Mode S/TCAS/PCAS/Flarm/ADS-b Whats going on!?

[TheClaggy]

I dont understand these and how the relate to each other (if at all)

What do they do? Who needs them and when (I hear Mode S is compusory from March 2012?):confused:

[BackPacker]

It is really simple. Once you cross the threshold for departure you turn them all on. And if during the flight they go BEEP you look outside to see what's going on.

But on a more serious note, a transponder emits a bunch of signals once it's being hit with an interrogation signal. These signals, in civil aviation, can be one or more of the following:
- The mode-A signal: The four-digit code you enter on the transponder.
- The mode-C signal: Your flight level
- The mode-S signal: This one is a bit more complicated. It's a bunch of signals coming from various sources, including the FMS if so equipped, and is encoded in (or using) ADS-B. This will always include your callsign and ICAO ID but may also include things like indicated airspeed, altitude selected in the AP and so forth.

The interrogation signal that activates all this can be a basic radar sweep, or an interrogation signal from an airborne TCAS unit, or a specific Mode-S selective interrogation, to which only the interrogee should respond (based on ICAO ID).

PCAS is a completely passive unit which just receives all these replies, and tries to compile a list of aircraft close by, mostly based on signal strength and mode-C (flight level) returns. TCAS does the same thing but is active (it sends out an interrogation signal itself) and is therefore much more precise and more expensive.

ADS-B is a data encapsulation standard, to the best of my knowledge. It specifies how all this info should be encoded in a transponder return, but in the US I think it's also used to upload traffic information to the aircraft as part of the interrogation signal.

Flarm is an anti-collision device that's compleely non-related to the above. It works by getting position and altitude data from a built-in GPS, and broadcasting this on a specific frequency. Other Flarm units pick up this broadcast, compare it to its own position and warn the pilot if necessary. Flarm is mostly (exclusively?) used in the Glider world.

PowerFlarm is a Flarm device and a PCAS device in one unit.

[Rod1]

Ok, that is a big question so this is a summary only.

First up, from March 2012 mode s is not compulsory for VFR flight. Right now there is no plan to make mode s compulsory for VFR.

TCAS

A traffic collision avoidance system typically costing $10k plus fited to high end GA +. I have no direct experience but you can find more;

Traffic collision avoidance system - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Traffic_collision_avoidance_system)

PCAS

Portable Collision Avoidance System. A proprietary system costing £400 for an entry level system. It is intended as an assist to see and avoid. Assisted see and avoid is said to be 8 times more effective. I have such a system and am very pleased with it.

FLARM ('flight alarm) is a system originally intended for gliders. It works on GPS location and contains collision avoidance algorithms. It is light and portable and ideal for the lighter end of GA.

FLARM - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/FLARM)

ADS-b

Automatic Dependent Surveillance-Broadcast

Automatic dependent surveillance-broadcast - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Automatic_dependent_surveillance-broadcast)

In my opinion ADS-B is the future but the time frame for this is unclear. If you have a mode s transponder (other than a Garmin 328), and an approved panel mount GPS you can “do” ADS-B now. The problem is that the CAA will charge you a major mod fee to do this so 99% of suitably equipped aircraft do not have the wire connected. This system could give us a known traffic environment out to 40 miles assuming we have a suitable receiver like Power Flarm, which combines many of the above tecs into one box.

Big word of warning, the above devices is a big step forward but will only detect suitably equipped aircraft. Only about 50% of the things you are likely to bump into have the kit you need to see them. If you research just how bad see and avoid is you will appreciate that this is a big step forward!

I have been campaigning to raise awareness of collision avoidance for some time and have given a number of talks and written a article in Flyer. I will be “testing” Power Flarm in the near future when mode C support is added. If you want a chat through all this send me a PM.

Rod1

[bookworm]

Good summary of most of it Backpacker.

ADS-B and Flarm are actually quite similar in principle. Instead of waiting to be asked, the aircraft broadcasts its position (derived from GPS), level and other info in a data packet. Both ground stations and other aircraft can receive that signal. In the case of other aircraft, they can derive the relative position, and display the traffic to the receiving aircraft. I think Flarm might even do some track processing.

