Tag Archives: racechrono garmin glo qstarz 818 818xt

Garmin GLO vs QStarz 818XT Part 2

RC-GLOIn my previous post on the Garmin GLO vs QStarz 818XT I looked at the two units from a physical perspective without having done a real world test. In this article I’m going to look at and analyse the data produced by RaceChrono using both devices riding on Silverstone’s GP circuit.

HeadstockProtectorFor the test, I installed both GPS receivers on the bike. The Garmin Glo was strapped to the rear of the bike on the tail piece while the QStarz 818XT was taped between the top yoke and the instrument console on the head stop lock bracket (circled in yellow).

The test was conducted for one session with a detailed analysis of the fastest lap from that session. These are the raw numbers produced by RaceChrono for lap 4 of the test session:

  QStarz 818XT Garmin Glo
Lap Time 3:02.12 3:02.02
Distance (m) 5846.835 5854.346
Data Points 911 1346
Sample Rate 5Hz 7.5Hz
Trace Colour Red Blue

Taking the raw numbers from RaceChrono and plotting them against a schematic for the track is quite illuminating. The diagram below shows the two traces obtained from the GPS devices. The red trace is the QStarz 818XT while the blue trace is from the Garmin Glo.

Vale-Club

The first thing to notice is how both traces cut the right handed corner between Vale and Club. Neither trace is correct as I’m pretty sure that I was close to the right hand side kerb but definitely not over it. This isn’t totally unexpected because the reported accuracy of these devices is down to around 0.5 – 2m depending on the quality of the satellite signal and fix.

The second thing to notice is that the Garmin Glo (blue plot) records my line through those two corners more accurately than the QStarz 818XT. If you look carefully, you’ll also notice that the red trace (from the QStarz 818XT) looks smoother than that from the Garmon Glo. You can see this better in the detailed view below.

ValeTrace

The QStarz 818XT delivers a data point to RaceChrono every 1/5th of a second (hence 5Hz) without fail. The Garmin Glo is supposed to deliver a data point every 1/10th of a second (ie 10Hz) but fails to do so, often skipping one or more data points within each second – look for the gaps in the purple dots on the blue plot. Over the course of the 3 minute lap, the Glo delivered a data point every 0.133 of a second resulting in an average frequence of 7.5Hz – about 25% below the quoted 10Hz. As noted in my previous article, this could be a flaw in the Garmin or more likely the Bluetooth stack in the Android operating system used to run RaceChrono.

Comparing the two traces, it looks as though the QStarz 818XT (red) has come kind of filter or smoothing algorithm because of the smooth nature of its trace. The Garmin Glo’s trace (blue) shows some “jumping” between two consecutive data points; almost as if the bike jumped one bike width’s to the side. Rather than being a flaw, this indicates to me that the Glo doesn’t attempt to compensate, smooth and adjust fixes but just reports the fix as calculated.

TraceLuffield1Over the course of the lap, both devices report pretty similar traces, except for the trace recorded by the QStarz 818XT at Luffield. You can see this in the red trace that leaves the track completely for a while before finally rejoining the circuit. A more detailed view is show below.

TraceLuffield2

It’s pretty obvious that the QStarz 818XT was unable to accurately locate itself possibly due to a temporary loss of view to the overhead satellites. It takes the device almost 8 seconds to re-establish its position back on track. This could have occurred because the QStarz 818XT was mounted in a position on the front of the bike where my helmet (and upper torso) could have been shielding the device from a decent view of the satellites. I checked the laps before and after and this problem didn’t recur so it’s likely to have been a one off that lap. What’s interesting is that a seemingly large error like this has relatively impact on the overall lap time and both devices report lap times within 1/10th of a second of each other for that particular lap.

Lap QStarz 818XT Garmin Glo Difference
1 3:11.55 3:11.52 0.03 3/100ths
2 3:06.13 3:06.12 0.01 1/100th
3 3:05.64 3:05.64 0.00 Same
4 3:02.12 3:02.02 0.10 1/10th
5 3:03.94 3:04.00 0.06 6/100ths

With the exception of the excursion off track described above, both devices show fairly similar traces around the circuit. In terms of position reporting, the Garmin Glo looks to be more accurate than the QStarz 818XT but that could be because the QStarz 818XT was located less favourably than the Glo. If you’re looking to plot your lines around the track, the Glo would be a better unit to use than that QStarz 818XT. If however, you’re just interested in your lap times, then both devices do a great job reporting very similar lap times as you can see above.

In my next article, I will analyse the speeds reported by the two devices at different parts of the circuit.

