# Presentation: 749358

## TRMM Coincidence with Other Satellites

***Steve Bilanow******1******, Michael Hensley******2******,  Kwo-sen Kuo******3****** and Hezekiah Carty******4***
***1,2******RS Information Systems, LLC (RSIS), SESDA2 support of  NASA, Goddard Space Flight Center (GSFC) Code 610.2, ***
***3******Caelum Research Corporation, GSFC Code 613.1, ******4******Dept. of ******Atmospheric and Oceanic Science,*** U***niversity of Maryland***
***1******e-mail:  Steve.Bilanow@nasa.gov ***

**Satellite-to-satellite coincidences, also called satellite ground track path intersections or crossings (or by other names?), will be computed for the Precipitation Processing System (PPS) for TRMM and other satellites.  PPS is also planning a web interface to provide information on path intersections.  **

## This poster presents general information on the locations, frequency, and time delay between the path crossings:

**Specific results illustrated for TRMM and Cloudsat, an A-train satellite*****1**
**Similar results apply for TRMM and other EOS satellites*****2**** (e.g. AQUA)**
**Tools apply to any other satellites*****3****, and will be used for GPM**

- *1  The A-train satellites, including Aura, PARASOL, CALIPSO, Cloudsat, Aqua, and OCO follow the same track within minutes.

- *2  The A-train satellites all intersect the TRMM ground track at nearly the same places each day.  The crossing event distributions are similar, with various different timing details, for TERRA, Windsat, DMSP, ICEsat and other satellites in near polar orbits.

- *3  Our general tool works for any satellites that fly in different orbit planes where the paths will cross regularly..

## AbstractTitle: TRMM Coincidence with Other SatellitesAuthors:  Stephen Bilanow, Michael Hensley, Kwo-sen Kuo, and Hezekiah Carty Poster presented at 3rd International TRMM Science Conference, February 4, 2008

**      ****Tools for the calculation of near-in-time swath crossings between TRMM and other satellites are being developed at the Precipitation Processing System (PPS) at Goddard Space Flight Center (GSFC). The tools will be described, and general results about the spatial and temporal distribution of the coincident data will be presented in this poster, with a focus on specific results for TRMM and Cloudsat path crossings. For typical sun-synchronous polar orbiting satellites (like Cloudsat), the locations of the path crossings with TRMM move between higher and lower latitudes on a 46 day cycle with the orbital precession. The two satellite paths will cross these intersection locations within a couple minutes only a few times each day. The time delay between crossing the intersection points grows up to a half an orbit (47 minutes) and shrinks again daily with the beat frequency between the two orbit periods. While some science algorithms need the data to be within minutes, other algorithms can tolerate longer delay times and collect more events for analysis. The amount of coincident data collected versus time delay limits will be shown. The preliminary design for a web based tool for users to identify and predict path crossings between satellites will be described, and user feedback solicited. Existing web tools (such as one at Marshall Space Flight Center (MSFC), the Coincidence Search Engine) only show past events that are very close in time and ignores those with a time delay of more than a minute or two in passing the path-intersection point. The new tools could be of use to many more researchers, and the aim is to create outputs in a convenient format for users. **

## Orbit Planes and Precession

**Orbit Planes and Precession**

- Satellites in different orbit planes, like TRMM and Cloudsat, typically cross paths twice each orbit, however the relative phasing of the orbits determines a maximum time delay of up to 1⁄2 of an orbit period between overflights of the respective ground paths.  The latitude of the crossings will to drift north and south as the orbits precess about the Earth spin axis due to Earth oblateness.

- A sun synchronous orbit plane is near polar (typically about 98 degrees inclination) and precesses slowly about 1 deg./day to keep a fixed orientation relative to the sun, and keeps the same local time for equator crossings.

