Principals of Operation
6 -27
6.1.8.E
M Code (Military)
A major component of the modernization process, a new military signal called M-code
was designed to further improve the anti-jamming and secure access of the military GPS
signals. 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).
Unlike the P(Y) code, the M-code is designed to be autonomous, meaning that users can
calculate their positions using only the M-code signal. P(Y) code receivers must typically
first lock onto the C/A code and then transfer to lock onto the P(y)-code.
The M-code is intended to be broadcast from a high-gain directional antenna, in addition
to a wide angle (full Earth) antenna. The directional antenna's signal, termed a
spot
beam
, is intended to be aimed at a specific region (i.e. several hundred kilometers in
diameter) and increase the local signal strength by 20 dB (10X voltage field strength,
100X power). 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 full-Earth M-code signal is available on the Block IIR-M satellites, the spot
beam antennas will not be available until the Block III satellites are deployed.
Other M-code characteristics are:
•
Satellites will transmit two distinct signals from two antennas: one for whole Earth
coverage, one in a spot beam.
•
Modulation is Binary Offset Carrier (BOC) and occupies 24 MHz of bandwidth
•
Uses a new MNAV navigational message, which is packetized instead of framed,
allowing for flexible data payloads
•
There are four effective data channels; different data can be sent on each
frequency and on each antenna.
•
Can include FEC and error detection
•
The spot beam is ~20 dB more powerful than the whole Earth coverage beam
M-code signal at Earth's surface: -158 dBW for whole Earth antenna, -138 dBW
for spot beam antennas.
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