Garmin GLO vs QStarz 818XT Part 1

After hearing reading a lot of positive feedback on the Garmin GLO, I decided to get one and compare it with my QStarz 818XT which has worked perfectly for me over the past year. My primary interest in the GLO is to see whether it can provide faster updates and more accurate positioning information.

Garmin GLO and QStarz-818XT

Garmin GLO and QStarz-818XT

The Garmin GLO and QStarz 818XT are bluetooth GPS receivers and can be used to provide a much more accurate GPS signal for race track timing compared to the built-in GPS found in most mobile phones. The GPS in a mobile phone won’t typically update its position more than once a second (1Hz) while these units can update at up to 10 times per second (10Hz). I use RaceChrono to record my lap times and if you are new to this, you can find out how RaceChrono works in this detailed post on using RaceChrono.

Both of these bluetooth GPS devices are similar in size although the Garmin is slightly slimmer and longer than the QStarz-818XT. The battery in both is good with each device giving a minimum of 12 hours on a single charge – enough for a full day on track. In fact the battery life of the QStarz is actually a lot better than the GLO, supposedly lasting up to 40 hours (although I haven’t actually proved that).

Size and battery life isn’t the only difference between these two units – the technology used by them for determining their position is also different. The QStarz-818XT only uses the US GPS satellites in order to determine its position while the Garmin GLO uses both the US satellites and the Russian GLONASS system. This makes the GLO’s hybrid technology fairly unique in the market compared to most other makes and model of GPS unit, and it’s what gives the GLO an edge over the QStarz in terms of outright accuracy.

QStarz-818XT

This image is from RaceChrono showing the satellite information when connected to the QStarz-818XT. You’ll notice how the GPS unit has locked on to 7 satellites and is reporting its horizontal position accuracy as 1.1 – this is an excellent value indicating a high degree of precision.

The DOP values range from 1 to more than 20 where 1 is ideal, 1-2 is excellent, 2-5 good and values over 5 less increasingly less accurate . DOP stands for dilution of precision and you can get a detailed description of DOP values from Wikipedia.

Garmin GLO

Because the Garmin GLO is able to use both the US GPS and Russian GLOSNASS satellites for determining position, it has access to more satellites at any point in time which in theory allows it to determine its position more accurately. Under the satellite settings it reports its horizontal DOP as 0.7 which means greater accuracy than the 1.1 DOP value reported by the QStarz. This makes sense because the GLO is reporting a fix from 14 satellites compared to the QStarz’s 7 satellites.

To get these screenshots of RaceChrono, both GPS devices were placed side by side with each connected in turn to RaceChrono to grab the two screenshots. The two readings were therefore taken within a few minutes of each other and so should not really be affected by the fact that the satellites will have moved slightly during the test.

Startup times and operating speed

My feeling is that while the Garmin GLO gets its first fix a little faster than the QStarz-818XT there really isn’t that much in it, and since both devices lock on to satellites so quickly – you’re never stuck waiting for the first position fix. Both devices were also really easy to connect to RaceChrono running on my Android phone.

One major difference is the position output frequency of the devices. The QStarz is able to operate at 1Hz, 5Hz or 10Hz while the Glo only operates at 10Hz* – this means that it will send an updated position fix (over bluetooth) ten times a second or every 1/10th of a second.

While the QStarz-818XT is also able to operate at 10Hz, it will not output as accurate a signal as it will at 5Hz where the DGPS (differential GPS) capability can be enabled. In fact, the general consensus is that it is better to run the QStarz at 5Hz because although you only receive a new position fix five times a second each fix will be more accurate fix that the ones received at 10Hz. I’ve written another post that describes how to configure the QStarz-818XT to operate at 5Hz.

* When running RaceChrono with some Android 4.0+ devices using the Garmin GLO, the output frequency can switch from 10Hz to 5Hz and sometimes even down below 1Hz. This isn’t a problem with the GLO so much as a problem with the bluetooth stack in Android 4.0 and above. The device is sending too much data too fast for the OS to be able to process because some genius programmer decided to rate limit the receiving side of the Android bluetooth stack!

First impressions

My first impressions of the Garmin GLO are good. The unit is smaller than I through it would be, and it operates well with RaceChrono. It ships with a USB and a separate cigar charging lead which is good. The one thing I don’t like though is the On/Off switch which is too easy to knock. You could pack the unit away and find that the battery flat when you needed to use it because the On/Off switch got pressed by mistake in transit.

The QStarz-818XT is a solid performer. I’ve never had any problem with my unit. It locks on to satellites quickly and operates flawlessly at 5Hz with the DGPS enabled. I like the long battery life and the fact that the On/Off switch requires a more positive sliding action to switch it on or off which means that it’s unlikely to be switched on by mistake.

In my next post, I compare the operation of these two bluetooth GPS receivers on the race track.