GPS Elephant Tracking: Utilizing A-GPS, WAAS, DGPS and Understanding HDOP, NSAT, and ZDA

Question

*I'm using a GPS data logger (qstarz bt-q1000xt) to track the movement of zoo elephants, and I have a variety of technical questions. I've had limited success receiving answers from the vendor. You can see the GPS anklet on an elephant (links to flickr) here.

A-GPS:
In mobile phones, A-GPS refers to the ability to use cell towers to triangulate position when SV's are not visible. In GPS units, A-GPS means you can download almanac data via the internet for faster TTFF, rather than downloading the almanac from a satellite. Is this a correct description? If so, does this mean that A-GPS provides only improved TIFF, and has no effect on coordinate accuracy? The reason I ask is that I want to send these units out to other zoos to collect data, but don't want them to have to screw around with the software.

WAAS/DGPS:
There seems to be varying opinions about whether or not WAAS is a type of DGPS. For example, my data loggers state they have "DGPS(WAAS+EGNOS+MSAS) support." Yet my understanding is that WAAS is an SBAS that my device can access and utilize in real time, and DGPS is an GBAS which is strictly a post-processing analysis done with software. But perhaps this is wrong, and you can purchase fancy receivers that allow your unit to execute differential correction in real time? Even if this is true, for budgetary reasons I am more interested in post-processing with DGPS. What level of expertise is required to conduct DGPS analysis? Where would be a good place to start looking for more information on this? The reason I ask is that I hope to evaluate elephant herd dynamics, for which I need meter or sub-meter accuracy, which my data logger alone cannot seem to provide.

HDOP/NSAT:
Zoo researchers using GPS regularly remove fixes with an HDOP less than 2, and will then also remove fixes with an NSAT less than 6, "just to be safe." However, 4 satellites with perfect geometry should be able to make a highly accurate fix. Is there any reason to remove fixes with NSATS of 4 or 5? I also understand that an HDOP value of 1 is "perfect;" meaning the satellite geometry is multiplying the effect of other errors (atmospheric effects, satellite clock error, etc) by a factor of 1 (i.e., not at all). But my data loggers occasionally give an HDOP value of less than one. How can this value be interpreted?

ZDA:
Are the times provided by this NMEA sentence more accurate somehow than the times given by RMC? Why would you want to record ZDA?

An associate told me that the irony of GPS is that everyone knows enough to be dangerous, but nobody REALLY knows how it works. Prove him wrong?

Thanks!
Matty

Answer 1

Very interesting problem. There are many questions here so I'll give my shot at one. There is such a thing as real-time differential GPS. When we did NASA flight tests, we sent an engineer up to our landing site to setup up the GPS station ahead of the airplane. The station was placed at a known accurate location and broadcast corrections to a GPS receiver on the airplane correcting our position data during the flight. As the airplane approached the airfield (where the station was located), we could begin obtaining DGPS on the first approach. So, we could get good enough accuracy to auto-land an airplane.

If you really need meter/sub-meter accuracy, then DGPS is going to be (at least part) of your solution.

Answer 2

In my experience you need at least 5 of them to have a somewhat accurate sample. I see many with 4 who deviate visibly from reality when projected. The value of NSATS is also not enough to determine accuracy, the quality (gain) of the gps signal is as important. (I see many with NSAT=7 but with less than 30dBm signal gain, 1 decibel referenced to 1 milliwatt here.)

Also in my experience using WAAS/EGNOS SBAS I hardly see any improved accuracy at all on a unit with a GPS antenna which is positioned well and has a decent POV coverage.

I actually do know of a project in South Africa where they do this with rhino's. I think it is this organisation that initiated it, but for obvious reasons, a public realtime tracking page will never exist.

Hope this helps to shape your idea's.

Answer 3

I agree with @TreyA that you're going to need DGPS to get to sub-meter accuracy. Post-processing sounds like it would work fine for your application, so there is no reason to buy expensive hardware that can handle real-time DGPS.

Unfortunately, you will need a GPS logger that records the relevant information (range and satellite IDs). Some info is here: http://www.gpsinformation.net/main/poordgps.htm

Also, depending on where you are located, a public reference station may not be easy to come by even if it exists on paper. I would have liked to have DGPS data for post-processing in West Africa, but the station which nominally existed had been broken for several months.