GPS for interval training

So I bought a Garmin Forerunner 220 off ebay and used it for the first time this morning. The watch works great except for one thing......intervals. The speed meter on it is super slow catching up to bursts in speed. By slow I mean close to 30 seconds slow.

Everything else works great but this really bugs me because I like doing sprints throughout the week. Mt old 310xt (which is still lost in my house or garage somewhere) updated much more quickly. Problem was, as a watch to wear it sucked. Bulky, lopsided and not comfortable. Plus the battery life wasn't good.

Any suggestions on a better option that won't break the bank? Not looks to spend hundreds and don't mind getting it second hand.

BTW, if anyone wants a 220 for a good price and only wants it for overall paddle stats, I've got one. Love the watch but really would like this feature.

I think you will find that the Garmin Fenix 5x is a bit better than your 310xt. I use both on my workouts, the 310xt is mounted on my foot strap so that I can see it easily and I wear the Fenix on my wrist. The Fenix is pretty expensive, but the added GLONASS capability makes it a bit more accurate than the 310xt, especially when there are a lot of trees around.

A bit of a warning here. GPS is a subject where I have significant technical knowledge as a result of work. IMHO, the number one error of consumer level GPS users is to over estimate the speed accuracy of GPS. Caution: Geeky Math Follows! GPS calculates speed by determining the distance travelled between fixes and dividing by time. At surski speeds (~7 mph), you cover approximately 10 feet per second, so to get a good 2 sec speed reading, the unit has to be able to measure 20' distances accurately. The problem is, GPS is not nearly that accurate. According to the owner of the GPS system (US Government), consumer grade non-augmented GPS receivers have an accuracy of about +/- 16' (the spec accuracy is generally 30'). This means that when a GPS tries to give you a two second speed reading, it will measure the distance as 20' +/- 16', which results in a speed reading of 7 +/- 5.5 mph. GPS manufacturers recognize this limitation so almost all add speed smoothing algorithms to their processors so that you as a user do not see that mush error. This is also why GPS units significantly lag any changes like when you speed/slow while doing intervals. From a technical standpoint, you need to be at a steady speed for about 30 seconds before the speed displayed on your GPS can be considered statically reliable (<5% error).

Sorry, but I am new here and have not figured out how to post pictures. Here is a link to an interval workout recorded on my Fenix.

Fenix 5x Interval Workout

I've been using a Speedcoach SUP 2 GPS on my surfski and it's very good for intervals and stuff. Pretty easy to program complex intervals workouts into. Attaches with a gopro mount. Responds (almost) instantly to speed changes; quick enough to see each individual "run" that you catch when downwinding, for example, and quick enough to give a pretty accurate distance on a 30 second interval. Talks to your computer or phone with bluetooth so you can upload workout data to a fitness tracker pretty easily. Has a large display that can show speed, average speed, heart rate (if you're wearing a monitor), time, distance. Also has an accelerometer and can display a bunch of stroke rate / distance per stroke metrics, but the stroke rate based metrics aren't very reliable when using it with a surfski, because the acceleration / deceleration with each stroke is less pronounced than on a SUP board.

A friend of mine used to use a speedcoach on his sup, then lost it when surfing, has since been using a garmin watch with his Nelo 550, but is now about to get another speedcoach for his Nelo because he thinks it's worth it.

GPS calculates speed by determining the distance travelled between fixes and dividing by time.

I beg to differ. GPS devices can measure speed by using the doppler shift of the satellite signals, that means the speed measurement is independent of the position measurement. The device I'm playing with definitely does that. The error can still be quite high if there are few satellites in view, but it's not a position error. It's certainly possible that the garmin watches don't support this type of measurement.

I beg to differ. GPS devices can measure speed by using the doppler shift of the satellite signals, that means the speed measurement is independent of the position measurement.

I believe the more precise term is "differential GPS". When GPS is off, it is consistently shifted in the same direction for the same amount over the entire area. If you know your exact location, you can apply a correction factor, and all your reading will be almost dead on.

