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Comment Impeccable Timing (Score 1) 400

The timing of this announcement causes me to think there is something else going on. Not that I disagree with the announcement. I think the 100:1 price to earnings ratio of Facebook's IPO is overly speculative and the ability to produce valuable targeted advertising from Facebook's massive database is yet to be seen. However I strongly doubt GM had no consideration for the timing and the impact it might have to the IPO. Surely FB would know about the announcement before it was released. And they could have offered money to delay the announcement. I just can't figure out what business objective GM would have for announcing it now.

Comment Re:Why... (Score 1) 647

Thanks for posting this. When decrypting the P code the receiver is operating in AS mode (anti spoofing). The module that is used to decrypt it is called SAASM for Selective Availability Anti-Spoofing Module. All military receivers have them. So I had the same thought.

It's also possible it did not have current decryption keys.

Comment Re:Weak spot in FAA's "NextGen" system (Score 1) 386

The next generation of GPS, block III, will provide several new codes in different bands, and is designed for safety of life concerns such as airplane navigation. With up to 5 signals in five different bands, and some with large bandwidths and higher power, it will be significantly more difficult to jam. In such cases only a ground-based military jammer will work.

Comment Re:Vulnerable (Score 1) 386

Current military GPS users have an embedded module called SAASM. It decodes the P-code to prevent spoofing, which is the process of transmitting the code at a different delay to make the device believe it is in a different location. However the current GPS codes are susceptible to jamming where GPS just fails. The M-code, which is partially deployed and scheduled to be enabled in maybe 2014, will really help prevent jamming because of it's large spreading bandwidth.

Comment Re:CMDA (Score 1) 244

If a single transceiver transmitted one code and received another, but on the same frequency channel, the transmitted signal would overpower the received signal. I explain why in this post:

http://tech.slashdot.org/comments.pl?sid=1997010&cid=35213010

In a CDMA system multiple transmitters send different coded signals on the same frequency where a single receiver receives the sum.

Comment Limitations (Score 5, Informative) 244

The signals will only perfectly cancel when they are separated by a distance that is exactly one half the wavelength. Assuming you separate the two transmit antennas by this distance at the carrier frequency, then there will be a limitation on the available bandwidth. This is because the further you get away from center frequency, and away from the ideal antenna spacing, the less destructive interference you will have (and the more your transmit signal will leak into your receive signal). So you will double your capacity for only narrowband channels.

The pdf gives actual numbers. I just wanted to point out that there is a limitation on bandwidth.

You might also think, "If I know what I'm transmitting, why can't I just subtract it from what I receive?" This has to do with the dynamic range of the receiver, which is a function of the number of bits in your analog to digital converter. You must attenuate your received signal so that you don't saturate your converter. Have you ever turned the volume up so loud that you begin to hear distortion? It's the same thing.

So you are receiving this loud unwanted transmit signal, and this soft receive signal. You must lower the volume so that you are not distorting the highest signal. This lowers the volume on the desired signal as well. You can lower it so much that your analog to digital converter is not able to differentiate between a 1 and a 0 anymore.

I think if you could have an A2D with enough bits that you didn't care if you received the transmitted signal, then you could just carefully subtract out the unwanted transmit signal. Maybe I should patent that? Meh. I'm probably wrong.

Comment Re:The Second, If Not Both (Score 1) 466

They also come up in digital signal processing. You might be able to code a video compression algorithm without the math, but you won't be able to understand the code (somewhat overrated) and you won't be able to come up with new DSP algorithms. I used the gradient to look for the peak in a two dimensional signal search algorithm I designed. I also used partial differential equations to solve for the solution to a set of non-linear equations for a GPS application. I think if you can add them to your toolbox, you will not regret it unless you go into web development or something.

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