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IFD540 - first major bug, and a possible inability to use EGNOS

SBAS providers use the identical specification. There is a coded Service Provider ID that is broadcast by each GEO satellite that is intended to be used by the receiver to determine which SBAS provider is the best choice. A value in this field of 0 is for WAAS and 1 for EGNOS. The following quote is from RTCA DO 229D:

The operational concept for GNSS and Space-Based Augmentation Systems is predicated on the combination of the different GNSS elements without pilot intervention. As GNSS is a global system, there should be no flight crew interaction based on airspace, so that the flight crew should not be involved in the selection of different SBASs (e.g., WAAS, EGNOS, MSAS).

For LPV and LP approaches that require designating a particular SBAS service provider, the FAS data block contains an SBAS service provider ID that can be confirmed against the ID broadcast in a Type 17 message. This can be accomplished transparent to the flight crew, consistent with the operational concept.
KUZA, United States

From the US site I referred to earlier, it appears that the IFD540 has a user (installer) config for WAAS v. non-WAAS. It doesn’t appear to be automatically selected.

It has been suggested that this may refer to the type of antenna. WAAS/EGNOS installations need a higher grade antenna to comply with the STC, though according to one installer they will apparently work just fine with a non-WAAS antenna. The spec of the two is really very very similar as the two following examples (combined GPS and VHF, from my TCAS installation) show: – non-WAAS WAAS Of course the latter one is about 2x the price… I find it hard to understand why the GPS would need to be told the antenna type since the gain spec (the minimum gain, anyway) is the same.

Administrator
Shoreham EGKA, United Kingdom

From the US site I referred to earlier, it appears that the IFD540 has a user (installer) config for WAAS v. non-WAAS. It doesn’t appear to be automatically selected.

This is true, see also post #5. The GPS must know if it would be operating as WAAS or non WAAS. Else you could get GPS error messages as the WAAS signal could not be processed correctly / consequent. One is allowed to use the IFD540 as a NON WAAS unit, however this limits some functions such as GPS approaches.

It has been suggested that this may refer to the type of antenna. WAAS/EGNOS installations need a higher grade antenna to comply with the STC, though according to one installer they will apparently work just fine with a non-WAAS antenna. The spec of the two is really very very similar as the two following examples (combined GPS and VHF, from my TCAS installation) show: – non-WAAS WAAS Of course the latter one is about 2x the price… I find it hard to understand why the GPS would need to be told the antenna type since the gain spec (the minimum gain, anyway) is the same.

That is partially true. I would not recommend anyone to use a non WAAS for a WAAS installation. At first it does not meet the regulations, so your installation would be illegal. The GPS receiver will receive both GPS signals and WAAS signals. The WAAS signals can have a much lower amplitude the basic GPS signal. This is why the antenna (with amplifier) has to be corrected for this. The non WAAS and WAAS antenna’s confirm to different specifications (different TSO’s) due to the difference in demands.

JP-Avionics
EHMZ

Non-WAAS antenna

WAAS antenna

The minimum gain (and therefore the gain of any two randomly selected samples of the two antenna types) is the same: 26.5db.

The noise figure of the WAAS preamp is better and its VSWR is better but I can’t see why an installer configuration is required for these aspects. The lower noise preamp is simply better. The different power supply range is moot – the supply will be 5V because the GPS can be installed with either type of antenna.

To me, this looks like a Marketing product differentiation exercise, to get 800 quid from the latter customer for what could even be the same antenna but labelled differently

Administrator
Shoreham EGKA, United Kingdom

Peter,

The non WAAS antenna may be used with the WAAS GPS if it is limited to Gamma Class 1, which enables only LNAV approaches and does not have vertical guidance.

The WAAS/Non WAAS is a pilot selectable SBAS setting on the Garmin units. I use this sometimes to simulate a failed integrity for LPV when flight training so that the pilot gets a downgrade to LNAV on an LPV approach. Apparently this is a configuration item with the Avidyne and is more of an SBAS on/off. When SBAS is on, it selects the appropriate provider with the Avidyne. As previously stated, with the Garmin system after a certain SW version, it allows the user to enable or disable any of the current SBAS providers individually. The earlier software only had a WAAS choice. In some cases, it was advised in the NE to turn off EGNOS as it might be closer than WAAS, particularly during the time there was only a single WAAS GEO.