For speed, you only care about how far it is from one reading to another reading, and not the absolute location. For that purpose, GPS can be made to be very accurate. Wikipedia said down to 10cm in ideal condition.

A GPS device using the doppler shift to measure speed is something it can do on its own. It's a different processing of the signals coming in from satellites than the one to determine location. Differential GPS is something completely different, where GPS signals are compared to a known very precise ground reference, an error is calculated and then transmitted to GPS receivers in the area. What I'm talking about is speed measurement using doppler shift. See, for example, www.vboxautomotive.co.uk/index.php/pt/co...ucts/75-gps-accuracy or www.quora.com/How-does-GPS-calculate-our...t-speed-while-moving

Yes, TVE is correct that some GPS receivers use doppler shift to augment the speed calculation. Doppler works well with higher speeds but not at the low speeds we are talking about. At slow speeds doppler shift is very small and difficult to measure with consumer grade electronics. Doppler is also strongly affected by satellite geometry. For it to work, you need to be moving towards or away from a satellite, making satellites located near the horizon on the bow or stern the best. The further above the horizon or off to the side that a satellite is, the less doppler that there is to measure. I know that about 15 years ago I had a larger Garmin unit that used doppler. I doubt that the small wearable units are complex enough to use it, and it is unnecessary for the new highly accurate augmented units.

TVE is also correct that Differential GPS is something entirely different. It operates as he describes.

The 16' GPS accuracy that I quoted earlier comes straight from the US Government's GPS web page. It is true that there are units accurate down to the cm, but unless you spent extra money to buy that capability, then you do not have it. GPS.gov says "High-end users boost GPS accuracy with dual-frequency receivers and/or augmentation systems. These can enable real-time positioning within a few centimeters, and long-term measurements at the millimeter level."

Here in North America augmented (WAAS) GPS receivers are readily available in larger hand held units and not too awful expensive. I have one that I use for sailing and its speed readings seem to be better than the smaller non-augmented units. To the best of my knowledge, WAAS is not yet available in any of the small wearable units.

The Speedcoach unit that d0uglass mentions looks very promising because it has a 5 hz receiver. Most consumer level GPS units operate at 1 hz. This means that they calculate a new position once per second. At 1 hz a standard GPS uses 2 position fixes to calculate a 2 second average speed. At 5 hz, Speedcoach uses 10 fixes for the same 2 second average speed. This should increase both accuracy and responsiveness of the speed measurement. For comparison, higher quality marine units operate at 5 hz and aviation units operate at 10+ hz.

Thanks mcnye1for sharing your technical GPS knowledge.

David

Hello

This is my first post on this forum (or any other forum for that matter), but I have been reading it for a while, more or less attentively. I'm not a total newbie paddler – I have been paddling in the Med for 2 years now, most of them in a Swordfish (the original one, not the S with the huge bucket). Before that there were 2 years more in a sea kayak (Nigel Dennis Cadence). Due to geographical challenges (I live about 2 hours away from the sea) I can only paddle once a week, rarely twice.. With such schedule progress is slow, even though my average paddling session is about 3 hours long (trying to make all that fuel not be burned in vain) and I try to pick out the 15-25 knot days. My pb at linking runs is 5-6 runs for now.

Having gotten the intro out of the way here is my question:
If the GPS quality as a speedometer is contingent on the frequency of its receiver, why not use a Garmin Virb Ultra 30 camera which, according to Garmin website, has a 10hz "high sensitivity" GPS. It also has some thing called "accelerometer" which, if true to its name, should help with immediate speed calculations?

If all these statements are true, does anybody have any experience with using the Virb as a speedometer? Is its screen big enough to see if attached near the footstrap? Can it be attached to the footstrap?

If not, can a Garmin watch with a big enough screen (which one?) be made to show the speed based on the readings of a camera rather than its own (in which case one can use this set-up simultaneously as a camera and a visible immediate speedometer)?

