Help me understand WAAS

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poltergeisty

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It was my understanding that WAAS used a terrestrial land based signal that was received by WAAS capable receivers. But in my reading from Wikipedia and a web site it appears a WAAS station (Can't remember the correct term) uplinks corrections to the GPS satellite. Is that how WAAS is used?

Another question: I typed in the GPS frequency of 1575.42 into my scanner connected to an outside antenna, but didn't hear anything. I guess either that is due to the signal being so weak or the wrong modulation which I was using NFM. Maybe it's like DSS? So you can't hear the signal?
 

mmckenna

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The WAAS system uses a number of ground GPS receivers. These receivers pull the GPS data off the L1 GPS signal. This location is compared to the real location of the receiver. The difference between the slightly inaccurate GPS L1 derived location and the highly accurate known position of the receiver is used to compute the error of the GPS system.

The error is then used to compute a correction. That correction information is uplinked to two non-gps satellites, one over the west coast, one over the east coast. That correction information is then broadcast out from those satellites to WAAS enabled GPS receivers. The GPS receiver takes it's GPS derived location, which will normally have a bit of error in it. It uses the WAAS correction information to increase the accuracy.

DGPS is a similar sort of system. Instead of uplinking the correction data to satellites, the correction data is instead broadcast out over long wave transmitters located at different locations. Again, a DGPS enabled receiver picks up the correction data and uses it to produce a more accurate GPS signal.

Sort of strange, when you think about it. The DOD purposely degrades the consumer GPS signal so it can't be used for military use. The US Government (FAA and US Coast Guard) spend more money sending out correction signals to make it more accurate.

As for why you can't pick up the GPS signal on your scanner, it has a lot to do with your antenna. A GPS antenna is a very high gain antenna specifically designed for the 1575MHz signal. I've got GPS timing antennas at work, some are 28dB gain, others are 40dB gain. The antennas use a circular polarization to match what the GPS satellites use. Your scanner antenna is much lower gain, probably vertically polarized, and oriented to pick up signals from the horizon. To top that off, 1575MHz is going to suffer a whole lot of loss in your feed line.
 

mmckenna

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It was my understanding that WAAS used a terrestrial land based signal that was received by WAAS capable receivers.
DGPS would fit the description of "land based" signal. There are other correction systems that are run by private companies. They basically do the same thing, but charge for the service. These tend to be more local systems, sometimes on UHF. Often surveyors will uses these services since they need accuracy a lot better than what navigational users need.
 

kayn1n32008

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DGPS would fit the description of "land based" signal. There are other correction systems that are run by private companies. They basically do the same thing, but charge for the service. These tend to be more local systems, sometimes on UHF. Often surveyors will uses these services since they need accuracy a lot better than what navigational users need.

High accuracy(sub centimetre) Surveyors do not use WAAS or DGPS, we use Real Time Kinematic(RTK). This uses a known reference station, that sends out correction data over 400MHz LMR, 900 ISM or 2.4GHz ISM with either a known or autonomous reference, There are also companies that have VRS(virtual Reference System) that have a network of reference stations, that are used to send correction data over cellular, that is then post processed using the static data stored in the reference receiver, and other reference stations that constantly receive and store this static data. Both WASS and DGPS are not suitable for surveying to sub centimetre work. WAAS and DGPS will only come down to sub 0.5 metre accuracy. For construction or legal survey this is just not accurate enough.

As a side note, majority of survey gear(Trimble Topcon) now uses GPS L1, L2, and I believe L5c as well as GLONASS. A GNSS survey receiver will see between 15 and 20 sats, allowing users fast RTK fixes, and the ability to work closer into vegetation and area that GPS only receivers will not work. A GPS only receiver will only see between 5 and 10 sats.


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krokus

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The DOD purposely degrades the consumer GPS signal so it can't be used for military use. The US Government (FAA and US Coast Guard) spend more money sending out correction signals to make it more accurate.
Selective Availability (SA) has been turned off for years, but can be enabled as needed. Which means, while SA is off, the consumer gear has the same signal accuracy as the others, and errors will be due to inherent tendencies of the equipment and propagation.

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