SBAS
(Satellite-based augmentation systems)
Contents: Satellite-based augmentation systems
What is a ‘satellite based augmentation system’?
How does a SBAS work?
How many SBASs are there?
A real world test
What is a satellite-based augmentation system?
That catchy umbrella term covers many different satellite-based systems that make compatible GNSS receivers in our phones and GPS units more accurate. Different SBAS systems broadcast error-correction signals to different regions on earth. A SBAS-capable GNSS receiver being used in those regions can use those error correction signals to improve the accuracy of it’s location fix.
An overview of the different SBAS around the world.
If you’ve ever looked through the settings on a Garmin handheld you may have noticed the option to switch on WAAS/EGNOS. These are two different SBASs. The SBAS for North America is WAAS and the European SBAS is EGNOS. Both were originally developed to make navigation more accurate for demanding applications like aviation. If your GNSS receiver will process the SBAS error-correction signals from those systems, then it will be more accurate when using it in North America and Europe. SBAS-capable GNSS receivers will offer ± 3 metres accuracy under an open sky. Amazing!
SBAS satellites are completely separate from the fast-moving satellites in the GNSS constellations. They are geo-stationary. ie They stay ‘parked’ over a particular region. Three geo-stationary satellites provide error-correction signals for North America. This SBAS is called the Wide Area Augmentation System (WAAS). Another three geo-stationary satellites provide corrections for GNSS receivers in Europe. This SBAS is called the European Geostationary Navigation Overlay System (EGNOS)
Not all GPS receivers take advantage of these corrections. Garmin handhelds and the SatMap Active 20 offer some SBAS corrections. Some phone GPS receivers do too. However, as I write this, the current crop of TwoNav handhelds don’t process SBAS signals. You need to check the specs of your device to see if…
a) It has SBAS processing.
b) Which SBAS regions are included.
SBAS corrections will remove most of the following errors in one hit. Ionospheric, tropospheric, orbital/ephemeris and clock errors.
The EU’s Space Agency answers the question ‘What is SBAS?‘.
How does a SBAS work?
The essence of SBAS is simple. Ground stations compare positions given by GNSS satellite signals with known fixed positions on the ground. Corrections for any differences are sent to the geo-stationary SBAS satellites for re-broadcast to earth.
Corrections for any errors are broadcast from geo-stationary SBAS satellites
If your GPS receiver will process SBAS signals, it can incorporate those error corrections in it’s calculations and you can enjoy that excellent ±3 metre accuracy. The SBAS transmissions use GNSS-like signals so our GNSS receiver aerials will also work well for the SBAS signals. No extra receiving equipment required.
How many SBASs are there?
WAAS has been around since 2003. EGNOS since 2005. Unless a Garmin GPS handheld is very old it will probably have both as a switchable option. The only downside to switching these on is a slight impact on battery life as more number-crunching is required.
Some phone GNSS receivers contain this tech and some don’t. However, on a phone you’re unlikely to find any user-adjustable options and it’s hard to find out if a phone’s receiver benefits from any SBAS corrections. If you have an Android phone you can install the free GPStest app to learn exactly which tech your phone’s receiver uses.
There are other SBAS systems too. More modern GNSS receivers may incorporate more than just WAAS and EGNOS.
| SBAS | Region | Augments | Active |
| WAAS | United States | GPS | 2003 |
| EGNOS | Europe | GPS & Galileo | 2009 |
| MSAS/QZSS | Japan | GPS | 2007 |
| GAGAN | India | GPS | 2015 |
| SDCM | Russia | GPS & GLONASS | Not yet |
| SNAS/BDSBAS | China | Beidou | Not yet |
| SACCSA | South America & Carribean | GPS | Not yet |
| A-SBAS | Africa & Indian Ocean | GPS | Not yet |
| KASS | Korea | GPS | Not yet |
| SouthPAN | Austraila & New Zealand | GPS | Not yet |
A real world test
The Garmin website says…
“A WAAS-capable receiver can give you a position accuracy of better than 3 m, 95 percent of the time.”
The EU’s Space Programme website says…
“An EGNOS-enabled receiver provides location accuracy to within 3 metres”.
They are impressive claims. Both my ancient Garmin 76CSx (2006) and my newer eTrex 20x (2015) can use WAAS and EGNOS. I know from seeing lots of recorded tracks that GPS is reliably good but could these older models really be that accurate? I took them out for a walk on familiar ground and took two smartphones along too.
I was walking along the tarmac path by a river in a part of town where most of the sky is visible. There are no nearby buildings. The tarmac path is only about 2 metres wide. The eTrex was attached to a shoulder strap on my rucsac and was never more than 3m out. The 76CSx was in the lid of my rucsac and did slightly better. The section pictured above was typical. The tracks I recorded confirmed the claims. You can see the lone tree upset the phones slightly.
Below is another track recorded with the eTrex with WAAS/EGNOS enabled. The red dashes are the track, shown on top of an OS 1:25,000 Explorer map in a hilly part of the Lake District. Great accuracy. It’s not unusual to be able to see which side of a dry stone wall I was walking along.
A GNSS receiver that can process SBAS signals for your adventure playground will be more accurate than one that can’t.