMEA ID
120 = 33
121 = 34
122 = 35
123 = 36
and so on to
135 = 48
136 = 49
137 = 50
138 = 51
157 = 70
158 = 71
In trying to understand the reasoning behind this, and again without the benefit of being able to actually see the NMEA-0183 specification, it appears as though the PRN numbering was transformed in order to cause it to become less than 64 for most possible PRN numbers. This may have been necessary to accommodate a limit in the NMEA-0183 specification with regard to the size of binary numbers it could send. This explanation seems plausible because the initial GPS configuration only suggested there would be PRN codes of 63 or less. When WAAS was introduced, there may not have been a specification in NMEA-0183 which provided for identification of signals with PRN numbers higher than 63.
I also speculate that in the NMEA-2000 specification there may be a new specification that provides for identification of the signal source by PRN, using a field size that can directly accommodate larger numbers. Or, perhaps there is an automatic re-translation of the numbers in the range 32 to 71 back to their original PRN of 120 to 158.
When initially looking at the relationship between PRN and NMEA SATELLITE ID, the offset of 87 did not make any sense. The value 87 does not seem intuitive as it is not a power of 2 (binary) or otherwise a natural number. However, once I discovered the notation of SBAS PRN beginning at 120, the translation by subtraction of 87 makes perfect since. It moves most all of the PRN codes into the range 32 to 63, a natural container based on binary math with a limited number of fields available.
If anyone with access to the NMEA-0183 or NMEA-2000 specification would care to comment on this, it would be appreciated.
CALCULATING LOOK ANGLES
The equations to calculate the look angles (azimuth and elevation) to a satellite from a ground position is demonstrated in a nice tutorial at