Read the latest eNEWS:

The 'Transit' Satellite Navigation System

Back to all articles

27.11.2010

The Birth of Satellite Navigation

Before we had GPS, there was an earlier satellite navigation system, known as 'Transit'.

This was the first that was made available to the public by the US government back in the 1960s.

It was designed to allow US submarines to surface, determine their position and then dive, returning to inertia navigation below the waves.

This early system was totally different from GPS from the technical point of view and the constellation of satellites only numbered 5-6 in low polar orbits at an altitude of around 600 miles and with an orbital period of 106 minutes. They all encircled the earth passing over the north and south poles and one of the peculiarities of the system was that you only needed radio reception from one satellite to determine your position. Fixes were only obtained every 1-2 hours or even longer if the satellites bunched up in space – bit like buses!

The radio transmissions were on a frequency of around 400 MHz although a second frequency of 150 MHz was also used, just below the marine VHF radio channels. If one imagines the orbits as a bit like a birdcage surrounding the earth with the earth revolving anti-clockwise (when viewed from the north pole) beneath or inside the birdcage giving us our days and nights. How does it work?

When a single satellite came over the user's horizon, radio reception started and the boat's receiver locked on to the 400 MHz transmission. Two things happened: firstly over the whole of the time that a specific satellite was being received, its orbital ephemeris was digitally downloaded and stored, thus defining very accurately the orbit and hence the satellite's position in space at a given time. The second thing was that the received radio frequencies were
repeatedly measured by the receiver and stored for the duration of the satellite pass. The measurements had to be very accurate as the received frequencies differed from those which the satellite had transmitted due to its speed travelling through space (let's assume the boat/receiver was stationary for this explanation).

Doppler shift, as it is known, is what you hear when a fire engine or police car dashes passed you at the roadside and the note of its siren changes as it approaches and then as it passes the observer, it drops almost instantly. It is caused by the sound waves from the siren being compressed in the air as it approaches you and then stretched out as the vehicle proceeds away. This same phenomena applies to radio waves (or light) so with a satellite climbing in the sky until it reaches a zenith, its range from the boat is decreasing and the radio waves have been compressed to raise the received frequency. Then, as it moves towards your horizon, the waves are stretched out to lower the apparent frequency. By temporarily storing these received frequencies in the satnav receiver computer, one has a record, or 'profile' of the various changing frequencies. If the user has keyed into the computer the boat's DR (or assumed) position and the computer has stored the orbital data, the boat's satnav computer is programmed to calculate a profile of the received frequencies, based on that satellite pass, the boat's DR position on the earth's surface and for that specific satellite orbit. Now, in the real world, the boat is unlikely to be in the DR or assumed position so when the computer compares its calculated Doppler profile with what it actually received, it will find there is a difference.

What does it do? It uses what computer people call an iterative programme to keep re-calculating the profile for slightly different lat/long positions until it gets the best match between the calculated profile and what it actually received. When it has done this for the lat/long it has used to get this good match, it displays that position as the boat's position – bingo! It is really comparing two graphs of received radio frequencies – one calculated and the other real as received from that satellite. You could look at these satnavs as a sort of electronic DR calculator. The user has typed in the DR position, the computer has done the calculations and found the typed-in position is wrong and moved the boat's position around over short distances until it is a 'happy bunny' with these two 'electronic' graphs or profiles matching in shape.

There are all sorts of corrections to be done behind the scenes as the radio waves get diffracted or bent as they come through the atmosphere so that has to be allowed for. Also they slow down – yet another effect to be taken into account. The very special radio receiver in the satnav must have very, very stable tuning so the heart of the radio part is put in a small oven that is very tightly temperature controlled and allowed to warm up for perhaps an hour before the next expected satellite pass. Even the electronic components in the oven have to be carefully selected to get the require frequency stability for the receiver's tuning. Another thing of interest is that the signals from the satellites was so weak that if one had connected a loudspeaker to the radio receiver instead of the computer, you wouldn’t hear anything at all, the digital data being so quiet it was below the random noise level.

Accuracy of this old system was at its best around 100 metres give or take but when one thinks of using an old D/F radio, handheld compass bearings on a rocking boat or a sextant, this was considered marvellous. Finally, with the boat under way while the satellite measurements were being made, its DR position was changing – another adjustment to the DR position that the computer uses, and to some extent the boat's speed must upset the relative speed of the satellite. By using an electronic compass and log to feed the boat speed and heading into the satnav computer, these effects were automatically taken into account.

Those members who are familiar with celestial running fixes may see a very good likeness between traditional methods and the satnav. If you used a sextant to take the altitude of the sun several times during a morning, then a noon sight followed by a few more afternoon sextant altitudes and repeatedly transfer the boat's position on your chart by the appropriate distances and directions to finally have a position in the afternoon with lots of crossed position lines, the result is very similar to the satnav. The big difference is that the satellite's 'day' is only some 15 minutes compared to the sun's day being around 12 hours. The US government made all the technical data available to the public in around 1970 but the costs of designing such satnavs then was very high, the electronics and computing kit took up something like the space of a three drawer filing cabinet. Navstar Ltd as it became known, based in Daventry, designed what was hailed as the first yachtsman's satnav and it was launched at the London Boat show in January 1980 on the Walker log company stand as they were well known world-wide. Yours truly had been out with a BBC TV team, helping them write the script, then out in a power boat off Poole – my finger got on TV asthey didn't know how to operate the kit and the programme was screened the night before the London Show opened. The trade and opening days of that show were out of this world in terms of hype. A colleague and myself spent nearly two days being interviewed by both radio and TV crews at Earl's Court, the word 'satellite' at that time being so emotive. This was followed by me doing a live TV presentation at Pebble Mill in Birmingham. I met the programme producer and director and they said something like 'you're obviously competent to talk about it, we'll do it live'! I nearly wet myself but I took a satnav to the studio together with an old towing log and I think a sextant to demonstrate the comparison and it all went off well - a bit worrying at the time. Transit was shut down in 1996 and now of course, we have pocket-sized GPS receivers with accuracies down to around 4-5 metres – how both times and technologies have developed.

READ LESS ∧

Back to all articles

05.01.2024
Hydro Motion - H2 from NL to England in 2024

The TU Delft Hydro Motion Team wants to cross to…

Read more >

05.01.2024
The Challenges of Unpredictable Marine Energy

From military to superyacht, it is clear there is an urgent…

Read more >

Speed@Seawork 2024

Dates:
10th June 2024
Location:
Cowes, Isle Of Wight,

Read more >

Foiling and Flying RIBs

Foiling powerboat designed to meet military needs - fast, stable, silent, fuel-saving. Collaboration by SEAir Foiling Systems and Sillinger RIBs.…

Read more >