- The TRMM orbit plane, at 35 degree inclination precesses ~ -6.7 deg/day making a full cycle relative to a sun-synchronous plane once every 47 days

- Earth Spin Axis

## TRMM and AQUA paths for 1 day

**TRMM and AQUA paths for 1 day**

- AQUA, like Cloudsat, is in a near polar, 98 degree inclination, sun-synchronous orbit with a 1:30 p.m. ascending node (northbound equator crossing).  The TRMM orbit, with a 35 degree inclination, stays closer to the equator.  There are hundreds of path intersections over a whole day, but we will be interested here mainly in those that occur within about a 1⁄2 orbit period.

## TRMM and Cloudsat coincidences (path crossings) within 60 minutes on sample day, February 10, 2007

**TRMM and Cloudsat coincidences (path crossings) within 60 minutes on sample day, February 10, 2007**

- Color code indicates time period within which paths cross.

- Cloudsat ascending (northbound), daytime crossings

- Cloudsat descending (southbound), nighttime crossings

- Note:  The latitudes of the crossings are fairly constant over a single day, but they change gradually over a 47 day cycle.

## 11 days later, crossings close to equator

-  &11 days more, crossings at max. latitudes

- Cloudsat ascending (northbound), daytime crossings

- Cloudsat descending (southbound), nighttime crossings

- After 23 days day and night reverse in northern and southern crossings

##  &12 days later, crossings back near equator, and more nearly perpendicular

-  &12 days later, crossings back near equator, and more nearly perpendicular

- This pattern repeats on a 47 day cycle due to the orbit precession rates, with some variation in where the crossings with minimal delay times occur as driven by the beat frequency between the two orbit periods.
- The pattern is very similar for TRMM crossing all the A-train satellites.

## Latitude Distribution of Crossings

**Latitude Distribution of Crossings**

- For TRMM and Cloudsat, or TRMM and any near-polar orbiting spacecraft, a sharp peak in crossing locations occurs around 35 degrees South or North

## LatitudeTrends; ~ 7.7 Cycles/Year

**LatitudeTrends; ~ 7.7 Cycles/Year**

- ~47 day cycle is due to the TRMM orbit precession relative to Sun Synchronous orbits

## Annual Distribution of Crossing Locations

- Locations with < 2 minute time delay shown for day, +, and night, o, crossings

- Locations with up to 1⁄2 orbit time delay

- Estimated 2007 distribution.  Note that the distribution is not completely uniform!

## Frequency of Annual Crossings by Area

- For a 20x20 degree area around equator, expect ~ 60 / year within 1⁄2 orbit delay.

- For a 5x5 degree area at 30-35 latitude, expect ~ 30 / year within 1⁄2 orbit.

- For passes with delay times within ~ 5 minutes, expect about 1/10th as many.
- This figure provides a rough idea of how many crossings occur per unit area.

- For a 10x10 degree area around equator, expect ~ 15 / year within 1⁄2 orbit delay.

## TRMM/ A-train Delay Time Daily Cycle

- For crossings that are nearest in time, the time delay between TRMM and Cloudsat (or Aqua) passing over the same place varies as shown due to the different orbit periods:
- TRMM, 92.5 min
- Cloudsat, 98.9 min
- Difference, 6.4 minutes per orbit

- The orbits synchronize to crossing nearly at the same time about once each day.

## Beat Frequencies Between Orbits

| Orbit Feature
\
Satellite | Altitude (TRMM  at 350, then 402.5 kilometers) | Period§1 (TRMM at 91.3 then 92.4 minutes) | Beat Frequency Period with TRMM at 350 | Beat Frequency 
Period  with TRMM at 402.5 |
| --- | --- | --- | --- | --- |
| A-train (Aqua, Cloudsat, Aura, etc.) or TERRA | 
705 km | 
98.9 min | 
0.84 days | 
0.997 days§2 |
| SSMI, SSMIS,Windsat | 
~830 km | 101 to
102 min | 
~0.6 days | 
~0.7 days |

