[Diddley]

Big brother?

[lelio]

Signals from cellphone masts can be used to track aircraft, monitor traffic congestion and spot speeding motorists without tipping them off that they are being watched.

Huh, I thought of this idea several years ago (I guess I should of applied for a government grant!). If you can figure out from a signal what phone it is coming from, all you need is a single receiver mounted by the side of the road that notes when the signal from the phone was at maximum power. That's most likely when it was closest to your receiver.

From that you can tell how many cars go by in a certain time frame, which is really all you need to monitor traffic flow. Put a couple of more sensors on the side of the road to get a more accurate picture. Granted the phone can change its signal's power during the call but you can get an idea of the number of cars passing by.

[Boot Hill]

From the article:   "From the Doppler shift in the signal it can also calculate the target object's speed."

This story is pure BS and here's why.

An automobile traveling at 65 mph will only produce a 90 Hz Doppler shift in a cellphone frequency of 900 MHz. Even if their "Celldar" instrument was measuring the frequency shift of an unmodulated cellphone transmission (and that will never be the case), the primary frequency accuracy of cellphones is, at best, only about 1 ppm or +/- 900 Hz (most cellphones actually have only 2.5 ppm accuracy). In other words, the 90 Hz of velocity information from the cellphone signal, will be drowned out in a sea of noise from the +/- 900 Hz variation due to primary frequency inaccuracy.

Worse yet, we haven't even begun to add in the additional noise from cellphone frequency instability due to temperature, battery voltage, vibration, etc. Nor have we considered the difficulty of accurately resolving the cellphone frequency to within +/- 1.5 Hz (+/- 1 mph) while the carrier frequency is being modulated (changed) by the voice communication on the phone.

To overcome these major "aw shucks" in their plan, they probably will attempt to use differential Doppler detection, but that would require the installation of multiple receivers in and around the cell tower. In differential detection, each receiver would first acquire the cellphone frequency and share that data with the other receivers so that a difference frequency could be determined.

In this scenario, the precise cellphone carrier frequency is no longer important, only the difference in frequency received by two Celldar systems. Moreover, the difference frequency will be 2X that from a single receiver (or about 180 Hz for 65 mph).

But here's the hole in that method. Each receiver must be able to resolve the cellphone frequency down to +/- 1.5 Hz (out of 900 MHz) in order to get any accuracy that a court would accept (+/- 1 mph) and that just is not possible to do with a signal whose frequency is constantly and rapidly changing because of the encoded voice (or data) communication.

Another problem is that whether you use the Celldar system in either single or differential mode, the data is only accurate if the vehicle is driving straight at, or away from the receiver. Any other path would result in an decrease in signal at a cosine rate.

And lastly, the current advent of third generation cellphones using the CDMA frequency-hopping, spread spectrum technology, will put the final nail in the coffin of the Celldar idea. In summary, while this proposal is interesting, it would be too expensive, require too many installation sites beyond each cell station and would not provide enough accuracy or resolution to convince a court to take the data seriously.

Thanks to its shortcomings, this technology offers no big-brother threat, but it does provide a welcome and humorous respite from life's real problems.

Regards,

Boot Hill

[_Jim]

Signals from cellphone masts

NOTHING comes off/out of 'the mast'.

One can expect quite a bit, however, to be seen coming out of the antennas found facing (usually) three different 120 degree sectors ...

[_Jim]

One would gain some appreciation for what these folks are attempting to achieve if they had some knowldge of the earliest, primitive use of broad-beam, illuminating sources with separate receivers -

- as used in the British Chain Home system. Though not a Doppler-based system, the elementary principles of RADAR are utilized in this system.

[_Jim]

Home page: www.roke.co.uk/sensors/stealth/celldar.asp

Concept: www.roke.co.uk/sensors/stealth/cell_phone_radar_concept.asp

Field Trials www.roke.co.uk/sensors/stealth/celldar_trials.asp

Initial CELLDAR Trials

The first CELLDAR trials were conducted back in November 2001. It was from the success of these trials that a CELLDAR development programme was started. During these trials both aircraft and cars were successfully detected and tracked.

CELLDAR Trials Equipment

Equipment                   Comments

2 x Siemens GSM Phones      Acting as receivers for the base station 
                            signal and the reflected signal from the target.

2 x Yagi aerials

1 x PC                      Standard desktop machine.

AD Converters               200KHz AD converters to digitise signals receive from the GSM phones.

[KantianBurke]

- shrugs shoulders - not using a cell phone at all should do the trick. Never understood the need for one. If the calls important they'll leave a message on your regular machine.