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GlobalSat's BT-335 functions as both a Bluetooth-equiped GPS receiver (compatible with most NMEA-compliant mapping/ navigation software on Bluetooth SPP-compliant PDAs, smartphones, and computers) and as a stand-alone data logger. It can perform both functions simultaneously, saving coordinates, time stamps, altitude, and speed to a log which can be wirelessly downloaded for use in geotagging photos or in other location-related applications. This review compares the BT-335 to my previously purchased and evaluated Holux M-241, focussing on Mac compatibility and geotagging utility.

Disclosure: A sample BT-335 unit was provided by USGlobalSat, Inc. for the purposes of this review.

Those unfamiliar with the basic concepts of geotagging may wish to read this introduction first.

What's in the box?

BT-335 packaging and accessories

• BT-335 unit;
• CD-ROM (with PDF manual, Windows-only software);
• Set-up/ quick start card;
• 12V DC car charger;
• US-style 110/ 240V AC wall charger;
• US-to-Euro wall adapter.

The BT-335 ($US120) is a compact 75g matchbox-style unit in silver plastic that includes anti-slip strips for horizontal placement (e.g. on a dashboard). Unlike the cylindrical M-241 (which has to be kept vertical) the BT-335 doesn't come with a lanyard, although use of one was obviously a consideration—the unit has a lanyard attachment. I do find myself using the lanyard with the M-241 around my neck, mainly so I can keep a close check on battery status. One of the LEDs on the BT-335 lights red when the battery needs charging, but with a claimed 25 hour capacity the unit could easily be turned on and "forgotten" in a pocket (e.g. jacket, camera bag). As well as the power button there are two more LEDs gracing the front of the device, indicating GPS fix and Bluetooth connectivity. The back of the device comprises the battery compartment lid, while the charge port is protected by a rather flimsy and fiddly soft plastic cover. There is no mini USB port and no waypoint marker.

A UK adapter was not supplied (the review unit was for the North American market) but this is the case when sold through retail channels in the EU.

How compatible is the BT-335?

As an independent logger the BT-335 is fully compatible with whatever digital camera you're using, from an expensive dSLR to an inexpensive point-and-click. It also allows first generation iPhone users to accurately geotag images taken with that device's camera (the second generation iPhone 3G introduced built-in GPS and geotagging).

The visible absence of a mini USB port is a clear indication that compatibility with computers for log retrieval—or with any device in GPS receiver mode—depends upon Bluetooth (built-in, or added via a USB module).

Mac compatibility is listed on the packaging in terms of GPS receiver functions (e.g. use in conjunction with navigation software), but this excludes data extraction which allegedly necessitates a Windows machine. It's only when you view the USGlobalSat website that there is any indication of Mac compatibile data extraction (i.e. the GlobalSync download). I expect this will change in subsequent packaging/ manuals) given that Mac utilities for specific GPS units are uncommon and therefore worth promoting.

Once you've extracted the data it becomes automatically usable with a good range of geotagging software for Mac. No geotagging software is included except for Windows, so for other platforms it's a case of BYO.

The quick start guide does refer to a "test utility program for either Windows or Mac operating systems" being available from the USGlobalSat website, and although you can download the Data Logger PC Utility that reads live data from the device (also on the CD-ROM) there is no such test utility for Mac OS.

If you don't already have GPS-aware mapping software for your Mac such as RouteBuddy, after pairing (see below) you can test your connection to the BT-335 using the free third-party GPSUtility (268KB .dmg).

How do I pair the BT-335 via Bluetooth on the Mac?

With the Mac's Bluetooth turned on and the BT-335 powered on, choose Set up Bluetooth Device from the Bluetooth menu (or click the + in System Preferences > Bluetooth if Bluetooth isn't shown in your menu bar). Run through the Assistant, entering the specific passkey 0000 when prompted. On successful pairing the BT-335 should be listed in System Preferences > Bluetooth along with any other Bluetooth devices you may be using:

Bluetooth pairing is a breeze under Mac OS X Leopard

A comprehensive Bluetooth Setup Guide is also available for download, covering not just earlier and newer iterations of Mac OS X, but also Windows XP/ Vista/ Mobile, Pocket PC and Palm. This information is also available on the quick start card.

