SATCOM AT SEA

The first important use of satellites support tactical naval operations was in the
Falklands War in 1982. Satellite systems had two important advantages. One was the
obvious one that intelligence information available in London could quickly be
transmitted to the deployed fleet in the South Atlantic. Another was that ships taken
up from trade for emergency use did not need specialised naval communications
equipment, at least not the extent usually envisaged. Instead they were given
commercial maritime satellite terminals. The British found some problems.

The most-publicised, helped cause the loss of missile destroyer Sheffield. Satellite
transmission is at radar frequencies, which is why satellite dishes look a lot like fire
control dishes. Just before she was hit, Sheffield was communicating with London via
satellite. The side-lobes of her satellite dish would have been detected by her
Electronic Support Measure (ESM) system. To avoid sensing non-existent targets, the
ship's ESM receiver was turned off during the communication. The British suspected
that the Argentinians might themselves be able to pick up such side-lobes, so
Sheffield, rather than the nearby carrier Hermes, transmitted.

The Argentinians used a pair of attackers, a targeter (a Neptune using a large radar)
and a shooter (the Super Etendard launched the Exocets) which hit the Sheffield.
Other ships in the British formation detected the targeting radar, which flashed on and
off as the airplane rose and dove to avoid consistent detection. Sheffield herself did
not detect anything, because her ESM receiver had been turned off to allow free
satcoms. It later turned out that she ignored the tactical picture data that would have
shown that other ships had detected and even understood the Argentine approach.

The first warning of the inbound Exocet was the light of its exhaust.
This is not just ancient history. Maritime satellites still use the frequencies otherwise
employed for fire control, so using them still interferes with electronic warning.
There was a much more basic problem: SATCOM did not deny all information to the
enemy. When the British task force arrived in the South Atlantic, it was met first by
an Argentinian reconnaissance plane (an ex-civilian Boeing 707 airliner) and then by
a mass air attack. The air attack was unsuccessful, so the British may have ignored its
implication. Somehow the Argentinians had been able to predict the position of the
task force as it approached the Falklands in the apparently trackless South Atlantic.
The track of the task force was by no means obvious, and in fact the British had
chosen a somewhat evasive one. What had happened?

It turned out that an Argentinian university had solved the problem of exploiting
satellite down-links. Typically the down-link uses a very wide beam, because the
positions of the moving receiving ships are unknown. In 1982 all satellites used
analogue signals. The down-link signal is essentially an amplified version of the up-
link. The key Argentinian perception was that the geo-stationary satellites are really not fixed in the sky. They follow a small figure-eight path. The movement of the
satellite imposes a Doppler shift on the up-link from a ship. The amount of the shift,
moreover, depends on where the ship is. The connection between satellite movement
and ship position was already being exploited by first-generation navigational
satellites such as the U.S. Transit. The Argentinians were the first to realise that this
phenomenon could be used to locate moving ships at sea.

Satellite positioning was not precise. Strike aircraft could not simply be sent out on
that basis. Hence the airliner scout. But the combination of the location and the scout
was far more than anyone outside Argentina had imagined. After the war, the idea
was demonstrated when a U.S. formation was tracked across the Atlantic.
The U.S. Government took the new phenomenon seriously. Its solution was digital
satellite that received an up-linked signal, in effect read and re-coded it, and then
transmitted a new signal which did not include any Doppler from the up-link. The
result was effective but very expensive. Most satellites are still analogue, and
formations can still be tracked (the Russians tracked the build-up in the Gulf in 1990-
91 that way). Had the Cold War continued, governments would presumably have
found it worthwhile to adopt digital satellite technology, but that did not happen, at
least not on a large scale.

The above article courtesy of the magazine Naval Forces.  Thanks Jim D