On 7/23/13 11:03 PM, Chris Albertson wrote:
You can prototype a system with off the shelf parts get a few computers, old notebook computers, Raspburry "pI' or repurposed routers, what ever you have. Connect a Trimble Thunderbolt GPS to each one. Each one runs NTP. Connect them all to a isolated network. It could be wired, WiFi or whatever.
This is at work. No junker old laptops.. there isn't enough time in the world to try and figure out their idiosyncracies.
Let's say you have three systems. When all three can se the sky they have good timing. Now cover lone, two or all three GPS antenna and you are stil doing OK for a while. With all thre antennacovered you ar will still have the system time sync'd even it it drifes from real UTC by a few tens of milliseconds. But if just one of the three can see the sky for a little while the time gets corected in all systems
I think that's overkill for this system. 1 millisecond relative timing is an easy bar to meet with almost any comm link; that's more of an integration challenge: what off the shelf widget comes with the ability to do the sync. The challenge is in the frequency control, without resorting to OCXOs or atomic standards.
You can't use Eithernet in the real system but you can rig some other media. Blue Toth works over a short distance but you have radar. It seems like you should be able to use the radar for data communications. Radars if they ca hear the echo at all would be senitive for very long range lateral communications.
It's not a pulsed radar. CW, at a milliwatt or ten. the "bandwidth" of the communications channel, if it can be called that, is on the order of 10-50 Hz.
The way the system works is to look at the received signal reflected from the rubble and victim. That signal has a very strong fixed component (the rubble isn't moving) and a very weak component (heartbeats cause a phase shift of around 10 degrees). With a sufficiently large dynamic range receiver one could just digitize, calculate the baseline, subtract it, and look for the changes. However, this is impractical for a variety of reasons.
So we coherently subtract a sample of the transmitted signal from the received signal to reduce the overall dynamic range requirement. Then we digitize and process. It's sort of like an nulling interference canceller in the RF comm world, or a flavor of Wheatstone bridge.
If the transmitter and receiver are separated (say on opposite sides of a collapsed building, with no direct path), the problem becomes "how do I get that copy of the transmitted signal to subtract". We also use the transmitted signal as a coherent LO for the demodulation. (Called homodyne detection in the radar world, and widely used in familiar applications like radar speed guns)
You say you don't want more radios on the devices so just use the radar signal for low data rate local communications. Put TCP/IP stack on it
Hmm.. TCP/IP at 10 bits/second? I think we can go simpler. We already have to have some sort of data link to send the sampled data for processing, but it's in the 1-10kbps range. Whatever that winds up being, I think that 1 millisecond sync won't be a problem. The challenge is the "frequency control" (or perhaps "frequency knowledge".
One additional challenge with the separated Tx and Rx is that we now have two phase noise sources, and we can't leverage the inherent cancellation with homodyne detection.
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