Sat nav the early years.

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..... The geoid WGS-84 was agreed in 1984, it was and still is the reference Geoid for GPS.......

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Being pedantic, WGS84 is not a geoid it is a geodetic datum. Its origin is the best estimate of the centre of the earths mass and its axes are defined by definitions of the zero meridian and polar axes. Associated with this is the WGS84 ellipsoid. This has semi-major and minors axes aimed at providing the best global estimate of mean sea level. In practice, and due to variations in density and topography, MSL can vary more than 80m above or below this ellipsoid.

The geoid is a model of the deviations of MSL from the reference ellipsoid. Most GPS receivers will output WGS84 latitude and longitude plus MSL height and the geoid separation as part of the NMEA GGA message.

Until very recently the global geoid model associated with WGS84 was EGM96. Last month this was updated by a refined model known as EGM2008.
 
I used the Magnavox MX2102 in about 1980. From memory we got a fix about every 1.5 - 2 hours. In between it dead-reckoned using inputs from gyro compass and log. It was state of the art at the time and gave position to about 1/10th mile.
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MX2102.jpg
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I always thought that Decca was the standard before GPS. It was good enough for `survey´ standards even if the charts were a bit difficult to follow due to the transmission patterns. The later sets, after electronics moved on, were small and quite accurate? Loran was limited by areas. Consol I tried, and it was clumsy and not very accurate (60s)
We fitted a transat system to the co. yacht. Just before they set off for the caribbean it refused to find a fix. Generated loads of confidence... This was about 83'?
A
 
Yep I remember the Magnavox system, although I seem to remember it being a lot bigger than the one pictured. Used it down the Falklands when I was comercial fishing, it used to get a reliable fix abour every 6-8 hours then run on DR. Back up was the 'new DECCA' (Mk 50??) which automaticlly converted the raw red/green/purple lines to Lat & Long. Magic stuff it was. Although it still account for thr fixed and variable errors.................or night time.

All of it a lot better than the DECCA Mk 12 which needed half a boat to store the 'magic boxes'.
 
I bought a 'Navstar' Satnav (they also made Decca units) at Southampton Boat Show in 1988 for (I think) GBP 500 incl VAT, and it seemed to be very modern and fast at the time, and good value.....
It was a bit cranky, could (did) take ages to acquire satellites, but we were very happy with it.
Bought a Garmin GPS72 almost exactly 20 years later in St Maarten for approx US$ 150....
 
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The only satnav available to civilians before GPS was Transit.
Loran, Decca, Chayka, Consul, RDF etc were/are all terrestrial systems (i.e. nothing to do with satellites). Glonass is operational (after a fashion) but post-dates GPS, and other GPS-like systems such as Galileo are not yet operational.

Transit used about half a dozen satellites in fairly low polar orbits. A receiver calculated its position from the Doppler shift -- the difference between the frequency transmitted by the satellite and the frequency received by the receiver.

The same principle is used in 406MHz epirbs, except that it is reversed: the epirb does the transmitting and the satellite does the calculating.

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Glonass was "re-aligned" to aid the fight in Chechnya and other small pockets and so delayed in it's final set-up for global use. It is now reported as being put back on track for global solution.

Transit had more than 6 sats ... I believe it was nearer 18 sats in all ... but of course I'll stand corrected.
 
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Glonass was "re-aligned" to aid the fight in Chechnya and other small pockets and so delayed in it's final set-up for global use. It is now reported as being put back on track for global solution.

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The biggest problem with Glonass has been reliability of the satellites. They have had a far lower life expectancy than their GPS equivalents and have had trouble launching them faster than they fail.
These difficulties seem to have been partly resolved and as of today there are 17 Glonass satellites in orbit with 3 more due to be launched on Christmas Day. Although this is still not sufficient to provide GPS-like 24 hour coverage, there are several survey grade GNSS (Global Navigation Satellite System) receivers on the market that track both GPS and Glonass satellites and provide a combined position solution.
The main benefit of the combined solution is a stronger fix due to more satellites being in view and better geometry. I suspect that in the not too distant future leisure marine type receivers will available that combine GPS and Glonass with Galileo joining them sometime later.
 
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The final Transit constellation comprised 6 satellites in approx 1125 km high polar orbits. The orbital period of each satellite was 107 min.
Transit_Orbits.jpg


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Operational - but I'm sure there was like GPS system - redundancy built in that they switched on other sats to take over failed ones that they could track / data acquire and switch off if needed.

The biggest problem with Transit was in areas as we were - China Seas - the sats would be spread out in reasonable time periods, but over a tim of 10 - 14 days they would bunch up so we got a load of fixes in a run ... then wait ages for next.... they would then over days spread out again.
We were running 85,000 m3 Methane Gas tankers that needed accurate pinpoint nav and satnav was fitted at great cost to provide it from Brunei to Japan on liner service. Part way up Palawwn passage we still used echo-sounder to determine one turn point. It was a reef that had a wreck sunk on it, we would steam up and watch that KH sounder trace for that tell-tale rise in bed ... it would reach xx fthms and we would turn.

The machine we ended up with on that work when I left in '80 was the Magnavox 1102 - not the one above with Omega addition - but the Transit only version.

Back to Glonass .... the info passed to me unofficially is that re-aligment to aid Russian military effort took precedent over any further development or deployment of system. I prefer not to go into source of my info - suffice it to say that I live and work with people that were involved. I could say a lot more about various statements made on these forums about Russia and military but choose not to for good reason - I live here !
 
Transit always gave best coverage closer to the poles with big gaps between fixes nearer the equator. Additionally the satellites were not all launched into perfect circular polar orbits with the result that the orbital planes precessed from their planned positions.
Transit_Orbits_2.jpg

Diagram above was situation in 1978 when there were only five satellites in orbit. There were also periods where satellite signals were disabled because they were too close to another and causing interference.

I'm sure you're correct about the Glonass priorities. Just like Transit and GPS, it was initially a military system.
 
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