Electronically Scanned Thinned Array Radiometer

The potential of low frequency microwave radiometry in remote sensing of soil moisture has been recognized for quite some time. Recent advances in antenna technology have opened up a development path that will likely see a soil moisture mission in five years. In preparing for future L band (1.4 GHz, 21 cm) soil moisture satellite missions, investigators have employed ground, aircraft and satellite sensors. Of the satellite sensors, there has been only one instrument that has provided any heritage at L band, the Skylab S-194 instrument. This sensor was part of the Earth observation program on Skylab in the 1970s. Data have been analyzed and reported in a few applications (Eagleman and Lin 1976, Lerner and Hollinger 1976, Wang 1985). These results, especially those of Eagleman and Lin (1976) have been valuable. However, they utilized only a portion of the remotely sensed data collected.

Efforts to recover Skylab S-194 data revealed that no permanent archive was ever created. Whatever data exists would reside with investigators that were involved in the original mission. Our search revealed that the only comprehensive data that existed was that collected by Eagleman and Lin (1976). This data set was recovered in a digital format and is now available through a data archive. This document describes the Skylab S-194 and the data sets. The data are of particular relevance to SGP97 and SGP99 because they focus on the same region.

Skylab and the S-194 Sensor

Skylab was launched on May 14^th, 1973 in a near-circular Earth orbit. Orbital inclination was 50^o and the nominal altitude was 435 km. The orbit period was 93 minutes. Upon completion of the project, Skylab was positioned into a stable attitude and shut down. On July 11^th, 1977 Skylab impacted the Earth.

Data collection required astronaut operations. Astronauts were onboard during the following periods May 14-June 22, 1973, June 28-September 25, 1973, and November 16, 1973-February 8, 1974. During these periods Earth observation accounted for the following amount of the mission time; 71.4 hours-18.2%, 223.5 hours-20.6%, and 274.5 hours-17.6%

A variety of remote sensing instruments operated on Skylab. These included the S-190 multispectral photo facility, S-191 infrared spectrometer, S-192 10-band multispectral scanner, S-193 13.9 GHz active and passive microwave instrument, and the S-194 1.4 GHz microwave radiometer. Much of the photography is archived at the EROS data center (http://edc.usgs.gov/Webglis/glisbin/guide.pl/glis/hyper/guide/manned). No archives of the S-193 or S-194 data exist.

The S-194 microwave sensor was a nadir viewing L-band H polarization radiometer. It utilized a fixed planar array antenna oriented toward nadir, recording thermal radiation at a frequency of 1.4 GHz (21-cm wavelength) and measuring the absolute antenna temperature. The sensor used a calibration scheme referenced to fixed hot and cold load input. The precision of the temperature measurement was 1 deg K.

Half-power beam width of the sensor was 15 degrees, which provided a swath width or resolution footprint of approximately 110 km at an orbital altitude of 435 km. Data were recorded approximately three times per second, which resulted in a distance between centers of two consecutive resolution cells on the ground of 2.5 km. There was a minimum of 97% ground coverage overlap.

Lerner and Hollinger (1976) analyzed several aspects of the data quality of the S-194 sensor, which included an evaluation of the absolute accuracy and stability. Over the course of the entire series of data collection passes (7 through 98) they found that the standard error was approximately 1.3 deg K with close to zero bias.

S-194 Earth Observation Data Sets

As noted above, there were several Earth remote sensing experiments conducted using the S-194 instrument. These included coverage of specific targets and also larger regions that were not the focus of the specific experiment. Lerner and Hollinger (1976) state that they had data from 27 passes for their water studies. Wang (1985) used data from 12 passes over Texas. There were probably other data sets collected, however, none of these could be located.

Efforts at recovery revealed that all S-194 data from the Skylab experiments were distributed to the investigators and that there was never a permanent archive. The only data we were able to obtain were those used by Eagleman and Lin (1976). Figure 1 summarizes the tracks of the S-194 data available to us. These data sets were collected for either soil moisture (June 5, June 13, August 5, August 8, September 13, and September 18, 1973) or snow investigations (January 11, January 14, and January 24, 1974). Original data tapes could not be located and we concluded that they were no longer available. Only paper copies of the original data products were available.

