GPS System Segments - Understanding GPS: Principles and Applications - page 73

Crosslink
subsystem

TT&C
subsystem

Navigation
(mission)
data unit

L-band
subsystem

Frequency
synthesizer

Atomic
frequency
standards

Figure 3.3 Satellite navigation payload.

reception of the predicted navigation data and other control data from the CS via
the tracking, telemetry, and control (TT&C) links. The navigation payload is only
one part of the spacecraft, with other systems being responsible for such functions
as attitude control and solar panel pointing. Figure 3.3 is a generic block diagram of
a navigation payload. Atomic frequency standards (AFSs) are used as the basis for
generating the extremely stable ranging codes and carrier frequencies transmitted
by the payload. Each satellite contains multiple AFSs to meet the mission reliability,
with only one operating at any time. Since the AFSs operate at their natural frequen-
cies, a frequency synthesizer, phase-locked to the AFS, generates the basic
10.23-MHz reference that serves as the timing reference within the payload for
ranging signal and transmit frequency generation. The navigation data unit (NDU),
known as the mission data unit in the Block IIR design, contains the ranging code
generators that generate the C/A code and P(Y) codes (plus new civil and military
signals in later payloads) for modulo-2 addition with the navigation message data.
The NDU also contains a processor that stores the uploads received from the CS
containing multiple days of navigation message data, and it assures that the current
issue of navigation message data is provided for this modulo-2 addition. The com-
bined baseband ranging signals are then sent to the L-band subsystem where they
are modulated onto the L-band carrier frequencies and amplified for transmission
to the user. (Chapter 4 describes the signal-generation process in detail.) The L-band
subsystem contains numerous components, including the L1 and L2 transmitters
and associated antenna. The NDU processor also interfaces to the crosslink
receiver/transmitter for intersatellite communication, as well as ranging, on Block
IIR and later versions. This crosslink receiver/transmitter uses a separate antenna
and feed system. (It should be noted that the intersatellite ranging is functional on
the Block IIR, Block IIR-M, and Block IIF space vehicles; however, the U.S. govern-