GPS replacement

[wonkywinch]

I understand that most jamming is by blitzing the GNSS frequencies with a rogue transmission. The frequency the satellites use (1200~1500 MHz) is very much line of sight so a bad actor in an adjacent country could turn on a transmitter that would radiate up into the sky and even reach the moon (I used to do moon bounce with 145MHz amateur radio transmissions).

At sea level though, line of sight is very much reduced. Even if the transmitter was on top of a 10,000 foot mountain, then range is only c 125 miles. For sea level activities in continental Europe, you are very unlikely to experience the effects of GPS jamming whereas a plane at 40,000 feet could be affected 400 miles away (by that mountain top transmitter).

PBO May 2026 has an article (page 6) on GNSS security issues.

 

[Dockhead]

My point was that I've never seen anyone who grew up with GPS worrying about it, they generally understand all of the tools and just get on with it. Every thread I've ever seen on the subject on this forum was started by older folk who don't trust the new fangled satellite stuff, often accompanied by their declaration of love for a sextant and archaic speed log.

I can't really comment.

The young sailors I'm around were mostly trained by me personally, and they know their way around a HBC.

Don't know about others.

But neither do I know any old sailors who don't "trust the new-fangled satellite stuff". How old would you have to be, for GNSS to be "new-fangled"? It's been around since my childhood, and I've been using it since the mid-90's -- that's 40 years!!

Garmin GPS-50 came out in 1994. I think I still have mine in an attic somewhere.

 

[jbweston]

I can't really comment.

The young sailors I'm around were mostly trained by me personally, and they know their way around a HBC.

Don't know about others.

But neither do I know any old sailors who don't "trust the new-fangled satellite stuff". How old would you have to be, for GNSS to be "new-fangled"? It's been around since my childhood, and I've been using it since the mid-90's -- that's 40 years!!

Garmin GPS-50 came out in 1994. I think I still have mine in an attic somewhere.

I'm not yet in my dotage, but when I first tried my hand at coastal navigation and pilotage in around 1970 our only aids (apart from the ship's compass, charts, Reeds and pilot books) were a Seafarer whirligig echo sounder (excellent), a Sestrel hand bearing compass (not bad), and a Dutchman's log (bits of torn up envelope to throw overboard by the bow and a wristwatch seconds hand to time them as they floated past). The first satnav I saw was in around 1978 on board a Dutch salvage tug that had state-of-the-art equipment. I'm not sure but think this was Transit, and it gave fixes only every few hours.

So I think there are plenty of us out there that cut our teeth in navigation long before GNSS was available. That doesn't mean we haven't moved with the times - I seem to have more GPS receivers on my boat than I can count. And I'm sure many of us oldies have learned to use GPS and rely on it. Why wouldn't we. It's fun learning new stuff and being able to know where we are whenever I want still amazes me.

 

[st599]

I understand that most jamming is by blitzing the GNSS frequencies with a rogue transmission. The frequency the satellites use (1200~1500 MHz) is very much line of sight so a bad actor in an adjacent country could turn on a transmitter that would radiate up into the sky and even reach the moon (I used to do moon bounce with 145MHz amateur radio transmissions).

At sea level though, line of sight is very much reduced. Even if the transmitter was on top of a 10,000 foot mountain, then range is only c 125 miles. For sea level activities in continental Europe, you are very unlikely to experience the effects of GPS jamming whereas a plane at 40,000 feet could be affected 400 miles away (by that mountain top transmitter).

PBO May 2026 has an article (page 6) on GNSS security issues.

There's quite a problem in countries which apply tolls via GPS or monitor driver hours. In the US, you can't rely on GNSS near interstates.

Local spoofing has also been seen in London and the south coast

 

[lustyd]

Local spoofing has also been seen in London and the south coast

And in every case it was quickly corrected because it’s trivial to trace and it’s considered critical infrastructure.

 

[jbweston]

I’ve been thinking about how dependent we all are on GPS these days, especially offshore.


Out of interest:


• How many of you are actually concerned about GPS being unavailable or unreliable at sea?
• Does anyone here regularly use a sextant as a backup?
• For those who don’t — is that down to cost, complexity, or just not feeling the need?


Genuinely curious how people are approaching backup navigation these days.

Going back to the original questions, I think our answers will depend on where and when we are likely to be at sea. Now that my activities are coastal in the UK and usually in the light in good-ish weather, my answers are:

How many of you are actually concerned about GPS being unavailable or unreliable at sea? For myself, not at all as (a) it's unlikely to happen or if it does it will be short-lived and (b) if I lose GPS position data I still have electronic charts as well as paper backups, and can fix position adequately by eye using bearings, recognising landmarks and that very helpful feature in UK waters (but not in some other countries) the names or numbers painted on navigation buoys.

Does anyone here regularly use a sextant as a backup? I don't.

For those who don’t — is that down to cost, complexity, or just not feeling the need? It's because they're of no practical use for coastal navigation in most circumstances. Using a sextant for astro navigation isn't as accurate as a visual fix. In theory a sextant be used to provide visual fixes by horizontal or vertical angles from charted features but I've never done that and I can't imagine many have unless surveying or trying it out for fun. It would be different if I were making ocean passages when astro is the only practical alternative to GPS.

