gps spoofing - researchers feed false gps signals into a ships navigation system
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GPS distortion
Most cargo ships i worked on had very basic autopilots. Usually only capable of holding a heading fed from the gyro-compass with a back up to switch over to the magnetic compass should the gyro fail (happend to me once in the Singapore Strait, ... ). Some autopilots were capable of holding a GPS course which was seldom used, only very few ships at the time had ECDIS (Electronic Charts) and were capable of following a Track, like we do in aircraft. Within lets say 24nm from the coast GPS is not normaly used for primary navigation but the radar is. Plotting radar bearings on a paper charts always got us through even the most challenging waters. Also compass bearings is a great way to get a good position, simple and reliable. Out on the ocean we generally put the ship on a heading and plotted a GPS position every 4 hours. Als long as your within about 10 to 20 nm from you intended track, it will do. Besides on the ocean you dont run the risk of GPS spoofing cause you need to be fairly close to the ship. I used to keep my skills in celestial navigation up, though mostly for the fun of it. A practiced navigator can get a position by using a sextant within 2nm of the GPS position so its more than accurate enough to get across an ocean.
Sometimes US Navy convoys would jam the GPS signals, which is a non event really, some areas is the Mediterranean have bad GPS coverage, we always got through, amzazing... Bottomline its a nice experiment, but of very little value for real life provided we still have professionals on the bridge who know how to navigate.
Sometimes US Navy convoys would jam the GPS signals, which is a non event really, some areas is the Mediterranean have bad GPS coverage, we always got through, amzazing... Bottomline its a nice experiment, but of very little value for real life provided we still have professionals on the bridge who know how to navigate.
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I dont believe that is correct either.
ADS-B data comes from the IRU. Remember that GPS tells the aircraft where it was, not where it is. The latency in the GPS system is run through a Kalman filter and coupled with other data from the aircraft, such as airspeed.
The GPS signal itself, it 300 bits transmitted over 6 seconds. So, each satellite broadcast takes 6 seconds for the entire string. (think of how far the aircraft has traveled in 6 seconds)
The filters have the ability to weed out erroneous GPS signals or ones that dont match between broadcast as the aircraft is moving, based on the estimation of where it is.
The aircraft navigation systems need a certain number of sats depending on what you are doing. The minimum of 4 sats will give you horiz and vert location with no error checking. Most aircraft systems will require at least 5 sats for navigation with redundancy. If this minimum is temporarily not met, the IRU has some ability to span a gap, but them will go into a degraded mode with drift.
Surface ships are slow moving and don't require vertical, so fewer sats are necessary. The IRU is set up with a much larger outage time and IRU drift rates were never set up for precise navigation. Therefore, for a surface ship, it is far easier to broadcast 2 sat signals with a pseudorange correction because the ship is moving so slow, and doesnt likely have a very tight filter for the error corrections.
Trying to spoof an aircraft, you would have to follow it in the air, and replace the precise signal and pseudo range error that each individual sat has. If you are not able to do that, the aircraft will disregard the signal.
Back to aircraft and ADS-B. With the Kalman filter balancing the aircraft location, rapid changes in direction and speed will affect the position accuracy. Remember back when each sat signal takes 6 seconds, and you need at least 4, so, the latency of the estimation can be upwards of 30 seconds to get all of the sat signals, balance them and estimate with aircraft speed where the aircraft is. A rapid decrease in speed or a tight radius turn while descending on final can really throw things for a loop.
BTW, some ADS-B systems have their own independent GPS antennas and are not even coupled with the aircraft FMS
ADS-B data comes from the IRU. Remember that GPS tells the aircraft where it was, not where it is. The latency in the GPS system is run through a Kalman filter and coupled with other data from the aircraft, such as airspeed.
The GPS signal itself, it 300 bits transmitted over 6 seconds. So, each satellite broadcast takes 6 seconds for the entire string. (think of how far the aircraft has traveled in 6 seconds)
The filters have the ability to weed out erroneous GPS signals or ones that dont match between broadcast as the aircraft is moving, based on the estimation of where it is.
The aircraft navigation systems need a certain number of sats depending on what you are doing. The minimum of 4 sats will give you horiz and vert location with no error checking. Most aircraft systems will require at least 5 sats for navigation with redundancy. If this minimum is temporarily not met, the IRU has some ability to span a gap, but them will go into a degraded mode with drift.
Surface ships are slow moving and don't require vertical, so fewer sats are necessary. The IRU is set up with a much larger outage time and IRU drift rates were never set up for precise navigation. Therefore, for a surface ship, it is far easier to broadcast 2 sat signals with a pseudorange correction because the ship is moving so slow, and doesnt likely have a very tight filter for the error corrections.
Trying to spoof an aircraft, you would have to follow it in the air, and replace the precise signal and pseudo range error that each individual sat has. If you are not able to do that, the aircraft will disregard the signal.
Back to aircraft and ADS-B. With the Kalman filter balancing the aircraft location, rapid changes in direction and speed will affect the position accuracy. Remember back when each sat signal takes 6 seconds, and you need at least 4, so, the latency of the estimation can be upwards of 30 seconds to get all of the sat signals, balance them and estimate with aircraft speed where the aircraft is. A rapid decrease in speed or a tight radius turn while descending on final can really throw things for a loop.
BTW, some ADS-B systems have their own independent GPS antennas and are not even coupled with the aircraft FMS
