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Old 29 August 2013, 04:51   #1
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ICOM M-91D GPS accuracy

I have an Icom M-91D handheld VHF radio (v1.01sw?) with built-in DSC and GPS. I have been trying to use it to measure speed (SOG) for keeping within the 6.5kt speed limit.

Unfortunately, I have found that its SOG was rather inconsistent so fired off a question to Icom tech support to see if my expectations were too high on its accuracy. Having waited quite a few weeks for a reply and hearing nothing, I post the question to a wider audience.

Would one expect a GPS device to have a variance of over 2kts at around 6kts? ie give readings varying from 4.5 to 7 knots when maintaining as steady a speed and course as is possible?

I know that accuracy of GPS is not wholly accurate for individual readings, but different techniques such as differential GPS and Kalman filtering can be used to come to a pretty accurate stream of continuous steady kts.

I work with GPS speed monitoring of individual items at various sports events and once movement is under way, readings are pretty constant, although I must admit that the items that we are monitoring are typically faster than 6.5kts. However, even though some of these items use differential GPS, others do not and even programatically taking a raw feed from an iOS device, speeds soon settle down to pretty accurate measurement.

How accurate/consistent are other peoples GPS devices at the sorts of speed limits within harbours etc? Can you accurately measure your speed or is one moment, you going 5kts and next moment you going 6.8kts?

Im not convinced that Icom is doing enough filtering of the GPS signal to provide an accurate speed reading at what would be typical boating speeds.

I have also undertaken walk tests and found speed variance of well over 2kts although I am happy that the variance of up to 1.5kts when stationary is simply limits of GPS technology as we assume that any speed under 1m/s (<2kts) is likely to be stationary even if using differential GPS. However, once underway, we take readings accurate to 2dp although we filter out deltas of <0.15mph.
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Old 29 August 2013, 06:16   #2
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taking out the fluctuations of GPS remeber SOG is affected by tide/wind too so moving in and out of a channel will change the SOG, as will moving in and out of wind shelter.
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Old 29 August 2013, 06:42   #3
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These were constant conditions on a tidal river travelling both with and against the flow and some tests were conducted during slack water. I admit that there may be slight discrepancy, but fluctuations in walk tests were consistent with readings taken on water.

Im talking about one second a reading of 4.0kts, next second 6.0kts then back to 5.0kts etc next second. I pretty sure that I would feel that soft of buffeting let alone noticing that one second I was walking at 1.0kts and next second I was walking at 2.5kts.
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Old 29 August 2013, 09:01   #4
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I do not have any knowledge of your particular equipment, but it does sound like you need the average speed "dampening" for want of a better description.

On some units for commercial use you can set the time period for averaging speed, such as over 10 seconds, 30 seconds 1 minute etc. The longer the period the more stable the speed, but if you change speed quickly, it will take a few seconds to display the correct speeds.

On our units on my vessel the speeds are very accurate down to 0.1 or 0.2 knot, and that is in areas where we do not have differential GPS coverage.

Try looking in the manual to see if you can set this averaging, or dampening period, and try increasing the period slightly to iron out the highs and lows. If you dont have that facility, sorry I cannot help!
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Old 29 August 2013, 11:57   #5
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Other thing to remember is that the GPS on that RADIO is there for when you need to hit the "oh poo" button.

A couple of metres of accuracy when the SAR 'copter is looking for you aren't going to matter that much.


A bit like I'm not bothered that my Garmin 12 has no WAAS / EGNOS - if I need that level of accuracy I'll have my crew looking over the bow prodding the bottom with an oar!
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Old 29 August 2013, 14:17   #6
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Quote:
Originally Posted by tonto View Post
I do not have any knowledge of your particular equipment, but it does sound like you need the average speed "dampening" for want of a better description.

On some units for commercial use you can set the time period for averaging speed, such as over 10 seconds, 30 seconds 1 minute etc. The longer the period the more stable the speed, but if you change speed quickly, it will take a few seconds to display the correct speeds.

On our units on my vessel the speeds are very accurate down to 0.1 or 0.2 knot, and that is in areas where we do not have differential GPS coverage.
Unfortunately there appears to be no similar setting. I suspect that this determines the parameters of a filter such as the Kalman filter and either Icom have the parameters not at an appropriate value, of no filtering is being done.

Quote:
Originally Posted by 9D280 View Post
Other thing to remember is that the GPS on that RADIO is there for when you need to hit the "oh poo" button.

A couple of metres of accuracy when the SAR 'copter is looking for you aren't going to matter that much.
I see where you are coming from, but having dedicated screens with the SOG in big numbers is a bit pointless if its not accurate better than 2kts. There are plenty of other GPS devices, of similar size and price point which achieve better consistency.

I agree that a couple of meters out in rescue is hopefully not going to make too much difference, but when its the difference of 5kts and 7.5kts and you are trying to maintain about 6.5kts and the harbour speed gun is keeping a watch, you are either having to be unnecessarily cautious (not good when the flow of the river can be 5kts in its own right) or face a talking to.
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Old 30 August 2013, 06:00   #7
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AH, OK, fair call. I didn't realise it was that big a feature on the screen!
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