A GPS fix is actually a very complex series of calculations. A GPS receiver on the ground picks up timing signals from several GPS satellites and calculates your position in three dimensions, using what is known as “time of flight” triangulation. Each satellite has an atomic clock, accurate to 100 billionths of a second.
Your GPS picks up that signal and computes the distance to the satellite-based on the time it left and the time it arrived. When your GPS has locked onto at least four satellites, it is able to compute your position in three dimensions (latitude, longitude, and altitude). Usually, your fix will be based on many more satellites, improving the confidence in your position.
In a perfect world, each calculation would agree, and you would be exactly where your GPS says you are. However, satellite clocks have noise, gravitational, relativistic, and drift errors, the exact position of each satellite may be up to two hours old, and solar radiation pressure can affect the satellites. The distribution of satellites in the sky matters and the signals from each satellite are delayed by ionization, humidity, and atmospheric pressure differences on each path. Signals can then reflect off buildings, bridges, and other objects, resulting in further errors.
Your GPS computes all this, 1-10 times per second, and provides an error estimate, usually Estimated Position Error (EPE) or Horizontal Accuracy (HA), or Horizontal Dilution of Precision (HDOP), depending on the manufacturer. This figure normally represents a 50% confidence in your position. The 2DRMS circle, which is used by the CG, represents a 95% confidence in your position.
In addition, the coordinate system used is another factor. The Earth is not exactly round, and there are two primary coordinate systems in use in the United States, called datums. There is the North American Datum of 1983, referred to as NAD 83, and the World Geodetic System of 1984, referred to as WGS 84. They are very similar but can vary as much as 1 meter across the United States, which is our Area of Responsibility (AOR).
Android and iOS devices use WGS 84 exclusively. If you use an external GPS and there is a setting for the datum, you must set it to WGS 84 per ALCOAST 301/20 – AUG 2020 DATUM TRANSITION FROM NAD 83 TO WGS 84 FOR POSITIONING AIDS TO NAVIGATION (ATON).
Today’s phones can also utilize signals from cell towers and the GLONASS (Russia) and Galileo (Europe) satellite constellations.
The calculations are complex and the models to correct for each of these errors are also complex. It is actually amazing that we can know our position within a few meters for as little as $100.
http://gpsinformation.net/main/errors.htm https://en.wikipedia.org/wiki/Error_analysis_for_the_Global_Positioning_System
AVA GPS
AV Assistant (AVA), by default, uses your internal GPS, if the GPS hardware is present. All phones have GPS, but some iPads and Android tablets do not (WiFi only). These will require an external GPS to provide location information.
Even if you have an internal GPS, you may choose to use an external GPS on a pole to avoid hanging over the side of the boat with your device to make contact with the aid – which is hazardous to both you and your device.
In this case, you will most likely use a Bluetooth GPS such as a Garmin GLO 2, Dual Electronics XGPS150 or XGPS160, or Bad Elf Pro (multiple models). AVA will use the GPS provided by these devices when connected.
Position Fix
Let’s look at how AVA calculates a position fix. The ideal scenario, to get the best possible position fix, is to come to a full stop and make contact with the aid for 15 seconds, allowing the GPS reading to settle. Admittedly, conditions may not permit getting this close or coming to a full stop. Historically, most Auxiliarists probably haven’t done so. Let’s look at the impact of that.
In the diagram above,
HA = Horizontal Accuracy (also known as EPE) – generally 50% confidence
2DRMS = 95% Confidence circle
CA = Closest Approach (distance from GPS to aid)
SE = Speed Error (speed in passing aid)
PF = Position Fix
AP = Assigned Position (Light List)
While your position fix may be off the bow in contact with the aid, your actual position is likely to be anywhere in the blue HA circle (due to GPS timing errors).
Your 2DRMS circle (95% confidence in position) is 2.4022 times bigger than your HA. This is the standard the Coast Guard uses – 2DRMS.
You do have a little control over your GPS error. When you are moving, your GPS error is larger. When you stop moving, your error starts dropping due to refinements in position. Come to a complete stop by the aid and hold your position for 15 seconds or up to a minute if conditions permit.
Closest Approach
With a non-zero closest approach (CA) distance between your position fix and the aid, your actual position is now likely to be anywhere within the green circle, assuming you do not know the bearing from your position fix to the aid (not usually observed).
You have little control over HA (and subsequently 2DRMS) – that is mostly a function of your GPS hardware. For phones and tablets, the best you’ll get is about 5 meters or 16 feet.
You DO have control over your closest approach. Keep it tight – contact is best. If that’s not possible, try to be within 1-5 feet, or wait for better conditions. You can use a Bluetooth GPS fastened to a boat hook to avoid holding your phone or tablet over the water (and possibly banging it out of your hand).
Impact of Speed
If you approach the aid at speed, a speed error (SE) is added since the GPS updates once per second (usual) and you will travel a certain distance between two readings near the aid. As you can see, doing a drive-by observation can significantly reduce the accuracy of your position fix, not only due to the uncertainty of your position but also because of increased GPS timing errors.
As mentioned before, you have little control over HA/2DRMS, but you DO have control over your speed. Keep it tight – come to a dead stop at the aid (which also improves your HA). If that’s not possible, go as slow as possible or wait for better conditions.
AVA Calculations
Every time you take a position fix, AVA calculates the distance between your position fix (PF) and the assigned position (AP, from the Light List) of the aid. AVA also calculates your position fix error as HA * 2.4022 + CA + SE.
This information is automatically inserted into the comments of your 7054, so that reviewers can see how tight your position fix was and what credence to give to your report. Clearly, if you drove by 30 feet away at 10 knots, an off-station report is simply not going to be credible. While that might be acceptable for reporting a missing dayboard, you’re not going to know if the aid was also off-station or not marking best water.
Light List AP: 41-01-28.560N, 072-07-59.460W
Fix is within 43.0 ft (2DRMS(38.0) + CA(1-5) + SE(0.0)), PT used is 120.0 ft (floating)
Observed distance from Position Fix to AP: 66.8 ft
Above is a sample report showing a fix within 43.0 ft and a distance of 66.8 ft from the fix to the assigned position.
Your actual position could be anywhere in that 43 ft radius circle – or in other words – you could be as close as 23.8 ft or as much as 109.8 ft away. Your goal is to make that error as small as possible.
What is the best fix you can get? With iOS, that’s an HA of about 15 feet, regardless of whether it is using a more accurate external GPS. Converted to 2DRMS, that’s 15 * 2.4022 = 36 feet, if you’ve come to a full stop and the GPS is in contact with the aid for 15-30 seconds. This will change with dual-band GPS.
Some phones today have dual-band GPS, which can receive additional timing information, enabling position fixes within 30cm or one foot. Over time, more phones and tablets will include this feature as costs come down.
AVA takes the distance, fix error, and positioning tolerance (fixed or floating, in Settings), and if your fix is far enough away, may advise that the aid is off-station. If you confirm that, then AVA adds a comment that the aid is Not on Assigned Position.
This is a complex topic, and there will be another post to further cover how AVA determines an aid may be off-station.
AVA makes short work of complex calculations and does them instantly so you know before you leave the aid that it might be off-station, in which case you can then investigate immediately whether it is marking best water, saving another trip back out. You may find to your surprise that many aids in your area are, in fact, not quite where the Light List thinks they are.
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