CHAPTER 2:  TYPES OF GPS RECEIVERS

AND THEIR INTENDED APPLICATIONS

2.1  GPS RECEIVER ARCHITECTURES

Modern military GPS receivers use predominantly a continuous satellite tracking

architecture. However, some receivers use alternative architectures, either

sequential or multiplex tracking to reduce hardware complexity.

2.1.1  Continuous Receivers

A continuous tracking receiver has five or more hardware channels to track four

satellites simultaneously plus other channels to acquire new satellites.  Due to their

greater complexity, these receivers were traditionally the most expensive but offer

the best performance and versatility.  The multi channel receiver uses the fifth

channel to read the NAVigation (NAV) message of the next satellite to be used

when the receiver changes the satel lite selections.  It also uses the fifth channel in

conjunction with each of the other four channels to perform dual frequency

measurements as well as differential channel delay measurements.  Individual,

dedicated tracking channels enable the receivers to maintain accuracy under high

dynamics, provide the best anti jamming (A J) performance, and have the lowest

TTFF.  This type of receiver is best suited for high dynamic vehicles such as fighter

aircraft, vehicles requiring low TTFF such as submarines, plus any user requiring

good A J performance.

2.1.2  Sequential Receivers

A sequential GPS receiver tracks the necessary satellites by typically using one or

two hardware channels.  The set will track one satellite at a time, time tag the

measurements and combine them when all four satellite pseudoranges have been

measured.  These receivers are among the least expensive available, but they

cannot operate under high dynamics and have the slowest time to first fix (TTFF)

performance.

2.1.2.1  One Channel Sequential Receivers

A 1 channel sequential receiver makes four pseudorange measurements on both

the L1 and L2 frequencies in order to determine a position and compensate for

ionospheric delay.  The NAV message from each of the satellites must also be read

to obtain ephemeris data.  To determine an initial position, the receiver must

perform the following operations, 1) C/A  code  search for a SV, 2) C/A code/carrier

center, 3) data bit synchronization, 4) frame synchronization and Z count, 5) HOW,

6) P code carrier center, 7) data demodula tion and 8) ionospheric measurements.

Once these operations are complete for one SV, the receiver must perform them

again for three other SVs.  The four pseudorange measurements must

be propagated to the same reference time before a navigation solution is

generated.  Any movement of the Host Vehicle (HV) during the time the receiver

collects the four pseudoranges will reduce the accuracy of the position, velocity,

and time

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