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From the Listening Post #3

Questions from readers and answers from a specialist on electronic surveillance and related topics.

With this issue, we’re discussing a controversial change in cellular telephone service which will have an effect on many of you who use GPS-based covert vehicle tracking.

Will GPS Covert Vehicle Tracking Stop Working?

A reader asks:

  • Steve, what’s this I hear about something changing in the cellular telephone service which might prevent our agency’s recently-purchased GPS vehicle tracking system from functioning? Can you give some insight on this please – it was my recommendation to purchase the system!  ... Lt. Tommy P., Nebraska
  • Tommy, the scuttlebutt is genuine. Whether it will affect you or not depends on some factors.
  • Here’s the story:

    Cellular phones (cell phones) have been around since 1984.  In the early days, all cell was analog. Analog is not a very efficient use of spectrum. But everything started that way and a lot of communications traffic still is analog.

    Analog cellular traffic left a lot of ‘empty’ space in the airwaves. An approximate analogy is the dead space between words. Add all this ‘empty’ space and multiply by tens of thousands of users, and you have a significant chunk of bandwidth not earning any money for the cellular service providers.

    For the record, the analog cell network is called AMPS, for Analog Mobile Phone Service. The terms AMPS and analog are used interchangeably both in the industry and by me and mean the same thing.

    Enter CDPD. Cellular Digital Packet Data.

    CDPD is an analog service.

    CDPD sandwiches digital blocks (packets) of data in this ‘empty’ space in the analog cell network. This provides a low cost data capability, one which is easy to implement and required little modification to the existing infrastructure. That data capability was sold to users who needed a reliable, widely deployed, inexpensive (compared to the cost of building their own network) method to flow data to and from a mobile environment.

    CDPD originally was created by scientists at IBM and was used for the company's own field force. AT&T was and AT&T Wireless now is one of the two major CDPD network providers. Verizon is the other. These two represent the vast majority of CDPD capability nationwide.

    CDPD is a low speed, 19.6Kbps service – snail speed by today’s standards. In its heyday it was used by public safety organizations, health care and transportation industries and in similar markets where just getting the data transmitted was more important than high speed or high performance.

    Now step into the future. Digital cellular is deployed. Both analog (AMPS) and digital cellular exist side by side.

    As we know, digital provides more efficient use of the limited spectrum than analog.

    More efficient use = more users who can be accommodated in the same space.

    More users = more revenue.

    Of course the cellular carriers want more revenue, so they encourage the migration to digital. If they had their druthers, they’d turn analog off NOW, force everyone to upgrade to digital, and make impre$$ive numbers. But there is a lot of analog equipment out there. The FCC reacts, sticks their nose in and mandates indefinite support of the analog network. Cellular service provider beancounters and stockholders are not happy.

    Aging analog is expensive to maintain. The inefficiency compared to digital makes the overloaded cellular spectrum problem worse. With 2003 digital technology, either three voice channels (TDMA) or six channels (CDMA) can fit in the space of one analog channel.

    So carriers file a Petition for Reconsideration with the FCC requesting the mandate to keep analog alive indefinitely be relaxed. It is. The FCC lifts the mandate for the AMPS network to be maintained, and allows it to be knocked down effective February 2008.

    However, there is the matter of the CDPD users, who are on the AMPS network.

    CDPD can be considered a subset of AMPS. AMPS must be kept on life support until 2008, but nothing is said about CDPD.

    CDPD is a primary user of this AMPS infrastructure everyone would like to see go away.

    So the carriers decide to get rid of CDPD, a bastard child using precious bandwidth more profitably used elsewhere. Off with its head. And the CDPD users are out of luck.

    Key Point – CDPD is going away. Soon. Quite soon.

    The primary users of CDPD are public safety agencies, for their Mobile Data Terminals aka MDTs.

    Now this part is important. Pay attention.

    GPS Covert Vehicle Trackers

    Virtually all GPS-based realtime covert vehicle tracking systems use CDPD to telemeter their information back to the listening post.

    As mentioned, CDPD is simple and inexpensive to implement as a data pipe, and at the time GPS trackers were introduced, was the only practical option for relaying the target’s coordinates back to the listening post.

    Now here’s the kicker:

    The cellular companies have decided to dismantle CDPD. Except in very unusual circumstances such as a recently installed network where the contract calls for X years of availability, the carriers have ceased any maintenance whatever of the CDPD portion of their network.

    Without CDPD, your GPS-based covert vehicle tracker probably won’t work. It will have no way to communicate back to you.

    AT&T and Verizon are the primary CDPD carriers nationwide. Both carriers have given several different and conflicting dates as to when they will discontinue service. Dates given to me by the carriers are June 2004 for AT&T, and November 2003 for Verizon.  The CDPD format is long obsolete, extremely inefficient, and equipment to support the infrastructure has not been made for years. The carriers are under no obligation to keep CDPD up except in the very few instances where they are under long term contract commitment to do so.

    Verizon representatives told me the company will try to support it while public safety migrates to some replacement for their MDTs. But, the truth is the equipment manufacturers stopped making CDPD equipment some time ago, and according to a source familiar with CDPD industry equipment suppliers, the carriers had been scrambling around for spare parts, cannibalizing one system to fix another.

