NOAA response to Vendor Questions regarding the SpWx Pilot RFP.pdf

NE-EP1000-22-00858: NOAA Commercial Weather Data Pilot (CWDP) Program

Space Weather Pilot Questions

(Blue indicates contractors’ questions)

1.1 RFPs for other Space Weather Products

Since this RFP is very specific for ionospheric TEC, do you anticipate that there will be other RFPs in the future for other space weather data products?

NOAA Response: Yes. NESDIS posts General Requests for Information (RFIs) which cover all NESDIS Level Requirements, including space weather requirements. The next general RFI is expected to be posted in the 4QFY2022. We look forward to receiving capability statements that may include other space weather data products.

1.2 Data Sources

The RFP specifically enumerates GPS L1+L2, Galileo E1+E5 and GLONASS L1+L2 dual-frequency measurements. The BeiDou system (BDS) began world-wide operation with the BeiDou3 satellite launches in 2018. BDS3 currently provides world-wide coverage with 24 middle-earth orbit (MEO) satellites operating similarly to GPS as well as higher-density regional service with an additional 6x satellites in geostationary (GEO) or inclined geostationary (IGSO) orbits over East Asia. GNOMES is currently tracking the 24x MEO and 3x IGSO BDS3 for both neutral atmosphere and space weather purposes on the B1C and B2A signals.

Our analysis shows that they are performing on par with Galileo. We would like to include this data in our proposal. Is NOAA interested in assimilating these signals? We are prepared to assist NOAA in obtaining the auxiliary data streams (transmitter differential code bias (DCB), and orbits) necessary to process these data in realtime.

NOAA Response: At this time, NOAA is soliciting only measurements using the GPS, Galileo, and GLONASS GNSS systems. Observations using other GNSS systems may be of interest in future studies.

1.3 Reference Signal Tracking

Section II.3.1 calls out a request for data from a high-rate reference satellite to support phase analysis of all scintillating occultations. GNOMES receivers are equipped with a high-stability master reference oscillator with an allan deviation of 5e-13 s/s at up to 100 seconds. This oscillator was chosen specifically to support radio occultation phase processing without differencing against reference satellites. Our neutral atmosphere processing system operates without reference signals today, and shows performance in the stratosphere*** similar to

COSMIC-2.

Is it NOAA's intention to require reference tracks for scintillation data assimilation independent of the receiver's oscillator stability?

NOAA Response: No, not at this time.

1.4 Altitude Range

Section II.2.1 specified an observational altitude range of 90 to 800 km straight-line height. As is common for low-earth-orbit (LEO) satellites, the GNOMES receivers are operating at 520 and 650 km altitude today. Space weather tracks collected by GNOMES continue across the antenna's entire field of view, including directly overhead where applicable. This aids in phase leveling, determining the total electron content in the plasmasphere above GNOMES, and in estimating receiver differential code bias. However, strictly speaking the straight-line tangent height geometry is degenerate above zero degrees of elevation.

The section II.3.2 requirement for scintillation indices calls out a range of 90-500 km. Was this also intended to be the range for the II.2.1 requirement?

NOAA Response: The requirements on track extent have been updated. Please see the revised Statement of Work.

1.5 Median Data Latency

The altitude range of interest also affects data latency. Since whole tracks include the zenith hemisphere, they are 20 minutes long (or longer). For example, on March 11, GNOMES-2 generated 2250 tracks 10 minutes long (or longer) 1730 tracks over 20 minutes long, 315 tracks over 40 minutes long, and 34 tracks over one hour long. The entire track is relevant for phase-leveling. The shortest tracks are viewed directly in front of or behind the receiver, and take only a few minutes to traverse between 90 km and the receiver's altitude, while also including a longer-duration portion above zero degrees elevation. The longest tracks are side-looking tracks which slowly perform a scan of the ionosphere which includes both vertical and horizontal motion, and may never exceed zero degrees elevation.

What is NOAA's intention regarding these longer tracks? Our belief has been that any observation epochs which fall within a strict assimilation time limit may be used to update the model's state. However, anything which is older than the assimilation time limit would still be usable for leveling carrier phase to pseudorange.

For example, a 20 minute track's most recent 12 minutes falls within the assimilation time limit.

However, all 12 of those minutes are zenith-looking (ie, above zero degrees elevation). The model would assimilate the track, but it would impact only the uppermost layers of ionosphere and plasmasphere model state. More favorably, perhaps the most recent 12 minutes of a 30 minute track include from zero degrees elevation to the horizon (90 km straight-line height) and fall within the assimilation time limit. The remainder of the track is still used for phase-leveling, but does not inform the model state.

Another example: A side-scanning track lasts 1.5 hours in total**. Portions of the track are downlinked on successive ground-station contacts. On each contact, the entire track up to the downlink time is delivered to NOAA. The model assimilates the track on each contact, using the most recent observation epochs which fall within the assimilation time limit. The track's leveling error bias (and absolute TEC accuracy) was changed and improved by incorporating more data on the subsequent contact(s).

In this world, what exactly does it mean to assimilate "500 tracks" per day with a median latency of 30 minutes? Is it also acceptable for us to propose that at least 150,000 data epochs per day will have a median latency less than 30 minutes? We'll have to do some analysis to see what we can reliably deliver, but "X data epochs per day with median latency < Y" is closer to operational reality in LEO than "X tracks per day with median latency < Y".

NOAA Response Part a: What is NOAA's intention regarding these longer tracks? The longer a track, the better we are able to level phase to pseudorange when deriving absolute TEC.

NOAA intends to, where possible, combine tracks that are split between downlinks. The latency definition and requirement in the Statement of Work have been revised. This should remove any ambiguity.

NOAA Response Part b: What exactly does it mean to assimilate "500 tracks" per day with a median latency of 30 minutes? We have revised the definition and requirement for daily median latency in the Statement of Work to reflect a more appropriate measure.

NOAA Response Part c: Is it also acceptable for us to propose that at least 150,000 data epochs per day will have a median latency less than 30 minutes? Yes. Please see the revised Statement of Work.

2.1 Straight Line Altitude Definition

The straight line altitude definition is no longer valid for altitudes above the satellite orbit altitude.

If our satellites are situated between 450 and 600 km, how should we interpret the coverage between 600 and 800 km? Should we substitute this with an elevation angle with respect to the orbit local horizon instead?

NOAA response:

The track extent requirement has been updated to account for this. Please see the revised Statement of Work.

2.2 Latency of Ionospheric Track

The SOW states that “the latency of an ionospheric track is defined as the difference between the time of each observation epoch in the track and the receipt of the data by NCCF”. This seems to imply that there will be multiple latency values computed for each track. Is this correct? Which latency value should be used to meet the requirements?

NOAA response:

The latency definition and requirement have been updated. Please see the revised Statement of Work.

2.3 Altitude Range

Is Orbit altitude acceptable for maximum altitude for on-orbit scintillation data?

NOAA response:

Yes, please see the revised Statement of Work.

2.4 Latency for onboard S4

For the On-board S4 measurements, is latency imposed on the raw scintillation index data or does the 30 minute latency requirement apply to the high-rate data as well?

NOAA response:

Yes, latency is critical for scintillation data and the 30 minute or better median daily latency applies to high rate data as well.

2.5 Reachback Support

Can the government further define their request for additional reachback support needed for Phase 3?

NOAA response: NOAA requires that the Contractor remain available during this period to potentially field technical questions, supply additional documentation, or provide engineering technical support that could aid in the development of our processing software and the analysis of the data.