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High resolution and wide swath, which are related to imaging quality and observation efficiency, are the key specifications for spaceborne synthetic aperture radar (SAR). Owing to the restrictions of the Nyquist sampling theorem, it is difficult to improve both specifications simultaneously. The increase of the swath width often leads to the decrease of the spatial resolution, e.g., in scanning SAR and terrain observation with progressive scan SAR. For a sparse scene, an image containing only a few targets has massive data but little useful information. This paper proposes a novel SAR observation mode, AgileSAR, which is based on the time–space sampling method and can overcome the limitations of the Nyquist theorem. It also increases the swath width while preserving the resolution of the sparse scene. AgileSAR steers the antenna beam towards a different sub-swath, generally after one or two pulse intervals, and the average pulse repetition rate corresponding to every sub-swath is much lower than that determined by the Nyquist theorem. Compared with Sentinel-1, which can achieve 5-m resolution and 80-km swath, a single azimuth-channel AgileSAR system can achieve 5-m resolution and 300-km swath for a sparse scene, once the corresponding system parameters are designed. The $\text{l}_{1}$ relaxation method is used to reconstruct sparse SAR images, and the reconstruction performance is quantitatively analyzed based on the estimation error. The simulation results validating the proposed method with sub-Nyquist samples can achieve approximately similar performance as conventional SAR with Nyquist samples.

We adapted time-space sampling to enroll men who have sex with men (MSM) off Craigslist.org for face-to-face interviews. Men responding to our ads ( n = 322) were instructed to either complete an online pre-screening survey (to determine preliminary eligibility) or call our office directly. Of those taking further initiative to enroll, 29 % ( n = 41) called directly and 71 % ( n = 101) opted to first complete the online survey. Participants scheduled via online pre-screening were more likely to present for their face-to-face assessment than men deemed eligible directly via phone screening (72.3 vs. 47.1 %). Online pre-screening was a useful tool to offer potential participants when recruiting on Craigslist and improved study enrollment. Resumen: Hemos adaptado un muestreo de 'time-space' con el propósito de inscribir a hombres que tienen sexo con hombres (HSH) usando Craigslist.org, para hacer entrevistas en persona. Los hombres que respondieron a nuestros anuncios ( n = 322) fueron instruidos a completar una encuesta por internet, que ayudaría a determinar la elegibilidad preliminar, o llamar directamente a nuestra oficina. De los que tomaron la iniciativa de inscribirse, 29 % ( n = 41) nos llamaron directamente y 71 % ( n = 101) decidieron en completar la encuesta por internet. Los participantes programados para una cita usando la encuesta por internet fueron más propensos a presentarse para su entrevista en persona que los hombres considerados elegibles directamente por la entrevista de teléfono (72,3 vs 47,1 %). Usando la encuesta por internet fue una herramienta útil de ofrecer a los posibles participantes al reclutar por Craigslist.org y también fue algo que aumento inscripción a esta investigación. [ABSTRACT FROM AUTHOR]

Data are presented from the Baltimore Young Men’s Survey, a cross-sectional, venue-based sample survey of risks associated with HIV and report of a prior HIV test, conducted between 1996 and 2000, and enrolling 843 young men who have sex with men (MSM) aged 15–29 years. HIV prevalence was 12.1% overall and racial disparities in HIV prevalence were pronounced (range, 2.9% among non-Hispanic whites to 27.1% among non-Hispanic blacks). Risks independently associated with being HIV-positive were: being between 26 and 29 years of age, being non-Hispanic black or of other/mixed race, having had 20 or more lifetime male sex partners, having been diagnosed with a sexually transmitted disease (STD), and not being currently enrolled in school. The majority of participants (78.9%) reported a prior HIV test. In multivariate analysis, being older, having had five or more lifetime male sex partners, having had anal intercourse with males, reporting an STD diagnosis, and reporting recent unprotected anal sex were associated with report of a prior HIV test. Prevention efforts must address high HIV prevalence among young non-Hispanic black MSM and must make testing and effective counseling for young MSM readily available. [ABSTRACT FROM AUTHOR]

The Club Drugs and Health Project was supported by a grant from the National Institute on Drug Abuse (R01-DA014925-02, Jeffrey T. Parsons, Principal Investigator). Christian Grov was supported as a postdoctoral fellow in the Behavioral Sciences training in drug abuse research program sponsored by Public Health Solutions and the National Development and Research Institutes, Inc. (NDRI) with funding from the National Institute on Drug Abuse (T32 DA07233). The authors recognize the contributions of the Club Drug and Health Project team—Michael Adams, Virginia Andersen, Anthony Bamonte, Jessica Colon, Armando Fuentes, Sarit A. Golub, Chris Hietikko, Eda Inan, Juline Koken, Jose E. Nanin, Anthony Surace, Julia Tomassilli, Jon Weiser, Brooke E. Wells, and the recruitment team. An earlier version of this paper was presented at the 2008 meeting of the College on Problems of Drug Dependence (CPDD). Though some researchers have indicated club drug users are more likely to be polydrug users, there remains little known about the prevalence and specific combinations of the substances they use. Between 2004 and 2006, and using time-space sampling, a stratified sample of 400, 18-29-year-old New York City club-going, drug-using young adults were recruited into the Club Drugs and Health Project. Most participants (91.7%) had engaged in polydrug usage and 1,670 combinations of drugs were reported. Ecstasy (86.6% of users) and cocaine (85.7% of users) were the two most-frequently reported club drugs used in combination with other substances. In terms of poly-club drug combinations, ecstasy appeared to be the “universal compliment” as this drug was most often cited in combinations with other club drugs (specifically ecstasy + ketamine, ecstasy + cocaine, ecstasy + gamma hydroxybutyrate or GHB). Other frequently cited drug combinations included cocaine and marijuana, ecstasy and marijuana, LSD and marijuana, and cocaine and alcohol. These data highlight the need to develop drug health-education and prevention messages targeted at polydrug usage. [ABSTRACT FROM AUTHOR]

Background: 'Hard-to-reach' is a term used to describe those sub-groups of the population that may be difficult to reach or involve in research or public health programmes. Application of a single term to call these sub-sections of populations implies a homogeneity within distinct groups, which does not necessarily exist. Different sampling techniques were introduced so far to recruit hard-to-reach populations. In this article, we have reviewed a range of ap-proaches that have been used to widen participation in studies., Methods: We performed a Pubmed and Google search for relevant English language articles using the keywords and phrases: (hard-to-reach AND population* OR sampl*), (hidden AND population* OR sample*) and ("hard to reach" AND population* OR sample*) and a consul-tation of the retrieved articles' bibliographies to extract empirical evidence from publications that discussed or examined the use of sampling techniques to recruit hidden or hard-to-reach populations in health studies., Results: Reviewing the literature has identified a range of techniques to recruit hard-to-reach populations, including snowball sampling, respondent-driven sampling (RDS), indigenous field worker sampling (IFWS), facility-based sampling (FBS), targeted sampling (TS), time-location (space) sampling (TLS), conventional cluster sampling (CCS) and capture re-capture sampling (CR)., Conclusion: The degree of compliance with a study by a certain 'hard-to-reach' group de-pends on the characteristics of that group, recruitment technique used and the subject of inter-est. Irrespective of potential advantages or limitations of the recruitment techniques reviewed, their successful use depends mainly upon our knowledge about specific characteristics of the target populations. Thus in line with attempts to expand the current boundaries of our know-ledge about recruitment techniques in health studies and their applications in varying situa-tions, we should also focus on possibly all contributing factors which may have an impact on participation rate within a defined population group.