    So the vast majority of the remotely interrogable GPS-based tracking systems manufactured in the last fifteen years and still sold today by the spy shops will stop working very soon. Only covert tracking systems using GSM or another pure digital format as the communications link will continue to work, and that's a small percentage. GPS tracking is of limited value anyway due to severe technical limitations, regardless of legal status. However, they are heavily advertised and hyped, so they find their way into law enforcement circles, private investigator hands, and the hands of wannabee civilian 007s.

    Over the last few weeks, I have been researching GPS covert vehicle trackers advertised on the web. I wanted to see if the peddlers of the systems were aware of the imminent dismantling of CDPD, if their systems used CDPD for data transmission, and if they had plans to redesign their products to use some other form of datalink.

    What I found was pretty disappointing.

    I learned apparently there are only a few actual manufacturers of these systems. There are, however, a very large number of places who resell them. There’s nothing inherently wrong with this, except not one of the places I emailed or telephoned had any idea what I was talking about with CDPD. A single vendor acknowledged his system used AMPS however, and this is a clear red flag if you see it as a reference. One would think resellers at least would have a rudimentary understanding of how their products operate. One spy shop in Texas referred me to a brand new tracking product they allegedly ‘manufacture’ and are marketing heavily to the public. On visiting the webpage, I was able to discern the system used CDPD. As the system is brand new, even the basement operator building the thing apparently don’t know he’s producing a product which will become useless within a short time if in fact it’s still supported at all by the time the customer/victim receives it. Oh, no returns on tracking equipment by the way.

    The good news is I was able to identify several professional (generally law enforcement only) covert GPS vehicle trackers which use the GSM network for communication. GSM is a reliable digital format, the standard cellular data system in most countries outside the U.S. However, within the U.S., GSM is in the minority. The Sprint network started with GSM, and other carriers are adding it.

    Moral: If you use a GPS-based realtime covert vehicle tracking system, check and see if it uses CDPD or AMPS as the data link. If it does, you will need to replace it soon. If you’re in the market for a new system, make sure it uses one of the several digital formats to transmit its data (GSM, CDMA, TDMA amongst others). Do not buy any GPS-based tracking system if it uses CDPD or the AMPS/analog network to telemeter its data.

    Be proactive. Don’t expect a phone call from the cellular service provider the morning before they discontinue CDPD in your area. One morning you’ll wake up and the service will be gone. Don’t expect them to tell you the truth either.

    The above discussion does not apply to the recording – only versions of GPS covert tracking systems as they do not relay their locations in realtime and do not use any sort of data link.

    For the agencies using MDTs which link via CDPD, your radio shop almost certainly will be aware of this situation and should already be well along the way towards implementing changes to your network to ensure seamless transition to the new data transmission network.

    Wavecom Wireless Video Frequencies?

    Here’s another inquiry, a fairly common one as a matter of fact:

  • Steve, what are the 4 ‘standard’ 2.4 gig frequencies used by the Wavecoms and similar wireless video systems? ........ Sgt. Jerry McH, New Hampshire
  • Jerry, much of the consumer-grade low cost wireless video on 2.4 gigs are built around the same modules. These modules have 4 frequencies hard coded into them and are relatively standard throughout the industry.
  • Make a note of these:

    Channel 1  2433.75 megacycles

    Channel 2  2452.75 megacycles

    Channel 3  2472.75 megacycles

    Channel 4  2410.75 megacycles

    Yes, the frequencies are not in ascending order. I believe this is to try to prevent co-channel interference from two units operating in the same area on different channels.

    Channel 1 is by far the most common frequency used, as most of the Wavecom-generation devices power up on channel 1.

    You don't need to be exactly on frequency to copy the things with most equipment other than high end stuff due to the slop from the wide open front end. If you're within 5 megacycles or so you'll see a picture. If you're within 15 megs, you'll see a raster, enough to know there's a transmitter on the air.

    Old Bob Doms Newsletters now online

    Many of you will remember with affection Bob Doms, formerly of AID. Bob is well known and respected in the law enforcement and military intelligence circles. After his retirement from AID, Bob started a virtual (electronic-only) law enforcement surveillance – related newsletter and many of you were and are subscribers.

    Bob ultimately decided to hang up his spurs for the final time, and transferred ownership of the newsletter to me. I’ve been putting together newsletters as time permits (not very often, to be honest). The newsletter is named ‘Tips For Techs’. (Note -the newsletter has no affiliation with Police & Security News Magazine).

    Bob’s work was valuable, and most of it doesn’t age. Bob has known everyone and everything in his days, and he shared his insight with us via the newsletter.

    I’ve now placed all back issues of the newsletter on my company website behind a password protection. Visit:


    for access after requesting a user name and password from me via email only, to:


    Note much of the information here is law enforcement sensitive and access is tightly restricted to law enforcement. The 660 subscribers need to feel secure enough to discuss sensitive issues with other technical surveillance officers without fear of the information falling into the wrong hands. Therefore, please indicate a verifiable public law enforcement affiliation in your first email.

    There is no charge for subscriptions to the newsletter.

    Back issues of this column and other articles originally published in Police and Security News by me are available to all at the above URL. Some of them date back to the very beginning of this esteemed organ! You’ve come a long way, baby!

    Please email me at:


    if you have any questions to ask or information to share which are in keeping with the mission of this column. I will attempt to answer your question and, if pertinent, may use it here with your permission.

    See y’all next time ...... Steve

    Copyright (c) Steve Uhrig, SWS Security, October 2003