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HP hw6515, First with (A)GPS
Posté le 02 novembre 2005 à 15:53:57 par gpspassion.

1. HP hw6515, First PDAphone with (A)GPS

UPDATED 03/16 - new v2.02 GPS drivers available, improved accuracy - >> LINK <<

Introduction/Best PDA I've Owned ?/AGPS on Board
GPS Performance: A need for AGPS/Indoor Testing/On the Road
Conclusion/Picture Gallery (next page)/Discussion forums

We started hearing about the HP6500 back in February just before the 3GSM show in Cannes where it was not launched after all. Although we got some early pictures, what with several missed deadlines, it looked as though it was going to follow in the tracks of the HP6340, HP's first PDAPhone released in late 2003 a year after it was announced. Fortunately this did not happen with the 6500 since it arrived in Europe during the summer and is now showing up in the US on HP's site. In Europe, it is bundled with a "trial" version of TomTom (actually an unlimited version with a single map of a town) and in the US you can purchase a special version of the HP iPaq Navigation Software (see pictures on page 2). This article will focus on the GPS aspects of the hw6515 but since it is now my main PDA I'll say a few words about that. The novelty here is the A-GPS module for "Assisted GPS" so we'll spend some time trying to understand what this is about before some side by side comparisons with a "traditional" PDA+SiRFstarIII Bluetooh GPS combo to see how it stacks up.

Best PDA I've Owned ?
I have used the hw6515 daily since early August as my "mobile data platform", and I will say that combined with an unlimited Edge (speeds up to 160kbps) data subscription, it is the most useful PDA I have owned since I started with a Palm Pro in 1997 later moving to the PocketPC platform in 2001, not in the least thanks to its keyboard that lets you type emails at an unexpected speed and I can even perform some maintenance on the site. The two memory card slots are convenient too as the miniSD can be used as permanent storage and the SD to pop in a WiFi card if need be. I do not find the square 240x240 screen to be a problem (I don't play games though) and while a 320x320 screen would have been nice, the larger pixel size lets them capture more light for better outdoor readibility. As with all modern PDAs, battery life can feel short (about 5 hours with GMS, Bluetooth and GPS on), so I always carry an extra battery (this one at $38 works) and...a regular GSM phone for making voice calls. Some might say a Smartphone will do a better all around job ? Possibly, but the flexibility of the PocketPC OS is much better and you can't run the excellent nPOP email program on a Smartphone ;-)

AGPS on Board
Why AGPS ?
In AGPS there is "GPS" of course, so we're in familiar territory and the hardware is standard (antenna and RF and baseband chips - see picture), the novelty is the "A" for Assistance or Aiding and there is a good reason to have it here. Without going in to too much detail, acquiring a GPS position, the "Fix" or Time to First Fix, see "About TTFF" here for more details, requires a minimum level of signal of (about 28dB) so that the navigation message broadcast by each satellite can be decoded, but once the fix has been obtained, that level can go down significantly to about 12dB for a SiRFstarIII based GPS. Since the satellite signals are quite weak when they reach us, a decent sized antenna is required as well as a lot of computation power, the famous "correlators", so that means a lot of energy is needed each time you start your GPS after turning it off, as is required on a phone to preserve battery life. Three good reasons to have AGPS aboard the hw6515: to make up for the tiny antenna (black "bit" top right), reduce the power requirements as much as possible and allow for fast TTFF when then GPS is turned on again. So at this point, AGPS should not be viewed as giving an advantage to the 6515 over autonomous GPS solutions with no assistance, except maybe for indoor TTFF when the satellite level is very weak, but rather as a clever way to make up for the constraints of its form factor. Let's move on to the way AGPS is implemented on the 6515 :

How AGPS works on the 6515

The three main screens in French of the Quick Connect software developed by Globallocate for HP

Resetting erases all previous AGPS data

"User Plane" AGPS on the 6515
The idea behind AGPS is to provide "help" to the GPS receiver to improve its performance. There are two forms: "Assistance" that would include the navigation message (ephemeris, almanac), time, position and doppler corrections and "Aiding" that limits itself to the navigation message. "Assistance" is the most efficient form of help and requires "control plane" systems with data being broadcast in real time over a network such as CDMA, but these are costly to deploy and are only available in the USA as far as I know, where they are used by iDEN phones for GPS navigation by TeleNav or Garmin (see this article) for instance. SiRF have such a solution called SiRFloc. With GPRS, Edge and now UMTS, mobile "user plane" systems are becoming possible, whereby the user decides when to download the AGPS data, but that would be "Aiding" data for now at least. Focusing on "Aiding", what happens is that the navigation data broadcast by each GPS satellite is decoded in a favorable environment (ground station with large antennas for instance) and then sent over the network so that it can be "injected" in the GPS receiver without the need to download it locally.