How do I download logs and manage the BT-335 from the Mac?

USGlobalSat announced GlobalSync 1.0 in May 2008, a Universal Binary and free download (1.4MB .zip). It sure is good-looking (compare the screen captures to BT747 as used with the Holux here). After the relative complexity and ugliness of BT747's Java-based interface, GlobalSync.app truly is a breath of fresh air. In contrast to the poorly translated BT-335 User Manual this app refreshingly boasts grammatically-correct English, having been written by the folks at RouteBuddy Ltd. in the UK.

In my testing (under OS X 10.5.3 on an Intel MacBook) most of the time everything worked well. I wasn't able to connect to the BT-335 every time, reflecting my general experience juggling Bluetooth devices on the Mac. However, either turning the logger off then on again or restarting the Mac would resolve the issue—it's hard to know where the exact problem lies.

On every launch of GlobalSync you need to specify which Bluetooth port corresponds to the BT-335; a means to set a default port would be a nice touch:

Once connected there's just no comparison between this simple yet elegant interface (below) and that of the cluttered Windows-based Utility:

Most of the detail is in the Configure panel, a match for the same panel in the Windows-based tool. If the devil is in the detail, however, that demon must be behind a few issues I experienced using it.

The first option set (Active Features) determines whether the BT-335 operates as a GPS receiver, a logger, or both. I've no idea what changing the mode does for battery life, memory usage, etc.

Under GPS Features you can enable WAAS/ EGNOS/ MSAS. These augmentation standards are enabled by default. WAAS is supposed to improve accuracy from 10m to 5m; the other acronyms are European and Japanese equivalents (not explained in the User Manual).

Memory Usage shows just that, turning green as you consume storage space. To see a numeric percentage (as displayed in the Windows Utility) mouse-over the green area to reveal a tooltip (this makes guesstimating your typical day-by-day usage that bit easier).

The empty space to the right of Memory Usage would ideally report firmware version, as without this Mac users have no means of determining whether they have the latest firmware. Even if it won't be possible to flash the firmware from a Mac, users will at least know to borrow a Windows machine for bringing their firmware up-to-date.

In a similar vein, reporting live UTC time on the device in this location would be a useful aid to synchronise the clock in your digital camera, ensuring time stamps for track points and images are as closely matched as possible (the M-241's display has a Time Mode for this purpose). If any discrepancy is outside a geotagging program's tolerance for automatic matching, you will need to determine the correct offset (more on that here) which is not always easy.

Under Data Logger, Record Data offers the option to specify what variables are included in a track point record. Together with Recording Interval this will determine the size of the track log. Here's where things became unstuck.

Opening downloaded GPX files in a text editor revealed that Position outputs exactly the same track point data as Position, Time, Speed (that is, both give coordinates and time stamp). Position, Time, Speed, Altitude does include elevation with each track point, but none of the options produce GPX files including speed (which was dropped in the GPX 1.1 schema anyway). Why mention speed at all here if it's an on-the-fly calculation made during log manipulation by the Windows utility?

A more significant "gotcha": On opening the initial GPX file I downloaded via GlobalSync I noticed that the track points were 30sec apart (the device default, according to the Manual)—despite the Recording Interval showing Every 10 seconds (see screen capture below). It seems GlobalSync correctly reported a 10sec interval only after I had used it to update the device. But then I set it to 5sec, updated and restarted the device, yet the track points were still 10sec apart. Similarly, if you click Use Defaults , the Recording Interval as shown in GlobalSync does not reflect the actual 30sec default interval (it shows 10sec). Some work clearly needs doing to fix this.

You can also change recording behaviour based on your velocity, optionally turning off track point logging if the distance covered between track point intervals is below a certain threshold (in metres, or km/hr). Geotaggers are unlikely to enable these options, since most photos will be taken while stationary and automatic geotagging preference may require a short time/ distance interval—especially when taking multiple shots from the same location.

