loco wrote:
Their products up to G3000 go bananas when GPS signal is lost. Red crosses on everything, including AHRS.
Hmm… I flew LLHA (Haifa, Israel) to mid Anatolia, more than 500 nmi, must have been close to 3h, under constant loss of GPS signal (under vectors over the Mediterranean and then mostly along VOR radials over Turkey), starting from the takeoff roll (I admit I had GPS signal at the parking position). My Garmin G500 TXi, and the backup AI Garmin GI275, were completely fine throughout.
But I completely agree that I really really would like to have DME/DME and VOR/DME and etc multisensor RNAV integrated into what will would then replace my Garmin GTNs.
use DME/DME for INS correction but as a navigation system in its own right. If the purpose was only for INS correction, there would be no need to sprinkle the airport neighbourhood with DME stations, would it?
Yes, wires totally crossed, because obviously I know that 
I mean I know what a TDME is used for!
lionel wrote:
My Garmin G500 TXi, and the backup AI Garmin GI275, were completely fine throughout.
Sorry. I should not generalize. I only have experience with an older G1000, a G3000 and G5000. There are more types and software versions within a single type.
I’d have thought that anyone with the right EE background (and plenty of time on their hands) could produce a functioning DME, though it wouldn’t of course be approved or approvable. The frequency range is probably a bit high for SDR so you’d need a “proper” RF design, but everything else can be done in software.
Modern Inertial Navigation Systems (INS) normal operation is to operate in a hybrid mode (multi sensor), where the primary reference is updated primarily from valid GPS and secondary from DME/DME.
In the GPS spoofing areas the operator may choose to de-select GPS and de-select INS hybrid mode from updating reference position from GPS receivers and consequently the update will be from valid DME/DME only.
johnh wrote:
The frequency range is probably a bit high for SDR so you’d need a “proper” RF design, but everything else can be done in software.
johnh, according to SparkFun at https://www.sparkfun.com/products/13001:
This is the HackRF One, a Software Defined Radio (SDR) peripheral capable of transmission or reception of radio signals from 1MHz to 6GHz.
johnh wrote:
The frequency range is probably a bit high for SDR so you’d need a “proper” RF design, but everything else can be done in software.
DME is like a reverse transponder, the aircraft transmits a pulse sequence on one frequency and the DME ground station, replies with the pulse train on a paired frequency after a fixed delay. The DME receiver locates the same pulse train and establishes the round trip time minus the delay and determines the distance. The DME has a built in random jitter so that two DME’s hitting the same ground station won’t stay in sync with each other. AIUI, the ground stations can handle about 100 aircraft.
NCYankee wrote:
DME is like a reverse transponder, the aircraft transmits a pulse sequence on one frequency and the DME ground station, replies with the pulse train on a paired frequency after a fixed delay. The DME receiver locates the same pulse train and establishes the round trip time minus the delay and determines the distance. The DME has a built in random jitter so that two DME’s hitting the same ground station won’t stay in sync with each other. AIUI, the ground stations can handle about 100 aircraft.
NCYankee, what surprise me a lot is that why not to build a ground-based network similar to GNSS (timing-based across multiple stations)?
You can even add public-key encryption to that (with the key updates sent to the world with AIP every cycle if needed), so that the spoofing will be damn near impossible.
DME is fairly old tech I’m pretty sure something new can be invented for a backup…
arj1 wrote:
why not to build a ground-based network similar to GNSS (timing-based across multiple stations)?
Propagation of typical GNSS frequencies is limited to the line of sight, so either a whole lot of ground stations will be needed, or the frequency needs to be much lower. The latter kind of system does exist and is called LORAN-C. Most stations in the world were decommissioned around 10 years ago, but there are some efforts to establish an enhanced variant called eLoran. In particular, a few months ago the UK issued a licence for an eLoran transmitting station in Swingate near Dover.
Ultranomad wrote:
Propagation of typical GNSS frequencies is limited to the line of sight, so either a whole lot of ground stations will be needed, or the frequency needs to be much lower. The latter kind of system does exist and is called LORAN-C. Most stations in the world were decommissioned around 10 years ago, but there are some efforts to establish an enhanced variant called eLoran. In particular, a few months ago the UK issued a licence for an eLoran transmitting station in Swingate near Dover.
Ultranomad, I’m fine with both options – even with LoS if you limit the reception down to a height of 1500ft, then for the most aircraft it should be OK to place stations every 60km…
