The solution to this problem is to transmit the pseudolite data in very short pulses so

the user will appear to be "jammed" during short time inter vals.

One proposed differential service is to broadcast GPS like data on L1 from geo  

synchronous satellites.  Compatible receivers would interpret these signals as an

additional GPS satellite and read the differential correction data for all other satellites. 

The receivers would require a channel dedicated to receiving these corrections.  This

data link would provide coverage to wide areas of the earth and have the added benefit

of providing additional satellite ranging methods to improve satellite availability.

11.3  GPS USED AS AN ATTITUDE REFERENCE SYSTEM

11.3.1  Introduction

Angular orientation in 3 dimensions is frequently determined using inertial sensors.  A

GPS receiver with two or more antennas has the capability to be used for real time

angular reference.

Simulations and studies indicate that it will be cheaper and more accurate to use GPS

for attitude reference than to use inertial sensors.  Also, GPS attitude accuracy will not

degrade with time.  Combining GPS and inertial sensors for attitude reference would

give the user the best of both worlds.   Firstly, GPS will give very precise angular

measurements under normal conditions and could provide updates to the inertial

sensors, both for the wander in the gyros and the platform tilt error.  Secondly, if the

GPS receiver is jammed, the inertial sensors would still provide position, velocity and

attitude.  It could also be used to initialize the GPS receiver when the jamming is over.

11.3.2  Concept of Operation

The basic concept of operation for using GPS as an attitude reference system involves

using various types of differencing techniques in conjunction with interferometry. 

Single differencing virtually eliminates the sensitivity of the antenna position errors to

ephemeris, satellite clock, ionospheric and tropospheric error since they are common to

both antenna positions.  Double differencing eliminates the sensitivity to receiver clock

biases.

A GPS interferometer measures the satellite carrier signal phase difference as it arrives

at two different antenna locations.  The two antennas, placed a distance "d" apart will

receive the carrier signal at a different time and therefore with a different phase, see

Figure 11 1.

11 3