It seems after clicking Update you must turn the logger off and then on again in order for the changes to take effect; the need to do so needs to be made clear (e.g. a confirmation to the effect of "Settings were updated. Please turn your device off then on again to apply changes").

If you click Download the rotating "candy cane" animation that corresponds to "busy" quickly changes into an informative progress bar (as per the Safari downloads window):

You can download and convert your track logs in one step into either of the ubiquitous GPX or KML formats. I prefer to do both—GPX for geotagging (in GPSPhotoLinker) and KML for viewing routes (in Google Earth).

A bug discovered during this review involved GlobalSync taking account of the Mac's local time zone, but omitting to account for daylight savings (summer time), so that time stamp conversions to UTC were out by an hour in downloaded GPX files. Release of a fix is pending.

Curiously when you save the data to a single GPX or KML file GlobalSync reports something like "34 items were downloaded from the device". This actually refers to the number of track segments, not separate track downloads, so is redundant information that could cause confusion.

Unfortunately the download is all-or-nothing. Using BT747 with the Holux I appreciated being able to download and convert one file per log. This enables the saving of separate track logs representing distinct activities (e.g. photos taken at a particular attraction can be archived with the relevant GPX file; a morning walk could be viewed separately from an afternoon walk via KML in Google Earth).

The Clear button immediately wipes the device memory. There's no confirmation so if you accidentally hit it before downloading, you've lost your valuable data and will sadly be forced to fly back to Venice in order to re-acquire it. I think this is an instance where an "Are you sure you want to erase the data on the device?" dialogue box would warrant an enabled-by-default Preferences checkbox. Photographers don't like loosing bits or bytes at any processing stage!

After you've cleared the data the trash icon still looks full: there's no visual indication either here or on the device (as with the M-241) that the logger's memory has been cleared. However, the paranoid can click Configure to examine the Memory Usage indicator, which should now be devoid of green.

How does GlobalSync compare to the Windows software?

Whilst GlobalSync may be gorgeous, it's also limited. The in-box CD-ROM includes two software tools for Windows users, which together handle not only device management, receiver mode testing, and track log download, but offer a fairly reasonable "beginner" geotagging and photo-sharing solution.

GlobalSync is able to read the native log format and convert to KML or GPX. The Data Logger PC Utility additionally offers export to CSV and TXT, and can save to the device's native DataLogger (GSD) file format—although I've no idea what practical value these formats offer. I support the developer's informed decision to omit them; those folk needing data in alternative formats can create them using the format conversion tool in GPSBabel+ (see below).

Data Logger PC Utility's Configuration Wizard allows the user to directly set preferences for units (metric or Imperial) and determine recording interval based on speed of locomotion (i.e. walk/jog, bicycle, or vehicle). GlobalSync cleverly sets unit preference automatically according to System Preferences, although it offers no such guidance on an appropriate activity-related track interval. Data Logger PC Utility can read device data such as fix and UTC (which GPSUtility can do for Mac users) and firmware/ SiRF version. The built-in display of the M-241 means most settings can be configured without the aid of a computer: some devices are evidently more "stand alone" than others.

Additionally in Data Logger PC Utility you can view selected track points in Google Earth, optionally displaying the velocity recorded at each point—perhaps useful for analysing your sporting activities, but a mere curiosity for typically pedestrian geotaggers. There is a similar viewer for plotting using Google Maps. Mac users can get similar functionality after loading a GPX export into a third-party tool e.g. GPSPhotoLinker.

The CD-ROM also contains locr GPS Photo software (requiring a download and install of the .NET framework) or you can obtain a more recent version here. This free geotagging tool is fairly basic, unable to deal with GPX files and of no use to those who shoot in raw formats or value IPTC metadata—but its inclusion does mean Windows-based geotagging novices purchasing the BT-335 have a start-to-finish hardware and software combination. It would be nice to see a similar approach completing the package for Mac-based geotaggers, bundling not only GlobalSync but one of these tools on a hybrid CD-ROM (the HoudaGeo demo would be my suggestion; it's easy to use and includes photo sharing).

Are there Mac alternatives to GlobalSync?