Each day of data collection was contained in a book that consisted of sheets produced by an impact printer. Each book was made up of three parts; radiometric antenna temperatures, raw data tabulations (Engineering units), and Ephemeris data. The radiometric antenna temperature data consisted of numerous columns of data used to compute the brightness temperature that were referenced to time. The second part of the book contains the raw data tabulations. The final section of these books contained a record of time versus geolocation information on the satellite and the sensor footprint. The data set that was useful for further geophysical analyses is a record of time, latitude, longitude, and T_B. Therefore, we focused our efforts on recovering the first and last portions of the data books.

The original paper copies were all scanned and edited. Copies available to us were reproductions of impact printer outputs typical of computer products of the era and required extensive editing. It should be noted that only the time and T_B of section 1 and the time, latitude and longitude of section 3 of the data books were quality controlled.

As noted, the final data set desired was one of time, latitude, longitude and T_B. This required the merger of information from sections 1 and 3 of the data books. The time intervals of the records in these two sections were not identical and, therefore, it was necessary to interpolate values of latitude and longitude for each T_B observation based upon time. Satellite position data were available on a less frequent basis than the T_B data. For each day of data, a second order polynomial was fit to the time-latitude and time-longitude data. These equations were then used to predict latitude and longitude at the time of each T_B observation.

The performance of the geolocation method described above was evaluated for each date according to curve fitting statistics. Based upon a nominal estimate of one degree being 100 km at mid latitudes, the expected geolocation error would be less than 5 km on average. For a 110 km sensor footprint this was judged to be negligible.

Soil Moisture Ground Truth During the Skylab Missions

Eagleman and Lin (1976) conducted ground based soil moisture sampling to coincide in space and time with specific S-194 data takes. The sites were 300-400 km in length and approximately 125 km in width. Samples were collected at 2.5 cm depth intervals to a maximum of 15 cm.

Of the Skylab soil moisture experiment data takes plotted in figure 1, two were not useable in analyses of T_B and soil moisture. On August 5 there was an error in the satellite positioning information that resulted in no S-194 data being collected over the area that was sampled on the ground. Immediately following the September 18 S-194 data take there was a rainfall event over the ground-sampling site that makes comparisons between the ground and satellite data invalid. It should be noted that there is nothing wrong with the quality of the S-194 data on these two dates and that they can be used for soil moisture retrieval, they just can't be compared to the ground data sets available.

General features of each useable soil moisture data set are described below. Note that on September 13 two separate ground sites were sampled.

June 5, 1973: The test site was located in Texas between Lubbock and San Angelo. Landscape features were different in the northern and southern portions of this area. The northern portion is part of the High Plains, which is characterized by a very flat topography. The area is well-drained and has relatively fertile sandy soil, which has permitted agricultural development over most of the region. The dominant crop was cotton and to a lesser degree grain sorghum. The southern portion has rolling and broken topography. There are deep steep-sided arroyos and small steeply sloping mesa. Most of the area was uncultivated and the vegetation was generally shrubs, with some trees. The shrubs varied in height from 50 cm in dry areas to 500 cm in wet areas. The Skylab overpass was between 2:00:30 pm and 2:02:00 pm local time. Gravimetric soil samples were collected between 8:30 am on June 5 and 12:30 pm on June 6, 1973. There was no rainfall after Skylab passed this area before the soil samples were taken.

June 13, 1973: The data were from a transect between Concordia and Emporia, Kansas across the Flint Hills region. The landscape is characterized by rolling topography with some substantial local relief in some parts of the site. The soil is very rocky with gravel-sized material on the surface and throughout the soil profile. Vegetation in this area consists mainly of grass prairie with heights ranging from 50 cm to 200 cm. Most of the site is used for grazing with a small portion in cultivation. The Skylab data over the Kansas site were collected between 7:51:22 am and 7:57:57 am local time. Soil samples were taken between 8:30 am and 4:30 pm on June 13 1973. Several thundershowers occurred on June 12 within the test site before the Skylab and soil sample data collection, resulting in very high soil moisture levels.

August 8, 1973: The coverage area was parallel to the June 5 Texas data take, offset by approximately 60 km. Ground conditions are typical of cultivated and grassland areas. Skylab passed over the test site between 10:04:00 am and 10:05:30 am local time. The soil samples were collected between 8:30 am on August 7 and 12:30 pm on August 9, 1973. There was 0.1 to 0.2 cm of rain in the early morning of August 8 in the northwest portion of the test site.

