How to achieve 1 meter accuracy with Android Was: Re: Dropgang vulnerabilities

jim bell jdb10987 at
Sun Feb 24 14:22:10 PST 2019

 On Sunday, February 24, 2019, 12:28:28 PM PST, Steve Kinney <admin at> wrote:

On 2/24/19 2:01 AM, jim bell wrote:
> [partial quote begins]
>> Recent changes in hardware and standards make one-meter accuracy
>> possible, in some cases as soon as this year. The transcript of a talk
>> given to Android developers earlier this year, this article gives a
>> short overview of location in smartphones, introduces Wi-Fi round-trip
>> time technology and standards, and then explains the Wi-Fi application
>> programming interfaces.
>> /By Frank van Diggelen, Roy Want and Wei Wang, Android Location, Google/

>That's very relevant indeed - one-meter accuracy would remove the need
>to put any kind of 'flag' on top of a dead drop container.  Without
>retroreflectors or etc. marking packages, broad searches of likely drop
>areas would not produce results worth paying for. 
Yes, I have been considering the retroreflector idea, and while I still think it would be a good idea for 'early' deaddrops, any technique in common use would eventually be sufficiently well-publicized that opponents (cops; thieves) would gradually catch on.  A retroflector that can be seen much more than 5 meters away would be convenient, but also risky.   Retroreflectors which simply lay on the ground, retroreflective surface up, maybe 1/2 inch in diameter, would still be a useful idea.  I imagine that they wouldn't be easily visible from more than 2-3 meters away.
Yet another marker would be a sprinkling of a modern powdered clothes-washing detergent around the area of the dropped idea, perhaps up to  10 centimeters away.  Such detergents have long (50+years) had phosphorescent materials in them, designed to make clothes brigher.  There are UV and near-UV LEDs, say 405 nanometers and shorter, which would make those phosphors glow brightly, visible at least at night.    This would have the advantage that they would not be visible more than a few meters away.  Another advantage is that they would gradually be washed away, leaving little permanent record.  
Yet another idea would be to place a relatively prominent object in an area, and then place the drop a defined distance and direction away from that object.  
 >Also buyers would
>have no need for a means of finding inconspicuous retroreflectors - just
>a 'really good' mobile phone.
I have been trying to get an answer to what should be a simple question:  Do modern cell phones receive and use WAAS corrections?   ×
WAAS is a type of differential GPS in which the corrections are pre-computed and broadcast by geosynchronous satellites at the same frequency as L1 GPS.  This on-frequency transmission has the advantage that no separate correction-signal path should be necessary: a receiver which receives the L1 GPS would, in principle, be able to receive the WAAS corrections.
Whereas GPS alone is supposed to be accurate to 5 meters, WAAS corrections are said to result in 1 meter (or less) accuracy.  What I haven't heard whether cell phones use this?  You'd think you'd get the answer with a google search of 'android waas', but I simply haven't found it.  I find discussions of this as early as 2010, but very little is discussed over the years.  
My next step is to find a 'tracker' program for my phone, and then set the phone down and put it in 'track' mode for a few hours.  The size of the scatter-plot should show, not the accuracy, but at least the stability of the signal.  This might correlate with the accuracy,  At least, if I see a 2-meter diameter scatter, I will be able to expect 1 meter stability.  
The next step is the phones' addition of the Broadcom BCM47755  GPS chip,    that chip which uses both L1 and L5 frequencies.  It is already working in a Chinese cell phone, the   Xiaomi Mi 8, although reading of the first reviews over a half-year ago revealed some early bugs.  Accuracies of 30 centimeters are espected.  

I'm come to the conclusion that anyone _placing_ these dead-drops should do so solely with a stand-alone dedicated GPS+WAAS (or GPS-L1+L5) receiver.  It looks too mysterious what a typical Google phone can do to track itself, even turning on 'airplane mode' and taking other precautions.  A person hunting these dead-drops could still use a smartphone to find the material.
               Jim Bell


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