The difference is that Flarm is an uncertified system that works in the Industrial, Scientific and Medical band (garage door openers usually get mentioned at this point). ADS-B can be implemented over different datalink types. The one that will be used in Europe is the 1090 MHz Extended Squitter system, the same one that transmits that data in response to a Mode S interrogation. In fact the formats differ very little. In principle many Mode S transponders will simply be able to do ADS-B over 1090ES. However some, like the GTX328, don't connect to the avionics bus, so won't be able to.

In the US, a new datalink technology was developed, called UAT. It has the advantage that in principle it has higher bandwidth than 1090ES, so the intention is to use it for other data (like weather). However, IIRC there isn't available spectrum at same frequency (978 MHz) in Europe. There's also no technical reason why it should be a cheaper box than a 1090ES box.

Mandates for Mode S and ADS-B are undoubtedly on the way (Mode S has been required for IFR in some controlled airspace for a long time now), though who will have to carry what and when is not yet clear.

[Daysleeper]

Rod1 speaks the truth,

See-and-avoid is unwise in 21st century aviation. A system designed for merchant shipping and, if you look at all the collisions there, it never worked very well anyhow.

P-FLARM seems to offer the holy grail, receiving transponders and ADS-B from powered aircraft and FLARM from gliders (and other P-FLARM users) while transmitting its own FLARM position. Portable and runs off AA bats.

The only drawback IMHO is the price tag of about £1,500 (ish) and Mode C not yet being enabled. If they can get C to work, and reduce the price by 50% then the CAA should mandate it. I can't see them (the CAA) actually doing it as the roadmap seems to be Mode S leading to ADS-B eventually (years? decades?).

I look forward to Rob1's "test" results.

[robin]

First up, from March 2012 mode s is not compulsory for VFR flight. Right now there is no plan to make mode s compulsory for VFR.


Strictly speaking that is true.

As I understand it we are still operating under an exemption issued by the CAA until April 2012. The rules state that in designated airspace Mode S will be required and that includes Class D and TMZs. There is no mention that I can find that indicates the exemption is valid beyond April. Although we may get a further extension nearer the date that allows those of us with serviceable Mode C to operate in designated airspace.

[TheClaggy]

Ok, I think i've got it...

'Mode S' is a transonder (when interrogated), sends out

your squark,
your PA, (derived from your altimeter)
your callsign
your IACO ID
other information

ADS-b (i'm still not quite sure) is like (or is it just an add-on to?)a mode S transponder but

gives the information out without being interrogated,
information is 'digitalised' and is encoded in a specific standardised way,
information is partially GPS derived,
is compatable with mode S transopnders,
may be used to replace RADAR?

Does the information from incoming signals get displayed on a screen?
mode s can work without ADSb but ADSb can not work without modes?

TCAS sends an interrogation signal (that only other TCAS equiped aircraft can reply to? or does it send an interrogation to mode S transonders? or both?) and uses the information (that mode S sends?) to display nearby aircraft on a screen.

PCAS is the same as TCAS but does not send an interrogation signal (just waits for another TCAS to send a signal) and displays the information on a screen.

FLARM is constantly sending gps derived information over short distances to other FLARM equiped aircraft (usually gliders) Power FLARM does the same but also combines the informaion with PCAS. Information is displayed
on a screen.



Tell me if i'm wrong!!!

[ShyTorque]

TCAS sends an interrogation signal (that only other TCAS equiped aircraft can reply to? or does it send an interrogation to mode S transonders? or both?) and uses the information (that mode S sends?) to display nearby aircraft on a screen.

Not quite. TCAS: Transmits a signal which sets off a response from transponder equipped aircraft. No need for the other aircraft to have TCAS or mode S.

If the other transponder has Mode C (Alt) the relative altitude will be displayed on the TCAS, otherwise just a "target" diamond will be shown. This means the pilot has no idea of the other aircraft's altitude, only some idea of relative position.