**Beat Frequencies Between Orbits**

**§1**** Approximate values given.   Varies slightly with orbit adjustments.  The period of interest is the nodal period—the average between orbit ascending node crossings—and this is different from the anomalistic or classical orbital period due to orbit precession effects.**
**§****2**** Beat frequency between TRMM and Cloudsat observed as very close to 1.0, as verified by regularity in short-delay crossing locations**

## TRMM/Cloudsat crossings with very brief observing delays are located along diagonal bands which drift gradually over the year

- A somewhat limited geographic extent per month for the short-time-delay coincident events for TRMM and the A-train satellites is a result of a relative orbital beat frequency period being very close to 24 hours.

- Night Crossing Locations

- Day Crossing Locations

- Crossing Locations with < 5 minute delay shown for January through March 2007

- Note that over a three month period, no close-in-time crossings occur over the North Atlantic or Australia

## Evolution of < 2 min. Delay locations 2007

- Jan.

- Dec.

- Feb.

- Mar.

- Apr.

- May

- Jun.

- Jul.

- Aug.

- Sep.

- Oct.

- Nov.

- Note:  Based on 2007 predicts, but general trends verified by actual data.

## Crossing Events Count vs. Delay Time

- For a longer delay acceptable in the observation of the same location, we get more “coincident” events counted as indicated in the following graphs:

- * 47 day cycle is span where latitudes cover a full range for day and night crossings

- *

- *

## Sample output listing for 2007, January 1st showing TRMM and Cloudsat path crossing times and locations.

-    TRMM and Cloudsat Path Crossing Times         Crossing Location
-  ------------------------------------------ Day/ -----------------
-  orbit    TRMM     delay to CLOUDSAT orbit/ Night (~1:30 pm or am)
- granule day time   lead/lag   time  granule Flag latitude longitude
-  #####  DDD HH:MM   minutes  HH:MM   #####   D/N  degrees  degrees
-  52012    1 00:46    -39.4   00:07    3606   D       17.4   -158.7
-  52013    1 01:33    -36.4   00:56    3607     N    -17.1      9.3
-  52013    1 02:18    -32.3   01:46    3607   D       18.1    176.5
-  52014    1 03:05    -29.2   02:35    3608     N    -17.9    -15.5
-  52014    1 03:50    -25.3   03:25    3608   D       18.7    151.7
-  52015    1 04:37    -22.1   04:14    3609     N    -18.7    -40.4
-  52015    1 05:22    -18.3   05:04    3609   D       19.4    126.9
-  52016    1 06:09    -15.0   05:54    3610     N    -19.4    -65.3
-  52016    1 06:54    -11.3   06:43    3610   D       20.0    102.1
-  52017    1 07:41     -8.0   07:33    3611     N    -20.1    -90.2
-  52017    1 08:27     -4.3   08:22    3611   D       20.6     77.3
-  52018    1 09:13     -0.9   09:12    3612     N    -20.9   -115.0
-  52018    1 09:59      2.6   10:01    3612   D       21.2     52.5
-  52019    1 10:45      6.2   10:51    3613     N    -21.6   -139.9
-  52019    1 11:31      9.6   11:40    3613   D       21.8     27.7
-  52020    1 12:17     13.2   12:30    3614     N    -22.2   -164.8
-  52020    1 13:03     16.6   13:19    3614   D       22.4      2.9
-  52021    1 13:49     20.3   14:09    3615     N    -22.9    170.4
-  52021    1 14:35     23.5   14:58    3615   D       22.9    -21.9
-  52022    1 15:21     27.3   15:48    3616     N    -23.5    145.5
-  52022    1 16:07     30.5   16:37    3616   D       23.5    -46.7
-  52023    1 16:53     34.3   17:27    3617     N    -24.1    120.7
-  52023    1 17:39     37.5   18:16    3617   D       24.0    -71.5
-  52024    1 18:25     41.3   19:06    3618     N    -24.7     95.8
-  52024    1 19:11     44.4   19:55    3618   D       24.5    -96.3
-  52025    1 19:57     48.3   20:45    3619     N    -25.3     71.0

- Sample output listing for 2007, January 1st showing TRMM and Cloudsat path crossing times and locations.
- Note that a key output included here is the identification of the orbit numbers (granule IDs)  so that users can more readily obtain the corresponding data files for further examination.
- A data flag is included to show if the crossing occurs in day or night—which is convenient for many Sun-synchronous orbits where one side is fixed toward a daytime equator crossing.
- Is there any additional information users would request included in output?

