GRAM/SAASM Receivers for the AN/ASN-128B Doppler/GPS Navigation System

GPS is a widely accepted highly accurate navigation system that provides not only extremely accurate position but velocity and time as well. Military users of GPS in airborne applications often combine it with a back-up dead-reckoning navigation system such as a Doppler radar or an Inertial Navigation System (INS). This approach provides a ride-through or back-up capability should GPS data become unavailable due to accidental or intentional signal interference or operation in areas where the signals are blocked by man-made or natural obstructions. One such integrated system, the AN/ASN-128B Doppler GPS Navigation Set (DGNS), is currently in production for the U.S. Army's large fleet of utility and cargo helicopters - the UH- 60A/Ls and the CH-47Ds. The AN/ASN-128B is an AN/ASN-128 Lightweight Doppler Navigation System with an embedded military quality 1-card GPS receiver. The AN/ASN-128 has been in the inventory of the U.S. Army and in military aircraft and helicopters of over 20 countries for over 25 years. Trimble Navigation LTD developed the GPS receiver embedded in the ASN-128B. This receiver is a one of a large number of GPS receivers successfully developed by them for both military and civilian users. GPS position data are used to continuously update position data computed from Doppler radar velocity and aircraft heading; the latter is obtained from the existing magnetic compasses on UH-60s and CH-47s. Should GPS data not be available, navigation and guidance continues using Doppler derived position data starting at the last valid position provided to the Doppler by the GPS. The AN/ASN-128B has enabled the U.S. Army to incorporate GPS with minimum impact on the airframe and hence minimum helicopter retrofit costs. Logistics, pilot and maintenance training, and other system support functions have been in place for the Doppler for many years. Embedding of GPS into the existing Doppler navigation system takes advantage of these capabilities, thereby further reducing retrofit costs. The paper will briefly describe the ASN-128B. Operation and performance of a Doppler radar navigation system will be reviewed. The performance advantages obtained by combining the two sensors into a hybrid system will be discussed. The development of the GPS Receiver Application Module (GRAM) will be described and also its integration into the DGNS. Contractor tests and Government flight tests of the upgraded system or DGNSU that were performed to verify its performance will be described. The integration of the GRAM/SAASM receiver, the GPS JPO’s latest embedded GPS receiver, will be discussed including its impact on the ASN-128B. The current developmental program for this configuration will be described including the potential fielding of production systems starting in 2003.