September 13, 1973 (Texas): The test site was perpendicular to previously described Texas sites. A small portion of these sites overlapped. A major difference between the data set for this site and the others was the stage of crop growth. In June there was a good deal of bare soil as opposed to September when there was a dense canopy. The southwestern portion of the site had a greater amount of natural vegetation. Topography is flat across the entire site. The soil is sandy. It becomes sandier towards the southwest with dunes and sparse vegetation. Vegetation was short drought resistant grass. Skylab passed over the site between 11:57 am and 12:00 pm September 13. Soil moisture sampling was conducted between 8:30 am on September 8 and 12:00 pm on September 10, 1973, which is a rather long interval. There were varying amounts of rainfall and evaporation in the different locations within the test site during the period of September 8th to 13th.

September 13, 1973 (Kansas): The transect extended from Wichita, Kansas to Springfield, Missouri. The topography is generally rolling with moderate local relief. The area was largely uncultivated and dominated by natural vegetation, consisting of short to medium grasses on the west side, shrubs and some large deciduous trees. The density of trees increased to the east. No photos were available for this data take. The soil in this site is more variable than in the other sites. Skylab data were collected between 11:59:21 am and 11:59:51 am local time. The soil samples were obtained between 10:00 am on September 13 and 5:00 pm and September 14, 1973. There were heavy thunderstorms in the region prior to the Skylab and soil sample data collection.

The Data and Files

There are two directories. The S194Lbanddata directory contains the combined Time, Brightness Temperature, Latitude and Longitude S-194 data files for each day. There is a single ASCII file for each date labeled S194MDDYY.xls where M=month, DD=Day, YY=year. The variables in each file are

The S194soilmoisturedata directory contains data scanned and digitized from the final report to NASA by J. R. Eagleman titled "Detection of Soil Moisture and Snow Characteristics from Skylab", October 1975. The gravimetric soil moisture data tables were scanned and converted to ASCII files. Plots of the locations of the sample points were digitized to determine the geographic coordinates of each. These two data sets were combined to generate the files included here. Each file is labeled SMXMDDYY.prn where X=T(Texas) or K=Kansas, M=month, DD=day, and YY=year. The date information corresponds to the Skylab pass date. The data included are as follows:

Data Access and Contacts

FTP Site

The SKYLAB data files reside on DAAC anonymous FTP. You may access them from this document:

ftp disc.gsfc.nasa.gov
login: anonymous
password: < your enail address >
cd /data/sgp99/ancillary

Contact

T. Jackson or A. Hsu
USDA ARS Hydrology and Remote Sensing Lab
104 Bldg. 007 BARC-West
Beltsville, MD 20705
(301) 504-8511
Fax (301) 504-8931
tjackson@hydrolab.arsusda.gov

EAGLEMAN, J.R., and LIN, W.C., 1976, Remote sensing of soil moisture by a 21-cm passive radiometer. Journal of Geophysical Research, 81, 3660-3666.

JACKSON, T.J., HSU, A.Y., VAN DE GRIEND, A., and EAGLEMAN, J. R., 2004, Skylab L band microwave radiometer observations of soil moisture revisited. Int. J. Remote Sensing, 25, 2585-2606.

Lerner, R. M., and HOLLINGER, J.P., 1977, Analysis of 1.4 GHz radiometric measurements from Skylab. Remote Sensing of Environment, 6, 251-269.

MCFARLAND, M., 1976, The correlation of Skylab L-band brightness temperatures with antecedent precipitation. Preprints of the Conference on Hydro-meteorology, AMS, 20-

Wang, J.R., 1985, Effect of vegetation on soil moisture sensing observed from orbiting microwave radiometers. Remote Sensing of Environment, 17, 141-151.

S-194 Coverage of Test Site				Ground Sampling of Test Site
Start Day	Site	Start Time (UTC)	Stop Time (UTC)	Start Day	Start Time Local	Stop Day	Stop Time Local
6/5/73	TX	18:00:49	18:01:38	6/5/73	08:30	6/5/73	12:30
6/13/73	KS	13:51:22	13:51:57	6/13/73	08:30	6/13/73	16:30
8/7/73	TX	16:04:31	16:05:14	8/7/73	08:30	8/9/73	12:30
9/13/73	TX	17:57:25	17:58:01	9/8/73	08:30	9/10/73	12:00
9/13/73	KS	17:59:21	17:59:52	9/13/73	10:00	9/14/73	17:00

Figure 1. Ground tracks of S-194 data collected for Eagleman and Lin (1976) and available for the current study. To distinguish overlapped ground tracks, different symbols were used.

Skylab S-194 L band Microwave Radiometer Observations