[time-nuts] Coalition to Save GPS

Chuck Harris cfharris at erols.com
Sun Mar 13 10:58:45 EDT 2011

I'll quote this from a wiki, since it is already out in the open:

Military (M-code)

A major component of the modernization process is a new military signal. Called the 
Military code, or M-code, it was designed to further improve the anti-jamming and 
secure access of the military GPS signals.

Very little has been published about this new, restricted code. It contains a PRN 
code of unknown length transmitted at 5.115 MHz. Unlike the P(Y)-code, the M-code is 
designed to be autonomous, meaning that a user can calculate their position using 
only the M-code signal. From the P(Y)-code's original design, users had to first lock 
onto the C/A code and then transfer the lock to the P(Y)-code. Later, 
direct-acquisition techniques were developed that allowed some users to operate 
autonomously with the P(Y)-code.
[edit] MNAV Navigation message

A little more is known about the new navigation message, which is called MNAV. 
Similar to the new CNAV, this new MNAV is packeted instead of framed, allowing for 
very flexible data payloads. Also like CNAV it can utilize Forward Error Correction 
(FEC) and advanced error detection (such as a CRC).
[edit] M-code Frequency information

The M-code is transmitted in the same L1 and L2 frequencies already in use by the 
previous military code, the P(Y)-code. The new signal is shaped to place most of its 
energy at the edges (away from the existing P(Y) and C/A carriers).

In a major departure from previous GPS designs, the M-code is intended to be 
broadcast from a high-gain directional antenna, in addition to a full-Earth antenna. 
This directional antenna's signal, called a spot beam, is intended to be aimed at a 
specific region (several hundred kilometers in diameter) and increase the local 
signal strength by 20 dB, or approximately 100 times stronger. A side effect of 
having two antennas is that the GPS satellite will appear to be two GPS satellites 
occupying the same position to those inside the spot beam. While the whole Earth 
M-code signal is available on the Block IIR-M satellites, the spot beam antennas will 
not be deployed until the Block III satellites are deployed, tentatively in 2013.

An interesting side effect of having each satellite transmit four separate signals is 
that the MNAV can potentially transmit four different data channels, offering 
increased data bandwidth.

The modulation method is binary offset carrier, using a 10.23 MHz subcarrier against 
the 5.115 MHz code. This signal will have an overall bandwidth of approximately 24 
MHz, with significantly separated sideband lobes. The sidebands can be used to 
improve signal reception.

-Chuck Harris

bg at lysator.liu.se wrote:
>> The military is the one service that won't be affected by this
>> interference.
>> They run on a different band, and their modulation is more robust than
>> the civilian side.
> Could you deliberate a bit?  I suspect that military receivers use L1 C/A,
> L1 P(Y) and L2 P(Y). This is the exact same signals civilian geodetic
> receivers has been using since about the Ashtech Z12 and its z-tracking.
> http://www.prc68.com/I/DAGR.shtml#GPSs
> It seems two out of three signals used by the military is affected by
> interference close to L1 (1575MHz.)
> --
>     Björn
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