I noticed the Data Logger PC Utility also allows configuration of the DG-100, and Richard noted that both LoadMyTracks and GPSBabel+ were working on support for that device. I successfully convinced LoadMyTracks 1.3.1 that the BT-335 was a DG-100, converting and wirelessly downloading a less segmented GPX file (at 448KB vs 508KB for the same data).

It isn't possible to change device settings using this tool, and GPSBabel+ (using gpsbabel 1.3.5) could not be so tricked (it likes only Garmin, Magellan, or Wintec devices).

Is this thing on?

The M-241 has a very visible Start and End menu button, so logging is normally manually initiated as a separate step from turning the logger on. With the BT-335 if it's on (and has a fix), it's logging.

I found this a bit annoying. Using the M-241 I could, for example, record a track for export to KML showing my jogging route. On return to my desk I could turn on the device and, at my leisure, download the track log specific to that activity. When returning to my desk with the BT-335, turning on the device turned on logging, and my "clean" jogging route was contaminated with "desk data"! This problem is compounded by GlobalSync's inability to separate out non-contiguous tracks (as BT747 does), since splitting them would resolve such issues.

How do I know if I've got a fix?

The specs suggest a 42sec average time for satellite acquisition from a "cold start". In practice it seems more on the ball than the M-241, with a stated 36sec cold start average.

The green LED is at least as effective an indicator as the satellite icon on the display of the Holux, changing from flashing (acquiring satellites) to steady (GPS fix)—the opposite of expectation. It would seem more natural to flash during acquisition, and light steady when locked.

What about waypoints?

Although the BT-335 is a capable device for automatic geotagging (matching photos to track points) it does not support manual geotagging (matching photos to waypoints). Waypoints are perhaps less attractive to photographers than relying on "set and forget" track points: do you remember to set them every time at almost the exact time you take a photo? But waypoints have other uses. For example, in geocaching you might want to mark an exact cache location, but this could be difficult to guess when all you have is a multi-point track log to work from. If you're a mapping enthusiast using software like RouteBuddy waypoints can be converted into placemarks/ points of interest for storage or sharing.

The M-241 records waypoints (while simultaneously recording a track log) in Position Mode.

Has it got stamina?

GlobalSat to Holux: "Mine's bigger than yours"

The rechargeable 1800mAh Li-ion battery is removable and thus replaceable ($US13 excluding shipping and tax), removing one of my personal concerns over diminishing usefulness of devices with non user-replaceable proprietary Li-ion batteries.

I turned on the logger and left it to die, then downloaded the GPX log:

Event	Time (UTC)
First time stamp	18 June 2008 11:53:13
Battery warning LED	19 June 2008 13:45 (approx.)
Last time stamp	19 June 2008 13:45:06
LEDs extinguished	19 June 2008 14:39 (approx.)

That's just shy of 26 hours continuous recording. Needless to say this is an improvement over the 12 hours of the M-241 (although under the same test conditions I did manage to eke out 13.5h). Interestingly as soon as the red battery LED lit the logger stopped recording, even though it had power for almost a further hour. The BT-335 thus has twice the stamina of the M-241 (even when the later lives up to expectations).

Can I still use it when the battery is depleted?

Even with a fully discharged battery at the end of a geotagging session (or no battery) logs can still be wirelessly downloaded from the M-241 if it is receiving power over USB (it can't recharge over USB). In the case of the BT-335 downloading logs is possible while recharging the Li-ion battery on an AC or DC charger, but the absence of a mini USB port means the BT-335 cannot act as a GPS mouse for a laptop with a depleted battery when AC or DC power is unavailable.

I like to keep a universal USB port DC adapter in the car since, with the appropriate USB device cable, the one socket can recharge my TomTom GPS, iPod/ iPhone, and my wife's mobile. With no USB input for charging the BT-335 that won't serve; I'll need a multi-socket DC adapter.

How do I know when the memory is full?

The BT-335 can record "up to" 60,000 track points (which the documentation erroneously refers to as waypoints) to 16Mb (i.e. 2MB) on-board memory, although presumably this refers to the minimum data set (i.e. coordinates and time stamp only). By way of comparison the M-241 can store "up to" 130,000 track points to its "capable" storage (the actual storage capacity appears undocumented).