KUZA, United States

The minimum gain (and therefore the gain of any two randomly selected samples of the two antenna types) is the same: 26.5db.

This is only partially true, with a signal within a certain dynamic range. The absolute level of the WAAS signal is typically much lower then that of the “regular” GPS signal.

The noise figure of the WAAS preamp is better and its VSWR is better but I can’t see why an installer configuration is required for these aspects. The lower noise preamp is simply better. The different power supply range is moot – the supply will be 5V because the GPS can be installed with either type of antenna.

You give the answer yourself. The WAAS preamp is better as it introduces less noise. As the WAAS signal is the lower amplitude signal this is critical. A WAAS antenna should therefore have a lower gain and a higher dynamic range than a non WAAS antenna.

Both the unit and antenna must meet the requirements of TSO C-129A for non WAAS GPS receivers and antenna’s (GNS-430 / GNS-530 for example with GA-56 antenna), and must meet the requirements of TSO C146 for WAAS GPS receivers and antenna’s. There are different certification standards that do apply for these, and different operation standards for both.

The unit needs to know it should take in account WAAS signals or not, and limit or expand options based on these signals. A WAAS GPS receiver shouldn’t do WAAS if no WAAS antenna or correct coax cable is used. As you describe yourself it can not “feel” if that is the case. In the IFD450 it therefore becomes more like a compliant / non compliant selection

Last Edited by Jesse at 16 Nov 18:12
JP-Avionics
EHMZ

The absolute level of the WAAS signal is typically much lower then that of the “regular” GPS signal.

I don’t think that is a hard truth.

According to Navipedia, the C/A signal is > -158.5dBW at 5° elevation, and Navipedia lists -161dBW for the WAAS signal at 5° elevation. The GTN650 installation manual (190-01004-02, Rev. C) lists acquisition sensitivities as -134.5dBm (-164.5dBW) for GPS and -135.5dBm (-165.5dBW) for WAAS. So we end up with a difference of 1.5dB. In practice, however, the WAAS signal is mostly seen at a very high elevation angle, while the geometrically best normal satellites are at a very low elevation angle (at least for LNAV), therefore most of the time, the difference is even smaller than that.

The Comants Peter posted have even less difference, 1.3dB is just laughable. IMO there are other requirements for WAAS antenna, notably radiation pattern (how low elevations is the antenna capable of receiving), but since the TSO standards are just pointers to RTCA documents, and RTCA wants $$$ for their specs, I don’t know the details.

I personally wouldn’t want to entrust my life in a system which has less than 10dB margin. Commercial Receivers are nowadays capable of tracking GPS signals down to -160dBm, which is -190dBW, resulting in about 30dB margin. Tracking performance for the GTN650 is specified at -144dBm (-174dBW), so about 15dB margin. Also note that state of the art commercial receivers are 15dB better than state of the art aviation receivers – aviation again lags the rest of the world (but then again aviation receivers aren’t expected to track signals within buildings, most of the time).

BTW, you don’t want too much gain in the preamplifier. The preamplifier should only have enough gain to overcome the noise figure of the GPS receiver and the cable loss. Any excess gain eats into the AGC range of the GPS receiver. I would expect 26.5dB to be good for 40m RG400 cable, thus A320 size installations, but on the high side for typical GA aircraft.

That was a long way to say that I think Peter is spot on in that the Comant thing is an exercise in product differentiation, helped or at least enabled by the authorities (through certification), with very little base in technology or physics.

LSZK, Switzerland

Does the EGNOS trouble also apply to MSAS or GAGAN or SDCM (for SNAS & SACCSA too early to tell), so is it a general non-US-WAAS SBAS glitch?
Btw: Is there an update of the SBAS global status, this one here is almost 5 years old

FAA link

local copy

especially interesting to see the actual status of the multiple frequency and when we all need new panel mount GPS devices.

Last Edited by at 10 Jan 09:01

This has been long fixed. It was a teething problem with the first IFD installations in Europe. Either EGNOS wasn’t behaving per specs or Avidyne was misreading the specs with the result that certain situations were problematic. In any case, there is no longer an issue.

LSZK, Switzerland
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