Ephemeris data being valid for about 4 hours, it would be inconvenient to have to download it that often, so instead the hw6515 relies on the Globallocate LTO (Long Term Orbit) solution that predicts ephemeris data very reliably for up to 4 days with 2 days being the default setting on the 6515. As can be seen above, Quickconnect downloads 4 files that weigh about 40Kb and remain valid for two days. Quick and easy ! Let's see how it performs in the field.

GPS Performance - 6515 vs SiRFstarIII
A need for AGPS
As expected in light of the form factor, getting a "fix" without aiding is very difficult even with an open sky (7 minutes in the example below while driving). On the other hand acquisition is very fast once the aiding data has been downloaded. The aiding data remains valid for two days but Globallocate updates it every 30 minutes so downloading "fresh" data might improve acquisition performance a bit.

7 long minutes witout aiding, on the right, you can see an unusual $PGLOR NMEA message (aiding related)

Indoor TTFF, Signal Tracking and Accuracy
AGPS helps by downloading the navigation data but you still need to acquire and track the signal to get a "fix" and the hw6515 can't seem to compete with a GPS based on the powerful SiRFstarIII for these last two stages. In my test environment inside a house on the top floor, the signal level is low but still sufficient for the SS3 GPS to download the navigation data.

Simultaneous TTFF comparison (First fix after reset on the 6515 and "Cold Reset" on the SS3 GPS)

Quick fix on the Roylatek RBT2001 - On the right the 6515 struggles

Royaltek RBT2001 - SiRFstarIII : 47/35/47/38
HP6515 - AGPs - Globallocate : -/330/100/80

A series of 4 tests in a row with a simultaneous reset on the SS3 based RBT2001 and the 6515, in seconds. The signal level is sufficient for the SiRFstarIII GPS to download the navigation data. The result might have been different with a lower signal level, but it's not certain the 6515 would have acquired a fix either judging by the results below.

Signal Tracking
To get a sense of performance in extreme conditions, the two receivers were placed in a metallic drawer while already locked on a signal. Needless to say the signal level dropped drastically inside the drawer as can be seen below. The RBT2001 was able to hold the fix, but the 6515 lost it, reacquiring it as soon at it was taken out. Same results in my semi-underground garage.

Royaltek RBT2001 - SiRFstarIII : 100% locked
HP6515 - AGPs - Globallocate : 60% locked

Significant drop in signal level inside the metallic drawer!

Another simultaneous comparison with the RBT2001 connected to a Qtek S100 and placed next to the 6515. Significant difference in accuracy, likely due to the different type of antenna, standard sized patch antenna for the RBT2001 and tiny antenna for the 6515.

On the left, the scratter plot of the 6515 shows a lot of dispersion, on the right, the RBT2001 manages a good accuracy in spite of the indoor environment

After this indoor testing, a difficult and "unnatural" environment for a GPS receiver, but where I hoped the 6515 would perform better in order to be used for LBS (Localization Based Services), let's see what happens on the road, the most common use for GPS.

On the Road
As was done for the Chipset comparison article, some side by side testing and logging of the raw data for later track analysis. Three SiRFstarIII based receivers (Globalsat BT338, Semsons iTrekM3/GS BT339, Royaltek RBT2001) vs the HP6515.

Some drifting for the 6515, but good overall performance in this favorable environment and navigation software should give good directions

The 6515 loses the fix 210 meters before the SiRFstarIII receivers in this tunnel

It locks on again a mere 50 meters later

Notable drifting (20 meters) in this semi-tunnel

A bit more cause for concern here, with drifting that would have likely confused navigation software and produced erroneous guidance as quite a few European users have noted over the past few months.

The HP hw6515 is a very capable "mobile data platform" with its fast mobile internet Edge module and convenient keyboard and it breaks some new GPS ground, with the availability of the first consumer "user plane" AGPS solution to improve the user experience with fast TTFF and longer battery life. The Globallocate LTO (Long Term Orbit) aiding does work very well to get a "fix", but the actual chipset (RF and baseband modules) appears to be far less capable than the SiRFstarIII chipset to acquire and track GPS signals. This could come down to the use of a tiny antenna on the 6515 but absent any other Globallocate consumer solution, it's hard to tell. I, for one, appreciate the convenience of having a GPS available at all times with the option of using a SiRFstarIII Bluetooth GPS for improved accuracy when required, but quite a few European users have complained that they were getting poor guidance. My 6515 is running v1.19 of Quick GPS while more recent models have v1.24 installed with users reporting "better" behaviour, no update is available at this time to verify this claim. Looking forward, it's pretty clear the end user would greatly benefit from a combination of Globallocate's AGPS LTO "user plane" solution and SiRF's powerful SiRFstarIII chipset. It will be interesting to see how the upcoming "tiny antenna" SiRFstarIII based PDAs (Loox N500, HTC Galaxy and Mio A701 for instance) perform without any aiding available. If you have questions or comments, you can use this discussion thread in the forums.

On to the Picture Gallery on the next page...

  2. Picture Gallery >>

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