Two specific types of BRNAV/PRNAV equipment are available for older aircraft (without FMS fitment, such as some early models of the L1011/business jets etc), and these are...
Honeywell HT9100 GPS navigator
Universal UNS-1M GPS navigator
Having personally used both units in the past (and now), they are very reliable units and operate superbly....even transAtlantic/trans Pacific, for which they are both approved as a sole means of naviagation.
Pony up the cash for dual units...a done deal.
Both units have their advantages.
The HT9100 is a color (colour, for you folks in the UK
) display and is more user-friendly, IMO.
However, the Universal UNS-1M comes equipped with a worldwide database...it even provides various ATC frequencies, runways lengths, elevation, magnetic variation etc etc of
allthe airports in the database (and in the USA there are 9100 of them, all public use, with instrument approaches, SIDS/STARS included...the whole nine yards.
About the only thing missing is a Google Earth view...you have to go to the internet for that.
And, speaking of Google Earth, their new views actually shows the registration number of my private airplane parked in Arizona...if you look
very carefully.
NB.
Google Earth, again.
Before the availability of this (and many other military sat.. views, and even before the U-2 aircraft looking at the Russkies...there was the B-36J.
Yes, the 'ole B-36 with ten engines, stripped down to the 'featherweight' model (look on U-tube for views) and equipped with superb cameras that could (and I have personally seen the photos, photographed from 70,000 feet...yes the 'ole B-36 could get this high...just imagine)..my neighbor, Colonel Jim flew some of these flights over China...and the golf balls could clearly be seen, on a golf course, in these photos.
Says Colonel Jim...'you could nearly count the dimples.
1950's technology, folks.