The reason for all this line of interrogation is that the Virb costs exactly the same as a Speedcoach, and, at least on paper, has a better GPS, and, naturally, it is also a camera…

Thanks in advance

does anybody have any experience with using the Virb as a speedometer?

Interesting question.

I use a Forerunner 910xt and do find it quite frustrating in that the speed (presumably because of the low frequency calculations) jumps by between .5 and 1kph to the point where I don't watch it closely any more as I paddle i.e. I'll glance at it on a wave, but I don't try to work with the speed when training on the flat. Heart Rate yes, speed no.

In contrast I was fascinated to watch the speed on my videos shot with the VIRB. One takeaway was just how radically the boat speed goes down if you wallow off the back of a wave... Do what you have to - but keep the boat speed up between waves!

But to your question - in the manual I don't see any way either of linking the VIRB speed to the Forerunner screen OR displaying speed on the VIRB display.

The question for me though is: how important is it to have very accurate speed anyway while training? Isn't it more important to have HR, cadence and perhaps stroke distance? Not challenging, just asking.

> how important is it to have very accurate speed anyway while training?

I think you answered that yourself:

> One takeaway was just how radically the boat speed goes down if you wallow off the back of a wave...



On my device I'm working to display min & max speed in the past few seconds so I can see how well/poor I did. I don't know how useful it is in the long run (other than bragging about top speed, haha). I assume that after a few outings you get to associate the feeling with the accelerations and wallowing. But that's more or less true with HR as well...

I think you answered that yourself:

> One takeaway was just how radically the boat speed goes down if you wallow off the back of a wave...

Right! But I wouldn't have noticed that necessarily on the water anyway - it was only when I was reviewing the footage on video afterwards!

If I were doing downwinds, then speed wouldn't really be an issue but I'm a 99.9% flat water paddler (small lakes and rivers). Touring around can be fun and good exercise but I didn't buy a V14 for it's lounging comfort

I judge my improvements by my ability to obtain a certain speed over a certain distance. For the most part, My Garmin 220 will work just fine. Over 6-10 miles, it will give me an accurate rate.

However I do like to sprint sometimes and would love to see what kind of speed I can achieve. I have a dream of being able to touch 10mph on the flat, even if for a second. On my Nelo 550, I have gotten close to 9mph but that was using the 310xt I lost.

Ouch! For some reason I kind of assumed that at the very least the Virb would show its own speed on its own screen in the moment and not just on youtube, weeks later, once you've forgotten entirely what was it that you did wrong on that particular wave (because why wouldn't it? The data is there, just not the output). Does it synchronize with a smartphone in such a way as to see the speed, even if it's in a corner of a video feed, or is the speedometer entirely a post-production affair?

And yes, I've gotten the idea from watching the clips by the notorious Oscar and Robin (I actually thought that yours were shot on a GoPro, was wondering how did you get them to look so good, now I know), with the amazingly responsive speedometer in the corner, hoping it didn't come from some image analyzing algorithm but from the hardware itself.

As for your question: my reasoning is that since, ultimately, speed is what we're there for, every other measurement than speed itself is indirect and therefore potentially misleading (maybe except the stroke distance, but even then you have to multiply it by cadence and divide it by time which isn't feasible in real time). For instance, if my HR is the same as yours (because our fitness levels are the same) and so is my cadence you might still move faster than me because of superior stroke technique, or different hull construction, or paddle shape. And that's before we consider the water condition..

Besides, even if I don't compare myself to you, my own HR will fluctuate according to how much sugar and bacon I'm eating. And that's without counting coffee and alcohol (I'm a fan of both). Therefore, in order to eliminate as many variables as possible I should be able to measure my speed after each stroke and that's the only way for me to know whether for my given fitness, boat, and weather conditions that particular stroke was efficient or not (setting cadence aside, for the sake of the general argument).

That's more or less what I had in mind when fantasizing about a responsive speedometer after watching yours and Oscar's Miller Runs…