So for what I do at sea loss of GPS would be a minor inconvenience. Interestingly, loss of GPS when driving my car on a longer journey would be much more of an inconvenience, and when trying to find an address in a strange town it would be a major hindrance particularly if I lost the signal without warning.

 

[st599]

Going back to the original questions, I think our answers will depend on where and when we are likely to be at sea. Now that my activities are coastal in the UK and usually in the light in good-ish weather, my answers are:

How many of you are actually concerned about GPS being unavailable or unreliable at sea? For myself, not at all as (a) it's unlikely to happen or if it does it will be short-lived and (b) if I lose GPS position data I still have electronic charts as well as paper backups, and can fix position adequately by eye using bearings, recognising landmarks and that very helpful feature in UK waters (but not in some other countries) the names or numbers painted on navigation buoys.

Does anyone here regularly use a sextant as a backup? I don't.

For those who don’t — is that down to cost, complexity, or just not feeling the need? It's because they're of no practical use for coastal navigation in most circumstances. Using a sextant for astro navigation isn't as accurate as a visual fix. In theory a sextant be used to provide visual fixes by horizontal or vertical angles from charted features but I've never done that and I can't imagine many have unless surveying or trying it out for fun. It would be different if I were making ocean passages when astro is the only practical alternative to GPS.

So for what I do at sea loss of GPS would be a minor inconvenience. Interestingly, loss of GPS when driving my car on a longer journey would be much more of an inconvenience, and when trying to find an address in a strange town it would be a major hindrance particularly if I lost the signal without warning.

For personal navigation, I'm not worried at all.

But as has been linked to, the effect of a GNSS outage would be catastrophic elsewhere with power, water, telecommunications, finance and logistics all reliant - the effects would be felt. The government reckon a 24 hour outage would cost the UK more than £1bn.

 

[lustyd]

For personal navigation, I'm not worried at all.

But as has been linked to, the effect of a GNSS outage would be catastrophic elsewhere with power, water, telecommunications, finance and logistics all reliant - the effects would be felt. The government reckon a 24 hour outage would cost the UK more than £1bn.

None of those things are reliant on GNSS. Finance was about 25 years ago but has long since stopped relying on GPS for timing signals (the only bit they ever did use!). As for the others, that’s just absurd to even suggest.

 

[st599]

None of those things are reliant on GNSS. Finance was about 25 years ago but has long since stopped relying on GPS for timing signals (the only bit they ever did use!). As for the others, that’s just absurd to even suggest.

Yep, but the UK government doesn't agree with you.

https://assets.publishing.service.g...derived-time-and-position-blackett-review.pdf

Financial services
GNSS, and specifically GPS, has become the primary time source in financial services. It is
used for:
Clock synchronisation. Stock exchanges check the GPS timestamp of received electronic share trading transactions against their own clocks to ensure that the orders are “fresh”. If the time difference is above a given threshold, they are rejected.
Performance monitoring and service level tracking. In recent years, latency of trading (the time taken to perform an operation) became a key competitive tool. The ability to measure your own and others’ performance accurately attained a new value, driven by minimising latency and the “race to zero”.
Regulatory controls. Perhaps the single largest cause of increasing GNSS dependence has been European and international regulation. From the need to timestamp all reportable events, such as trades, to the forensics of market abuse, accurate time is becoming essential to the operation of a stable financial market.
ATM and credit card transactions. ATM networks need accurate time to provide a clear record of transaction and to aid in fraud investigations, correlating to the time on local CCTV and other devices. One documented case where clocks were not synchronised led to a wrongful arrest

They also list critical GNSS requirements for road logistics, energy supply, telecommunication, food production etc.

They have also just spent £155m on a resilience programme

UK Commits £155 Million to eLoran, Timing and GNSS Monitoring in Major PNT Resilience Push

And the latest UK register of risks says loss of PNT, of which GNSS is the major one in the UK:

There would be a significant disruption or complete cessation of transport
(including aviation and maritime services), communications networks,
financial services, energy and emergency services within a few hours of the
incident taking place.

https://assets.publishing.service.g...e/1175834/2023_NATIONAL_RISK_REGISTER_NRR.pdf

 

[S04]

None of those things are reliant on GNSS. Finance was about 25 years ago but has long since stopped relying on GPS for timing signals (the only bit they ever did use!). As for the others, that’s just absurd to even suggest.

An alternative view is held by others, for instance, Professor Washington Yotto Ocheng.

"Professor of Engineering at Imperial College London, and President of the Royal Institute of Navigation. Over a career bridging industry and academia, he has helped shape the movement of urban transport; how satellites guide us and locate us; and how governments manage the technologies underpinning so much of modern life."

In the most recent edition of "A Life Scientific" on BBC4 he claimed that the cost to the UK of widespread GNSS denial would be around £70bn for a 5-day outage (c. 6 minutes into the programme on BBC Sounds) due to disruption in transportation, energy, communications etc. He gave the impression to me at least of being someone who knew what he was talking about having been involved in the government PNT resilience initiatives.