I'll quote this from a wiki, since it is already out in the open:
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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.
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-Chuck Harris
[email protected] 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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