calculations in the receiver.  One channel sequential receivers are limited to low 

dynamic or stationary applications.

2.1.2.2  Two Channel Sequential Receivers

Two channel sequential receivers have been developed for use on medium dynamic

vehicles such as helicopters.  During initial power up each channel operates like a 1 

channel sequential receiver.  After four SVs have been acquired, one channel is

dedicated to navigation (pseudo range measurements, carrier tracking, etc.) while the

other channel reads the NAV message from each satellite.  Both channels are also

used to perform dual frequency measurements to compensate for ionospheric delay

and to measure differential channel delay.  Two channel sequential receivers decrease

the time it takes to start navigating by better than one minute when compared to 1 

channel sequential receivers.

2.1.3  Multiplex (MUX) Receivers

A MUX receiver switches at a fast rate (typically 50 Hz) between the satellites being

tracked, continuously collecting sampled data to maintain two to eight signal

processing algorithms in software.  In addition, the 50 Hz NAV message data is read

continuously from all the satellites. In single channel MUX receivers the hardware

channel is time shared and only one code generator and one carrier synthesizer is

required to track the satellites.   However, a multiplex receiver s measured carrier to

noise ratio (C/N) for any satellite signal will be 10 log (n) (where n is the number of

satellites being tracked) decibels (dB) below that of a continuous tracking receiver.

Consequently, for military receivers, the MUX technique has the disadvantage of lower

resistance to jamming and interference when compared to continuous tracking

receivers. The MUX technique is more commonly found in commercial receivers where

the reduced hardware cost can result in a less expensive product and where

interference may be less of a concern.

2.2  "ALL IN VIEW" RECEIVERS

Traditionally, GPS receivers choose the four satellites of those available that give the

best geometry to perform a position fix.  However, in situations where one or more of

the satellites are temporarily obscured from the antenna s view, the receiver will have

to acquire additional satellite signals to generate a continuous PVT solution.  The PVT

solution degrades until the new satellites are acquired.  One solution is to have a

receiver which uses all available satellites in view to generate a solution.  The inherent

advantage of this receiver is that if it is tracking six or seven SVs and a satellite

becomes obscured, the receiver will continue to provide a PVT solution with little, if

any, degradation.  In general, over determined solutions improve accuracy of the

receivers.  If the receiver does not dedicate one hardware channel per satellite, then

the receiver must use some sort of continual re acquisition strategy (see MUX receivers

paragraph 2.1.3).

2 2