## Selecting Subsets by Various Criteria

-  ------------------------------------------ Day/ -----------------
-  orbit    TRMM     delay to CLOUDSAT orbit/ Night (~1:30 pm or am)
- granule day time   lead/lag   time  granule Flag latitude longitude
-  #####  DDD HH:MM   minutes  HH:MM   #####   D/N  degrees  degrees
-  52018    1 09:13     -0.9   09:12    3612     N    -20.9   -115.0
-  52034    2 09:53      0.6   09:54    3627     N    -16.4   -124.8
-  52065    4 10:29     -1.0   10:28    3656   D        5.2     48.3
-  52081    5 11:10     -0.3   11:09    3671   D       -0.8     38.8
-  52129    8 12:26     -1.0   12:25    3715     N     17.6   -157.6
-  52145    9 13:06      0.6   13:07    3730     N     21.9   -167.4
-  52160   10 13:01     -0.9   12:60    3745   D      -25.8     15.1
-  52192   12 13:35      0.5   13:35    3774     N     31.1   -172.8
-  52207   13 13:29     -0.0   13:29    3789   D      -32.9      9.2
-  52223   14 13:22     -0.3   13:22    3803     N     34.1   -168.9
-  52238   15 13:16     -0.4   13:16    3818   D      -34.8     12.8
-  52254   16 13:10     -0.3   13:10    3832     N     35.0   -165.6
-  52270   17 13:03      0.0   13:03    3847   D      -34.7     15.9
-  52285   18 12:57      0.4   12:57    3861     N     33.9   -162.8
-  52301   19 12:51      1.0   12:52    3876   D      -32.8     18.4
-  52347   22 11:46     -0.3   11:45    3919   D      -26.5     33.8
-  52362   23 11:39      0.6   11:40    3933     N     23.6   -145.4
-  52393   25 10:41     -1.0   10:40    3962   D      -16.2     48.4
-  52408   26 10:35     -0.0   10:35    3976     N     12.1   -131.0
-  52424   27 10:29      0.9   10:30    3991   D       -7.7     49.6
-  52454   29 09:32     -0.4   09:31    4020     N     -1.9   -117.0
-  52470   30 09:26      0.5   09:26    4034   D        6.5     63.5

**Selecting Subsets by Various Criteria**

- e.g. for crossings with < 1 minute delay

- Initial Release Design:
- 	by Date Span
- 	by Delay/Lag limit
- 	by Lat/Lon Region
- 	by Day or Night
- Possible Future Criteria:
- 	by Rain/No Rain
- 	by Land/Ocean
- 	by availability
- And other suggestions?

## Tools, Data Storage and User Interface

**Prediction Tool**:  Simplified orbit model in IDL for crossing estimates which is tuned using information from the other tools

**TLE Coincidence Computation Tool**:  A raw search in c code using TLE data

- Coincidence Database,  Predict Files and Confirmation files

**PPS web interface**:  Selection Options

**PPS web output**: Events Lists, Swath Plots, etc.

**Selection criteria**

**Combined Product Tools**:  e.g confirming TRMM/Cloudsat coincident data availability

**Selected data**

**User**

- Two-line element (TLE) files

- TRMM and other Products

**Product Orders**

***Tools are still under development.  If  interested when new PPS satellite-to-satellite coincidence information becomes available, send e-mail to Steve.Bilanow@nasa.gov***