The BT-335 doesn't have the decrementing track point counter as per the M-241 display. If I was away on a week-long trip, with no computer to hand for downloading tracks, I'd be worrying in the absence of visual reassurance whether the device was almost full. Perhaps the battery LED could flash yellow if say less than 20% storage remains (alternating red if the battery is also low)?

Using the "out of the box" settings (recording coordinates and time at a 30sec interval) here's what nearly 26 hours of continuous recording does to the on-board storage:

Mouse-over the green bar to reveal percentage

Reassuring, isn't it? That's 3081 track points producing a compact 296KB GPX file and utilizing about 4% of storage, meaning you should be good for several weeks in the field without having to download. That's assuming you can power it continuously—and that the GlobalSync indicator and my figuring are accurate...

Recording coordinates, time, and altitude at a 10sec interval continuously for 12 hours (a days worth of photography) generated a 508KB GPX file containing 4300 track points, consuming about 10% of available storage on the BT-335. By turning off the logger when not needed to conserve memory, in the real world with a more typical interval you should therefore be able to record for at least a week before needing to download to a computer.

What about accuracy/ sensitivity?

The BT-335 employs a 20 channel SiRF III chipset in contrast to the 32 (51) channel MTK used in the M-241, both with the same sensitivity (-159dBm). Enable the WAAS/EGNOS/MSAS on the BT-335 to acquire a "more precise position", apparently. But all this technical mumbo-jumbo is virtually meaningless to me. I performed two real world comparison tests, the first walking our jogging route which is readily identifiable in Google Earth (some tree cover, no urban canyons). Any poor tracking should therefore be obvious:

BT-335 (red) vs M-241 (blue) track logs converted to KML

A high degree of concordance. The second test involved placing the BT-335 and M-241 next to each other on my desk (indoors, but by a window), and comparing how many satellites they could each lock onto:

BT-335 (top) vs M-241 (bottom) satellite acquisition

Cut from the same cloth, I'd say.

Summary and conclusion

As comparable wireless data loggers the Holux M-241 has a useful display, mini USB, and stores waypoints whereas the GlobalSat BT-335 has much better battery life and a Mac-friendly utility. The two devices seem to perform similarly—if you keep the M-241 upright.

GlobalSync needs to be celebrated and promoted, but several issues in the Configure panel merit attention and it's too easy to accidentally delete your valuable data. Ideally it should allow splitting of non-contiguous tracks, and more information about memory status (in GlobalSync and via a device LED) would be useful—as would live UTC.

Additional (typically free) third-party Mac software can largely fill the gaps in testing the device's GPS receiver functions and geotagging via its logger functions, matching (or exceeding) the software package provided for Windows users. At the moment you have to go looking for it—but we can expect this situation to improve as GlobalSat press new CD-ROMs.

Any other quibbles are rather minor, and congratulations are due to GlobalSat for thinking of Mac users. The BT-335 is doubtless a welcome addition to the still relatively small number of Mac-compatible GPS data loggers. Unlike some of its peers, however, using the BT-335 with a Mac doesn't require hacks, Terminal commands, or challenging clunky software—paired with the GlobalSync utility data extraction is a seamless Mac-like experience. AMOD's AGL3080 may mount on the OS X desktop, but GlobalSync lets you manage your device too, from the comfort of a stylish interface. In true Mac spirit the GlobalSat BT-335 plus GlobalSync software together offer a comfortable road into track log acquisition, bringing automatic geotagging to the rest of us.

Update 12.08.08: Further details on storage capacity:

• Only 1.4MB is actually available for track point storage;
• A total of 100,000 position-only track points can be stored;
• A total of 62,000 position, time, date, and speed track points can be stored;
• A total of 43,000 position, time, date, speed and altitude track points can be stored;
• Track points are retained in memory for 20mins when the battery is removed.

Update 21.08.08: The BT-335 does in fact log scalar speed; GlobalSync 1.0 ignores it.